Materials Selection and Design (2010) Part 17 docx

Chandra, Concurrent Technologies Corporation Computer Prediction of Residual Stresses In recent years, the finite element method has become the preeminent method for computer predictio

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In the second case, P is increased from 0 to 534 kN (120 kips) and then brought back to zero The entire history of stresses in the three bars is shown in Fig 1(b) When P exceeds 400 kN (90 kips), the two outer bars deform plastically

and, because of the reduced modulus, begin to share less load The stress in the two outer bars follows the path ABCD,

whereas that in the middle bar follows the path ABEF It can be seen that when P is again zero (unloading), the stresses in

the three bars do not go back to zero Instead, the middle bar has a residual tensile stress of 78.8 MPa (11.4 ksi), and each

of the two outer bars has a residual compressive stress of 39.4 MPa (5.7 ksi) Because there is no external load on the assembly, the residual stresses in the three bars are in self-equilibrium A comparison of the two loading histories indicates that the presence of inhomogeneous plastic deformation in the three bars is responsible for the generation of residual stresses Similarly, mechanical residual stresses occur in any component when the distribution of plastic deformation in the material is inhomogeneous, such as the surface deformation in shot-peening operation

Thermal Loads. A similar three-bar model explaining the generation of residual stresses due to inhomogeneous plastic deformation caused by thermal loads is discussed by Masubuchi (Ref 6, presumably adopted from Ref 7) In this model, three carbon-steel bars of equal length and cross-sectional area are connected to two rigid blocks at their ends The middle bar is heated to 593 °C (1100 °F) and then cooled to room temperature, while the two outer bars are kept at room temperature Some of the details are not clearly explained in Ref 6, but the problem is very similar to the previous example When the temperature in the middle bar is raised, the requirements of compatibility and equilibrium imply that a compressive stress be generated in the middle bar and tensile stresses in the two outer bars; the stress in each of the two outer bars being half of that in the middle bar If the temperature in the middle bar is so high that its stress exceeds yield but in the two outer bars the stresses are still below yield, residual stresses will occur in the three bars when the temperature of the middle bar is brought back to room temperature (i.e., on unloading) Similarly, if the stresses in all three bars exceed yield but by different amounts, residual stresses will still occur when the temperature of the middle bar

is brought back to room temperature Indeed, this case is very similar to that of a cylinder immersed vertically in a quenchant where, during the initial stages of quenching, the temperature in the outer layer is much lower than that in the inner core

The three-bar model can be further utilized to explain the generation of residual stresses due to the mismatch in coefficients of thermal expansion For example, suppose the two outer bars represent the layers of matrix in a composite lamina and the inner bar represents a layer of fibers The coefficient of thermal expansion of the two outer bars is equal but, in general, different from that of the middle bar It is assumed that the initial temperature of all the three bars is equal, which corresponds to a certain processing temperature much higher than room temperature When the assembly is brought to room temperature, the requirements of compatibility and equilibrium will be satisfied if a system of forces (residual stresses) is established such that the sum of the forces in the two outer bars is equal and opposite to that in the middle bar In this case, the presence of unequal plastic deformation is not a prerequisite for the generation of residual stresses This explains why, while selecting the constituent materials for a composite or for a coating, the designers try to minimize the mismatch between their coefficients of thermal expansion

Solid-State Transformation. In quenching, welding, and casting processes, many metals such as steels undergo one

or more solid-state transformations These transformations are accompanied by a release of latent heat, a change in volume, and a pseudoplasticity effect (transformation plasticity) All of these affect the state of residual stresses in the part The release of latent heat during solid-state transformation is similar to that during the liquid-to-solid transformation, albeit of a smaller amount The change (increase) in volume occurs due to the difference in mass densities of the parent phase (e.g., austenite) and the decomposed phases (pearlite, ferrite, bainite, and martensite) In steels, the volumetric change due to phase transformation is in contrast to the normal contraction or shrinkage during cooling (Ref 8)

A simple example of transformation plasticity is shown in Fig 2, which is based on the results of a constrained dilatometry experiment (Ref 9) The figure shows that during cooling in the phase transformation regime, the presence of even a very low stress may result in residual plastic strains Two widely accepted mechanisms for transformation plasticity were developed by Greenwood and Johnson (Ref 10) and Magee (Ref 11) According to the former, the difference in volume between two coexisting phases in the presence of an external load generates microscopic plasticity

in the weaker phase This leads to macroscopic plastic flow, even if the external load is insufficient to cause plasticity on its own According to the Magee mechanism, if martensite transformation occurs under an external load, martensitic plates are formed with a preferred orientation affecting the overall shape of the body

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Fig 2 Transformation plasticity Source: Ref 9

Material Removal. A fact that is often overlooked in discussing residual stresses caused by various manufacturing processes is the effect of material removal on the state of stresses in the product Consider, for example, that a casting mold must be finally broken and removed, or a forging die must be retracted Likewise, in making a machined part some

of the material has to be removed All of these operations change the state of stress in the part In order to fully understand this concept, three examples discussed in Ref 5 should be considered

The first example entails an assembly of two concentric springs of slightly different lengths, Li and Lo, as shown in Fig 3(a); the subscripts i and o refer to inner and outer springs, respectively The bottom ends of the two springs are fixed Then, the upper ends are tied to a rigid block that is free to move only in the vertical direction The two springs adopt a

compromise length, L, which is in between Li and Lo, as shown in Fig 3(b) As a result, the two springs develop equal and opposite forces: compressive in the longer inner spring and tensile in the outer shorter spring The assembly of the two springs may be viewed as analogous to the assembly of a cast part and its mold or to the assembly of the forged part and the die, or to a machined part before some portion of it is removed Then, the removal of the outer spring becomes analogous to removal of material during machining (Ref 5, 12), of the casting mold (Ref 13, 14, 15, and 16), or of the forging die (Ref 17) Two cases are considered In the first case, the stresses in both springs are assumed to be within their elastic limits When the outer spring is removed, the force acting on it is transferred to the inner spring in order to satisfy equilibrium and the inner spring returns to its original length In the second case, it is assumed that the inner spring has undergone a certain amount of plastic deformation When the outer spring is removed, the inner spring does not return to

its original length, Li In either case, because the two springs and, therefore, the forces, are concentric, the residual stress

in the inner spring becomes zero when the outer spring is removed

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Fig 3 Residual stresses in an assembly of two springs with unequal initial lengths Source: Ref 5

For the second example, reconsider the three-bar model from the section "Mechanical Loads" in this article After creating residual stresses in the three bars by loading and unloading the assembly, bar 3 is removed, by (for example) machining As shown in Fig 4(a) and 4(b), a redistribution of stresses in the remaining two bars takes place The resultant stresses at the centroids of the two bars become -14.8 MPa (-2.14 ksi) in bar 1, and 14.8 MPa (2.14 ksi) in bar 2 Also, the assembly rotates (distorts) by an angle of 4.3 × 10-3 radians

Fig 4 Effect of asymmetric material removal in the three-bar model of Fig 1 Source: Ref 5

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The third example in Ref 5 is of a thick-walled cylinder with an internal diameter of 101.6 mm (4 in.) and an outer diameter of 203.2 mm (8 in.) as shown in Fig 5(a) Both ends of the cylinder are restrained axially, and the cylinder is subjected to an internal pressure A 25.4 mm (1 in.) thick (along the axis) slice of the cylinder is analyzed by subdividing

it into 10 equal finite elements (5.08 mm, or 0.2 in., thick each) in the radial direction (Fig 5b) The residual stresses are created by increasing the pressure from zero to 345 MPa (50 ksi), and then back to zero The elements 1 and 2 are removed successively The variation of the three stress components along the radius is shown in Fig 6, before material removal (i.e., the residual stresses) and after removing the two layers It may be noted that in an overall sense, the level of residual stresses goes down as the material is removed However, this is not necessarily true in a local sense Consider, for example, the circumferential stress at the centroids of elements 3 and 4 in Fig 6(b); it increases as the material is removed

Fig 5 Cylinder with internal pressure and its finite element mesh Source: Ref 5

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Fig 6 Effect of removing layers of elements of material from the inside of the cylinder Source: Ref 5

Important conclusions from the three examples discussed above can be summarized as follows:

• When the material removal is symmetric with respect to the stress distribution (Fig 3), the residual stresses in the remainder of the assembly or part are very small or even zero

• When the material removal is not symmetric with respect to the stress distribution (Fig 4, 6), the residual stresses in the remainder of the assembly or part are not necessarily small

• Material removal may result in an increase in stresses at some locations of the assembly or the part (Fig

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6)

References cited in this section

5 U Chandra, Validation of Finite Element Codes for Prediction of Machining Distortions in Forgings,

Commun Numer Meth Eng., Vol 9, 1993, p 463-473

6 K Masubuchi, Analysis of Welded Structures, Pergamon Press, 1980, p 94-96

7 W.M Wilson and C.C Hao, Residual Stresses in Welded Structures, Weld J., Vol 26 (No 5), Research

Supplement, 1974, p 295s-320s

8 W.K.C Jones and P.J Alberry, "The Role of Phase Transformation in the Development of Residual Welding Stresses," Central Electricity Generating Board, London, 1977

9 J.-B Leblond, G Mottet, J Devaux, and J.-C Devaux, "Mathematical Models of Anisothermal Phase

Transformations in Steels, and Predicted Plastic Behavior," Mater Sci Technol., Vol 1, 1985, p 815-822

10 G.W Greenwood and R.H Johnson, The Deformation of Metals under Small Stresses During Phase

Transformation, Proc Royal Soc., Vol 283, 1965, p 403-422

11 C.L Magee, "Transformation Kinetics, Microplasticity and Aging of Martensite in FE31 Ni," Ph.D Thesis, Carnegie Institute of Technology, 1966

12 U Chandra, S Rachakonda, and S Chandrasekharan, Total Quality Management of Forged Products

through Finite Element Simulation, Proc Third International SAMPE Metals and Metals Processing Conf.,

Vol 3, F.H Froes, W Wallace, R.A Cull, and E Struckholt, Ed., SAMPE International, 1992, p M393

M379-13 U Chandra, Computer Prediction of Hot Tears, Hot Cracks, Residual Stresses and Distortions in Precision

Castings: Basic Concepts and Approach, Proc Light Metals, J Evans, Ed., TMS, 1995, p 1107-1117

14 U Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput Model Simul Eng., Vol 1, 1996, p 127-174

15 U Chandra, R Thomas, and S Cheng, Shrinkage, Residual Stresses, and Distortions in Castings, Comput Model Simul Eng., Vol 1, 1996, p 369-383

16 A Ahmed and U Chandra, Prediction of Hot Tears, Residual Stresses and Distortions in Castings Including

the Effect of Creep, Comput Model Simul Eng., to be published

17 U Chandra, S Chandrasekharan, and R Thomas, "Finite Element Analysis of the Thread Rolling Process," Concurrent Technologies Corporation, submitted to Knolls Atomic Power Lab, Schenectady, NY, 1995

Control of Residual Stresses

U Chandra, Concurrent Technologies Corporation

Computer Prediction of Residual Stresses

In recent years, the finite element method has become the preeminent method for computer prediction of residual stresses caused by various manufacturing processes A transient, nonlinear, thermomechanical analysis software is generally employed for that purpose Some of the mathematics that form the basis of such software is common for all manufacturing processes Such common mathematics is summarized by this section However, because every process is unique, some mathematical requirements are, in turn, dependent on the process Also, for the simulation of certain processes a sequential thermomechanical analysis is adequate, whereas for others a coupled analysis may be preferred or even essential Such subtleties are pointed out later when individual processes are discussed

Ignoring convection, the following conduction heat-transfer equation is solved with appropriate initial and boundary conditions:

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(Eq 1)

where T is the temperature at an arbitrary location in the workpiece at time t, k is the thermal conductivity of the material,

c is the rate of heat generated per unit volume, is the density, Cp is the specific heat, and is the differential operator; all material properties are assumed to vary with temperature The term c accounts for the release of latent heat during liquid-to-solid transformation in casting and welding processes or during solid-state phase transformation in quenching, welding, or casting processes It also accounts for the heat of plastic deformation in forging and other bulk deformation processes The initial and boundary conditions are process dependent Details of converting Eq 1 into its finite element form and of numerical solution are available in a number of technical papers and textbooks and are not repeated here For

a general treatment of the subject, the reader is referred to Ref 18, 19, 20, and 21

The transient temperatures computed above are used as loading for the subsequent transient stress/displacement analysis Using the incremental theory, the total strain increment { } at time t can be divided into various components (Ref 22,

23, 24, 25, and 26):

{ } = { e} + { t} + { p} + { cr} + { v} + { tr} (Eq 2)

where superscripts e, t, p, cr, v, and tr refer to elastic, thermal, plastic, creep, volumetric change, and transformation plasticity components, respectively The first three strain terms are needed in the simulation of every manufacturing process discussed here, whereas the use of the other three terms is dependent on the process and are pointed out as appropriate Also, mathematical details for the first four strain terms are discussed in most standard references (Ref 22, 23), whereas the details for the last two terms are discussed often in the context of the simulation of quenching and welding processes (Ref 24, 25, 26)

In forging and other large deformation processes, the term c in Eq 1 represents the heat of plastic deformation and leads

to a coupling between Eq 1 and 2

At present, no single computer code is capable of predicting residual stresses caused by all manufacturing processes However, several general-purpose finite element codes are capable of predicting these stresses to a reasonable degree of accuracy for at least some of the manufacturing processes (Ref 27, 28, 29) In addition, some of these codes permit customized enhancements leading to more reliable results for a specific process Before attempting to predict residual stresses due to a manufacturing process, it is advisable to compare the capabilities of two or three leading codes and use the one most suited for the simulation of the process in consideration Examples of such comparisons are given in Ref 12 and 13 for forging, quenching, and casting processes It must be noted that, due to continuous enhancement in these codes, it is always advisable to compare the capabilities of their latest versions

18 G Comini, S Del Guidice, R.W Lewis, and O.C Zienkiewicz, Finite Element Solution of Non-linear Heat

Conduction Problems with Special Reference to Phase Change, Int J Numer Methods Eng., Vol 8, 1974, p

613-624

19 R.W Lewis, K Morgan, and R.H Gallagher, Finite Element Analysis of Solidification and Welding

Processes, ASME Numerical Modeling of Manufacturing Processes, PVP-PB-025, R.F Jones, Jr., H

Armen, and J.T Fong, Ed., American Society of Mechanical Engineers, 1977, p 67-80

20 A.B Shapiro, "TOPAZ A Finite Element Heat Conduction Code for Analyzing 2-D Solids," Lawrence

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Livermore National Laboratory, Livermore, CA, 1984

21 B.G Thomas, I Samarasekera, and J.K Brimacombe, Comparison of Numerical Modeling Techniques for

Complex, Two-Dimensional, Transient Heat-Conduction Problems, Metall Trans., Vol 15B, 1984, p

25 S Das, G Upadhya, and U Chandra, Prediction of Macro- and Micro-Residual Stress in Quenching Using

Phase Transformation Kinetics, Proc First International Conf Quenching and Control of Distortion, G.E

Totten, Ed., ASM International, 1992, p 229-234

26 S Das, G Upadhya, U Chandra, M.J Kleinosky, and M.L Tims, Finite Element Modeling of a

Single-Pass GMA Weldment, Proc Engineering Foundation Conference on Modeling of Casting, Welding and Advanced Solidification Processes VI, T.S Piwonka, V Voller, and L Katgerman, Ed., TMS, 1993, p 593-

600

27 "ABAQUS," Version 5.5, Hibbitt, Karlsson and Sorenson, Pawtucket, RI, 1995

28 "ANSYS," Release 5.3, ANSYS, Inc., Houston, PA, 1996

29 "MARC," Version K 6.2, MARC Analysis Research Corporation, Palo Alto, CA, 1996

Measurement of Residual Stresses

It is generally not possible to measure residual stresses in a product during its manufacture; instead, they are measured after the manufacturing process is complete Smith et al (Ref 30) have divided the residual stress measurement methods into two broad categories: mechanical and physical The mechanical category includes the stress-relaxation methods of layer removal, cutting, hole drilling, and trepanning, whereas the physical category includes x-ray diffraction (XRD), neutron diffraction, acoustic, and magnetic The layer-removal technique as originally proposed by Mesnager and Sachs (Ref 4) is only applicable to simple geometries such as a cylinder with no stress variation along its axis or circumference,

or to a plate with no variation along its length or width Thus, whereas it could be used to measure quench-induced residual stresses in a cylinder or a plate, it is not suitable for measuring complex stress patterns such as those caused by welding The layer removal and cutting techniques, however, have been applied to pipe welds in combination with conventional strain gages and XRD measurements The layer-removal technique is also used to measure residual stresses

in coatings

Hole-drilling and trepanning techniques can be used in situations where the stress variation is nonuniform, but they are generally restricted to stress levels of less than one-third of the material yield strength Also, these two techniques can be unreliable in areas of steep stress gradients and require extreme care while drilling a hole or ring in terms of its alignment

as well as the heat and stress generation during drilling (Ref 31) For such reasons, and others, these two techniques have found little application in the measurement of weld-induced residual stresses

Of the methods in the physical category, XRD is probably the most widely used method, the neutron diffraction method being relatively new These two methods measure changes in the dimensions of the lattice of the crystals, and from these measurements the components of strains and stress are computed The XRD technique has undergone many improvements in recent years With the development of small portable x-ray diffractometers, the technique can be used for on-site measurement of residual stresses It should be noted, however, that this technique is capable of measuring strains in only a shallow layer (approximately 0.0127 mm, or 0.0005 in., thick) at the specimen surface To measure subsurface residual stresses in a workpiece, thin layers of materials are successively removed and XRD measurements are

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made at each exposed layer For reasons discussed in the section "Material Removal" in this article, the measurements at

an inner layer should be corrected to account for the material removed in all the previous layers Reference 32 gives analytical expressions for such corrections in cases of simple geometries and stress distributions For more complex cases,

it still remains difficult to determine subsurface residual stresses accurately

In contrast to the x-rays, neutrons can penetrate deeper into the metals For example, in iron the relative depth of penetration at the 50% absorption thickness is about 2000 times greater for neutrons than for x-rays Only a few materials, such as cadmium and boron, absorb neutrons strongly However, to gain the advantage of greater penetration of neutrons requires the component to be transported to a high flux neutron source (Ref 30), which limits the use of the technique

4 R.G Treuting, J.J Lynch, H.B Wishart, and D.G Richards, Residual Stress Measurements, American

Residual Stresses Caused by Various Manufacturing Processes

Casting. In the past, little attention has been paid to the control of residual stresses in casting; much of the interest was focused on the prediction and control of porosity, misruns, and segregation A review of the transactions of the American Foundrymen's Society or of earlier textbooks on casting (e.g., Ref 33) reveals practically no information on the subject;

even the ASM Handbook on casting (Ref 34) provides little insight In a recent book, Campbell (Ref 35) has included a

brief discussion of residual stresses summarizing the work done by Dodd (Ref 36) with simple sand-mold castings Dodd studied the effect of two process parameters: mold strength, by changing water content of sand or by ramming to different levels, and casting temperature The conclusions of these costly experiments could have been more economically and easily arrived at by using the basic concepts discussed in the section "Fundamental Sources of Residual Stresses" and further amplified in the following paragraphs

When a casting is still in its mold, the stresses are caused by a combination of the mechanical constraints imposed by the mold, thermal gradients, and solid-state phase transformation Also, creep at elevated temperature affects these stresses Finally, when the casting is taken out of its mold, it experiences springback that modifies the residual stresses

As discussed in Ref 13, 14, 15, and 16, the computer prediction of residual stresses in castings requires a software that is capable of performing coupled transient nonlinear thermomechanical analysis (see the section "Computer Prediction of Residual Stresses" in this article) In addition, it should be able to account for the following:

• Release of latent heat during liquid-to-solid transformation, that is, in the mushy region

• Mechanical behavior of the cast metal in the mushy region

• Transfer of heat and forces at the mold-metal interface

• Creep at elevated temperatures under condition of varying stress

• Enclosure radiation at the mold surface to model the investment-casting process

• Mold withdrawal to model directional solidification

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• Mold (material) removal

The author and his coworkers have recently modified a commercial finite element code and have analyzed simple mold castings (Ref 15, 16) Computer simulation of these castings indicates that: (1) for an accurate prediction of transient and residual stresses, consideration of creep is important; creep is also found to make the stress distribution more uniform; and (2) just prior to mold removal the stresses in the casting can be extremely high, but after the mold removal they become very small (owing to the springback discussed in the section "Material Removal" ) except in the areas of stress concentration The residual stresses after the mold removal will not necessarily be small if the casting is complex and the mold removal is asymmetric with respect to the stress distribution Also, small variations in mold rigidity are not found to have any noticeable effect on residual stresses, which confirms the observations based on trial and error using green-sand molds with various water contents (Ref 35)

sand-Although very little work is published thus far on the subject of control of residual stresses in castings, finite element simulation methodology is now sufficiently advanced to enable the study of the effect of various process and design parameters on the residual stresses in castings, for example, superheat, stiffness and design of the mold, design of the feeding system and risers, and the design of the part itself Also, residual stresses caused by different casting practices such as sand-mold, permanent-mold, investment casting, and so forth, can be determined As the manufacturers and end-users of cast products become more aware of the status and benefits of the computer-simulation methodology, it can be expected to play a very important role in controlling residual stresses in complex industrial castings At present, the biggest limiting factor in the use of simulation is the lack of thermophysical and mechanical properties data for the cast metal and the mold materials

Forging. As with the casting process, little attention has been paid in the past to the control of residual stresses caused by forging; most of the interest was in predicting the filling and the direction of material flow Now, due to recent advances

in computer-simulation techniques, it is possible to predict and control the residual stresses in forged parts

Large plastic flow of the workpiece material is inherent in the forging process The material flow is influenced by a number of factors including the die shape and material, forging temperature, die speed, and lubrication at the die/workpiece interface Therefore, finite element simulation software used to predict and control residual stresses in the part should be capable of accounting for these factors Because a significant amount of energy is dissipated during forging

in the form of heat due to plastic deformation, a coupled thermomechanical analysis becomes necessary especially for nonisothermal forging Other factors contributing to the complexity of the finite element simulation of this class of problems are: temperature-dependent thermal and mechanical properties of the materials (especially for a nonisothermal forging); the choice of solution algorithm and remeshing due to large plastic deformation in the workpiece; and mathematical treatment of the die/workpiece interface that includes heat transfer, lubrication, and contact The last two terms in Eq 2 need not be considered in the simulation of the forging process

Finite element simulation of the forging process with simple geometries and of a two-dimensional idealization of the thread-rolling process (Ref 17) showed that, although the stresses in the workpiece are high during the deformation stage, the stresses after retraction of the die (residual stresses) are no longer high except in the regions of stress concentration Again, similar to the simulation of the casting processes, it is premature to generalize this conclusion but it is clear that the technique of computer simulation of forging and many other bulk deformation processes has advanced to a stage where it can assist in controlling the residual stresses in the part by performing a detailed parametric study with much less investment of time and capital than trial and error on the shop floor

Quenching involves heating of the workpiece to the heat treatment temperature followed by rapid cooling in a quenchant (e.g., air, water, oil, or salt bath) in order to impart the desired metallurgical and mechanical properties The choice of a quench medium is the key element; it should be such that it removes the heat fast enough to produce the desired microstructure, but not too fast to cause transient and residual stresses of excessive magnitude or of an adverse nature (e.g., tensile instead of compressive) The heat removal characteristic of a quenchant is known to be affected by a number of factors including the size, shape, orientation of the workpiece (even for simple shapes such as plates and cylinders, the heat removal is different at the bottom, top, and side surfaces); the use of trays and fixtures to hold the workpiece in the quenchant; composition of the quenchant; size of the pool and its stirring, and so forth (Ref 37, 38, 39) Additional difficulties arise when, due to economic reasons, quenching is performed in a batch process

In the past, using trial and error, shop-floor personnel have come up with some interesting strategies to control the residual stresses (and warpage), for example, air delay or an intentional delay while transporting the workpiece from the

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heating furnace to the quenchant, and time quenching or performing the quenching operation in two steps In the first step, the part is quenched in a medium such as a salt bath until the part has cooled below the nose of time-temperature transformation curve, followed by quenching in second medium such as air to slow the cooling rate Obviously, perfecting the quenching operation by trial and error can be an extremely time-consuming task

At first glance, computer simulation of the quenching process may appear to be simple It involves an uncoupled transient nonlinear small deformation thermomechanical analysis (as outlined in the section "Computer Prediction of Residual Stresses" in this article), with due consideration to solid-state transformation effects (Ref 9, 24, 25); creep is generally ignored However, the major difficulty lies (for reasons discussed in the preceding paragraph) in a lack of knowledge of the heat removal characteristic of various quenchants, which is mathematically represented as the convective heat transfer coefficient at the outer boundary of the workpiece Other difficulties arise due to the lack of thermophysical and mechanical properties of the workpiece material at elevated temperatures Still, at least in the United States, major aircraft engine manufacturers and their forging vendors have been using computer simulation to control quench-related cracking and residual stresses for some time One such example involving a turbine disk is discussed in Ref 40 The reported work was performed without the benefit of sophisticated simulation software that could account for solid-state transformation effects For proprietary reasons, few such cases are published in the open literature

Machining. Many complex parts in aerospace and other key industries are made by machining forgings, castings, bars,

or plates to their net shapes The presence of residual stresses in the workpiece affects its machinability and, on the other hand, the machining process also creates residual stresses and undesired distortions in the part and alters the already existing stress state In order to minimize or eliminate these adverse effects, machine-shop personnel often experiment with a number of process parameters, for example, depth of cut, speed of the cutting tool, and coolant For single-point turning, they frequently flip the workpiece in order to balance the distortions and stresses evenly on the two sides This trial and error is frequently combined with statistical process control

A serious problem associated with machining and residual stresses is often manifested in the form of part distortion For example, consider the example in Table 1 (Ref 12) The table shows the results of a dimensional check on 30 samples of

an aircraft engine part that was made by machining heat treated forgings procured from three different vendors (10 samples each) The location at which the dimensional check was performed is identified on the figure included in the table It was found that: (1) for all forgings from any one vendor, the drop was almost identical; (2) the drop in forgings from vendor B was within the specifications, but not so in the case of the other two vendors; and (3) the drop in forgings from vendors A and C was on the two opposite sides of that from vendor B It was recognized that all heat treated forgings contained residual stresses When these forgings were machined to net shapes, distortions occurred for two reasons: the release of residual stresses from the removed portion of the workpiece and the machining process itself The former can be easily modeled using the finite element method if the magnitude of residual stresses in the forgings prior to machining is known (Ref 5) However, to this author's knowledge, no serious attempt has so far been made to predict residual stresses in a workpiece due to the machining process itself If an attempt of this type is to provide reliable results,

it must take into account such factors as: depth of cut, speed of the cutting tool, interaction between the tool and the workpiece (heat and force), coolant, and clamping/unclamping of the workpiece It must also recognize the fact that the location of contact between the tool and the workpiece moves as the machining process progresses If such technology could be developed, it would become possible to predict and control the overall distortions and residual stresses in a part after machining, thereby reducing scrap

Table 1 Dimensional check of a machined part

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Welding. The residual stress distribution in welded joints depends on a number of process and design parameters such as the heat input, speed of the welding arc, preheat, thickness of the welded part, groove geometry, and weld schedule Welding engineers have long used trial and error to obtain a suitable combination of these parameters in order to control the residual stresses

The role of computer simulation in the prediction of residual stresses in weldments is the subject of a recent review (Ref 14) Major elements of computer simulation of the process are:

• Mathematical representation of the heat input from the welding source

• A transient thermal analysis

• A transient stress/displacement analysis; the flow of molten metal and thermal convection in the weld pool are generally ignored

Following Rosenthal (Ref 41), a semisteady state approach is often used, although some attempts at full dimensional analysis have also been made As mentioned earlier, it is now possible to account for volumetric change and transformation plasticity effects Because of the short time periods involved, creep is ignored In the case of a single-pass weld or a weld with few passes (e.g., four or five), it is now possible to predict residual stresses with reasonable accuracy But, as the number of passes increases (e.g., 20 or 30), it becomes computationally intractable to model each pass The scheme of lumping several passes into one layer has been employed with less than satisfactory results In addition to excessive computation time, other major difficulties with the simulation of a multipass weld are: the numerical errors tend

three-to accumulate with each pass, and the changes in metallurgical and mechanical properties of material in previously deposited layers during deposition of a subsequent layer are difficult to quantify and to account for in the finite element analysis The technique of lumping several layers together aggravates these problems

Between the mid 1970s and early 1980s, the Electric Power Research Institute (EPRI) in the United States sponsored a program to systematically study the effects of various process and geometric parameters such as the heat input, welding method (gas tungsten arc welding, submerged arc welding, laser, and plasma), speed of the welding arc, diameter and thickness of the pipe, and groove geometry, on residual stresses in pipe welds (Ref 42, 43, 44, 45, 46, 47, 48, 49, 50, and 51) In addition, various thermal processes such as heat-sink welding, backlay welding, and induction heat treatment were investigated to verify if the residual stresses on the inner surface of the pipe could be changed from tensile to compressive

to avoid intergranular stress-corrosion cracking Both experimental and finite element methods were used in the study The results of this effort are summarized in Ref 52

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A very interesting effort related to in-process control and reduction of residual stresses and distortions in weldments is being pursued (Ref 53, 54) The effort aims at moving beyond mere analysis of residual stresses and distortions to aggressively controlling and reducing them To accomplish this objective, the effort is subdivided into the development of the following three primary capabilities: prediction, sensing, and control For prediction purposes, a series of computer programs have been developed, include simple but fast one-dimensional programs that analyze only the most important stress component, that is, the one parallel to the weld line Sensing capability refers to a set of devices including a laser interferometer to measure minute amounts of distortions, a laser vision system to measure large amounts of distortions, and a mechanical system to measure radi of curvature Finally, to control the residual stresses, various techniques including changes in heating pattern and application of additional forces have been attempted References 53 and 54 provide further examples of the application of this methodology in reduction of residual stresses in weldments in high-strength steels and girth-welded pipes

Coating. Coatings are being used extensively in aerospace, marine, automobile, biomedical, electronics, and other industries For example, in modern jet aircraft engines, approximately 75% of all components are coated Some of the reasons for the application of coatings are: thermal barrier, wear resistance, corrosion resistance, oxidation protection, electrical resistance, and repair or dimensional restoration of worn parts A variety of methods are used for the deposition

of coatings on a substrate; the following discussion is limited primarily to the thermal spray process

The prediction of residual stresses in a coating/substrate system is in its infancy These stresses result from the difference

in the coefficient of thermal expansion of the coating and substrate materials and from plastic deformation of the substrate material The limited number of numerical studies conducted thus far have been related to small button-type specimens where the coating material was assumed to be fully molten and deposited instantaneously These efforts have ignored several important factors, for example:

• The presence of partially molten particles in the spray

• A nonuniform deposition of coating material normal to the axis of the plasma jet

• A liquid-to-solid and solid-state transformation

• Imperfect bond between the coating and the substrate

• The relative motion between the spray and the substrate

Also, because a layer of coating consists of several successive passes, the effect of any new pass on its adjacent previously deposited pass in terms of partial remelting, additional material buildup, solute diffusion, and redistribution of residual stresses could be important and should be accounted for If more than one layer is involved, for example, in functionally graded coatings, modeling the effect of a whole new layer of material on the previously deposited layer would be computationally prohibitive Also, due to the morphology of the coating material on deposition, its thermal and mechanical properties are extremely difficult to measure and, thus, are generally unavailable for simulation purposes Due

to such reasons, end-users of coated products still rely on the methods of trial and error and statistical process control for the selection of an optimal combination of process parameters in order to control the residual stresses in coated parts (Ref 55)

14 U Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput Model

Trang 14

Simul Eng., Vol 1, 1996, p 127-174

24 S Sjöström, Interactions and Constitutive Models for Calculating Quench Stresses in Steel, Mater Sci Technol, Vol 1, 1985, p 823-829

33 R.W Heine, C.R Loper, and P.C Rosenthal, Principles of Metal Casting, Tata McGraw-Hill, New Delhi,

India, 1976

34 Casting, Vol 15, ASM Handbook, (formerly Metals Handbook, 9th ed.), ASM International, 1988

35 J Campbell, Casting, Butterworth Heinmann, Oxford, U.K., 1991

36 R.A Dodd, Ph.D Thesis, Department of Industrial Metallurgy, University of Birmingham, U.K., 1950

37 H.E Boyer and P.R Cary, Quenching and Control of Distortion, ASM International, 1988, p 11

38 T.V Rajan, C.P Sharma, and A Sharma, Heat Treatment Principles and Techniques, Prentice-Hall of

India Private Ltd., 1988

39 S Segerberg and J Bodin, Variation in the Heat Transfer Coefficient around Components of Different

Shapes During Quenching, Proc First International Conf Quenching and Control of Distortion, G.E

40 R.A Wallis, N.M Bhathena, P.R Bhowal, and E.L Raymond, Application of Process Modelling to Heat

Treatment of Superalloys, Ind Heat., 1988, p 30-33

41 D Rosenthal, The Theory of Moving Sources of Heat and Its Application to Metal Treatments, Trans ASME, Nov 1946, p 849-866

42 R.M Chrenko, "Residual Stress Studies of Austenitic and Ferritic Steels," Conference on Residual Stresses

in Welded Construction and Their Effects, London, Nov 1977

43 R.M Chrenko, "Weld Residual Stress Measurements on Austenitic Stainless Steel Pipes," Lake George Conference, General Electric, 1978, p 195-205

44 W.A Ellingson and W.J Shack, Residual Stress Measurements on Multi-Pass Weldments of Stainless Steel

Piping, Exp Mech., Vol 19 (No 9), 1979, p 317-323

45 W.J Shack, W.A Ellingson, and L.E Pahis, "Measurement of Residual Stresses in Type-303 Stainless Steel Piping Butt Weldments," Report NP-1413, Electric Power Research Institute, June 1980

46 E.F Rybicki and P.A McGuire, "A Computational Model for Improving Weld Residual Stresses in Small Diameter Pipes by Induction Heating," 80-C2-PVP-152, Century 2 Pressure Vessels and Piping Conference, San Francisco, CA, Aug 1980, American Society of Mechanical Engineers

47 A.F Bush and F.J Kromer, Residual Stresses in a Shaft after Weld Repair and Subsequent Stress Relief,

Exp Tech., Vol 5 (No 2), 1981, p 6-12

48 F.W Brust and R.W Stonesifer, "Effect of Weld Parameters on Residual Stresses in BWR Piping Systems," Report NP-1743, Electric Power Research Institute, March 1981

49 F.W Brust and E.F Rybicki, A Computational Model of Backlay Welding for Controlling Residual

Stresses in Welded Pipes, J Pressure Vessel Technol (Trans ASME), Vol 103, 1981, p 226-232

50 R.M Chrenko, Thermal Modification of Welding Residual Stresses, Residual Stress and Stress Relaxation,

E Kula and V Weiss, Ed., Plenum Publishing, 1982, p 61-70

51 E.F Rybicki, P.A McGuire, E Merrick, and J Wert, The Effect of Pipe Thickness on Residual Stresses

due to Girth Welds, J Pressure Vessel Technol (Trans ASME), Vol 104, 1982, p 204-209

52 U Chandra, Determination of Residual Stresses due to Girth-Butt Welds in Pipes, J Pressure Vessel

Trang 15

Technol (Trans ASME), Vol 107, 1985, p 178-184

53 K Masubuchi, In-Process Control and Reduction of Residual Stresses and Distortion in Weldments, Proc Practical Applications of Residual Stress Technology, C.O Ruud, Ed., ASM International, 1991, p 95-101

54 K Masubuchi, Research Activities Examine Residual Stresses and Distortion in Welded Structures, Weld J., Dec 1991, p 41-47

55 R.V Hillery, Coatings Producibility, The Leading Edge, GE Aircraft Engines, Cincinnati, Ohio, Fall 1989,

p 4-9

Stress-Relief Methods

The basic premise of a stress-relief method is to produce rearrangement of atoms or molecules from their momentary equilibrium position (higher residual stress state) to more stable positions associated with lower potential energy or stress state These methods can be classified into three broad categories: thermal, mechanical, and chemical (Ref 4, p 134) The following concern the methods in the first two categories

Thermal stress-relief methods include annealing, aging, reheat treatment (e.g., postweld heat treatment), and others In general, a stress-relief operation involves heating the part to a certain temperature, holding at the elevated temperature for

a specified length of time, followed by cooling to room temperature Primary reduction in residual stresses takes place during the holding period due to creep and relaxation Thus, computer simulation of a thermal stress-relief method generally entails a thermal-elastic-plastic-creep analysis of the part A simple, one-dimensional computer analysis of residual stresses in thin plates along with experimental verification is discussed by Agapakis and Masubuchi (Ref 56) More sophisticated thermal-elastic-plastic-creep simulations of the annealing of single pass and multipass girth-butt welds

in pipes are presented in Ref 57 and 58

A number of subcategories of mechanical stress-relief methods are listed in Ref 4 Of these, the methods in the stressing subcategory such as stretching, upsetting, bending and straightening, and autofrettage are common, and these should not pose much difficulty in simulation by the finite element method Similarly, in the mechanical surface treatment subcategory, it should be possible to model the surface-rolling method However, within the same subcategory, shot peening (a frequently used stress-relief method) is likely to be difficult to simulate; and to this author's knowledge, no realistic attempt has yet been made to do so The obvious reason is that, whereas it should be possible to model a single impact, modeling multiple impacts will be difficult, just as it is for modeling multipass welding

static-In recent years, the method of vibratory stress relief (especially in the subresonant region) has received considerable attention (Ref 59, 60) The basic premise of this method is that the presence of residual stresses in a part changes (increases) its natural resonant frequency When the part is subjected to vibrations below its new frequency, the metal absorbs energy During this process, the stresses redistribute gradually and the resonant frequency shifts back to the point corresponding to a residual stress-free (or almost free) state The process does not change the metallurgical or mechanical properties of the material The technique has been found successful in relieving residual stresses induced by thermal processes such as welding and casting, but not those induced by cold working It has also been applied to reduce residual stresses in parts prior to machining in order to minimize distortions It has been found particularly beneficial in low- and medium-carbon steels, stainless steels, and aluminum alloys, but not in copper alloys In view of the fact that the technique is much simpler, quicker, and more inexpensive than the thermal-relief methods, it merits further study

4 R.G Treuting, J.J Lynch, H.B Wishart, and D.G Richards, Residual Stress Measurements, American

Society of Metals, 1952

56 J.E Agapakis and K Masubuchi, Analytical Modeling of Thermal Stress Relieving in Stainless and High

Trang 16

Strength Steel Weldments, Weld J Res Suppl., 1984, p 187s-196s

57 B.L Josefson, Residual Stresses and Their Redistribution During Annealing of a Girth-Butt Welded

Thin-Walled Pipe, J Pressure Vessel Technol (Trans ASME), Vol 104, 1982, p 245-250

58 B.L Josefson, Stress Redistribution During Annealing of a Multi-Pass Butt-Welded Pipe, J Pressure Vessel Technol (Trans ASME), Vol 105, 1983, p 165-170

59 R.A Claxton, Vibratory Stress Relieving Its Advantages and Limitations as an Alternative to Thermal

Treatments, Heat Treat Met., 1974, p 131-137

60 A.G Hebel, Jr., Subresonant Vibrations Relieve Residual Stress, Met Prog., Nov 1985, p 51-55

6 K Masubuchi, Analysis of Welded Structures, Pergamon Press, 1980, p 94-96

7 W.M Wilson and C.C Hao, Residual Stresses in Welded Structures, Weld J., Vol 26 (No 5), Research

Supplement, 1974, p 295s-320s

8 W.K.C Jones and P.J Alberry, "The Role of Phase Transformation in the Development of Residual Welding Stresses," Central Electricity Generating Board, London, 1977

10 G.W Greenwood and R.H Johnson, The Deformation of Metals under Small Stresses During Phase

Transformation, Proc Royal Soc., Vol 283, 1965, p 403-422

11 C.L Magee, "Transformation Kinetics, Microplasticity and Aging of Martensite in FE31 Ni," Ph.D Thesis, Carnegie Institute of Technology, 1966

14 U Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput Model Simul Eng., Vol 1, 1996, p 127-174

Trang 17

18 G Comini, S Del Guidice, R.W Lewis, and O.C Zienkiewicz, Finite Element Solution of Non-linear Heat

Conduction Problems with Special Reference to Phase Change, Int J Numer Methods Eng., Vol 8, 1974, p

613-624

19 R.W Lewis, K Morgan, and R.H Gallagher, Finite Element Analysis of Solidification and Welding

Processes, ASME Numerical Modeling of Manufacturing Processes, PVP-PB-025, R.F Jones, Jr., H

Armen, and J.T Fong, Ed., American Society of Mechanical Engineers, 1977, p 67-80

20 A.B Shapiro, "TOPAZ A Finite Element Heat Conduction Code for Analyzing 2-D Solids," Lawrence Livermore National Laboratory, Livermore, CA, 1984

21 B.G Thomas, I Samarasekera, and J.K Brimacombe, Comparison of Numerical Modeling Techniques for

Complex, Two-Dimensional, Transient Heat-Conduction Problems, Metall Trans., Vol 15B, 1984, p

26 S Das, G Upadhya, U Chandra, M.J Kleinosky, and M.L Tims, Finite Element Modeling of a

Single-Pass GMA Weldment, Proc Engineering Foundation Conference on Modeling of Casting, Welding and Advanced Solidification Processes VI, T.S Piwonka, V Voller, and L Katgerman, Ed., TMS, 1993, p 593-

600

27 "ABAQUS," Version 5.5, Hibbitt, Karlsson and Sorenson, Pawtucket, RI, 1995

28 "ANSYS," Release 5.3, ANSYS, Inc., Houston, PA, 1996

29 "MARC," Version K 6.2, MARC Analysis Research Corporation, Palo Alto, CA, 1996

30 D.J Smith, G.A Webster, and P.J Webster, Measurement of Residual Stress and the Effects of Prior Deformation Using the Neutron Diffraction Technique, The Welding Institute, Cambridge, UK, 1987

31 C.O Ruud, A Review of Nondestructive Methods for Residual Stress Measurement, J Met., Vol 33 (No

34 Casting, Vol 15, ASM Handbook, (formerly Metals Handbook, 9th ed.), ASM International, 1988

35 J Campbell, Casting, Butterworth Heinmann, Oxford, U.K., 1991

36 R.A Dodd, Ph.D Thesis, Department of Industrial Metallurgy, University of Birmingham, U.K., 1950

37 H.E Boyer and P.R Cary, Quenching and Control of Distortion, ASM International, 1988, p 11

38 T.V Rajan, C.P Sharma, and A Sharma, Heat Treatment Principles and Techniques, Prentice-Hall of

India Private Ltd., 1988

39 S Segerberg and J Bodin, Variation in the Heat Transfer Coefficient around Components of Different

Shapes During Quenching, Proc First International Conf Quenching and Control of Distortion, G.E

40 R.A Wallis, N.M Bhathena, P.R Bhowal, and E.L Raymond, Application of Process Modelling to Heat

Treatment of Superalloys, Ind Heat., 1988, p 30-33

41 D Rosenthal, The Theory of Moving Sources of Heat and Its Application to Metal Treatments, Trans

Trang 18

ASME, Nov 1946, p 849-866

42 R.M Chrenko, "Residual Stress Studies of Austenitic and Ferritic Steels," Conference on Residual Stresses

in Welded Construction and Their Effects, London, Nov 1977

43 R.M Chrenko, "Weld Residual Stress Measurements on Austenitic Stainless Steel Pipes," Lake George Conference, General Electric, 1978, p 195-205

44 W.A Ellingson and W.J Shack, Residual Stress Measurements on Multi-Pass Weldments of Stainless Steel

Piping, Exp Mech., Vol 19 (No 9), 1979, p 317-323

45 W.J Shack, W.A Ellingson, and L.E Pahis, "Measurement of Residual Stresses in Type-303 Stainless Steel Piping Butt Weldments," Report NP-1413, Electric Power Research Institute, June 1980

46 E.F Rybicki and P.A McGuire, "A Computational Model for Improving Weld Residual Stresses in Small Diameter Pipes by Induction Heating," 80-C2-PVP-152, Century 2 Pressure Vessels and Piping Conference, San Francisco, CA, Aug 1980, American Society of Mechanical Engineers

47 A.F Bush and F.J Kromer, Residual Stresses in a Shaft after Weld Repair and Subsequent Stress Relief,

Exp Tech., Vol 5 (No 2), 1981, p 6-12

48 F.W Brust and R.W Stonesifer, "Effect of Weld Parameters on Residual Stresses in BWR Piping Systems," Report NP-1743, Electric Power Research Institute, March 1981

49 F.W Brust and E.F Rybicki, A Computational Model of Backlay Welding for Controlling Residual

Stresses in Welded Pipes, J Pressure Vessel Technol (Trans ASME), Vol 103, 1981, p 226-232

50 R.M Chrenko, Thermal Modification of Welding Residual Stresses, Residual Stress and Stress Relaxation,

E Kula and V Weiss, Ed., Plenum Publishing, 1982, p 61-70

51 E.F Rybicki, P.A McGuire, E Merrick, and J Wert, The Effect of Pipe Thickness on Residual Stresses

due to Girth Welds, J Pressure Vessel Technol (Trans ASME), Vol 104, 1982, p 204-209

52 U Chandra, Determination of Residual Stresses due to Girth-Butt Welds in Pipes, J Pressure Vessel Technol (Trans ASME), Vol 107, 1985, p 178-184

53 K Masubuchi, In-Process Control and Reduction of Residual Stresses and Distortion in Weldments, Proc Practical Applications of Residual Stress Technology, C.O Ruud, Ed., ASM International, 1991, p 95-101

54 K Masubuchi, Research Activities Examine Residual Stresses and Distortion in Welded Structures, Weld J., Dec 1991, p 41-47

55 R.V Hillery, Coatings Producibility, The Leading Edge, GE Aircraft Engines, Cincinnati, Ohio, Fall 1989,

p 4-9

56 J.E Agapakis and K Masubuchi, Analytical Modeling of Thermal Stress Relieving in Stainless and High

Strength Steel Weldments, Weld J Res Suppl., 1984, p 187s-196s

57 B.L Josefson, Residual Stresses and Their Redistribution During Annealing of a Girth-Butt Welded

Thin-Walled Pipe, J Pressure Vessel Technol (Trans ASME), Vol 104, 1982, p 245-250

58 B.L Josefson, Stress Redistribution During Annealing of a Multi-Pass Butt-Welded Pipe, J Pressure Vessel Technol (Trans ASME), Vol 105, 1983, p 165-170

59 R.A Claxton, Vibratory Stress Relieving Its Advantages and Limitations as an Alternative to Thermal

Treatments, Heat Treat Met., 1974, p 131-137

Design for Surface Finishing

Eric W Brooman, Concurrent Technologies Corporation

Introduction

AN AXIOM among surface-finishing industry professionals is that the quality of a finish is only as good as the quality of the substrate and its pretreatment That is, to obtain a finish of high quality, meeting all performance specifications and

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customer expectations, great care has to be taken in identifying and using the appropriate pretreatment The latter could be grinding, heating, buffing, cleaning, or a number of other processes to prepare the surface properly Although not articulated in quite the same way, the surface-finishing industry also recognizes that the design of the part (component or assembly) can have a significant influence on the ability to use satisfactory pretreatments and obtain quality finishes The overall part design, and the design of surface features and their size, can have an impact not only on the choice of pretreatments and finishes, but also on the efficacy of these processes and the results obtained This article provides some guidelines about general design principles for different types of surface-finishing processes, which include cleaning, organic coatings, and inorganic coatings applied by a variety of techniques

Many of the guidelines discussed here apply equally as well to other articles in this section and vice versa Therefore, although what is presented here is a fairly comprehensive summary of the topic of design for surface finishing, useful information can be found in other articles such as "Design for Machining" and "Design for Heat Treatment." As is stressed elsewhere and discussed in this article, design must be considered an integral part of the overall manufacturing process and cannot be considered in isolation

Definitions and more detailed descriptions of the processes discussed in this article can be found in Surface Engineering, Volume 5 of ASM Handbook

Design as an Integral Part of Manufacturing

In recent years, manufacturing processes have been evaluated in terms of life-cycle costs and their impact on the environment, and even ecology in general Several scenarios have been proposed for the life cycle of materials, which of necessity incorporates manufacturing processes In the article "Introduction to Manufacturing and Design" in this Volume, an example of one such scheme is given where a material flows through the extraction, refining (preparation), shapemaking and structural treatments (manufacturing), and surfacing (surface-finishing) stages before being assembled and placed in use Manufacturing also can be viewed as part of an "integrated product- and process-development" process, also described in that article This approach places emphasis on "product design" and "process design" in a concurrent-engineering environment and forces product developers to consider both simultaneously, rather than sequentially and separately (Ref 1) If these concepts are extrapolated, with a focus on design in relation to surface finishing, the iterative process shown in Fig 1 results The scheme presented in this figure is entirely consistent with another modern manufacturing concept, namely that of continuous improvement

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Fig 1 Flow diagram for incorporating design principles into surface-finishing operations

Having stressed the importance of considering design precepts as an integral part of the manufacturing process and product improvement, the following guidelines should be considered as just that guidelines Each application should be treated on an individual basis, and surface-finishing processes should be flexible enough to permit design improvements, and vice versa In addition, it should be obvious from Fig 1 that the earlier in the product-development cycle manufacturing-related design considerations are addressed, the more efficient will be the manufacturing process and the better the quality of the surface finish will be Communication with and input from the manufacturing and surface-finishing staff are very important in establishing a satisfactory product design

Design of the part or component and pretreatment selections are important, but in keeping with the concept of design being an integral part of manufacturing and surface finishing, process equipment design restrictions and fixturing design also are very important Both can influence the quality of the resulting finish Figure 2 shows their interrelated roles schematically In this overview, emphasis is placed on issues relating to part design and process equipment Fixturing must be designed and tailored for each individual application and is beyond the scope of this article

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Fig 2 Interrelation between part design, equipment limitations, and fixturing

Reference cited in this section

1 H.A Kuhn, Concurrent Technologies Corp., Johnstown, PA, personal communication, 1996

General Design Principles Related to Surface Finishing

There are a number of general design principles that apply to a variety of finishing processes, while others are specific to individual finishing techniques These general principles are discussed in this section, and the following three sections discuss design aspects relating to: (1) surface-preparation techniques, including cleaning; (2) organic finishing techniques; and (3) inorganic finishing techniques References 2 and 3 provide some background material on the various finishing techniques discussed, while Tables 1, 2, and 3 summarize the important design limitations for these three categories The subject of materials selection is covered elsewhere in this Volume The choice of materials can limit the choices for surface finishing Where appropriate, significant limitations are described

Table 1 Summary of design limitations for selected surface-preparation processes

Process Design limitations

Avoid recesses, holes, channels, and similar features (such as closely spaced ribs) that could trap blasting media

Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the blasting media

Blasting/deburring

Avoid intricate designs and surface features

Broaching/honing Typically used for inside diameters of tubes and other cylindrical parts, or for grooves, large holes, and

other cavities

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Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness

Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness that could deflect

Avoid sharp corners and edges

Brushing/burnishing

Avoid intricate designs and surface features Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air or evolved gases and prevent chemical action from occurring or cause uneven attack

Chemical milling

Mask areas not to be attacked Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining

Conversion coating

Mask areas not to be attacked Allow for electrical contact to be made on nonsignificant surfaces Avoid features that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air or evolved gases and prevent polishing action from occurring or cause staining

Electropolishing

Mask areas not to be attacked Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent etching action from occurring

Avoid sharp corners and edges Avoid shallow intricate designs and surface features

Etching

Mask areas not to be attacked Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness

Avoid sharp corners, edges, and protuberances

Grinding

Avoid intricate designs and surface features Surfaces must be accessible to tools (preferably flat or simple, curved contours Avoid very thin cross sections/wall thickness that cannot withstand the local pressure and heat buildup Avoid sharp corners and edges

Lapping/buffing

Avoid intricate designs and surface features that would trap the lapping/buffing compounds Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent pickling action

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent cleaning from occurring

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Avoid features (e.g., small recesses, blind holes, cavities) that would trap smut and process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating removal from occurring

Stripping, chemical

Mask areas not to be attacked Avoid recesses, holes, channels, and similar features that could trap blasting media Avoid thin cross sections or intricate designs that could be damaged by the stripping media

Table 2 Summary of design limitations for selected organic finishing processes

Process Design limitations

Allow for electrical contact to be made on nonsignificant surfaces Avoid features that could trap air and prevent wetting by process solutions Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Painting, solvent spraying

Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Allow for fixturing/racking on nonsignificant surfaces

Allow for electrical contact to be made on nonsignificant surfaces Avoid deep recesses and blind holes that cause the "Faraday cage" effect

Solution coating

Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle

Table 3 Summary of design limitations for selected inorganic finishing processes

Allow for electrical contact to be made on nonsignificant surfaces Avoid, if possible, sharp edges and corners, ridges, blind holes, etc., that would prevent uniform density distribution

Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Avoid features that could trap air and prevent electrochemical reactions from occurring Avoid features that could trap evolved gases and cause staining

Anodizing

Mask areas not to be anodized

Cementation/diffusion Surfaces must be thoroughly deburred and cleaned before cladding, so design principles for these

processes also apply

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Avoid thin cross sections or intricate designs that could become distorted during thermal cycling Mask areas not to be coated

Only for relatively simple shapes, especially with flat surfaces

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining

Electrophoretic plating

Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces Avoid, if possible, sharp edges and corners, ridges, blind holes, etc., that would prevent uniform current density distribution; or use current robbers and/or shields

Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Avoid features that could trap air and prevent deposition from occurring Avoid features that could trap evolved gases and cause staining

Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by internal stress in the coating

Electroplating (plating,

electrodeposition)

Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces for discrete, small parts Best for relatively simple shapes (e.g., tubing) and flat surfaces

Allow for excess coating material to drain quickly Allow for doctor blades or air knives to be used to obtain uniform coating thickness

Hot dipping, galvanizing

Avoid thin cross sections that could become distorted during thermal cycling Surfaces must be accessible (preferably flat or simple, curved contours) Allow for fixturing/racking on nonsignificant surfaces

Avoid features that would trap excess paint Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Avoid features that could trap air and prevent coating from occurring Avoid thin cross sections or intricate designs that could become distorted during drying/fusing cycle

Inorganic painting, slurry

coating

Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen Avoid features that would shield the surface from the beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam

Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach the bottom surfaces

Ion implantation

Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen Avoid features that would shield the surface from the beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam

Ion plating

Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid features that would shield the surface from the laser beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam

Avoid thin cross sections or intricate designs that could be damaged by local heating during glazing

Laser glazing

Mask areas not to be treated

Mechanical (peen) plating Allow for fixturing/racking on nonsignificant surfaces on large parts

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Avoid features that could trap air and prevent activation by the process chemicals from occurring Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover of activating solutions

Avoid recesses, holes, channels, and similar features that could trap peening media Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the peening action Avoid sharp edges and corners that could be damaged by the peening media

Avoid intricate designs and small surface features that cannot be reached by the peening media Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover

Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing

Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining

Passivation

Mask areas not to be attacked Allow for fixturing/racking on nonsignificant surfaces Design should allow for surface roughening to promote adhesion, so blasting design precepts also apply

Avoid features that would shield the surface from the spray (line-of-sight limited) unless multiple sprays are used or part is rotated/translated in spray plume

Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the spray to reach the bottom surfaces

Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the local heating and kinetic energy

Thermal spraying

Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by heating, if needed prior to coating deposition

Vacuum processes are line-of-sight limited, so similar design precepts to those for ion plating will apply

Vapor deposition (CVD, PVD,

RVD)

Mask areas not to be coated

(a) CVD, chemical vapor deposition; PVD, physical vapor deposition; RVD, reactive vapor deposition

Fabrication Processes. Some methods of fabrication such as the forging, extrusion, molding, and casting of metals and ceramics can lead to surface defects that must be removed by subsequent surface-finishing techniques, such as grinding, lapping, and polishing or electropolishing, or hidden by techniques such as applying a leveling copper deposit before a decorative plated finish Defects include laps, tears, cracks, pores, shrinkage cavities, gating and venting residues, ejection marks, and parting lines Careful design of the casting or molding operation including the dies, gates, vents, and overflows will minimize finishing problems by ensuring such defects are avoided, occur on nonsignificant surfaces, or are hidden by specially incorporated design features, such as steps or ridges at parting lines

When polymeric materials are being cast, molded, extruded, or formed, it is especially important that the design and tooling lend themselves to producing an acceptable surface finish because any finishing operation that removes surface layers could expose porosity and other defects and remove aesthetic qualities such as smoothness and luster Also, the selection of finishing tools and conditions is much more critical because of the physical properties (e.g., softness, plasticity) of the polymeric materials and the potential for damage (e.g., heat distortion) caused by heat generated during the finishing operation This topic is discussed more fully in Ref 4 As with metals and ceramics, some undesirable attributes of the fabrication process, such as parting lines, can be hidden by added design features

Whatever the type of material being cast or molded, dimensional and warpage allowances must be made in the design of the tooling (i.e., dies) to accommodate shrinkage and distortion during solidification and cooling Otherwise, parts may be undersized or require excessive machining to obtain the specified dimensional tolerances

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Control of fastening or joining processes also can influence surface finishing For example, two flat surfaces riveted together produce cavities that can entrap processing solutions, impair coating, and lead to corrosion (Ref 5) Spot or tack welding is no better in this regard However, a continuous weld with a smooth bead and no weld spatter will prevent this problem and make surface finishing easier Also, the elimination of sharp edges and corners will prolong the life of grinding, polishing, and buffing belts and wheels

Size and Weight. The design, dimensions, and weight of a part to be surface finished have a direct influence on fixturing and the size and type of equipment that is used As Ref 6 points out, size has little to do with production rate Large parts do not necessarily mean low production rates, but they will dictate the size of the cleaning tanks, spray booths, plating tanks, vacuum chambers, and the like Nevertheless, the end-use application often dictates the size of a part or component and materials selection will dictate its weight Consequently, as a variable, only design is left to influence ease of finishing, efficacy of finishing, and finishing cost Whatever can be done in terms of optimizing a design

to facilitate surface finishing will have a positive impact In this context, design optimization includes facilitating the handling of parts between the various surface-finishing steps, as well as during the processes themselves These operations will be most efficient when the parts are designed to prevent "nesting" or flat surfaces from sticking together (because of surface tension effects) and preventing those surfaces from being treated or coated Similarly, small parts may

be light in weight and unstable in sprays during pretreatment or painting The design of these parts should be such that clips or magnets can be used to secure and stabilize them during processing

Aesthetics and Function. Another general consideration is that not all surfaces may require the same high standard of surface finish While surfaces exposed to view must be aesthetically pleasing, and surfaces subjected to more aggressive conditions of exposure or use require durable coatings, hidden (internal) surfaces or less-exposed surfaces may not need such a high-quality finish Specifications for surface finishes for a part depend not only on the design and end-use application, but also must take into account that the requirements may differ for different areas or surfaces on that part A design should take this into consideration, as well as the fact that different types of equipment or equipment operation settings may be necessary for those areas and surfaces

Functional requirements of a part also influence the selection of surface preparation processes For example, grinding processes can introduce stresses that could have a negative impact on fatigue properties Choosing an alternative process, such as chemical milling, or mitigating the stresses by shot peening can alleviate the problem

Design Features. Shape and features such as recesses, holes, threads, keyways, slots, fins, and louvers can present problems to the finisher, and the severity of the problem can depend on the finishing technique For example, when holes are included in thin sections that require a finishing operation such as grinding, if too much pressure is applied edges and corners might be chipped If only a light pressure is used to avoid this possibility, then the desired finish might not be obtained Another example is when paint is applied by conventional solvent spraying or when a part is electroplated, bowl-shaped recesses, blind holes, and similar features can trap the paint or plating solution, leading to areas that sag or

do not cure properly (in the case of paint), or carry over trapped chemicals to subsequent processing steps (in electroplating) The latter can cause problems such as rinse-water contamination and increased waste-treatment costs Also, solutions that are trapped can lead to blistering or delamination of the plated coating, especially if there is a posttreatment step that requires the part to be heated (such as for electroless nickel, cadmium, and hard chromium deposition)

For parts that will be sprayed, especially with paint, another problem with deep recesses, closely spaced, large fins or partitions, and the like is the entrapment of air The back pressure of entrapped air causes incomplete coverage at the bottom of the recesses One way to avoid this problem, if a change of design is not possible, is to use an "airless" spraying technique (Ref 6) During electrostatic powder coating there is the problem associated with "Faraday cage" effect, in which the charged components of the powder-coating system are attracted by the high fields at the edges and corners of parts, causing excessive coverage there and incomplete coverage in other areas (Ref 7), as shown in Fig 3 Rounding corners and edges, tapering the sides and decreasing the depth of recesses, minimizing the use of louvers or fins, or changing their dimensions are ways to avoid the Faraday cage effect In electroplating, a similar phenomenon exists whereby the depositing metal or alloy ions are attracted to the high-current-density areas at edges and corners, and thicker coatings are obtained in those locations Rounding such edges, changing dimensions to allow for the excessive buildup, or using shields and current "robbers" or "thieves" will help the finisher to obtain the desired coating thickness distribution Reference 8 provides some examples of the use of such devices

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Fig 3 Faraday cage effect in powder coating Adapted from Ref 7

In conventional paint spraying and many vacuum-deposition techniques, such as ion plating, ion implantation, physical vapor deposition, and sputtering, attention has to be paid to the limitations imposed by the "line-of-sight" deposition process Certain features, such as ridges, flanges, and fins, can shadow or mask areas behind them leading to incomplete

or nonuniform coverage, as shown in Fig 4 and 5 Similarly, if the aspect ratio of holes and recesses is too high (i.e., the depth is much greater than the diameter of the opening), it is not possible with line-of-sight limited techniques to penetrate to the bottom surfaces and coat them (Fig 5) Decreasing the aspect ratio, providing rounded edges, and tapering the sides of ridges and fins or holes will help to facilitate finishing, as will lowering the height of features such as fins Of course, rotating or translating a part in the spray plume also will help to obtain complete and more uniform coverage, but this approach usually requires longer times and more sophisticated finishing equipment and fixturing; hence, it often leads to higher costs The same can be said for using multiple line-of-sight sources to obtain better coverage

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Fig 4 Some examples of line-of-sight limitations in spraying or ion-beam coating processes

Fig 5 Design features that cause shadowing or poor coverage because of line-of-sight limitations

Finally, as a general rule of thumb, parts of the same size, weight, design, and material should always be finished at the same time so that the finishing process(es) can be optimized for those parts Batches of mixed parts should be avoided unless they share some common features, such as shape and substrate material

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2 M.F Browning et al., "Applying Inorganic Coatings: A Vital Technology for Industry," Report 38, Battelle Technical Inputs to Planning, Battelle, Columbus, OH, 1983

3 E.W Brooman, "Plating, Finishing and Coating: State-of-the-Art Assessment," Report EM-4569, Electric Power Research Institute, 1969

4 C.A Harper, Ed., Handbook of Plastics and Elastomers, McGraw-Hill, 1975, Chap 12

5 M Henthorne, Corrosion Causes and Control, Chemical Engineering Series, McGraw-Hill, 1972, Part 7

6 D.L Stauffer, Ed., Finishing Systems Design and Implementation, Society of Manufacturing Engineers,

1993, Chap 1

7 S Guskov, Faraday Cage, Finish Quality, and Recoating: New Technology for More Effective Powder

Coating, Powder Coat., 1996, p 82-91

8 W.H Safranek and E.W Brooman, Finishing and Electroplating Die Cast and Wrought Zinc, Zinc Institute,

1973, Chap 7

Surface-Preparation Processes

To facilitate surface preparation including cleaning prior to subsequent coating operations, design precepts should have been considered during the product-design and manufacturing stages, as indicated in Fig 1 Abrupt changes in surface contours should be avoided, and features such as fine grooves, recesses, surface patterning, blind holes, and reentrant areas should be avoided because they will be inaccessible to polishing media or would trap polishing media, making subsequent cleaning more difficult Such features also would entrap cleaning chemicals, making rinsing more difficult, or could possibly entrap air, preventing cleaning of these areas

Sharp corners and edges or protrusions can cause excessive wear of polishing wheels and belts and lead to uneven polishing because the high areas are polished at the expense of the surrounding lower areas As mentioned earlier, rounding edges and corners is a good design precept, while minimizing the height of protuberances is beneficial, as is decreasing the aspect ratio of holes, grooves, and recesses

Large expanses of flat surfaces may be a problem if these are significant surfaces, especially if these surfaces must be polished to a reflective, mirrorlike finish Imperfections are exaggerated Minimizing the area of such surfaces and providing a slightly rounded contour will help to attain the desired finish and help with visual appearance

Simpler designs lend themselves to automatic finishing processes, while more complex designs may require manual surface-preparation techniques If parts are to be mass finished (e.g., by tumbling or vibratory finishing) significant flat areas should be avoided Otherwise, parts may stick together, and these occluded surfaces will not be finished Designs that prevent access by the deburring or polishing media (such as small recesses and holes) or that entrap the media (such

as narrowly spaced ribs) should be avoided as mentioned above

When it is impractical or impossible to use mechanical polishing, chemical etching, chemical milling, or electropolishing can be used The design principles for the latter are similar to those for electroplating, which is discussed later In electropolishing, the workpiece is the anode, which is the opposite of electroplating Current density distribution is extremely important, as is the original surface of the part being electropolished In high-current-density areas on susceptible materials, the surface layers may be removed and etching of the substrate can occur Polishing occurs on a microscopic scale, so macro features such as large grooves or scratch marks will not be removed but will receive a luster and become more noticeable Similarly, parting lines can be smoothed, but not removed; therefore, parting lines must be minimized by good die design and careful molding operations

Solvent cleaning is a fairly forgiving surface-finishing process, but part design can influence its efficacy, as already alluded to If agitation or other cleaning aids are used, such as ultrasonic energy, care must be taken to prevent soft materials or thin and fragile features or cross sections from being damaged The energy released during cavitation, for

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example, in ultrasonic cleaning is very large If techniques such as plastic media blasting are used, the blasting parameters should be tailored to the part material and design, and the part should be designed to allow easy access by the media and easy removal of the media once the desired finish (cleanliness) is obtained

If a power spray washing technique is used, the part design should allow for proper drainage to conserve chemicals and minimize carryover to the next process step Providing drainage holes may be necessary These should be either a natural feature of the design or located on nonsignificant surfaces As the design of a part becomes more complex, rinsing requirements become more stringent, and several rinsing stages may be necessary If an air knife is used afterward to remove excess water, the part must be capable of withstanding the pressure or must be fixtured such that the air pressure does not distort any delicate design features while holding the part steady

Table 1 provides a summary of the design limitations of some surface-preparation and cleaning processes and indicates which design features to avoid

Organic Finishing Processes

Organic finishes are applied by a variety of techniques, such as dipping, brushing, spraying, airless spraying, or electrostatic spraying In addition, some primers are deposited using electrophoretic techniques, while electropolymerization is being looked at for certain types of organic coatings Table 2 summarizes these techniques and the design limitations associated with each

Most of the techniques are line-of-sight limited, and the guidelines provided in the previous section, "Surface-Preparation Processes," will apply Allowance for drainage is important for processes that involve dripping or spraying Avoiding sags and runs on large, flat, vertical surfaces can be accomplished by applying good coating practices and by minimizing such surfaces in the design of the part

A few organic coating techniques use electric or electrostatic fields Designing the fixtures and electrical grounding, such that points of contact are on nonsignificant surfaces, will improve the appearance of the coated part and give the impression of a better quality product With spraying techniques, proper fixturing and racking of parts can improve the use of coating material because less empty space exists during a run However, the parts should not be racked so closely together that they shield some surfaces and prevent some areas from being coated

Avoiding thin cross sections and good fixturing will help prevent distortion during the curing and baking steps used after paint or powder is applied

Optimizing a design for surface finishing, such as painting, becomes very important as coating thickness is reduced to 30

m or less Access to all surfaces must be possible and any features that would prevent this should be avoided (see the section "Design Features" in this article) This is because the dimensions of the solid components in the coating formulation (e.g., powder particle) are similar to the dimensions of the desired dry film thickness (Ref 9) For example, during the first part of curing, when the particles liquefy, the surface tension of the film formed will tend to pull it away from sharp corners or edges, resulting in poor coverage If a design modification is not possible, the powder formulation should be changed to include higher-viscosity resins, and no, or only small amounts, of surfactants (Ref 9) Thin-film coatings are best applied to parts with simple geometries, with flat or curved surfaces, and few sharp edges

Earlier, the problem with the Faraday cage effect was mentioned This phenomenon is further complicated by ionization with traditional corona-charging systems (Ref 7) Not only does the design of a recess, hole, or channel control the distribution of coating thickness, but the buildup of back-ionization at the areas of high field intensity lowers the effective charge of the powder particles, further reducing their ability to reach the bottom surfaces Some possible design modifications were mentioned earlier but, if these are not possible, changing to a turbocharging system will help Back-ionization is greatly reduced, and the absence of free ions between the gun and the part promotes better coverage of all surfaces (Ref 7)

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back-References cited in this section

9 B Fawer, Thin-Film Powder Coatings: Design and Application Issues, Powder Coat., Vol 7 (No 7), 1996, p

56-63

Inorganic Finishing Processes

Inorganic finishes including metal- and ceramic-based coatings are applied by a variety of techniques, such as electroplating, electroless plating, thermal spraying, hot dipping, ion plating, and various vapor-deposition techniques Other techniques, such as ion implantation and laser glazing, modify surface properties Table 3 summarizes design limitations for these and other types of inorganic coating processes

Electroplating is widely used in industry to apply inorganic coatings, especially metals and alloys Like some organic finishing processes, satisfactory coatings are only obtained when a uniform current density can be established on all surfaces to be finished Phenomena like the Faraday cage effect occur when design features prevent the establishment of a uniform current density distribution As mentioned earlier, techniques relating to fixturing and racking can alleviate some

of the problems General design approaches are discussed in Ref 8 and 10 and summarized in Table 4

Table 4 Design features that influence electroplating

Convex surface Ideal shape Easy to plate to uniform thickness, especially where

edges are rounded

Flat surface Not as desirable as crowned surface Use 0.015 mm/mm (0.015

in./in.) crown to hide undulations caused by uneven buffing

Sharply angled edge Undesirable Reduced thickness of plate at center areas

Requires increased plating time for depositing minimum thickness of durable plate All edges should be rounded Edges that contact painted surfaces should have a 0.8 mm ( in.) min radius

Flange Large flange with sharp inside angles should be avoided to minimize

plating costs Use generous radius on inside angles and taper abutment

Slots Narrow, closely spaced slots and holes cannot be plated properly with some

metals (e.g., nickel and chromium) unless corners are rounded

Blind hole Must usually be exempted from minimum thickness requirements

Sharply angled indentation Increases plating time and cost for attaining a

specified minimum thickness and reduces the durability of the plated part

Flat-bottom groove Inside and outside angles should be rounded generously to

minimize plating costs

V-shaped groove Deep grooves cannot be plated satisfactorily; should be

avoided Shallow, rounded grooves are better

Fins Increase plating time and costs for attaining a specified minimum thickness

and reduce the durability of the plated part

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Ribs Narrow ribs with sharp angles usually reduce platability; wide ribs with

rounded edges impose no problem Taper each rib from its center to both sides and round off edges Increase spacing, if possible

Deep scoop Increases time and cost for plating specified minimum thickness

Spearlike jut Buildup on jut robs corners of electroplate Crown base and round

all corners

Ring Platability depends on dimensions Round corners; crown from center line,

sloping toward both sides

Note: Distribution of electroplate on design shapes is intentionally exaggerated by solid black outline Cross-hatched areas indicate part before plating

With the plating of fasteners, some special considerations apply, particularly in respect to threads (Ref 10) As might be expected, electroplated metals build up faster on apexes of the threads, and coverage can be minimal at the bottom of the grooves ANSI Specification B 1.1 states that, compared to flat surfaces, plating thickness builds up six times faster on the major diameter than the minor diameter and that this results in a fourfold buildup on the pitch diameter, as illustrated in Fig 6 This is known as the "Rule of Four and Six."

Fig 6 Rule of Four and Six as applied to coating external threads Source: Ref 10

Similarly, plating inside holes can be difficult The general rule of thumb is that if the hole diameter is x, the plating will occur to a depth of 2x However, for blind holes, plating will only occur to a depth of x Agitation, solution flow,

maximizing the throwing power of the plating bath, and other aids can improve the situation somewhat, but the best approach is to eliminate or minimize holes with high aspect ratios during the product-design stage

In plasma-coating processes, the part design will have considerable influence over the operating parameters of the coating-deposition equipment Complex shapes, blind holes, fins, slots, and similar features will dictate that a high vacuum pressure, low part temperature, and light plasma density be used (Ref 11) The converse will be true for simple geometries In plasma processing, consideration also must be given to heating of the part by the plasma itself Some design features with thin cross sections and low mass, such as fins, louvers, and bosses will heat up faster than the bulk material in the part For parts that have been heat treated, or otherwise finished to provide desirable mechanical properties, overheating could destroy those properties or at least change the values detrimentally Reference 11 provides some examples of process and equipment modifications to avoid such problems during plasma nitriding Reference 12 discusses the effect of part geometry on the growth of the nitride layer during ion nitriding and how coating uniformity can be improved for grooved surfaces As in electroplating, decreasing the aspect ratio (depth of groove to width of groove) has a positive effect

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8 W.H Safranek and E.W Brooman, Finishing and Electroplating Die Cast and Wrought Zinc, Zinc

Institute, 1973, Chap 7

10 L.W Flott, Quality Control: Becoming a Better Customer, Met Finish., Vol 94 (No 2), 1996, p 79-82

11 R Gunn, Industrial Advances for Plasma Nitriding, Ion Nitriding and Ion Carburizing, T Spalvins and

W.L Kovacs, Ed., ASM International, 1990, p 157-163

12 M.J Park et al., Effect of Geometry on Growth of Nitride Layer in Ion Nitriding, Ion Nitriding and Ion Carburizing, T Spalvins and W.L Kovacs, Ed., ASM International, 1990, p 203-209

Conclusions

From the above discussion, it may seem that design for surface finishing is an art rather than a science Recently there has been a trend to modeling some of the coating processes in order to predict coating composition or distribution, and these models can be useful in avoiding bad design features However, most of the principles in use and described in the article are derived from good common sense and have resulted from the cumulative experience gained by the finishing industry over many years

Some finishing processes may impose their own special design limitations, but for many the general precepts outlined above should provide some guidance Table 5 groups together the surface-treatment techniques discussed and a few others not included here because of their similarity to these processes that have similar design limitations associated with them It cannot be stressed enough that product design is not a stand-alone activity, but must be integrated with other manufacturing operations to provide the requisite surface finish quality

Table 5 Surface-finishing technologies grouped by similar design limitations

Allow for fixturing/racking on nonsignificant

surfaces

Painting, powder coating, sol-gel coating, solution coating, electroplating, electroless plating, anodizing, electrocoating, electropolymerization, electrophoretic plating, hot dipping, ion implantation, laser glazing, passivation, thermal spraying, electric arc spraying

Allow for electrical contact to be made on

nonsignificant surfaces or use a conductive screen

Elecroplating, electropolishing, anodizing, electrocleaning, electrocoating, electropolymerization, electrophoretic plating, ion implantation, ion plating, powder coating

Surfaces must be accessible (preferably flat or

simple, curved contours)

Grinding, polishing, lapping, burnishing, buffing, painting, electroplating, electroless plating, ion vapor deposition, cladding, ion implantation, hot dipping, slurry coating, laser glazing, mechanical plating, thermal spraying, electric arc spraying

Avoid intricate designs and surface features Blasting, peening, brushing, burnishing, polishing, lapping, buffing, grinding,

mechanical plating, hot dipping, cladding, cementation, slurry coating, mechanical plating, thermal spraying, electric arc spraying

Surfaces must be accessible to tools and withstand

the local pressure and heat build up

Broaching, honing, grinding, brushing, burnishing, polishing, lapping

Avoid sharp corners, edges, and protuberances to

prolong tool life

Blasting, lapping, brushing, burnishing, polishing, buffing, grinding

Allow for surface roughening to promote

adhesion

Thermal spraying, electric arc spraying, sputtering, ion vapor deposition, ion plating, mechanical plating

Avoid thin cross sections (such as fins, louvers,

walls) that could be distorted by heating or kinetic

energy

Brushing, burnishing, grinding, lapping, solution coating, polishing, thermal stripping, hot dipping, cementation, laser glazing, mechanical plating, thermal spraying, electric arc spraying, diffusion coating

Avoid features that would shield the surface from

beams or sprays (line-of-sight limited) unless

Chemical cleaning, chemical stripping, ion implantation, sputtering, ion plating, laser glazing, thermal spraying, electric arc spraying

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multiple sources are used, or part is

rotated/translated in beam or spray

Avoid high aspect ratio holes and recesses,

grooves, etc., that would not allow beams or

sprays to reach bottom surfaces

Spray painting, ion plating, ion implantation, chemical cleaning, sputtering, laser glazing, thermal spraying, electric arc spraying

Avoid features that could trap air and prevent

surface chemical reactions or coating from

occurring or cause staining

Chemical milling, chemical cleaning, chemical stripping, etching, pickling, electroplating, spray painting, hot dipping, diffusion coating, electrocoating, sol- gel coating, solution coating, anodizing, electroless plating, electrophoretic plating, chemical vapor deposition, passivation

Avoid features (e.g., small recesses, blind holes,

cavities) that would trap process chemicals or

prevent satisfactory rinsing

Chemical milling, chemical cleaning, pickling, electrocleaning, chemical stripping, etching, electroplating, electroless plating, anodizing, mechanical plating, dipping (painting), conversion coating, solution coating, passivation, electrophoretic plating, electropolishing, sol-gel coating, electrocoating

Avoid recesses, holes, channels, and similar

features that could trap materials

Blasting, peening, grinding, polishing, lapping, mechanical plating, thermal spraying, electric arc spraying

Provide good drainage or use drainage holes on

nonsignificant surfaces to minimize carryover of

Avoid, if possible, sharp edges and corners,

ridges, blind holes, etc., that would prevent

uniform coating thickness or metal removal

Chemical milling, etching, electropolishing, pickling, chemical stripping, sol-gel coating, solution coating, hot dipping, ion implantation, ion plating, thermal spraying, electric arc spraying

Avoid features that could trap evolved gases and

cause staining or uneven attack

Chemical milling, etching, chemical cleaning, chemical stripping, electropolishing, pickling, electroplating, electrocleaning, electrocoating, anodizing, electroless plating, electrophoretic plating, passivation

References

1 H.A Kuhn, Concurrent Technologies Corp., Johnstown, PA, personal communication, 1996

2 M.F Browning et al., "Applying Inorganic Coatings: A Vital Technology for Industry," Report 38, Battelle Technical Inputs to Planning, Battelle, Columbus, OH, 1983

3 E.W Brooman, "Plating, Finishing and Coating: State-of-the-Art Assessment," Report EM-4569, Electric Power Research Institute, 1969

4 C.A Harper, Ed., Handbook of Plastics and Elastomers, McGraw-Hill, 1975, Chap 12

5 M Henthorne, Corrosion Causes and Control, Chemical Engineering Series, McGraw-Hill, 1972, Part 7

6 D.L Stauffer, Ed., Finishing Systems Design and Implementation, Society of Manufacturing Engineers,

1993, Chap 1

8 W.H Safranek and E.W Brooman, Finishing and Electroplating Die Cast and Wrought Zinc, Zinc

Institute, 1973, Chap 7

9 B Fawer, Thin-Film Powder Coatings: Design and Application Issues, Powder Coat., Vol 7 (No 7), 1996,

p 56-63

10 L.W Flott, Quality Control: Becoming a Better Customer, Met Finish., Vol 94 (No 2), 1996, p 79-82

11 R Gunn, Industrial Advances for Plasma Nitriding, Ion Nitriding and Ion Carburizing, T Spalvins and

W.L Kovacs, Ed., ASM International, 1990, p 157-163

12 M.J Park et al., Effect of Geometry on Growth of Nitride Layer in Ion Nitriding, Ion Nitriding and Ion Carburizing, T Spalvins and W.L Kovacs, Ed., ASM International, 1990, p 203-209

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Glossary of Terms

o A

• accuracy

• In measurement, the lack of deviation of a reading from a known input A voltmeter that reads 10

V when subjected to 10 V is accurate Accuracy is related to and governed by statistical bias

• activity-based costing (ABC)

• A cost-accounting approach that assumes that products incur costs by the activities they require for design, manufacture, and marketing To implement ABC one must identify the major activities and their cost drivers, for example, hours of engineering services, number of production setups, etc This is in contrast to conventional cost accounting where overhead costs are allocated solely through hours of direct labor or machine time

• advanced ceramics

• Ceramic materials that exhibit superior mechanical properties, corrosion/oxidation resistance, or electrical, optical, and/or magnetic properties This term includes many monolithic ceramics as well as particulate-, whisker-, and fiber-reinforced glass, glass-ceramics, and ceramic-matrix composites Also known as engineering, fine, or technical ceramics

• The characteristics of exhibiting different values of a property in different directions with respect

to a fixed reference system in the material

• assembly

• A collection of two or more parts The assembly process is a series of joining processes (either permanent or nonpermanent) in which parts are oriented and added to the build It is a mass-increasing process in which the macrogeometry is established by the positioning of the components

• austempering

• A heat treatment for ferrous alloys in which a part is quenched from the austenitizing temperature

at a rate fast enough to avoid formation of ferrite or pearlite and then held at a temperature just above Ms until transformation to bainite is complete Although designated as bainite in both austempered steel and austempered ductile iron, austempered steel consists of two-phase mixtures containing ferrite and carbide, while austempered ductile iron consists of two-phase mixtures containing ferrite and austenite

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• availability

• In reliability theory, a measure of the degree to which an item is in operable and commitable state

at the start of a mission, when the mission is called for at an unknown (random) time

• B

• bake-hardening steels

• Very-low carbon, fully ferritic steels (0.001% C) that are very formable, but harden during the paint-curing cycle The strengthening results from the precipitation of titanium/niobium carbonitrides at 175 °C (350 °F)

• blow molding

• A method of fabricating plastics in which a warm plastic parison (hollow tube) is placed between the two halves of a mold (cavity) and forced to assume the shape of that mold cavity by use of air pressure The air pressure is introduced through the inside of the parison and forces the plastic against the surface of the mold, which defines the shape of the product

• blueprint

• The common shop term for a detailed engineering drawing giving dimensions and manufacturing details for a part The term comes from an early printing process that gave a drawing with white lines on a blue background

• brazing

• A group of welding processes that join solid materials together by heating them to a suitable temperature and using a filler metal having a liquidus above 450 °C (840 °F) and below the solidus of the base materials The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction

• break-even analysis

• An economic analysis in which costs are determined as a function of units of output or volume of production The break-even point (minimum batch size) occurs at the number of units for which the revenues equal the total costs

• break-in

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• The early period of operating an engineering system, during which early equipment failures may

be detected With electronic systems this is usually known as burn-in The break-in process is useful for weeding out design and manufacturing defects

• Forming processes, such as extrusion, forging, rolling, and drawing, in which the input material is

in billet, rod, or slab form and a considerable increase in surface-to-volume ratio in the formed part occurs under the action of largely compressive loading Compare with sheet forming

• An abbreviation for computer-aided design/computer-aided manufacturing

• Cartesian coordinate system

• A coordinate system in which the position of a point in a plane is determined by its distance and

direction from the x and y axis, which are perpendicular to each other In three dimensions a Cartesian coordinate system is defined by the x, y, and z axes

• casting

• (1) Metal object cast to the required shape by pouring of injecting liquid metal into a mold, as distinct from one shaped by a mechanical process (2) Pouring molten metal into a mold to produce an object of desired shape (3) Ceramic forming process in which a body slip is introduced into a porous mold, which absorbs sufficient water from the slip to produce a semirigid circle See also polymer casting

• central tendency of data

• In statistics this is described by the central limit theorem This theorem states the fact that if one

draws samples of size n from the population, and calculates the mean of these samples, the means

will form a distribution which tends toward normality regardless of the form of the original sample distribution

• ceramic(s)

• Any of a class of inorganic nonmetallic products that are subjected to a high temperature during manufacture or use (high temperature usually means a temperature above a barely visible red, approximately 540 °C, or 1000 °F) Typically, but not exclusively, a ceramic is a metallic oxide, boride, carbide, or nitride, or a mixture of compound of such materials; that is, it includes anions that play important roles in atomic structures and properties See also advanced ceramics

• Charpy test

• An impact test in which a V-notched, keyhole-notched, or U-notched specimen, supported at both ends, is struck behind the notch by a strike mounted at the lower end of a bar that can swing as a pendulum The energy that is absorbed in fracture is calculated from the height to which the striker would have risen had there been no specimen and the height to which it actually rises after fracture of the specimen Contrast with Izod test

• checklist

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• A tool used to ensure that all important steps or actions in an operation have been taken Checklists contain those items that are important to the situation

• Chvorinov's rule

• See casting modulus

• coefficient of thermal expansion

• (1) Change in unit of length (or volume) accompanying a unit change of temperature, at a specified temperature (2) The linear or volume expansion of a given material per degree rise of temperature, expressed at an arbitrary base temperature or as a more complicated equation applicable to a wide range of temperature

• coherent precipitate

• A crystalline precipitate that forms from solid solution with an orientation that maintains continuity between the crystal lattice of the precipitate and the lattice of the matrix, usually accompanied by some strain in both lattices Because the lattices fit at the interface between precipitate and matrix, there is no discernible phase boundary

• composite structure

• A structural member (such as a panel, plate, pipe, or other shape) that is built up by bonding together two or more distinct components, each of which may be made of a metal, alloy, nonmetal, or composite material Examples of composite structures include: honeycomb panels, clad plate, electrical contacts, sleeve bearings, carbide-tipped drills or lathe tools, and weldments constructed of two or more different alloys

• compression molding

• A technique of thermoset molding in which the plastic molding compound (generally preheated)

is placed in the heated open mold cavity, the mold is closed under pressure (usually in a hydraulic press), causing the material to flow and completely fill the cavity, and then pressure is held until the material has cured

• computational fluid dynamics (CFD)

• An area of computer-aided engineering devoted to the numerical solution and visualization of fluid-flow problems

• computer-aided design (CAD)

• Any design activity that involves the effective use of the computer to create or modify an engineering design Often used synonymously with the more general term computer-aided engineering (CAE)

• computer-aided materials selection system

• A computerized database of materials properties operated on by an appropriate knowledge base of decision rules through an expert system to select the most appropriate materials for an application See also knowledge base

• computer-aided process planning (CAPP) program

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• A computer program that extracts the relevant geometric features from a CAD drawing and produces a process sequence or set of tool paths (for machining) that is optimal with respect to some criterion such as minimum processing time

• computer modeling

• The use of computers to simulate a physical system Computers perform the numerical analysis and often graphically display the results

• concept selection method

• A group process of selecting a design concept or material in which the selection criteria and concepts are arranged in matrix form, and for each criterion the concepts are compared one at a time to a datum (reference) concept Often called the Pugh concept selection method, after its originator, Stuart Pugh

• conceptual design

• The initial stage of the engineering design process in which a physical concept of the product is developed The physical concept includes the principles by which the product will work and an abstract physical embodiment that will employ the principles to achieve the desired functions

• concurrent engineering

• A style of product design and development that is done by concurrently utilizing all of the relevant information in making each decision It replaces a sequential approach to product development in which one type of information was predominant in making each sequential decision Concurrent engineering is carried out by a multifunctional team that integrates the specialties or functions needed to solve the problem Sometimes called simultaneous engineering

• configuration design

• The stage after conceptual design in the engineering design process in which the features of a part and their arrangement and connectivity are determined Qualitative reasoning based on fundamental principles is used to make decisions between alternatives A sketch of the part and preliminary decisions on material selection and manufacturing methods are made in this stage Final dimensions and tolerances are not determined in this stage Also known as embodiment design

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• crack opening displacement (COD)

• On a KIc specimen, the opening displacement of the notched surfaces at the notch and in the direction perpendicular to the plane of the notch and the crack The displacement at the tip is called the crack tip opening displacement (CTOD); at the mouth, it is called the crack mouth

opening displacement (CMOD) See also stress-intensity factor for definition of KIc

• creative process

• A total problem resolution process that yields a truly novel solution to the problem A creative solution requires both vertical thinking (convergent thinking) and lateral thinking (divergent thinking)

• creep

• Time-dependent strain occurring under stress The creep strain occurring at a diminishing rate is called primary creep; that occurring at a minimum and almost constant rate, secondary creep; and that occurring at an accelerating rate, tertiary creep

• cumulative distribution function (CDF)

• A frequency distribution arranged to give the number of observations that are less than given values 100% of the observations will be less than the largest class interval of the observations

• customer

• The recipient or beneficiary of the output of an organization's work effort or the purchaser of a product or service A customer may be either external or internal to the organization A chief objective of total quality management is to exceed customer expectations, each and every time

• customer delight

• The result of delivering a product or service that exceeds customer expectations

• customer importance rating

• An indication of the priority of importance the customer places on a certain want Determined by dividing the number of times customers mention this need by the number of subjects interviewed

• The ability of a computer-aided engineering system to share design information among a variety

of computer-based applications (such as design, drafting, and numerical-controlled machining) without each application having to translate or transfer the data Associativity also requires that

Tiêu đề	Materials Selection and Design (2010) Part 17
Trường học	Unknown University
Chuyên ngành	Materials Science and Engineering
Thể loại	lecture notes
Năm xuất bản	2010
Thành phố	Unknown City

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