Handbook of Lubrication part 9 ppsx

A., Ed., Metal Deformation Processes: Friction and Lubrication, Marcel Dekker, New York, 1970... Both maximum and average pressures are a function of the d/h ratio Figures 2b andc.Stress

FIGURE 2 Interface pressures in upsetting a cylinder with (a) no friction, (b) high friction, and (c) high friction

and larger d/h ratio (From Schey, J A., Introduction to Manufacturing Processes, McGraw-Hill, New York,

1977 With permission.)

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FIGURE 1 Examples of the variation of frictional stress with normal pressure, (a) Variation of shear stress; (b) coef- ficient of friction; and (c) interface shear strength factor.

(From Schey, J A., Ed., Metal Deformation Processes:

Friction and Lubrication, Marcel Dekker, New York, 1970.

With permission.)

where x is the distance from the edge, d is workpiece diameter, and h is the workpieceheight Both maximum and average pressures are a function of the d/h ratio (Figures 2b andc).

Stresses set limitations: interface pressures cause an elastic deformation of the tool, ofsignificant proportions when precise forgings are to be made; maximum die pressure mayexceed the pressure rating of the tooling; total force required may be too high for any press

or hammer of reasonable size All these limitations are a function of τi and of processgeometry, characterized by the d/h ratio For this reason, forging of relatively thin workpiecescan become extremely difficult unless friction is kept very low with a suitable lubricant.Friction also affects the deformation process With low friction, resistance to sliding atthe interface results in barrelling (Figure 2b) When friction is high enough to immobilizepart of the end face, deformation becomes highly inhomogeneous, and some of the end face

is actually formed by a folding over of the original side surfaces (Figure 2c) Barreling andfolding over generate tensile stresses on the barrel surface These “secondary tensile stresses”may lead to surface cracking in moderately ductile materials

Impression Die Forging

When forging in shaped dies, a flash is generated which contributes to die filling bypreventing the free escape of material from the die cavity Therefore, high friction in the

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FIGURE 3. Interface pressure in upsetting a flat, rectangular workpiece with friction (From Schey, J A.,

Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977 With permission.)

FIGURE 6. Geometry of roll pass and associated velocities (From Schey, J A., Ed., Metal Deformation

Processes: Friction and Lubrication, Marcel Dekker, New York, 1970 With permission.)

Since the same material volume must enter and leave the rolls in unit time, the product

of velocities and slab thicknesses must be the same everywhere in the absence of spread.The slab moves with the rolls at the neutral point (Figure 6) but moves slower (backwardslip) towards the entry and faster (forward slip) towards the exit Since the position of theneutral point is governed by friction in the roll gap, forward slip sfis a very sensitive measure

of the efficiency of lubricants With decreasing friction, forward slip diminishes:

(5)

where the neutral angle depends on µ in the following relation:

(6)

Interface pressures reach a maximum at the neutral point, and the friction hill is similar

to that observed in the compression of slabs (Figure 3) When µp = σfsticking sets in andthe friction hill becomes rounded

Rolling of very thin strips presents difficulties because roll flattening (broken lines inFigure 6) becomes commensurate with strip thickness Interface pressures can be reducedwith small-diameter rolls and application of front and back tensions to the strip; even so,good lubrication is indispensible Some minimum friction is still needed, otherwise the stripmay skid in the rolls and lateral strip movement becomes difficult to control

Wire, Bar, and Tube Drawing

As the product is pulled through, deformation is attained by compressive stresses at thestationary die (Figure 7) Most drawing operations are performed on round wire (Figure7a), although shaped wire and bar are also drawn All sliding is unidirectional, and the drawforce is opposed by frictional stresses at the interface The draw stress is

(7)

If the strength of the drawn product is insufficient to carry the draw force, the product will

be torn off This sets a limit of 30% or less reduction in cross-sectional area per pass, evenwith a good lubricant Interface pressures are always below the compressive flow strength

of the material

As in forging, a large h/L ratio increases inhomogeneity of deformation Since this ratioincreases and friction forces decrease with increasing die angles, an optimum angle exists

at which draw force is a minimum Inhomogeneous deformation results in greater elongation

of the surface layers, thus putting the center of the wire in tension These secondary tensilestresses, combined with the drawing stresses, can lead to internal fracture (centerburst orarrowhead defect) in materials of limited ductility Friction promotes this defect by increasingthe drawing stresses

In tube drawing without an internal die (Figure 7b), frictional conditions are the same as

in wire drawing More frequently, however, a short plug (British) or mandrel (U.S.) controlsthe internal diameter (Figure 7c) Friction on this plug increases drawing stresses, thusreducing the maximum attainable reduction However, when the internal die element is along mandrel (British) or bar (U.S.), this die element moves together with the drawn product(Figure 7d) and some of the drawing stresses are transmitted to it by interface friction.Higher friction on the internal surface actually increases the attainable reduction

322 CRC Handbook of Lubrication

a flat face and the material, in seeking a minimum-energy path, shears at some angledetermined by the extrusion ratio Material behind the shear surface forms a dead-metalzone (Figure 8c).

To minimize friction in reverse extrusion of tubes, the container is kept as short as possibleand a short land is formed on the punch (Figure 8d) Lubricant between the workpiece andpunch end face must be gradually metered out to protect the freshly formed, highly extendedsurfaces The extrusion ratio and die pressures diminish as the punch diameter decreases.With a small diameter, however, the process changes to indentation and punch pressurescan never be less than p = 3σf As with all inhomogeneous deformation, lubrication isrelatively ineffective in reducing punch pressure; nevertheless it is still desirable to preventmetal pickup and punch wear

A special case is hydrostatic extrusion in which the extrusion pressure is supplied by ahigh-pressure fluid; container friction is eliminated and die friction reduced but the cycletime is long

As in wire drawing, a high h/L ratio can lead to centerburst defects In contrast to drawing,friction increases the pressure in the deformation zone, reduces secondary tensile stresses,and delays the onset of the defect

Sheet Metalworking

Sheet metaiworking is always a secondary process on previously rolled flat products such

as sheet, strip and plate The first such operation is usually shearing (or slitting, blanking,

or punching) Separation of adjacent metal parts occurs through highly localized plasticdeformation followed by shear failure (Figure 9a) and seems unaffected by friction Never-theless, lubrication is necessary to protect against rapid wear and die pickup Many partsare formed by bending, and bending forces are affected by friction when the workpieceslides over some die element (Figure 9b)

Friction becomes extremely important when shapes of three-dimensional geometry areformed through stretching, deep drawing, or their combination In pure stretching the sheet

is firmly clamped (Figures 10a and b) In the absence of friction, thinning is most severeand fracture occurs at the apex of the stretched part With increasing friction, free thinningover the punch nose is hindered and the fracture point moves further down the side of thepart

In deep drawing (Figure 11), blanks of large diameter-to-sheet thickness ratio wouldbuckle (wrinkle) and must be kept flat by applying pressure through a blankholder Frictionalstresses increase the force required for deformation, and when the force exceeds the strength

of the partially drawn product, fracture occurs Friction must be minimized to reach themaximum possible draw

Cups of large depth-to-diameter ratio must be produced with several redrawing steps.When the cup wall is to be reduced substantially, the drawn cup is pushed through an ironingdie, and wall thickness reduction takes place under high normal pressures and severe sliding,calling for a much heavier-duty lubricant Friction on the punch surface is again beneficial.Many sheet metal parts in the automotive and appliance industries are of complex shapesproduced by simultaneous stretching and drawing Lubrication is essential to prevent diepickup and surface damage To restrict free drawing-in of the sheet, a draw bead is incor-porated (Figure 10b); this imposes severe conditions on the lubricant

LUBRICATING MECHANISMS

Lubricants in deformation processes have to survive under an extremely wide range ofconditions Interface pressures range from a fraction of flow stress σfup to multiples of σf(up to 4 GPa or 500 kpsi) Sliding velocities range from zero (at sticking friction) to 50m/s (10,000 fpm) sometimes combined with approach velocities of up to 20 m/s (66 ft/sec)

324 CRC Handbook of Lubrication

Layer-lattice compounds such as MoS2and graphite are effective if deposited in a continousfilm Their shear strength is also pressure-sensitive In common with other solid films, theycannot protect surfaces if the lubricant film breaks down.

Boundary Films

Very thin films formed on the die or workpiece may prevent adhesion and also reducefriction Extreme-pressure (EP) compounds rely on reactions that take place only if sufficienttime is available at temperature and if substrate composition is favorable Contact time undermetalworking conditions is usually too brief, but if contact is repeated (as in cold heading,wire drawing, sheet metal working, and sometimes rolling), reaction is possible with thedie

Boundary films form almost instantaneously and are among the most important lubricantsfor reactive metals, particularly aluminum, copper, and to a lesser extent, steel Their shearstrength is pressure- and temperature-dependent Breakdown at elevated temperatures limitsthem to cold working

Full-Fluid Film Lubrication

In this regime, tool and workpiece surfaces are separated by a liquid film of sufficientthickness to avoid asperity interaction Plastohydrodynamic theory can account for the effects

of process geometry, sliding speed, and lubricant properties in maintaining such a film Thepressure and temperature sensitivity of viscosity must be considered, together with thepossibility of the lubricant becoming a polymer-like solid at higher pressures (Figure 12)

con-Surface Roughness Effects

Although asperities piercing through the lubricant film can create adhesion problems, tooland workpiece surfaces need not always be very smooth To avoid skidding in rolling, theroll surface is kept somewhat rough A moderately coarse, nondirectional (such as bead-blasted) surface finish is desirable in maintaining graphite or MoS2 supply in hot forging

A smooth, polished die surface is, however, desirable for liquid lubricants; any remainingroughness is preferably oriented in the direction of material flow

Moderate roughness of the workpiece surface helps carry liquid lubricants into the face, especially if the roughness is perpendicular to the direction of feeding If sliding ismultidirectional, a random (bead-blasted) finish is preferable (as on automotive body sheets)

inter-Lubricating Regimes

The range for various lubricating mechanisms is summarized in Figure 13 For an interfacepressure of p = σf, friction cannot attain a value higher than that corresponding to sticking.The presence of a solid film (other than metal) reduces the coefficient of friction to about0.05 to 0.1, apparently irrespective of the nature of solid film With a liquid, mixed lubri-cation is attained once velocity and viscosity combine to sustain the pressures required forplastic deformation With increasing velocity and/or viscosity, the proportion of surface arealubricated by the fluid film increases and the coefficient of friction drops to typically 0.03

to 0.05 True plastohydrodynamic lubrication is rare

With increasing interface pressures, the apparent coefficient of friction drops even for

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deformation makes the formulation of a single test technique virtually impossible Standardlaboratory bench tests (such as the four-ball test) occasionally give successful correlationwith metal working practice; these cases are, however, exceptions and are perhaps fortuitous.Most attractive are tests in which friction balance leads to a readily measurable change indeformation The ring compression test (Figure 4) is one which measures lubricant behaviorunder normal contact to simulate forging Forward slip (Equation 5), which directly reflectsfriction balance in the roll gap gives similarly obvious results under rolling conditions.

A second group of test measures interface shear strength Thus, a sheet drawing test(Figure 14a) reveals the magnitude of friction under moderate pressures for light-duty sheet-metalworking lubricants Higher interface pressures and testing for lubricant durability arepossible using a hollow specimen with an annular end face pressed and rotated against aflat anvil (Figure 14b); reasonable correlation is found with severe cold working such asextrusion

In a third group of tests, the magnitude of friction is judged from the force required toperform deformation such as upsetting of a cylinder, extrusion, or wire drawing Lubricantranking is made simply by comparing the magnitudes of forces

No single test provides information for all conceivable metalworking conditions For abroader evaluation at least three tests have to be performed, such as ring compression fornormal approach, plane-strain compression for deformation with extensive sliding, and twistcompression for lubricant starvation situations Die and workpiece material, surface prep-aration and roughness, interface velocity, and entry zone geometry should be the same or

as closely scaled as possible to the actual process

Testing for Staining

Lubricants are sometimes left on the deformed workpiece surface for corrosion protection,and testing in typical industrial atmospheres is necessary In other instances, the workpiece

is subjected to subsequent operations, such as annealing, joining, etc Testing for stainingpropensity can be done if air access is controlled to reproduce that typical of annealing incoils

Hot-Working Lubricants

Typical hot-working temperatures are in excess of 400°C for aluminum and magnesiumalloys, over 600°C for copper alloys, and over 900°C for steels and nickel-base alloys.Lubricants are limited to those resisting the workpiece (or interface) temperature, such asrecommended in Table 1

Oxides formed during heating the workpiece can fulfill a useful parting function, providedthey are ductile at the interface temperature This condition is partially satisfied only by ironoxides, cuprous oxide, and refractory metal oxides Other harder oxides (such as ZnO) areeffective only when they can break up into a powdery form Yet others (such as aluminumand titanium oxides) are hard, brittle, cannot follow surface extension at all, and do notprotect once broken up

Glasses of proper viscosity (typically 20 Pa·sec at the mean of the die and workpiecesurface temperatures) can act as true hydrodynamic lubricants If the process geometry isfavorable (such as in extrusion), a thick glass mat may gradually melt off to provide acontinuous coating on the deformed product Glass may be applied either as glass fiber orpowder to the die or hot workpiece, or in the form of a slurry with a polymeric bondingagent to the workpiece prior to heating In the latter case it may also protect from oxidationand other reactions during the heating period

Graphite is effective in forging steel or nickel-base alloys, if uniformly deposited on thedie surfaces from an aqueous or sometimes oily base Wetting a hot surface is difficult butspecial formulations (sometimes with polymeric binders) have been developed Application

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Table 2 TYPICAL LUBRICANTS USED IN COLD WORKING

a Chlorine is the most effective EP agent on stainless steel.

b Chlorine is avoided for Ti.

c Sulfur is avoided for Ni because of reaction and for Cu because of staining.

d Magnesium alloys are usually worked warm (above 200°C).

e Usually conducted hot.

Hyphenation indicates that several components are used in lubricant CL = chlorinated paraffin; EM = emulsion (the listed lubricants are emulsified and 1 to 20% dispersed in water); EP = “extreme pressure” compounds (containing S, Cl, and/or P);

FA = fatty acids, alcohols, amines, and esters; FO = fats and fatty oils, e.g., palm oil, synthetic palm oil; GR = graphite: MO

= mineral oil (viscosity in units of centistoke [ = mm 2 /sec] at 40°C); PH = phosphate surface conversion; PC = polymer coating;

and SP = soap (powder, or dried aqueous solution, or as a component of an EM).

for copper-base alloys, and is gaining some acceptance for steel rolling Chemical solutions(of rust preventatives, etc.) are often adequate when a protective oxide is present The waterfulfills the important function of heat extraction.

Oil-based lubricants represent a large proportion of the total used The viscosity of natural

or synthetic oils is chosen to give mixed-film or occasionally almost full-fluid film lubrication,but not so high to induce excessive surface roughening or to drop friction below an acceptablelimit Because of the impossibility of avoiding all asperity contact, lubricants always containboundary additives: typically fatty acids, alcohols, esters, or natural fatty oils When contact

is repetitive, EP additives may also be useful, particularly for metals (stainless steel, titanium)

on which fatty additives are ineffective When conditions are unfavorable to developinghydrodynamic films, grease may be used Whenever cooling is important, the oil is applied

in a recirculating system A flood of aqueous emulsions or dispersions is even more effective,but staining may be a problem (e.g., on Al or Mg) Removal of wear particles by filtration

is an essential requirement in all recirculating systems

REFERENCES

1 Bastian, E L H., Metalworking Lubricants, McGraw-Hill, New York 1951.

2 Schey, J A., Ed., Tribology in Mctalworking: Friction, Lubrication and Wear American Society of

Metals, Metals Park Ohio, 1983.

3 Tribology in Iron and Steel Works Publ No 125, Iron and Steel Institute, London, 1969.

4 Rowe, G W., Mech Mach Electr., 266, 20, 1972.

5 Schey, J, A., in Proc Triboiogy Workshop Atlanta, F F Ling, Ed., National Science Foundation

Wash-ington, D.C., 1974,428.

6 Schey, J A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977.

7 Wilson, W R D., in Mechanics of Sheet Metal Forming Plenum Press, New York, 1978, 157.

8 Proc 1st Int Conf Lubr Challenges in Metalworking and Processing, IIT Research Institute, Chicago,

1978

9 Proc 2nd Inst Conf Lubr Challenges in Metalworking and Processing IIT Research Institute Chicago,

1979

10 Schey, J A., in Metal Forming Plasticity, Lippmann, H., Ed., Springer-Verlag, Berlin, 1979, 336.

11 Wilson, W R D., J Appl Metalworking 1, 7, 1979.

12 Schey, J A., in Proc 4th Int Conf Prod Eng., Japan Society of Precision Engineering, Tokyo, 1980,

102.

13 Kalpakjian, S and Jain, S C., Eds., Metalworking Lubrication American Society of Mechanical

En-gineers, New York, 1980.

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It has been estimated that about 10% of the gross national product are spent in materialremoval operations in the U.S It is, therefore, important that such processes be understoodand efficiently performed if productivity (effective use of labor and capital) is to be achieved.Since friction, wear, and lubrication play important roles in material removal operations, it

is pertinent to consider these operations here

There are a wide variety of removal operations which use tools of different geometry andkinematic relationship between tool and work Some of the more important operations includethe following:

1 Turning to produce cylindrical surfaces

2 Milling to produce flat surfaces and surfaces of complex geometry

3 Drilling, boring, and reaming to produce round holes

4 A wide variety of grinding operations

In addition to these, there are a host of more specialized operations designed to do a particularjob more effectively or which are better suited to mass production However, all removaloperations involve tools that penetrate the work to peel off unwanted material What goes

on at the tip of these tools is essentially the same regardless of geometry or kinematics

CUTTING MECHANICS

Orthogonal Machining

Figure 1 is a photomicrograph of a partially formed chip.1This was produced by moving

a workpiece against a stationary two-dimensional tool, abruptly stopping the operation inmidcut and then sectioning and metallographically polishing the “hip root” The magnifiedview of the etched surface reveals a great deal concerning the action of a metal cutting toolwhen removing a chip As the material approaches line AB there is essentially no plasticflow until AB is reached At this point a sudden concentrated shear occurs and then thematerial proceeds upward along the tool face with essentially no further plastic deformation

In the case of Figure 1, the cutting edge was stationary and straight, extending perpendicular

to the plane of the paper This is called orthogonal cutting since the cutting edge is pendicular to resultant velocity vector (V) which is in the horizontal direction in Figure 1.Merchant2and Piispanen3first discussed orthogonal cutting in fundamental terms

per-Figure 2 is a diagrammatic representation of Figure 1 Line AB is the trace of the surface

on which the concentrated shearing action occurs and is called the shear plane The anglethe shear plane makes with velocity vector (V) is the shear angle (φ) while the angle betweenthe face of the tool (rake face) and the normal to the velocity vector is called the rake angle(α) Also shown in Figure-2 is the clearance angle (γ) The thickness of the layer removed

is the undeformed chip thickness (t) and the width of cut perpendicular to the paper will be

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fractures internally in shear leaving behind the stationary body of metal attached to the tip

of the tool (BUE) Another reason is associated with the fact that metals (notably steel)exhibit minimum ductility (strain at fracture) somewhat above room temperature This iscalled “blue brittleness”, since the minimum strain-at-fracture temperature for steel cor-responds to that where the thickness of surface oxide produced gives rise to a blue interferencecolor At relatively low-cutting speeds (low-tool face temperature), the temperature along

CD will be closer to the blue brittle temperature than along AC and then the strain at fracturealong CD will be less than along AC

The inherent instability of a large BUE is very troublesome relative to surface finish Ascutting proceeds, BUE tends to grow slowly until it reaches a critical size and then it leavesabruptly with the chip Since the BUE grows downward as well as outward (point D below

A in Figure 4), the surface produced by the periodic change in size of the BUE is as shown

in Figure 4 This is one of the sources of surface roughness when cutting at relatively lowspeed An unstable BUE can also decrease tool life due to abrasive action of pieces of BUE

on the tool face and wear land; on the other hand, a small, stable BUE can be beneficial inprotecting the tip of the tool from wear

The most important way of avoiding a BUE is to increase cutting speed Above a certain

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FIGURE 3 Photomicrograph showing large BUE and portions of BUE along finished surface and along face of chip.

AISI 1020 steel cut dry at 90 fpm (27.4 m/min) Undeformed chip thickness = 0.005 in (0.125 mm).

FIGURE 4 Diagrammatic representation of Figure 3 showing induced roughness on finished surface.