Numerical Model and X-Ray Residual Stress Measurement 1.. X-Ray Residual Stress Measurement To measure the residual stresses in the spray coating process, CrKa teristic x-rays were used.
Trang 1as ‘‘mushy zone’’ which is a mixture of solid and liquid phases.Figure 49illustrates an example of spray coating system, which is developed to fabri-cate laminated plate In most cases of the spray coating process, after thespray layer is solidified completely, the other kind of material is poured ontothe substrate so that the inner boundary grows and the interface of moltenstate moves toward spray layer direction.
In the spray coating process, solidifying material in the growingdomain undergoes a variation of mechanical quantities, such as mass,momentum and energy as well as the change of material properties due tophase transformation from liquid to solid state The complicated interactionbetween temperature and inelastic deformation, in this case, is to be takeninto consideration
B Numerical Model and X-Ray Residual
Stress Measurement
1 Modeling of Numerical Simulation
To verify the validity of the theory and the procedure stated above, tion of the fields of temperature and solidification mode and the stressFigure 40 Relation between stresses at the center of ingot and casting speed
Trang 2simula-distribution are performed over the course of the spray coating process oflaminated plates with two layers The, assumption is made that both ends
of laminated plate are constrained during the spray coating, and that thelaminated plates are large enough, so that a growing model of two-dimen-sional finite element is proposed to interpret the experimental phenomena
of the spray coating process and to compute the interfacial ical behavior between the impinging particles and the surface of substratejust beneath them A model of the spraying layer represented by a flat disk
thermo-mechan-of 30 mm diameter and 0.5 mm thickness thermo-mechan-of initially uniform temperaturewhich is put into contact with the substrate elements is shown in Fig 50.When the numerical analysis is started, the growth elements incorporatedwith the impinging particle from the spraying direction were put into thesubstrate In this analysis, the growth model is represented by an axisymme-trical problem Thermal flux entering and leaving each element as well as thelatent heat liberated within the elements themselves during the solidificationprocess is evaluated and the resulting element temperature is computed aftereach successive time increment
To simulate the spray coating processes, it is assumed that the ent materials are successively poured into the substrate, i.e., the differentFigure 41 Relation between stresses at the center of ingot and discharge of coolingwater
Trang 3differ-material is supplied after the differ-material in the outer layer is completely fied On the outer surface along spraying direction, the heat radiationboundary conditions was set on the initial step of the coating, and heattransfer condition was set on the cylinder surface of the axisymmetricalmodel, respectively.
solidi-C Properties and Coating Condition
of Specimen Materials
Laminated layers are deposited on a stainless steel (SUS304) substrate of
5 mm thickness In this work, the layer thickness and layer materials duced were: 0.50 mm for stainless steel The specified thickness was obtainedwhen spraying was performed 10 times.Table 2shows the components ofthese wires which are generally used for carbon dioxide arc welding.Table
pro-3shows the conditions of the spray coating process
The thermo-physical properties of the wire materials used for ture and stress calculation incorporated with solidification are shown inTables 4 and 5, respectively [69] It is assumed that the properties ofsubstrate are the same as those of the wire The heat transfer coefficientFigure 42 View of (a) twin-roll casting system and the (b) model for simulation
Trang 4tempera-for air on the surface of the model is chosen to be h¼ 2.78 103
(cal=(mm2
sec deg)) The thermal radiation coefficient G¼ 7.028 107
(cal=(mm sec K)) is used for the model
Depending on the inelastic constitutive model of Section 2, the tic strain rate can be given by a viscoplastic relationship Here, the viscositywhich described the viscoplastic model of wire material (stainless steel) isshown inFig 50
inelas-D X-Ray Residual Stress Measurement
To measure the residual stresses in the spray coating process, CrKa teristic x-rays were used Diffraction planes and angles were (2 1 1) and2y0¼ 1568 for the stainless steel SUS304 Surface roughness of the coatedlayer was about 6.5 mm These values might be too large for x-ray stress mea-surement to provide reliable results However, the parallel beam methodcould give stress values with sufficient accuracy on such rough surface.Before x-ray stress measurement, electropolishing conducted to remove anFigure 43 Mesh of finite element model
Trang 5charac-oxide-film from the layers Stresses were measured parallel and lar to the spray traveling direction The x-ray measuring conditions areshown inTable 6.
perpendicu-The full width at the half-maximum method was used to determinepeak positions We measured the residual stresses in a phase with 2 1 1 dif-fraction and g phase with 2 2 0 diffraction The stresses were obtained by thesin2c method The c-diffractometer method was used for the measurement
of residual stresses
E Verification and Discussion of Simulation Results
An example was used for simulating thermo-mechanical behavior and dual stress during the spray coating.Figure 51shows the variation of tem-perature distribution on the central element of spraying surface and outsidesurface of the layer From these results, we arrive at the temperature differ-ence between the central element and the outside surface of the sprayinglayer due to the solidifying process The distribution of temperature onthe total domain dependent on time is shown inFigs 52and53.The volumefraction of solid and the variation of the solidified thickness are depicted inFigs 54and55, respectively These figures, show that the temperature onthe central element tends to decrease slowly by the latent heat generationFigure 44 Distribution of temperature in strip and roll
Trang 6resi-due to solidification and also by the heat supply by the successively pouredmaterial followed by the rapid temperature decrease at the end of solidifica-tion Difference in heat conductivity due to the temperature differencereveals the influence on the cooling rate and mode of solidification.
As for the results of stress analysis, residual stress is represented in thefollowing figures, in which increasing stress reduces fluctuation or jumpdepending on the solidification and growing domain Distribution ofresidual stresses sron the radial direction is shown by the lines inFig 56.Figure 45 Temperature variations at center and surface of strip
Figure 46 Distribution of solid fraction
Trang 7The data are compared with the experimental results represented by thesame condition of the process Relatively reasonable agreement betweenboth values is seen even in the region with fluctuation of stresses on theinterface boundary between the spraying layer and the substrate The distri-butions of residual stress syand szare shown inFigs 57and58.Here, thejump behavior of stresses on the interface is presented by these simulatedresults Thus, it is important to reveal the damage of the spraying layerbased on the theory and numerical method.
VI DESIGNING OF FORGING PROCESS FOR
CONTROL OF INTERFACIAL STRESS
A Basic Description
A typical industrial metal component may be manufactured by forgingand heat treatment In the design of forging processes, information suchFigure 47 Distribution of stress sx
Trang 8Figure 48 Dependence of constitutive relationship on stress distribution.
Trang 9illustrate equivalent stress and axial residual stress distribution fromthe center to the surface in the middle of the rod after quenching,respectively.
In Fig 66,two cases are shown for axial residual stress distribution:one case is quenching without taking into account the residual stressfrom the forging process; and the other is quenching, taking account of
Figure 59 Outline of basic numerical model
Table 7 Material Properties of Heat Conduction
Heatconductivity
Specificheat
Heat transferwith ambient
Trang 10Figure 61 The rod in its (a) initial position and at the (b) end of ejecting stage.
Figure 62 Residual stress at the end of ejecting stage
Figure 63 Cooling curve
Trang 11the residual stress from the forging process Experimental results measured
by the Sacks method are also shown in the same figure In both cases, theaxial residual stress is in tension near the center and in compression nearthe surface Because of the effect of the residual stress from the forging pro-cess, less tension near the center and more compression near the surface wereobtained as seen inFig 66.The axial residual stress coming from the for-ging process does not make a big difference with the quenching calculation,Figure 64 Volume fraction of metallic structures
Figure 65 Effective stress after quenching
Trang 121 Comparing with calculated results and experimental data fortemperature, distortion and residual stress in these metallurgicalprocesses, the metallo-thermo-mechanical theory and simulationmethod proposed in Section II are verified.
2 Effects of cooling curves, distortion, and residual stresses on theoccurrence of the phase transformation in quenching are proved
by simulations of quenching and carburizing-quenching processes
3 It is important to identify the heat transfer coefficients ofquenchants with respect to the quenching process are obtainedfrom simulation results
4 The unified inelastic constitutive equation may describe the stressand deformation in the whole region of the solidifying processincluding liquid and solid state
5 In the simulation of continuous casting, the development ofstresses from solidifying domain is presented On the other hand,the effects of distortion on solidification also shown to be animportant factor
6 In the simulation of coating process, the jump behavior of stresses
on the interface between substrate and spraying layer is shown.Thus, it is important to reveal the damage of the spraying layer based
on the metallo-thermo-mechanical theory and numerical method
7 The advantage of finite volume technique over the finite elementmethod in the simulation of forging was shown From this point ofview, the finite volume method is expected to be a powerful tool inthe simulation of metal forming processes
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