Integrated modelling of deformations and stresses in the die casting and heat treatment process chain

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1 Perspektiven Integrted modelling of deformtions nd stresses in the die csting nd het tretment process chin Structurl high pressure die csting luminum prts re widely used in the utomotive industry. During csting nd susequent het tretment, the csting experiences thermlly induced stress formtion nd relted distortion. The design of the prt nd the die, together with the process control nd the choice of cooling nd het tretment prmeters, hve significnt impct on how the stresses nd deformtions evolve during the multiple mnufcturing steps. The rticle presents fully integrted pproch in Mgmsoft, to predict csting stresses nd distortions for the full mnufcturing process chin, which hs een pplied to different industril cstings. The enefits re significnt when dimensionl tolernce prolems re identified nd resolved systemticlly in the design phse of the component or efore tooling is mnufctured. Jesper Thororg, Jörg Klinkhmmer nd Heinz-Jürgen Gspers, Achen 1 Introduction Structurl luminum prts used in the utomotive industry re widely produced y the High Pressure Die Csting process, HPDC. The process is minly used ecuse of the high production rte nd the possiility to mnufcture complex prts with high requirements to shpe nd tolernces. Due to microstructurl requirements nd mechnicl performnce of the luminum prts, the HPDC process is in mny cses followed y sequence of het tretment steps, which govern the finl properties of the prts efore ssemling into lrger structures. During csting nd het tretment the cst mteril is thermlly loded in wide temperture rnge, strting from the csting temperture going through the solidifiction intervl nd during solid stte cooling down to room temperture. Depending on the chosen het tretment process, the temperture is susequently chnged in severl steps y reheting the prts nd holding the temperture for some time efore finlly cooling down to room temperture gin. Depending on the design of the prt, the process control, nd the choice of cooling nd quench prmeters, the level nd chnge in temperture led to therml grdients nd conditions which hve high influence on how the stresses nd deformtions evolve during the multiple mnufcturing steps. To nlyze nd predict the evolution of stresses nd deformtions quntittively, it is importnt to simulte the full sequence of mnufcturing steps in coherent nd consistent wy, where the full lod history of the mteril is considered. Tody, csting process simultion is widely used nd ccepted to e n efficient wy of optimizing the csting process. Industry is showing n incresed interest in extending the simultion cpilities to lso nlyze the susequent het tretment process. This rticle presents stte of the rt modelling pproch where results from the csting process re considered in the susequent het tretment clcultion. This fully integrted pproch in Mgmsoft supports the work flow of utonomous engineering, where virtul experiments re used to optimize mechnicl properties nd performnce, to improve qulity nd to reduce costs nd production time. The enefits from nlyzing the full mnufcturing process chin re significnt when dimensionl tolernce prolems re identified or cn e resolved efore tooling is mnufctured or even in the design phse of the component. 2 Process steps nd distortion control of structurl prts In the HPDC process, the mjority of the luminum solidifies inside the die nd even cools down elow the solidus temperture efore it is removed from the die. Therefore considerle level of stresses due to constrined contrction is formed in the cst mteril efore ejection. The result is complex in- 20

2 terction etween the cst mteril nd the die, i. e. some regions shrink onto the die nd other regions open up gps with no contct, [1]. Depending on the cooling conditions nd how long the prt stys in the die, the constrined contrction will led to stresses nd permnent deformtions in different regions of the prt. During the die opening sequence nd ejection, some of the stresses will e relesed due to elstic springck, nd the prt will deform s consequence of removing the constrints due to the die. The prt will freely contrct during the finl cooling/quenching step until it reches room temperture. At room temperture, the totl mount of deformtion is sum of the full therml contrction from solidus to room temperture plus the permnent deformtions which were minly generted during cooling in the die. The csting process is schemticlly shown in Figure 1, where the different process steps re indicted y smll icons. Csting of luminum structurl prts for the utomotive industry is often followed y het tretment process, where the min ojective is to improve the mechnicl properties y modifying the s-cst microstructure in sequence of therml steps. In this wy, it is possile to otin mteril with incresed ductility nd higher strength compred to the s-cst stte. The het tretment steps cn lso hve significnt influence on the stress level nd the distortion tht uilds up during e. g. solution tretment. To predict the finl distortion of structurl prt, it is necessry to consider ll relevnt steps of the mnufcturing process. This imposes some chllenges for the simultion of the integrted process. The importnce for hving n pproprite simultion technique origintes from reltively new trend in the utomotive industry to pply distortion engineering to structurl prts. Relevnt process stges nd the relted temperture history imposing stresses nd distortion during the het tretment process re illustrted in Figure 2. The following three lterntives to reduce/void possile finl distortion of the prt re usully pursued in prctice: Design support frmes used during het tretment to llow the prt to distort ck to the desired shpe. Trim or strighten the prt fter csting or het tretment. Compenste expected s-cst distortion y modifying the die cvity. 3 Simultion pproch Thermo-mechnicl modeling of the csting nd het tretment processes is chllenge. The min concern is to model the mechnicl response of the mteril t different temperture levels, on different time scles nd sometimes with different strin rtes. A unified creep model hs een chosen s n pproprite constitutive mode, [2] nd [3]; further detils cn e found in the Thermo-mechnicl constitutive model ox (Appendix 1). The simulted stresses results depend on prior comprehensive therml nlysis of the entire csting setup, including Figure 1: Schemtic temperture profile of the HPDC process with illustrtions of the ssocited process events resulting in stress formtion. Figure 2: Temperture profile for the different het tretment steps in typicl T7 tretment fter csting of luminum prts. SouRCe: MAGMA 21

3 Perspektiven filling of the die cvity, cooling nd heting of the die, sprying of die luricnts, etc. Furthermore, different process conditions hve to e considered such s die open tempertures, die constrints, shke-out conditions nd trimming opertions. While simulting stresses nd distortion for the HPDC process is rther well known procedure, it is not frequently done for the susequent het tretment process. In the integrted pproch presented here, prts re plced in the het tretment support frme fter csting. This requires n dditionl step in the simultion where the lredy deformed s-cst prt is positioned onto support frme. When the structurl prt is heted while positioned in the support frme, it cn experience considerle mount of deformtion due to grvittionl forces nd creep in the solution tretment step. During the finl rtificil ging step, the temperture level is only incresed to llow precipittion hrdening to tke plce, [2]. See further comments on the mechnicl ehvior during csting nd het tretment in Appendix 2 nd Appendix 3, respectively. Typicl oservtions in the different process steps re indicted with smll icon of n eye, nd interesting ehvior/fields to check nd vlidte re indicted y n exclmtion mrk,. 4 Thermo-mechnicl constitutive model Thermo-mechnicl modeling of the csting nd het tretment process hs to consider the complex ehvior of the mteril response nd the interction etween the csting mteril nd the surrounding dies nd support frmes. One of the min concerns is to model the response of the mteril t different temperture levels, on different time scles nd sometimes with different strin rtes, which is governed y different deformtion mechnisms. A unified creep formultion is used s the fundmentl constitutive lw, [4]. The model is sed on Norton s power lw nd includes y tht, strin rte sensitivity nd the possiility to descrie creep t elevted tempertures: nd The two properties A nd m descrie the strin rte sensitivity. The Arrhenius expression scles the response ccording to the temperture dependency nd the model is therefore pplicle over wide temperture rnge. The temperture dependency is governed y the ctivtion energy, Q. The reference stress σ ref descries the isotropic strin hrdening y clssicl power lw, where the inelstic strin, ε in, is used to cpture the effect of hrdening when e.g. disloctions re piling up nd nneling when the temperture is elevted, ccounting for diffusion processes. Clirtion of the thermo-mechnicl properties is sed on lrge rnge of tensile tests nd creep tests t different (1) (2) temperture levels. The tensile tests re typiclly performed t different strin rtes t intermedite nd high tempertures to get informtion out the strin rte sensitivity. Creep tests re performed t high tempertures to get informtion for the het tretment process nd for slow cooling csting processes, where stress relxtion is importnt. The hrdening response is governed y two temperture dependent properties, the initil reference stress σ 0,ref nd the hrdening prmeter n. The response of the creep eqution nd the hrdening lw cn e illustrted y clssicl creep curves nd the strin rte dependent tensile curves, see Appendix 1. 5 Appliction of the simultion pproch The integrted simultion pproch of Mgmsoft hs een pplied to different industril cstings. The primry exmple is shock tower, which illustrtes how simulted distortion from the csting process is used to design support frme for susequent het tretment. The ojective is to reduce the devitions in shpe compred to the reference geometry y using the distortion from solution tretment to correct the overll shpe of the prt. The results re compred to results from strightening simultion, where corrections re otined y pplying correctionl force t room temperture. The simulted results re compred to mesurements. Secondly, spce frme connection node is simulted nd virtully mesured distortion fter shke-out nd cooling is used s informtion to pre-shpe the die cvity dimensions to pre-compenste for nd to reduce distortion. Thirdly, the ejection process is evluted on the sme spce frme connection node. Simulted ejection forces re used to evlute the numer nd lyout of the ejector pins. Bsed on the results, reduced numer of pins re used for n optimized lyout nd the ejection forces re compred to the initil lyout. Finlly, virtul Design of Experiments (DOE) hs een pplied to third exmple, which is nother spce frme connection prt. Severl lyouts of the het tretment support frme nd different process conditions re utomticlly clculted nd evluted to minimize the distortion during solution tretment. 6 Csting distortion nd design strtegies for the het tretment support frme Significnt distortion nd unwnted stresses cn e consequence of the csting process. Even though the finl distortion is reltively simple to mesure, it is very hrd to control nd tckle in production. Simultion nd creful nlysis of the conditions fter csting cn e used s input to design strtegies for the support frme, to ctively correct deformtions during solution tretment. The front wheel shock tower (Figure 3) of pssenger cr hd initil prolems with distortion eing out of the llowed tolernces fter het tretment, [5]. Therefore it ws decided to nlyse the process with Mgmsoft. 7 Csting process The csting process ws simulted s first step, to predict the distortion efore het tretment. Figure 4 shows the evolu- 22

4 tion of the stress levels during the csting process nd how the displcements uild up t the sme time. The von Mises stress distriution is shown ove the process view nd the displcements re shown elow the process view. Strting from left, Figure 3: Front wheel shock tower of pssenger cr. Initil prolems with distortion eing out of tolernce fter csting nd het tretment were nlyzed virtully. the results show the conditions just efore die open, just fter ejection, t mient temperture nd finlly fter trimming. In the first result the stress level is governed y the constrints from the die nd the chosen die open time, Figure 4. Evluting this result mkes it possile to nlyze if criticl stress levels re reched, which could promote lrge permnent deformtions or even ffect the ejection process. After ejection the stress level is significntly reduced, which is seen in the second result, Figure 4. During the susequent cooling step to room temperture moderte stresses re generted due to the therml grdients, see the third result, Figure 4c nd only smll chnges re seen in the susequent trimming step Figure 4d. Evluting the distortion t the sme points in time shows how the min distortion evolves during the cooling step from die open to room temperture. Only limited mount of distortion is oserved just efore die open Figure 4, due to the constrints from the die. Just fter the ejection process some distortion is seen, minly due to elstic spring ck when the constrints from the die re removed, Figure 4. The free therml contrction from die open to mient temperture genertes significnt mount of distortion, Figure 4c. For this reson it cn e useful to mke vritions in the die open time to investigte how much the free contrction ffects the finl distortion level. For this exmple, the finl trimming step does c d Von Mises, MP Displcement Y, mm c d Y Figure 4: Evolution of von Mises stress (upper row of results) nd displcement in Y-direction (lower row of results) during the different HPDC process steps. From left, the results re shown t different process times just efore die open (), just fter ejection (), t mient temperture (c) nd finlly fter trimming of the gting system (d). 23

5 Perspektiven not chnge the distortion significntly, Figure 4d. However, depending on the gting system nd the design of the prt, this finl step cn contriute to the distortion level. 8 Het tretment nd support structure design The finl distortion fter csting ws used to design the support frme for the susequent het tretment steps. It ws cler from the clculted distortion, Figure 5, tht the upper left corner ends downwrds wheres the upper right corner ends upwrds compred to the reference geometry. The directions of ending re indicted y the two rrows. This type of unwnted distortion cn in most cses e reduced y llowing the structure to deform in controlled wy during solution tretment. As the tempertures re close to solidus, smll forces from grvity promote creep in regions where the support frme does not restrict the deformtion. The mount of otined creep, nd y tht distortion, in the structure depends on the temperture level nd the process time. The temperture must e sufficiently high to ctivte creep nd the frme must e crefully designed to llow wnted distortion nd restrict unwnted distortion. For the considered exmple the frme ws designed s shown in Figure 5. The zoomed-in view in the ox t the right shows how the prt initilly hs gp etween the upper right plte nd the r in the frme just elow it. This freedom to deform is designed to llow the two pltes to lign t the end of solution tretment. Devition, mm 1.5 Figure 5: Devition of the csting from the reference geometry fter trimming t room temperture () nd support frme designed to compenste for the csting distortion during het tretment () Von Mises, MP 2 Figure 6: Evolution of von Mises stress t different points in time during the het tretment process. Notice the different stress scles due to the different rnges of the stress levels Von Mises, MP 0 20 Von Mises, MP 0 24

6 Results from the csting process re mpped to the position of the prt in the support frme, Figure 5. This step is done utomticlly in Mgmsoft, i. e. ll relevnt mechnicl fields re trnsferred from the orienttion in the csting process to the orienttion in the het tretment process. 9 Stress development during het tretment During the het tretment process, the stress level in the prt significntly chnges due to the elevted temperture levels nd the cooling conditions. To illustrte the influence on the considered prt, severl von Mises stress results re shown in Figure 6, where the initil stress level is sed on the mpped results from the csting simultion. As expected, the stresses re relxed to lmost zero during solution tretment, where the temperture level is pproximtely 460 C. The susequent cooling only leds to smll increse in the stress level, nd in the finl ging step, t pproximtely 220 C, the stresses re gin relxed to n even lower level t the end of the entire het tretment process. 10 Distortion evlution fter het tretment The results in Figure 7 show the otined distortion fter het tretment nd the devition from the reference geometry fter csting nd fter het tretment. The designed gp in the support frme clerly llows the wnted deformtion to develop during solution tretment nd y tht to ctively compenste for the csting distortion. The level in devition from the reference geometry ws reduced y pproximtely 1.5 mm, see Figure 7 elow. 11 Vlidtion of results using opticl mesurements The otined correction to the csting distortion ws compred to mesurements. Figure 8 shows the distortion of the cst prt fter the full process chin of csting nd het tretment. The curves show the devition t multiple mesurement points from the reference geometry. The red curve shows the Mgmsoft-simultion result, where the lue, yellow nd green curve show mesurement results for 3 different specimens of the prt. The predicted results show very good greement to the mesurements in lmost ll res. Differences minly pper in the red mrked res, where the mesurement points re locted very close to the outer ounds of the geometry. In these outer regions, mechnicl influences from e. g. hndling nd trimming re very likely to hve influenced the mesured results. In one cse (detil on the right) n rtificil indenttion in the imported stl-geometry, contining the mesured shpe, is responsile for the shown devition to the simultion result. Overll, the greement etween simultion nd mesurements is very good nd the pplied simultion pproch hs een useful to nlyze the distortion prolem during csting nd the susequent het tretment processes. 12 Strightening nd the risk of deforming the prt t room temperture Distortion fter csting nd het tretment is typiclly corrected y different types of strightening processes. The needed corrections to get sufficient ccurcy in the finl shpe re otined y pplying high mechnicl loding to produce loclized permnent deformtion in different regions of the struc- c Y Displcement Y, mm Devition, mm Devition, mm Figure 7: Distortion in the direction of grvity (), devition from the reference geometry fter csting () nd devition from the reference geometry fter het tretment (c). 25

7 Perspektiven Figure 8: Mesurements compred to results of the simulted devition from the reference geometry for more thn 50 mesurement positions. Notice the mesurements contin spred in the level for the three considered prts. Figure 9: Setup for modeling the strightening process. The drk gry cylinders re constrints nd the red cylinder indictes the loction nd direction of the pplied lod. ture. Tody, stte-of-the rt strightening is done in fully utomtic process where severl steps of pushing, pulling nd twisting cn e pplied to the prt in different directions. The most dvnced systems re sed on self-lerning lgorithms to reduce nd optimize the required numer of correction steps. The strightening process provides high level of freedom to correct the prt, ut the mechnism ehind the process is to plsticlly deform the mteril t room temperture, which in the worst cse cn influence the mechnicl performnce during service loding. Especilly if severl ig correction steps re needed to otin the required tolernces, the risk of provoking crcks nd defects increses. To illustrte the impct of the strightening process, force is pplied to the shock-tower to compenste for the uneven ending of the two upper pltes. The setup is illustrted in Appendix 2 nd 3 nd Figure 9, where the drk gry cylin- 26

8 Y Y Displcement Y, mm 2.0 Figure 10: Deformtion during strightening (), otined deformtion fter strightening (). 0.0 Von Mises, MP 180 Figure 11: Von Mises stress during strightening (), von Mises stress fter strightening (). 0 ders re mechnicl constrints nd the red cylinder indictes the loction of the pplied lod. The displcement result in Figure 10 shows the distortion during loding, which is pproximtely 9 mm in the re close to the pplied lod. The otined distortion fter unloding is shown in Figure 10 nd is pproximtely 1.1 mm. During loding significnt stresses uild up in the prt, which cn e seen in Figure 11. Stress results during nd fter loding re shown in Figure 11 nd Figure 11, respectively. As consequence of the high loding, loclized permnent deformtion is generted inside the prt, which cn clerly e seen in the highlighted regions in Figure 12. The initil devition to the reference geometry fter csting is shown in Figure 13. The devition which ws possile to otin y designing the support frme for het tretment is shown in Figure 13, nd the devition otined from the strightening process is shown in Figure 13c. The devition in the two results Figure 13 nd Figure 13c re to lrge extent in the sme rnge. The exmple shows how simulted distortion cn e used in the design strtegy of the het tretment support frme to promote distortion during solution tretment, which ctively compenste for csting distortion. This pproch cn e used to reduce the required mount of strightening steps t the end of the process chin, which reduces costs nd the risk of generting defects nd incresed residul stresses in the finl prt. 13 Pre-shping the die to compenste for s-cst distortion In ddition to the different correction methods, which re pplied to the prt s shown in the previous sections, it is of course vitl to use resonle shrinkge fctor in the design of the die nd when possile pre-shpe the die to reduce the s-cst distortion. The procedure for using simultion to pre-shpe the die is illustrted in Figure 14. To generte the red pre-shped die cvity in Figure 14f the procedure is s follows: The lue reference geometry Figure 14 is scled up ccording to the shrinkge fctor of the luminum lloy, red rectngle in Figure 14. This is stndrd procedure to ccount 27

9 Perspektiven Effective plstic strin, % 1.0 Figure 12: Effective plstic strin on the inner side (), effective plstic strin on the outer side (), oth fter strightening. The results show the loclized permnent deformtion which ws generted during strightening. 0.0 c +1.5 Devition, mm Figure 13: Devition from the reference geometry fter csting (), devition from the reference geometry fter het tretment () nd devition from the reference geometry fter strightening (c) Reference geometry d Predicted shrinkge nd distortion Add shrinkge fctor to compenste for therml shrinkge e Apply the negtive locl csting distortion. Notice the geometry will expnd Figure 14: Procedure for pre-shping the die. Reference geometry (), enlrged geometry to compenste for therml shrinkge (), cvity geometry for the first clcultion (c), predicted shrinkge nd distortion (d), pplied locl negtive csting distortion (e) nd pre-shped die cvity for second simultion (f). c Cvity geometry used in the 1 st simultion f Downscle to mke the pre-shped die cvity. Run the 2 nd simultion 28

10 for the therml contrction of the csting during solidifiction nd cooling. The first simultion is sed on the up-scled die cvity in shown red in Figure 14c nd the results re indicted y the lck line geometry in Figure 14d. The four smll rrows in the corners of the two geometries in Figure 14d indicte the predicted distortion. This result is multiplied y negtive scle elow -1.0 nd pplied s correction fctor to the die cvity shown y the lck line geometry in Figure 14e. The correction is indicted y the four rrows. The negtively deformed geometry is scled down nd used s the new cvity shpe, red shpe in Figure 14f. The pplied negtive correction expnds the geometry nd lst down scle is pplied to predict the new pre-shped cvity. A second simultion is used to check to which extend the precompenstion corrects the originl distortion. Depending on the complexity of the geometry, it cn e difficult to correct distortion in ll regions of the prt even if the steps ove re repeted severl times. However, compred to mking chnges to the rel die, the ove simultion pproch is very ttrctive wy of evluting how fr s-cst distortions cn e reduced y pre-shping the die. To illustrte the die pre-shping procedure, spce frme connection node is used s n exmple. The considered connection node is shown in Figure 15. The devition from the reference geometry fter csting using the originl cvity dimensions is in the rnge of mm s shown in Figure 15. The negtive of the csting distortion shown in Figure 15 ws used to correct the dimensions of the die. In Figure 15c the grey colored prt illustrtes the dimensions for the originl cvity geometry, nd the purple color indictes the modified cvity geometry using the predicted distortion shown in Figure 15. After compensting the distortion y pre-shping the die, the level of devition from the desired geometry is reduced to e less thn 0.3 mm s shown in Figure Anlyzing the ejection process The loction nd numer of ejection pins governs the stility of the ejection process of the cst prt from the die. It is importnt to design lyout which distriutes the required ejection forces in wy tht the prt does not stick to the die or deform the prt. Simultion cn e used to evlute different lyouts, nd y tht dd pins where ejection forces rech criticl level nd remove pins where they hve little or no effect. Design constrints from e. g. the cooling system nd die inserts cn, of course, e considered when the loctions re evluted. The ejection process is evluted for spce frme connection node, where the initil lyout of 12 ejector pins ws simulted nd the results were used to check the force level on the different ejector pins. Bsed on the results, four pins were removed from the lyout nd results from new simultion were compred to the originl lyout Figure 16. The ejection process for the initil lyout is visulized in Figure 17. The three snpshots show the deformed prt during ejection with mgnifiction fctor of 5. The ejector pins re controlled y time dependent displcement input nd the pins will force the prt out of the die s function of time. The interfce etween the prt nd the ejector die is descried y Coulom friction model, which governs the rection forces depending on the shrinkge induced clmping Devition, mm -1.0 Figure 15: Pre-shping of the die. Devition from the reference geometry fter the first simultion () nd devition fter the second clcultion where the pre-shped cvity is used (). 29

11 Perspektiven R2 50 R1 Contct pressure, MP 0 Figure 16: Lyout of the ejector pins, where the red pins re not ctive in the second simultion. Figure 18: Contct pressure in the interfce zones etween the prt nd the ejector pins. The contct pressure is shown fter pproximtely 1 second. Due to symmetry, only hlf of the prt is shown. c Displcement X, mm X X X Displcement X, mm Displcement X, mm Figure 17: Deformtion of the prt during the ejection process (5x mgnifiction). The snpshots show from the left: The eginning of ejection (), the time where the forces re highest () nd the end of the ejection (c). The contct pressures etween the ejector pins nd the prt re evluted for the initil cse where ll pins re ctive. It is possile to visulize the pressure directly on the prt, which is shown s n exmple in Figure 18 fter pproximtely 1 second. Only hlf of the prt is shown due to the symmetry. Two points re identified s hving reltively low contct pressure nd hence used s cndidtes for eing removed (four points in totl only two re shown due to symmetry). They re highlighted s R1 nd R2 in Figure 18. The two points in the lower left corner of the prt lso show reltively low contct pressures. However, they re kept in the lyout due to their loction, since removing them could increse the risk of the prt tilting nd sticking during the rel ejection process. The two identified pins, R1 nd R2, were removed nd the second reduced lyout ws simulted. To evlute the consequences of removing the two pins, the contct pressure in three of the remining points were compred to the results from the first clcultion. The results re compred in Figure 19, where the normlized contct pressure is shown s function of time t the three points. The full lines show 30

12 Figure 19: Contct pressure s function of time in the 3 indicted ejector pins. Full lines re the originl lyout nd the dshed lines re the reduced lyout without the red pins. c d Figure 20: Vritions in the numer nd loction of the front support frme ems. Originl lyout (), dding em in the front left corner (), removing front center em (c) nd repositioning the pins in the front (d). the results for the first cse with ll pins ctive nd the dshed lines show the results for the second cse where four pins re removed from the lyout. The contct pressure is generlly incresed in ll three points, ut further evlution of the generl stress stte in the prt during ejection did not show criticl stress levels nd no significnt permnent deformtion ws detected due to the chnge in the numer of pins. The reduced numer of ejection pins in the second cse therefore seems to e resonle to ensure stle ejection process. 15 Design of virtul experiments for support frme optimiztion The finl exmple is nother spce frme connecting node locted in the rer prt of the chssis. The design of the het tretment support frme ws evluted y performing virtul DOE vrying the numer nd loction of the pplied front supporting ems in the frme; see Figure 20. The presented results re from the reserch project ProGRes, [6, 7]. The ojective ws to minimize the deformtion of the prt y chng- 31

13 Perspektiven ing the lyout of the support frme, while keeping the solution tretment prmeters fixed t 2 hours nd 485 C. The results of the DOE simultions re shown in Figure 21. For the frme in Figure 21, 3.2 mm is predicted in the direction of grvity in the left front re of the prt. Using n dditionl support in the front left corner of the frme, the deformtion is significntly reduced, Figure 21. The next vrition is done y tking wy one of the front support ems. This vrition hs only minor effect on the deformtion of the prt Figure 21c. Moving the front left support em further left nd to the ck of the geometry seems to led to the est result of the performed vritions, with the lowest displcements in the grvity direction, Figure 21d. The results from the DOE mde it possile to select design of the support frme tht could significntly reduce the distortion of the prt. The chnges in design were utomticlly generted nd simulted. The sensitivity to oven temperture nd tretment time ws evluted in seprte DOE nd the overview of the results is shown in Figure 22. On the x-xis the holding time of the solution tretment is plotted, nd the y-xis shows the mximum difference of deformtion in the prt compred to the reference geometry. The green colored dots show the simultion results for the solution tretment temperture of 485 C, red dots show the results for 535 C nd the lue ones show the results for 465 C. The digrm clrifies tht for the sme holding time (e.g. 2 hours) the solution tretment with the highest temperture shows the highest deformtions of the treted prt. At the sme time the digrm illustrtes the cler tendency towrds higher deformtions for longer tretment times for the sme temperture levels. The tendencies of different colored dots in the digrm show tht the vlues on the y-xis increse nlogous to the x-vlues. 16 Summry Integrted modeling of csting nd het tretment processes provides powerful tool for n upfront ssessment of criticl mteril conditions nd the influence of chnging process prmeters. Using virtul DOE in the design phse nd during process optimiztion provides unique possiility to c d Figure 21: Predicted distortion in the prt for different support frme configurtions. mx Deformtion, mm [-] Figure 22: Results of virtul Design of Experiments. Mximum prt deformtion during het tretment for different solution tretment tempertures nd times. 001 Solution Tretment - Time, s 32

14 control distortion nd meet tight tolernce requirements. Detiled simultions of specific process steps nd the integrtion of results from the different mnufcturing processes provide fundmentl understnding of the complex conditions in the csting nd het tretment processes. As this virtul pproch cn e pplied lredy during csting nd process design, it offers the opportunity to void most of the currently performed experiments nd mesurements from production, where expensive chnges to the process or design dely the production significntly. It lso helps reduce costly nd time consuming tool chnges or modifictions of het tretment supports. The systemtic implementtion of virtul Design of Experiments into the product nd process development chin is powerful methodology to estlish roust process conditions efore the first csting is mde. Dr. Jesper Thororg, Dr. Jörg Klinkhmmer und Dipl.-Ing. Heinz-Jürgen Gspers, MAGMA Gießereitechnologie GmH, Achen Literture [1] Klinkhmmer, J.; Thororg, J.: Modeling of mechnicl contct conditions in csting. Proc. 12th MCWASP, S.L. Cockcroft & D.M. Mijer, Pp [2] Thororg, J.; Klinkhmmer, J.; Heitzer, M.: Integrted modeling of trnsitions in mechnicl conditions during csting nd het tretment. Proc. Int. Conf. On Modelling of Csting, Welding nd Advnced Solidifiction Processes, MCWASP XIV, Awji Yumeuti, Jpn, [3] Simo, J. C.: Computtionl inelsticity. Springer Verlg, New York, [4] Frost, J.; Ashy, F.: Deformtion-Mechnism Mps: The Plsticity nd Creep of Metls nd Cermics. Pergmon Press, [5] MAGNA Cosm Soest. [6] Gspers, H.-J.; Thororg J.: Design optimiztion of het tretment support frmes for luminum lloy structurl cst prts using virtul experimenttion. 22. IFHTSE Europen conference on het tretment, Venice, [7] Herrmnn, C.; Pries, H.; Hrtmnn, G.: Energie- und ressourceneffiziente Produktion von Aluminiumdruckguss, Ergenisse des Verundforschungsprojektes ProGRess. Finl report, Bundesministerium für Bildung und Forschung, Appendix 1: Thermo-mechnicl constitutive model. Thermo-mechnicl constitutive model Thermo-mechnicl modeling of the csting nd het tretment process hs to consider the complex ehvior of the mteril response nd the interction etween the csting mteril nd the surrounding dies nd support frmes. One of the min concerns is to model the response of the mteril t different temperture levels, on different time scles nd sometimes with different strin rtes, which is governed y different deformtion mechnisms. A unified creep formultion is used s the fundmentl constitutive lw, [4]. The model is sed on Norton s power lw nd includes y tht, strin rte sensitivity nd the possiility to descrie creep t elevted tempertures: The two properties A nd m descrie the strin rte sensitivity. The Arrhenius expression scles the response ccording to the temperture dependency nd the model is therefore pplicle over wide temperture rnge. The temperture dependency is governed y the ctivtion energy, Q. The reference stress σ ref descries the isotropic strin hrdening y clssicl power lw, where the inelstic strin, ε in, is used to cpture the effect of hrdening when e.g. disloctions re piling up nd nneling when the temperture is elevted, ccounting for diffusion processes. Tensile curves Creep curves The hrdening response is governed y two temperture dependent properties, the initil reference stress σ 0,ref nd the hrdening prmeter n. The response of the creep eqution nd the hrdening lw cn e illustrted y clssicl creep curves nd the strin rte dependent tensile curves, see Figure. Clirtion of the thermo-mechnicl properties is sed on lrge rnge of tensile tests nd creep tests t different temperture levels. The tensile tests re typiclly performed t different strin rtes t intermedite nd high tempertures to get informtion out the strin rte sensitivity. Creep tests re performed t high tempertures to get informtion for the het tretment process nd for slow cooling csting processes, where stress relxtion is importnt. 33

15 Perspektiven Appendix 2: Csting process overview. Summry of the different process steps during csting nd the typicl oserved ehvior of the mechnicl fields. Csting Process Overview Thermo-mechnicl modeling of the csting nd het tretment process hs to consider the complex ehvior of the mteril response nd the interction etween the csting mteril nd the surrounding dies nd support frmes. One of the min concerns is to model the response of the mteril t different temperture levels, on different time scles nd sometimes with different strin rtes, which is Solidifiction nd cooling inside the die Stresses uild up during cooling due to constrints from the die Plstic strin is generted due to the constrined deformtion Check temperture level to evlute when the die should e opened Evlute the level of the shrinkge fctor pplied to the die Ejection process Stresses re relxed Distortion uilds up during die open nd ejection Check if the force levels on the pins re similr or if some pins experience less or higher forces thn others Compre the contct pressure when the numer of pins nd the loction of the pins re chnged Enlrge the design spce for the cooling system Cooling/Quenching outside the die Therml contrction governs the finl size nd shpe of the prt Stresses uild up for high therml grdients evlute how the die open temperture ffects the finl size due to different levels of free therml contrction Check how different cooling histories ffect the level of the stresses Trimming step Stresses re redistriuted when the gting nd other mterils re removed from prt Check if high stresses uild up when lod crrying mteril is removed 34

16 Appendix 3: Het tretment overview. Summry of the different het tretment steps nd how the therml lod influences the mechnicl ehvior. Het tretment overview Thermo-mechnicl modeling of the csting nd het tretment process hs to consider the complex ehvior of the mteril response nd the interction etween the csting mteril nd the surrounding dies nd support frmes. One of the min concerns is to model the response of the mteril t different temperture levels, on different time scles nd sometimes with different strin rtes, which is Positioning on support frme nd mpping results Designing n pproprite support frme for the het tretment process, e.g. to compenste for csting distortions during solution tretment Positioning of the deformed prt optimize the support frme to meet the requirements of the reference/ trget geometry Solution tretment Initil conditions from the csting process, i.e. stresses nd deformtions Relxtion of the stresses during heting nd solution tretment process, rottion nd trnsltion of mechnicl fields Deformtions due to grvity nd the influence of the loction of the support frme Check if stresses re relxed t the end of the process step nd the deformtion level is cceptle Quenching Stresses uild up during quenching, depending on the cooling rtes nd therml grdients Therml contrction reduce the size of the prt Check if the cooling conditions promote unwnted stresses due to the therml grdients Artificil geing Moderte stress relxtion due to the elevted temperture level The influence of the temperture level on finl stress level could e checked Cooling Therml contrction reduces the size of the prt The finl shpe of the prt cn e evluted nd compred to the reference geometry nd mesurements if ville 35