Effects of process parameters on quality of squeeze casting A356 alloy

Transcription

1 Effects of process prmeters on qulity of squeeze csting A356 lloy Q. M. Chng* 1, C. J. Chen 1, S. C. Zhng 1, D. Schwm 2 nd J. F. Wllce 2 This investigtion hs studied the influence of the vrious csting vriles on the qulity of indirect squeeze cstings primrily of luminium lloys. The vriles studied include gting design, filling velocity nd squeezing pressure. The qulity of the die csting ws ssessed y n nlysis of oth their surfce condition nd internl soundness. The cvity filling ptterns with fnned gtes nd stright gte re compred. Stright gtes re prone to cuse jetting of the metl strem even t low velocities while fnned gtes llow use of higher fill velocity without excessive jetting. A higher metl pressure provides more complete fill of the die including improved compenstion for solidifiction shrinkge. The gte velocity for cvity filling process is one of the most importnt fctors for ttining sound csting. Either too slow velocity or too rpid gte velocity cn cuse such defects s misrun, indictions, jetting vortex, ir entrpment, etc. Furthermore, the computer models using the UES ProCst softwre re conducted. Keywords: Squeeze csting, Gte velocity, Squeezing pressure, Filling pttern Introduction The squeeze csting of luminium lloys is rpidly developing technicl process tht offers the potentil for widespred utilistion nd growth. The process hs the cpility of producing ner net shpe cstings tht re essentilly pore free. 1,2 This permits the solution het tretment of the cstings so tht, fter selective ging, excellent mechnicl properties cn e ttined. It lso llows the squeeze cstings to e welded, s needed for different structures, without gssing or listering. 3 5 These dvntges nd the improved mechnicl properties result from severl fetures of squeeze csting. The steel dies used hve close dimensions nd detils of conventionl high pressure die cstings. However, thicker runner systems nd, prticulrly, lrger gtes re used compred to regulr pressure die cstings. 6 The proper loctions of the gtes nd mintining high pressure on the molten lloy nd the use of pressure pins (when needed) during solidifiction permits the csting to e solidified under sufficient pressure to void nerly ll shrinkge. 7,8 The high pressure during solidifiction keeps the molten metl in direct contct with the die surfce, providing true to die dimensions in the csting. 5 The filling speeds or gte velocities re reltively low, generlly less thn 5 ft s 21 so tht trpped gs in the csting my e usully voided with proper venting. The verticlly oriented shot sleeve on the equipment employed on this reserch 1 Key Lortory for Ferrous Metllurgy nd Resources Utiliztion of Ministry of Eduction, College of Mterils nd Metllurgy, Wuhn University of Science nd Technology, Wuhn , Chin 2 Cse Western Reserve University, Clevelnd, OH 44106, USA *Corresponding uthor, emil qmchng@163.com is filled efore the forwrd stroke of the plunger strts so tht entrpped gs from this source is voided, nd thus, the net result is pore-free or nerly pore-free prt. 1,9 The close contct with the die surfce during solidifiction results in rpid solidifiction of the csting. This rpid solidifiction produces fine secondry dendrite rm spcing in the cstings so tht good strength nd ductility cn e ttined. 3,8,9,10 These excellent properties re reltively high in the s cst condition nd re enhnced further in the het tretle lloys y the excellent response to solution het tretment. Since the process minimises oth gs porosity nd shrinkge cvities, excellent properties re ttined. These properties hve een shown to e equivlent to wrought lloys in mny instnces. 1,11 Although this process hs mny ovious dvntges in producing prts of light metls tht cn e utilised in structurl pplictions, the full potentil cn only e relised fter the process hs een optimised. 3 This reserch ws undertken to investigte the vriles tht will ffect the qulity of squeeze cst prts nd to indicte the est comintion of properties to provide cstings of the desired qulity. Approprite control of the process requires tht the influence of numer of fctors on csting qulity e evluted. 1,3 Some of this informtion is presently known for other ssocited csting procedures, such s permnent moulding nd high velocity, high pressure nd die csting. However, the direct trnsltion of informtion from these processes will not provide the optimum for squeeze csting ecuse of the different gting, die filling nd solidifiction ptterns tht pply. In this pper, the influence of the vrious csting vriles on the qulity of indirect squeeze cstings is investigted. 30 ß 2010 W. S. Mney & Son Ltd. Received 23 Mrch 2008; ccepted 28 April 2009 DOI / X Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1

2 12?7 mm plte with stright gte; 6?4 mm plte with fnned gte 1 Dimensions of squeeze csting plte Experimentl sets The experiment ws conducted on the 350 t UBE high pressure squeeze die csting mchine. The squeeze csting mchine is equipped with n electric heted 75 kw luminium melting furnce, hot oil die heter cple of heting the oil to 288uC nd Visi-Trk process monitoring system, which is redily to mesure the squeezing pressure nd the plunger tip speed. A portle nd utomtic die luriction sprying system is ville for use. The mchine is equipped with plte insert die shown in Fig. 1. The plte insert die llows the csting of pltes tht re mm with thicknesses of 6?4, 12?7 mm nd thicker pltes if required. The chnges in plte thickness nd gte dimension re ccomplished y removle inserts for the runner nd gte depending on the thickness of the csting. Results nd discussion Flow pttern The flow pttern of the molten metl into plte ws mesured using incomplete shots nd y mens of simultion with ProCst. This work ws conducted on oth 12?7 nd 6?4 mm pltes with the gte dimensions shown in Fig. 1. experimentl results; simultion results 2 Flow ptterns in 12?7 mm plte with stright gte t high filling velocity Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1 31

3 simultion results; experimentl results 3 Flow ptterns in 12?7 mm plte with stright gte t low filling velocity Figure 2 shows the filling pttern in 12?7 mm plte with the stright gte t velocity of 1?27 m s 21. At this high filling velocity, well formed molten metl jet hs reched the ck wll nd is strting to ounce ck. Two vortexes form in oth sides of the jetting strem, nd ir my e trpped in it. The simultion results re compred with the experimentl results nd show well greement with ech other, proving tht it is now possile to use ProCst to predict the ctul flow fter some djustments were mde to the progrm. When the filling velocity is reduce to 0?15 m s 21, the metl fills the cvity slowly while mushrooming from the gte inwrds, s shown from oth the experimentl work nd the computer simultion in Fig. 3. The mould cvity cnnot e completely filled efore the flow front solidified due to the lower filling velocity. The 6?4 mm plte ws lso tested with fn type of gte. The flow pttern with fst gte velocity of 1?02 m s 21 is illustrted in Fig. 4. In this cse, the die is filled much more uniformly through the fnned thin gte 4 Flow ptterns in 6?4 mm plte with fnned gte t high filling velocity 32 Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1

4 5 Effect of squeezing pressure on density of 12?7 mm pltes with stright gte even t this reltively high velocity, illustrting the dvntges of fnned gte for squeeze cstings. Effect of pressure nd filling velocity A series of 12?7 nd 6?4 mm plte cstings were cst from the A356 lloy using vrile metl velocities nd pressures. These cstings were exmined to determine their density using the Archimedes procedure. The surfce qulity ws evluted y visul exmintion, which ws rted on scle of seven for perfect csting nd one for defective csting, s shown in Tle 1, while the internl qulity ws evluted y shrinkge re frction exmintion of X-ry rted on scle rnge from seven to one, s shown in Tle 2. For the12?7 mm plte with stright gte, the effect of pressure on csting density nd shrinkge is illustrted in Figs. 5 nd 6 respectively. As expected, the density of the csting prts significntly incresed with higher pressure pplied. However, when the pressure pproches criticl vlue, e.g. 87?5 MP for density nd 86?2 MP for shrinkge rting in this cse, the density nd shrinkge rting of the prts remin constnt nd cnnot increse nymore. This mens tht the extr pressure cnnot tke ny effect on the molten metl in the cvity when pressure excesses the criticl vlue. The effect of pressure on the visul rte of csting prts ws similr to tht on the density nd shrinkge s shown in Fig. 7. The higher pressure mintins etter contct with the die surfce, nd it improves the surfce qulity in this respect. The effect of the gte velocity on the outside surfce condition (visul rte) is lso plotted in Fig. 8. The 6 Effect of squeezing pressure on shrinkge rting of 12?7 mm pltes with stright gte results illustrte tht it is necessry to fill the 12?7 mm plte csting firly quickly with gte velocities of out 0?18 m s 21 in order to void misrun. In other words, elow certin velocity threshold of out 0?18 m s 21, it ecomes difficult to completely fill 12?7 mm plte under the experimentl conditions tht were employed. A steep drop in the qulity rting s filling velocity increses is evident in the plot when the gte velocity excesses 0?26 m s 21 ; tht is, the gte velocity hs to e kept to reltively low levels to void considerle turulence in the die cvity nd ir entrpment. The drop of the visul rting t higher gte velocities is cused y the jetting of the molten metl into the cvity. This jetting of molten metl tends to ounce off the wll opposite to the gte nd crete two swirling strems of metl while trpping ir in the centre of the vortexes, s depicted in Fig. 2. Just like in the cse of the12?7 mm plte, the dominting fctors in the ttinment of sound cstings re filling nd jetting. Gte velocity need to e fst enough s to ensure complete filling efore the metl solidifies, yet slow enough to prevent jetting nd ounce ck. The est qulity pltes re mde with the optimum pressure nd gte velocity. Figure 9 shows tht the effect of the gte velocity on shrinkge hs the similr tendency, except the optimum gte velocity tht rnges from 0?18 to 0?61 m s 21. Figure 10 illustrtes the effect of pressure on csting shrinkge rte in 6?4 mm plte with fnned gte. Compred with tht in 12?7 mm plte with stright gte, the totl 30 tested csts hve etter shrinkge rte, even t lower pressure, e.g. 20?7 MP. The shrinkge rtings re improved grdully s the Tle 1 Definition of visul rting Rting scle Smples Description Prtil filling Significnt cold shuts Some cold shuts Significnt shrinkge Little shrinkge Sound csting With rough surfce Perfect csting Tle 2 Definition of shrinkge rting Rting scle Shrinkge re frction mesured y X-ry >9% 7% 5% 3% 2% 1% 0% Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1 33

5 7 Effect of squeezing pressure on visul rting of 12?7 mm pltes with stright gte squeezing pressure increses. The effect of the gte velocity on the visul rte shown in Fig. 11 is much different with tht s shown in Fig. 8. A higher gte velocity is required to void misrun due to the smller thickness of the cst. At the sme time, the criticl gte velocity for completely filled cst should e greter thn 0?28 m s 21. A fnned gte promotes jetting less thn stright gte. As result, smooth filling pttern cn e oserved, nd sound csting cn e otined even with higher gte velocity 0?97 m s 21. As indicted in Fig. 11, the window of gte velocities tht is conducive for sound pltes is quite rod. Microstructure The typicl microstructures of squeeze cst nd permnent mould cst in the s cst nd T6 conditions re illustrted side y side in Figs. 12 nd 13 respectively. Microscopic exmintion revels tht ppliction of the squeeze pressure during solidifiction of this lloy ffects the microstructure in oth the s cst condition nd T6 conditions. In the s cst condition, squeeze csting refines the primry phse nd the eutectic silicon phse. In the squeeze cst, the silicon hs firous morphology, while in the permnent mould cst, silicon hs plte or needle shpe. Very low porosity levels re encountered in squeeze cst prts. In the squeeze cst in the T6 condition, the Si prticles re smller nd rounder. The Si prticles of oth squeeze cst nd permnent mould cstings re spheroidised y the T6 tretment. The typicl impct frctogrphs in different conditions re shown side y side in Figs. 14 nd 15 respectively. Scnning electron microscopy frctogrphs 9 Effect of filling velocity on shrinkge rting of 12?7 mm pltes with stright gte show tht squeeze cst hs more ductile frcture thn permnent mould cst. The differences in the microstructures of squeeze cst nd permnent mould result indirectly from the pplied pressure. When the die is mde of metl, the die/csting interfce ecomes the gretest resistnce to het trnsfer. Due to the contrction of most metls nd expnsion of the mould during solidifiction, detchment of the csting from the die wll tkes plce once the initil solid shell of the csting hs sufficient strength to hold the remining molten metl. Consequently, n ir gp is formed etween the die wlls nd csting, which considerly increses resistnce to the het trnsfer. In squeeze csting, the pplied pressure on the csting forces the initil solid shell to remin in contct with the die. The intimte metl die contct is mintined y the plstic deformtion of the csting, throughout solidifiction. This leds to very fst het trnsfer rtes, high cooling rtes nd incresed temperture grdients in the csting. The cooling rte in squeeze csting cn therefore e much fster thn tht of permnent moulds, depending on pplied pressure, die prehet temperture, cooling line loction, csting thickness, die luricte nd other fctors. The rpid cooling rtes of squeeze cst prts gretly refines the microstructure nd chnges the silicon phse from the plte or needle shpe to firous morphology. The rpid cooling is potent Si modifier, equl to Si modifiction y dditives during metl melting. No significnt microporosity ws detected in the squeeze cst, ut some porosity nd interdendritic microporosity were found in the permnent mould cst s illustrted in Figs. 12 nd 14. This difference cn lso 8 Effect of filling velocity on visul rting of 12?7 mm pltes with stright gte 10 Effect of squeeze pressure on shrinkge rting of 6?4 mm pltes with fnned gte 34 Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1

6 11 Effect of filling velocity on visul rting of 12?7 mm pltes with fnned gte e explined y the ppliction of pressure to the squeeze cst. Corse intermetllic phses cnnot e found in either the squeeze cst or permnent mould csts ecuse this is high qulity A356 nd the impurity level is controlled, especilly the content of iron tht is less thn 0?2%. Conclusions A comprison of the results does show tht, generlly, squeeze cst properties re somewht etter thn some other csting processes ecuse of the soundness of the cstings nd the rpid solidifiction of the cstings producing the fine dendrite rm spcing nd the fine distriutions of the silicon phse of the eutectic. A higher metl pressure exerted on the solidifying metl through the piston provides more complete fill of the die, including improved compenstion for solidifiction shrinkge. The density of the squeeze cstings lso incresed with higher squeezing pressure. A proper gte velocity is necessry for ttining sound squeeze csting prts. Some defects, such s misrun, indictions, etc., cn e oserved if the gte velocity is too low. More rpid filling will rek up the strem of metl entering the cvity nd entrp more ir or gs into the molten metl. The density of the squeeze cstings lso incresed with slower filling velocities ecuse of the mintennce of solid prt fill with the stremers of molten metl entering the die tht reduce the mount of entrpped gs. permnent mould csting; squeeze csting 12 Microstructures of permnent mould nd squeeze cst s cst condition permnent mould csting; squeeze csting 13 Microstructures of permnent mould nd squeeze cst t T6 Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1 35

7 permnent mould csting; squeeze csting 14 Impct frctogrphs of permnent mould nd squeeze cst t csting condition permnent mould csting; squeeze csting 15 Impct frctogrphs of permnent mould nd squeeze cst t T6 condition Gte size nd shpe re very importnt, since it must remin open until the csting is solidified nd pressure is mintined on the solidifying csting. Fnned gtes, prticulrly on the smller section cstings, void jetting effects t the gte end. The fn type gte helps ccomplish rpid fill without high velocities. Stright gtes re prone to cuse jetting of the metl strem even t low velocities. Computer models using the UES ProCst finite element softwre cn, fter suitle djustments, predict the flow pttern nd solidifiction sequence in the die cvity. Acknowledgement The uthors cknowledge support of the Deprtment of Energy through the Cst Metl Colition progrmme, nd direction of the work ws provided y the High Integrity Cstings committee of the North Americn Die Csting Assocition. References 1. M. R. Ghomshchi, A. Vikhrov: J. Mter. Process. Technol., 2000, 101, M. S. Yong nd A. J. Clegg: Foundrymn, 1999, 92, (3), S. Rjgopl nd W. H. Altergott: AFS Trns., 1985, 93, P. Bln, R. M. Pilli, K. G. Stynryn nd B. C. Pi: Int. J. Cst Met. Res., 1993, 63, S. Kleiner, O. Beffort, A. Whlen nd P. J. Uggowitzer: J. Light Met., 2002, (2), D. M. Stefnescu: ASM hndook, Vo1?15, Csting, metls hndook, solidifiction of eutectic lloys, luminum silicon lloys ; 1988, Mterils Prk, OH, ASM Interntionl. 7. J. R. Morton nd J. Brlow: Foundrymn, 1994, 87, P. Vijin nd V. P. Arunchlm: J. Mter. Process. Technol., 2005, 170, G. A. Chdwick nd T. M. Yue: Met. Mter., 1989, 51, H. Hu: J. Mter. Sci., 1998, 33, J. N. Pennington: Mod. Met., 1988, 44, (1), Interntionl Journl of Cst Metls Reserch 2010 VOL 23 NO 1