Microstructure evolution of A1-Si semi-solid slurry by gas bubble stirring method

J. Cent. South Univ. Tecbnol. (2011) 18: 1789-1794DOI: 10.1007511771- -011-0903-3包SpringerMicrostructure evolution of Al-Si semi-solid slurry bygas bubble stirring methodZHANG Lei(张磊), DONG Xuan-pu(董选普), LI Ji-qiang(李继强}, LI Kan(李侃)',ZHANG Zong-kui(张宗奎)，WANG Wen-jun(王文俊)', FAN Zi-tian(樊自田)'1. State Key Laboratory of Material Forming and Mould and Die Designing,Huazhong University of Science and Technology, Wuhan 430074, China;2. School of Mechanical and Energy, Ningbo Instute of Technology, Zhejiang University, Ningbo 315100, ChinaCentral South University Press and Springer-Verlag Berlin Heidelberg 2011Abstract: A novel technique of introducing gas bubble strring during solidification was studied to prepare AIl-Si semi-solid slury.The microstructure evolution of the slurry during slow cooling process after string was investigated. The effects of the solidificationrate on the microstructure of the semi: solid sury were investigated under three dfferent solidification conditions. The results showthat fine non-dendritic slury can be obtained using the gas bubble sirring method. Ripening and coarsening of primary AI grains areobserved during the slow cooling process, and at last coarsened eutectic Si appears. Primary Al grains with different sizes andeutectic Si are obtained, corresponding to three different solidification rates.Key words: gas bubble sirring; semi- solid slurry; soldification rate; microstructurethe slurry happens, which directly relates to the1 Introductionmicrostructure and corresponding mechanical propertiesof the final rheoformed parts [13]. ATKINSON and LIUSemi-solid metal processing technology was first[14] had investigated microstructural coarsening oproposed by SPENCER et al [1]. Compared withsemi-solid aluminum alloys during slurry making. GUOtraditional casting and forging process, semi-solidet al [15] had studied pressurized solidification of A356forming has advantages such as lower processingsemi-solid aluminum in die casting, their results showedtemperature, smaller deformation resistance, lessthat higher density and mechanical properties of the finalmacro- segregation and solidification shrinkage [2- -4].parts could be obtained with the increase of fillingVarious techniques of semi- solid metal processing havepressure. However, there are not much researches on thebeen developed, such as mechanical stirring [5]，microstructure evolution of the slurry during holdingelectromagnetic stirring [6- -7]， strain-induced meltactivation [8], ultrasonic vibrations [9], cooling slopeevolution of eutectic silicon. Also few studies havecasting [10], liquidus casting [11] and melt conditionerfocused on the effects of different solidificationdirect chill casting [12]. However, each of theseconditions on the microstructure of the solidified slurry,techniques has its disadvantages such as high cost andincluding the morphologies of primary Al and eutecticenergy consumption, inhomogeneous or impure slurry, orsilicon.the process is too complicated and difficult to control.n the present work, the gas bubble stiringTherefore, novel methods of preparing semi-solid slurrytechnique was applied to achieving semi-solid slurry.still need to be explored, in order to reduce theThis process used gas bubbles as the medium to stir theproduction cost and simplify the process.melt during the initial stage of solidifcation. A rotatingFor rheoforming, two processes are involved, thetube-like graphite agitator was applied, which is differentfirst process is the slurry making, and the second processfrom the static graphite diffuser used by WANNASIN etis the forming of the slurry in the mold. Several studiesal [16]. Three different solidification conditions werehave shown that both the slurry making and the slurryobtained by pouring the semi-solid slurry into an ironforming processes are important because solidifcation ofmold, a copper mold cooled in water, and directlyFoundation item: Project(50775085) supported by the National Natural Scicnce Foundation of China; PBasic Recscarch and Operating Expenses of Central College, China; Projcct(2008A6100I中国煤化工Boe FoundationReceiveddate: 2010 09-5ceteate2011-02-05YHCNMHGCorresponding author:Xuan-pu, Professor, PhD; Tel: +86 27 -87558252; E-mail: xpdonghust@yahoo.cn1792J. Cent. South Univ. Technol. (2011) 18: 1789-1794Fig.5, which mainly consists of primary a-Al andeutectic Al+Si+Al3.2Sio.47.(a”一A!●一Al3;21Sio.47口一Si”d4010Melt:i。g:。(b)10 203040506070 80 9020/(°)Fig.5 X-ray diffaction pattemn of Al-Si semisolid slurry(quenched in water)3.2 Micrographs corresponding to different solidifica-tion ratesFigure 6 shows a schematic diagram of the moldsused in this study. Thermocouples were installed near theWatercenter of the molds to record the temperatures to plotsolidifcation curves of the slurry. Figure 7 represents the(solidification curves of the slurry in three different molds.Solidification rates of about 0.55, 1.5 and 5.25 °C/s canbe roughly estimated from the curves, corresponding toFigs.7(a), (b), and (C), respectively. The morphologies ofthe slurry solidified under these rates are shown in Fig.8.It is obvious that both the morphologies of the primaryAl and the eutectic Si are different. The eutectic Sicoarsens like laths and disorderly distributes in the Almatrix when the solidification rate is slow, as shown inFig.8(a). The eutectic Si is refined like needles, anddistributes along the primary Al grain boundaries, withFig.6 Schematic diagrams of molds (not to scale): (a) Iron mold;the increase of the solidification rate, as shown i(b) Copper mold cooled in water; (c) Water quenchingFig.8(b). With the solidification rate of 5.25 °C/s, as .700shown in Fig.8(c), the eutectic Si is extremely refined.(a)- - Water quenchingFigure 8(d) shows the morphology of the eutectic Si in600(b)一Copper mold cooled in water(c)- Iron moldFig.8(c), and small dendritic Si is observed. At the samee 50time, the primary Al grains in Fig.8(c) are smaller thanthose in Figs.8(a) and (b). The above results indicate that台400a mold with fast enough solidification rate is needed, in300 i(C)order to obtain final parts with semi-solid microstructure.The different microstructures under different官200.solidification conditions can be explained by the growth100of primary Al particles and the concentration of soluteelement Si in the bulk liquid. Studies have shown that Si10020300400500600_700 800will be rejected into the remaining liquid with the growth中国煤化工of primary Al particles, so the liquid becomes more andFig.7 SolidifcationfYHCNMHG"molds1794J. Cent. South Univ. Technol. (2011) 18: 1789-1794103-107.200s, 413/414: 186- -191.[9] JIAN X, XUH, MEEK TT, HAN Q. Effect of power ultrasound on[14] ATKINSON H V, LIU D. Microstucural coarsening of semi-solidsolidifcation of alumninum A356 alloy 0. Mater Lett, 2005, 59(2/3):aluminium aloys [] Mater Sci Eng A, 2008, 496(1/2): 439 446.190-193.[15] GUO H M, YANG x , WANG J x. Pessurized sidicaio of[10] HAGA T, KOUDAT, MOTOYAMA H, INOUE N, SUZUKI s. Highsemi-solid aluminum die casting alay A356 D J Alloys Compd,speed roll caster for aluminium alay strip [C] Proceedings of the :2009, 485(12): 812-816.ICAA7 on Aluminiumn Alloys: Their Pbysical and Mechanical[16] WANNASIN J, MARTNEZ R A, FLEMINGS M C. GrainProperties. Charlttesville, VA: Publ Trans Tech Publications, 1998:refinement of an aluminum alloy by introducing gas bubbles during327- -32.solidifcation [0 Scripa Mater, 2006, 5(); 15-118.[1] XIA K, TUASIG Q Liquidus casting of a wrought aluminum alay[1] NAFISI s, GHOMASHCHI R Effect of strring on solidifcation2618 for thixoforming [J]. Mater Sei Eng A, 1998, 246(1/2): 1-10.pattemn and alloy distribution during semi- solid-metal casting [0][12] HAGHAYEGHIA R, ZOQUIB E J, GREENC N R, BAHAIA H. AnMater Sci Eng A, 2006, 437(2): 388 -395.investigation on DC casting of a wrought aluminium alloy at below[18] GrIFITHS w D. MCCARTNEY D Q The efe ofliquidus termperature by using melt conditioner [U. J Alloys Compd,electromagnetic stirring during solidification on the structure of2010, 502: 382 -386.Al-Si aloysy [n]. Mater Sci Bng A 1996, 216(12):47 60.[13] JORSTADJL, PAN Q Y, APELIAN D. Solidifcation microstructure(Edited by he Yaun-bIn)affcting ductility in semi-solid-cast products [小Mater Sci Eng A, .中国煤化工MHCNMH G .