Effects of vibration and grain refiner on microstructure of semisolid slurry of hypoeutectic Al-Si a Effects of vibration and grain refiner on microstructure of semisolid slurry of hypoeutectic Al-Si a

Effects of vibration and grain refiner on microstructure of semisolid slurry of hypoeutectic Al-Si a

  • 期刊名字:中国有色金属学会会刊(英文版)
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  • 论文作者:ZHAO Jun-wen,WU Shu-sen,XIE Li
  • 作者单位:State Key Lab of Materials Processing and Die & Mould Technology
  • 更新时间:2020-11-03
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Available online at www.sciencedirect.com●CIENOEdoinectTransactions ofNonferrous Metals骂PSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 18(2008) 842-846Presswww.csu.edu.cnvysxb/Effects of vibration and grain refiner on microstructure of semisolid slurry ofhypoeutectic Al-Si alloyZHAO Jun-wen(赵君文),WU Shu-sen(吴树森), XIE Li-zhi(谢礼志), AN Ping(安萍), MAO You-wu(毛有武)State Key Lab of Materials Processing and Die & Mould Technology,Huazhong University of Science and Technology, Wuhan 430074, ChinaReceived 31 October 2007; accepted 15 February 2008Abstract: The efects of vibration and grain refiner on the microstructure of semisolid slurry of hypoeutectic Al-Si alloy werestudied. The impact of vibration on the convection of liquid was conducted by using a system of water-particle tracer. The 356 melt attemperature of 630-660 C with or without grain refiner Al-5%Ti-1%B was poured into a metal cup as the vibrating vessel, then itwas cooled to 590 -610 C in the semisolid zone and kept for some time, subsequently vibration with dfferenet frequencies wasapplied. The results show that the primary a(AI) particles become finer and rounder with the increase of vibration frequency. Theslurry with primary a(AI) equivalent particle diameter(EPD) of about 90 um and average shape coffcien(ASC) of about 05canbeprepared under vibration of 20 Hz. With the combined action of vibration and grain refiner Al-STi-B, even smaller and rounderspheroids with EPD of about 85 μm and ASC of about 0.6 are obtained.Key words: mechanical vibration; hypoeutectic Al-Si aly; semisolid metal; grain refinerWhat's more, the combined effects of vibration and1 Introductionrefiner on microstructure of semisolid slurry have beenlttle studied. Therefore, this work aims at studying theOver the last decade, a lot of attentions wereeffect of low-frequency vibration, grain refiner and theirattracted to metal rheoforming process[1-6], as it hascombined effects on microstructure of hypoeutectic Al-Simany advantages, such as high mechanical properties ofalloy.formed components, low cost and easy operation. As aresult, a number of novel slurry preparation methods and2 Experimentalrheoforming techniques have been invented, for example,semi -solid rheocasting[1], new rheocasting[7 - 8], liquidusThe self-developed equipment shown in Fig.1 wascasting[9], twin-screw rheomoulding[ 10], rotating barrelemployed in present trials, which consists of vibrationrheomoulding[11], swirled enthalpy equilbrationcontroller, vibrating apparatus(VA), heating fumace, PIDdevice[ 12] and controlled nucleation method[13].temperature controller, etc. The simplified schematic ofNevertheless, efforts were still made to develop newthe VA is shown in Fig.2. It consists of a vibration motorways of preparing semi-solid slurry with lower cost andattached to the vibrating vessel via a vessel holding tablesimpler process[14]. As vibration is easy to operate andsupported by a group of springs. The vibrating vesselneeds less expensive equipment, it could be anshall be fixed by bolts on the vessel holding table of thealternative for preparing semisolid slury.VA. The vibrating time, frequency and amplitude of theAlthough it was explored to prepare semisolidVA may be adjusted by the vibration controller. Theslurry with ultrasonic vibration[15], lttle study has beenpreheating temperature and isothermal holdingperformed on preparing slurry of semisolid metal btem中国煤化工the PID temperaturemechanical vibration with a frequency less than 50 Hz.YHCNMHGFoundation item: Prjec(50775086) supported by the National Natural Science Foundation of China; Project (2007AA032557) supported by the Hi-techResearch and Development Program of ChinaCorresponding author: ZHAO Jun-wen; TelFax: +86-27-87556262; E-mail: zhajunwen1982@yahoo.com.cnZHAO Jun-wen, et al/Trans. Nonferrous Met. Soc. China 18(2008)843where Lr is the overall length of measured lines used inthe software, N is the mumber of particles passed throughVibrating vesselby the measured lines.HeatingPID temperatureThe shape of a primary a(AI) particle iscontrollerVibrationcharacterized by shape coefficient, which is defined as口口口Isp=4n4(2)where A is the cross section area of a specific particleVibratingin the micrograph, and Lp is the circumference of theapparatuscorresponding particle. Sp varies in the range of 0-1.Fig,1 Equipment for mechanical vibrationThe greater the value of Sp is, the closer to a round theparticle is. The average Sp is regarded as the arithmeticaverage of shape coefficient of all the particles in ametallograph, which is named as average shapecoefficient(ASC).A group of physical simulation experiment utilizingVessel holding tablea system of water-particle tracer was conducted in orderto obtain some perceivable and qualitative informationregarding the impact of vibration on the convection ofVibration motorliquid. A transparent plastic beaker with 60 mm indiameter was fastened to the top table of the VA, inwhich water of 40 mm in depth and polystyrene particlesFig.2 Simplifed schiematic diagram of vibrating apparatusserving as tracing particles with diameter of about 360The raw material used in this study was commercialum and with density close to that of water were added.hypoeutectic Al-Si alloy 356 with a chemical3 Results and discussioncomposition (mass fraction) of 7.2% Si, 0.43% Mg,0.29% Fe and balance AL. According to Ref.[16], its3.1 Simulation with water and tracing particlessolidus and liquidus are approximately 554 C and 614C, respectively.The vibration amplitude of the equipment was aboutThe ingot was melted in a resistance fumace. After1.5 mm, and the vibration frequency of the VA variedhe melt was heated to 750 C, it was degassed forfrom8Hzto42Hz.15 min with argon gas through a graphite lance. ToFig.3 shows a series of streak photographs ofexamine the combined efects of grain refiner andparticles movement in water under vibration withvibration on the microstructure, 0.2% Al-5%Ti- 1%B wasdifferent frequencies. It is observed that flow isadded into the melt in one group of experiment. Thegenerated from the free surface of the fluid and spreadsmetal vessel was preheated to 530 C by the heatingdownwards, and the surface moves most intensively,furnace. After degassing the melt was cooled to awhich confirms the statement in Ref,[17]. When thetemperature of 630- 660 C. Then the VA was initiatedliquid is kept quiescent, i.e. at 0 Hz, parts of the particlesand a certain amount of melt was poured into the vessel.are uniformly suspended in the water while majority ofThe poured melt would be soon cooled down to 590-them float on the free surface. At 8 Hz, some particles610 C through thermal balance between the melt andare transmitted to the deep place in liquid whereas notthe vibrating vessel. At predetermined temperature andable to arrive to the bottorm of the container, as well asvibration time, some of the melt was subsequently drawnthe liquid surface fluctuates moderately. When thewith quarz tube in 6 mm diameter and quenched in roomfrequency is increased to 13 Hz, more particles aretemperature water.carried into the deeper liquid and the free surface movesThe quenched rods were cut, polished and etched byseverely. When the frequency reaches 17 Hz, the water0.5% hydrofluoric acid solution, and the microstructureand particles are completely mixed and the water gets soturbulent that sthe whnle vnlume nf licuid presents white,was observed and analyzed.The size of the primary a(AI) particles is estimatedand th中国煤化工g convection lastsover19by equivalent particle diameter(EPD) defined asC N M H Gins to move stably.u iMivaou UI loyuency, the surfacea=尔(1)goes more stable, and the particles carried to the bottom844ZHAO Jun-wen, et a/Trans. Nonferrous Met. Soc. China 18(2008)or lower parts of the container are reduced. Nevertheless,Fig.4 displays the comparison of the obtainedthe particles still move without any regularity in themicrostructure with different frequencies, and Fig.5whole volume of liquid, which implies a turbulentshows the analysis result of the EPD and ASC. It can beconvection for the bulk flow. In addition, the calculationseen that, when vibration is not imposed, the primaryof particle velocity was made by software based on thephase is dendrite with an EPD of more than 220 um andprinciple of Particle Image Velocimetry. And the averagea rather low ASC of0.18. With the increase of frequency,particle velocity above the frequency of 12 Hz wasthe dendritic structure converts to rosette and globular. Atestimated to be 60- 150 mm/s.12 Hz, the EPD is only half of that without vibration, i.e.110 um, while the ASC is more than twice of that3.2 Effects of vibration frequency on microstructurewithout vibration, i.e. 0.43. At the frequency of 20 Hz,of semisolid 356 slurrythe EPD of non-dendritic a(AI) crystals is 90 um, and theFrom the physical simulation experiment, it can beASC is 0.53. When the frequency is higher than 20 Hz,concluded that the vibration frequency is a criticalthe EPD increases a lttle while the ASC decreases a lttleparameter in preparing slury. In this group of experiment,compared with that at 20 Hz. Moreover, it is indicated infour frequencies were adopted, 0, 12, 20 and 35 Hz, andFig.4 that the non-dendritic primary a(AI) phasethe processing temperature and time were 605 C and 5homogenously distributes and is fine with introduction ofmin, respectively.vibration. .JCDh)Fig.3 Particles movement in water under vibration with dfferent frequencies: (a) 0 Hz; (b) 8 Hz; (c) 13 Hz; (d) 17 Hz; (e) 20 Hz; .(I) 25 Hz; (g) 30 Hz; (h)42 Hz(a中国煤化工MHCNMHG配大200um]Fig.4 Microstructures of semisolid 356 slurry processed by diferent frequency vibration: (a) 0 Hz; (b) 12 Hz; (C) 20 Hz; (d) 35 HzZHAO Jun-wen, et al/Trans. Nonferrous Met. Soc. China 18(2008)845ASC is the arithmetic average of shape fraction of all2400.55particles, it will accordingly become larger.220 t0.50.3.3 Effects of combined action of vibration and grain200十0.45区refiner on microstructure of semisolid 356 slurry160-0一Equivalent particle0.40Fig.6 shows the microstructure of semisolid 356slurry with only action of grain refiner. It can be seendiameter0.350一Average shapethat, compared with Fig.4(a), the primary particles have120|coefficient0.30much less dendritic. However, most of the particles arenot globular enough and not uniformly distributed. On100 t0.25 Qthe other hand, as can be observed in Fig.7, with the30 t-0.2combined action of vibration and grain refiner, the5048.15microstructure of the slurry distributes well, and theVibration frequency/Hzprhmary particles are fine and spherical.Fig.5 Effect of vibration frequency on EPD and ASCThe effects of combined action of vibration and grainrefineron microstructure of semisolid 356 slury areOHNO[18] examined the effect of low-frequencyshown in Fig.8. With only action of grain refiner (at 0 Hz),vibration on the metal solidification and argued thathe EPD is decreased by 36.4%, namely, from 220 μum tovibration affects the microstructure mainly from two.140 um, and the ASC is increased from 0.18 to 0.3.aspects, that is, enhancing the cooling efficiency throughWhen the combined action of grain refiner and vibrationresulted good contact of melt and wall of container, andis imposed, the EPD of the primary phase is reducedincreasing the number of nuclei formned on the wall andto100 um at 12 Hz. At 20 Hz the minimum of EPD andliquid surface, detached and brought to the bulk melt. Inmaximum of ASC are gained, ie. 85 μum and 0.61addition, the particles can be refined by thermalrespectively. In general, there is similar regularity of thefluctuation and nuclei multiplication in convectionASC and EPD for the semisolid microstructure eitherinduced by ocillation. For producing slurry by vibration,with only action of vibration or with combined action ofmultiplication and refinement of particles is furthervibration and grain refiner. In addition, it is also revealedundergone in the course of isothermal holding. With theincrease of vibration intensity, it is more effective thatthe nuclei generated at the wall and liquid surface wouldbe brought to the bulk melt due to more intenseturbulence in the melt. What's more, the temperature andmass distribution become uniform due to the convectionof the melt and isothermal holding, thus the dendriticformation and growth is depressed, which results in theround and refined particles.Further understanding of the effect of vibration on200umASC can be achieved by an equation proposed by WU etal[2], which relates the particle shape fraction with flowFig.6 Microstructure of semisolid 356 slury only with grainspeed of fluid relatively to the particle, that is3)where f is the particle shape fraction, and f=1 for acircle; R is the solidification rate, and v is the flow speedof fuid relative to the particle.Solidification speed is affected by several factorssuch as latent heat and heat release conditions. Inexperiment, it is reasonable to assume that thsolidification rate is constant. For the flow speed, with中国煤化工the increase of vibration frequency, it will becomeI 200 umincreasingly higher as the vibration becomes moreMYHCNMHGintense. Therefore, it can be deduced that f becomesFig.7 Microstructure of 356 slury with combined action oflarger from the above-mentioned equation. Since thegrain refiner and vibration (20 Hz)846ZHAO Jun-wen, et al/Trans. Nonferrous Met. Soc. China 18(2008)finer and rounder with the increase of vibration, and220slurry can be prepared with EPD of primary a(A1) about|(a)\o一Without grain refiner90 μm and ASC above 0.5 under vibration of 20 Hz.200-With grain refiner3) Rounder and finer spheroids, namely EPD of180about 85 μm and ASC of about 0.6, are obtained with the160combined action of vibration and grain refiner Al-5Ti-B.References120[1] MARTINEZ R A, FLEMINGS M C. Evolution of particlemorphology in semisolid processing [] Mtall Mater Trans A, 2005,36 (8); 2205 2210.0802] WU Shu-sen, WU Xue-ping, XIAO Ze-bui. A model of growth05101520 2530 35 40mophology for semi-solid metals []. Acta Materialia, 2004, 52:3519 -3524.Vibration frequency/Hz3] WU Shu-sen, ZHAO Jun-wen, WAN Li, LUO Ji-rong. Numericalsimulation of mould fllig in rheo-diecasting process of semi-solid0.60- (b)magnesium aloys [凹] Solid State Phenomena, 2006, 16/117: 554-0.55 t4] MAO Wei-min, ZHAO Ai-min, YUN Dong. ZHONG Xue you.0.50Preparation study of semisolid 60Si2Mn spring steel slurry [小. Acta0.45 tMetallurgica Sinica, 2003, 16(6): 83-488.5] WANG T, PUSTAL B, ABONDANO M, GRIMMIG T, BOHRIG-0.40-POLACZEK A, WU M, LUDWIG A. Simulation of cooling channel0.35●- Without grain refiner |rheocasing process of A356 aluminum aly using three phase0.30。一With gTain refinervolume averaging model [J]. Trans Nonferrous Met Soc China, 2005,15(2): 390-394.0.25 |6] GUO Hong-min, YANG Xiangijie. Preparation of semi-solid slurrycontaining fine and globular particles for wrought aluminum alloy2024 [] Trans Nonferrous Met Soc China, 2007, 17(4): 799-804.0.1505101520253035 407] UBE Industries Ltd. Method and apparatus for shping semisolidmetals. EPO 745694A1 [P]. 1996.8] TOSHIO H, KAPRANOS P Simple rheocastng proceses 0 JoumalFig.8 Effects of combined action of vibration and grain refinerof Materials Processing Technology, 2002, 130/131: 594- 598.9] PAN Ye, AOYAMA s, LIU Chi. Spherical structure and formationon EPD (a) and ASC (b)conditions of semi-solid Al-Si-Mg aly [C]/ SUN Guo-xiong, YUANHao-yang, YAO Rui-bo. Proceeding of the sth Asian Foundry Congress.in Fig.8 that the EPD and the ASC with combined actionNanjing: Southeast University Press, 1997: 443- 451.of vibration and grain refiner are decreased and increased[10]XIAOZe-hui,LUO Jirong, WU Shu-sen, LI Dong-nan.Performance of semi-solid slury produced by twin screw siringrespectively at the same frequency.mixer and rheo-diecasting process of AZ91D aly []. Journal ofSince many heterogeneous nuclei such as TiAl3 andWuban University of Technology-Mater Sci Ed, 20019(3): 81-85.TiB2 are created prior to the formation of a(AI) with the[1] KANG Yonglin, AN Lin, WANG Kai-kun, SUN Jan-lin, XIAOBang-guo, XU Chen-yang. Experimental study on theoforming ofaddition of Al-5Ti-B, and most of them become the basesemi-solid magnesium alloys [C]/ TSUTSUI Y, KIUCHI M,of heterogeneous nucleation, the nucleation probabilityICHIKAWA K. Proceedig of the 7h on Semi- solid Procssing ofof primary phase is enhanced. At a certain temperature,Alloys and Composites. Tsukuba, Japan, 2002: 287 -292.the solid fraction is relatively stable, therefore, the[12] DOUTRE D,HAY G WALES P. Semi-solid concentrationprocessing of metallic alloys. US6428636 [P]. 2002.increasing nuclei will result in smaller particles. When[13] WANG H, NING Z L, YAO X D, DAVIDSON CJ, JOHN D s.the melt is subject to vibration, the nuclei formed on theThixotropic structure formation in hypeutectic Al-Si alloys bycontrolled nucleation {J]. Materials Forum, 2004, 28: 1316-1321.wall will be broken off. Furthermore, those nucleiMARTINEZ R A, FLEMINGS M.C. A noveldistribute well in the bulk and are not easy totechnique to produce metal suries for semi solid metal poessingagglomerate. Consequently, the particles become finer[n]. Solid State Phenomena, 2006, 1/117: 366 -369.and rounder with the combined action of vibration and[15] GABATHULER 1 P, BUXMANN K. Proces for producing aliquid-soid metal aly phase for further processing as material in thegrain refiner.thixotropic state. us 5186236 [P]. 1993.[16] JIAN x, xU H, Meek T T, HAN Q Eftet of power ultrasound on4 Conclusionssolidifcation of aluninum A356 allay [小] Materials Letters, 2005,59: 190 193.1) Intense convection can be caused in melt by[17] ZAWLSKI K T CLAUDIA M, CUSTODIO C, DEMATTEI R C,中国煤化工-nixing applied to verticalvibration, which is generated from the free surface of theJrowth, 2003, 258(1): 211-bulk melt and spreads downwards, consequently leadingYH.CNMHG[18]ation of metals [M]. Tokyo:to the convection in the bulk.Chijinshokan Press, 2003. (in Japanese)2) Non-dendrite primary a(AI) crystals become(Edited by YUAN Sai-qian)

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