Effect of pouring temperature on semi-solid slurry of A356 Al alloy prepared by weak electromagnetic

Available online at www.sciencedirect.comsCIENGE @)DIRECT'Transactions ofNonferrous Metals骂Society of ChinaScienceTrans. Nonferrous Met. Soc. China 16(2006) 71-76Presswww.csu.cdu.cn/ysxb/Effect of pouring temperature on semi-solid slurry ofA356 Al alloy prepared by weak electromagnetic stirringLIU Zheng(刘政).2, MAO Wei-min(毛卫民)，ZHAO Zheng duo(赵振铎)'1. School of Materials Science and Engineering, University of Science and Technology Bijing,Beijing 100083, China;2. Faculty of Material and Chemical Engineering, Jiangxi University of Science and Technology,Ganzhou 341000, ChinaReceived 10 May 2005; accepted 15 September 2005Abstract: The semi-solid slury of A356 Al alloy was manufactured by low superheat pouring and weak electromagnetic stiring. Theeffects of pouring temperature on the slurry manufactured by weak electromagnctic stirring were researched. The results indicate that itis feasible to manufacture the slurry with particle-like primary phases by low superheat pouring and weak electromagnetic stiring, andthere is an important efct of the pouring temperature (superheat temperature) on the morphology and the size of primary a-AI in A356Al aloy. By the action of suitable weak electromagnetic stiring, increasing pouring temperature to put low superheat pouring in practiceis capable of obtaining semi-solid slurry of A356 Al alloy with particle-like primary phase. Compared with the samples made only bylow superheat pouring without stiring, raising pouring temperature by 15- :35 C above the liquidus temperature under condition ofweak electromagnetic stirring can ensure the same grain size and morphology of the primary phase.Key words: A356 Al aloy; semi-solid slurry; low superheat pouring; weak electromagnetic stiringof the a-Al phase, when they studied the evolution of1 Introductionmicrostructure in A356 Al alloy prepared by NRCprocessing. To obtain the semi-solid slurry to satisfyThe rheo-forming has been paid more attention tortheo forming, the pouring temperature and coolingduring the preparation of semi-solid metals to saverate of liquid alloy[4, 8, 9] are strictly controlled or thecost. In recent years, some of the new technologies,melt is held at the solid-liquid phase area[ 10] to ensuresuch as controlled nucleation method[1], controllingthe morphology of primary phase, or the alloy iscrystal method[2], liquidus casting[3], Nano casting[4],reheated again to the solid-liquid area to hold somecame forth. These new technologies are all based upontime[11]. In this way, the primary phase particle withcontrolling pouring temperature or ultilizing lowsound morphology is obtained, but it is difficult topouring temperature to control the morphology ofoperate in practice. On the basis of keeping thesolid particle, and results in fine grain size oreconomics of low temperature pouring technology, itimproved grain morphology[5]. .is very significant that the pouring temperature isBut the morphology of primary phase must besuitably raised so that the low temperature pouring isstrictly controlled if the semi-solid slurry prepared byconveniently operated. Based on the long-timelow temperature pouring is directly used to rheoresearches on SSM, the authors of this paper develop aforming because the morphology of primary phase innew technology- -low superheat pouring and weakthe slurry used to thixo forming can be improved byelectromagnetic stirring. The liquid aluminum alloythe remediation of the succedent reheating process[6].poured at low superheat is stirred for a short time by aSome research[7] reported that the final microstructuresimp中国煤化工equipment to realizewas very sensitive to the superheat of melt and lowtheenergy consumption,pouring temperatures promote spheroidal morphologyYHC N M H G increasing workingFoundation item: Project (G2002AA336080) suppurted by Hi-tech Research and Developmnent Program of China; Projeet (50374012) supported by theNational Natural Science Foundation of ChinaCorrespondence: MAO Wei-min; Tcl: +86- 10-62332882; E-mail: weiminmnao@ 263.net72LIU Zheng, et al/Trans. Nonferrous Met. Soc. China 16(2006) 71-76efficiency and ensuring the quality of slurry. Theffect of pouring temperature on the semi-solid slurryof A356 Al alloy under the condition of weakelectromagnetic stirring is studied in this paper.2 ExperimentalA356 Al alloy, as a kind of hypoeutectic Al-Sialloy, is extensively used to semisolid processingbecause of the wider solid-liquid range and goodfluidity. A356 Al alloy was used to study low superheatpouring and weak electro- magnetic stiring in this test.The composition of A356 was 7.46 Si, 0.49Mg andbalance Al.bThe liqudus temperature of A356 Al alloy isdetermined as 615.3 C by DTA. A356 Al alloy ismelted in an electric resistance fumnace. The meltingtemperature is 700 C. The mould, a cylinder made ofstainless steel with 102 mm in diameter and 220 mm indepth, is placed in an electromagnetic stirrer.During electromagnetic stirring liquid alloy, thestirring force is an important factor to present the qualityof slurry, but the actual stirring force is difficult tomeasure in practice. In this test, the stirring power isindirectly represented as the stirring force because(Cseveral of stirring power can be gotten to adjust theinput voltage of the stirrer at the same currentfrequency.To research the effect of pouring temperature onA356 Al alloy stirred by weak electromagnetic stirring,the pouring temperature is determined as 650, 630 and615 C.When liquid A356 Al alloy is prepared andpoured into the mould at the set temperature, the stirreris started and weakly stirs the liquid alloy at the samestirring power for a short time, respectively. Then the0.2 mmmould is quenched in order to maintain the structurestirred.Fig.1 Morphologies of primary phase in A356 Al alloy obtainedTo check the efect of weak electromagneticat different pouring temperatures (tirring power is 136 W): (田)stirring on low superheat pouring A356 Al alloy, the650 C;(b)630 C;(2)615 Csamples without stirring were poured at 650, 630 and615 C, respectively.A356 Al alloy obtained at the same stirring powerSome wafers with thickness of 10 mm were cut(136W) and strring time (8s) but different pouringfrom the same position of the ingots. The sectortemperatures. The microstructure of A356 Al alloysamples were fetched from the wafer (through thepoured at 650 C is shown in Fig.1(a). It can be seencircle center of the wafer). The samples were polishedthat the morphology of primary a-AI gives priority tousing standard metallographic practice, etched withrosette-like, and a few globular-like and particle-likegrains with coarse size. The microstructure of A356 Al0.5% aqueous solution of hydroflouric acid. Themicrostructure of the sample was observed on an opticalloy poured at 630 C is shown in Fig.1(b), whichmicroscope.nor primary a-Al withglot_中国煤化工orphology, and a few3 Results and Discussionwith:Y HC N MH Gy. Moreover, themicrostructure oI A550 AI alloy poured at 615 C is3.1 Effect of pouring temperatureshown in Fig.1(c), and basically consists of primaryFig.1 shows the semi-solid microstructures ofa-Al with globular-like and particle-like, with fineLIU Zheng, et al/Trans. Nonferrous Met. Soc. China 16(2006) 71-7673grain size. It can be seen from Fig.1 that under thecondition of not big stirring power of weakelectromagnetic sirring (i.e. 136 W), the morphologyof semi-solid primary d-A1 obtained from A356 AIalloy changes from rosette-like to particle-like, and the :grain size gradually decreases with the superheattemperature of liquid alloy (namely pouringtemperature). It is still sen from Fig.1 that under thecondition of weak electromagnetic stirring at 136 W,there is better morphology of semi-solid structure inA356 Al alloy poured at 615 and 630 C, and that the136 W stiring power shows a lttle semi -solidstructure of A356 Al alloy poured at 650 C. Inaddition, from the view of convenient operation,selecting 630 C as pouring temperature can ensureobtaining semi -solid slurry of A356 Al alloy suitablefor rheo-forming process.To further study the effect of pouring temperatureon the morphology of particles in A356 Al alloy shurry,the changing rule of the morphology and the grain sizeof primary a-Al was investigated at stiming power of352 W and different pouring temperatures. Fig.2shows the semi-solid microstructures of A356 Al alloyobtained at stirring power of 352 W and stirring timeof 8 s but different pouring temperatures. As thepouring temperature increases to 650 C, the primarya-Al gives priority to particle-like morphology andsome of primary a-Al grains in small amount stillpresents the rosette like morphology, even there existthe grains with fine size, as shown in Fig.2(a). Whenhe pouring temperature reaches 630 C， themorphology of primary a-Al gives priority toparticle-like and globular-like morphology, and thereis basically no primary phase with rosette-like0.2 mmmorphology, as shown in Fig.2(b). The morphology,besides giving priority to particle-like, presentsosette-like in small amount of primary a-Al as theFig.2 Morphologies of primary phase in A356 Al alloy obtainedpouring temperature decreases to 615 "C, as shown inat dfferent pouring temperatures (stiming power is 352 W): (2)Fig.2(c). It is inferred that this phenomenon relates to650 C;(b) 630 C;(c)615 Che pouring operation. Because the pouringtemperature of the melt is about 615 C, close to thespherical morphology of semi-solid microstructureliquidus temperature of A356 Al alloy, a layer ofexists in A356 Al alloy poured at 615, 630 and 650 Ceven though this stirring power can ensure to obtaindendritic crystal is formed on the cylinder wall due tothe chilling of the wall of the cylinder as the meltthe semi-solid slurry with particle-like primary phasefor A356 Al alloy poured at 650 C. In addition, frompoured into the cylinder. The dendritic crystal is notbroken by the strring force resulted from the weakthe view of convenient operation, the pouringelectromagnetic stiring but part of dendritic crystaltemperature selected as 630 and 650 C can ensure tofalls of as the solute concentrated on the root of theobtain the semi-solid slurry of A356 Al alloy satisfieddendritic crystal. There is not enough time for thewith rhen forming. Comnaring Fig.1 with Fig.2, it isdendritic crystal to change into the grain withknow中国煤化Iimary a-Al obtainedparticle-like morphology, so that the dendritic crystalat theYHCNMH Ges is btter than thatis reserved at the form of rosette-like grain. It is seenobtaincu al uIc LUwCI suling puwer (i.e. 136 W) andfrom Fig.2 that under the condition of weakhe grain size of primary phase obtained is fine andelectromagnetic stirring at stirring power of 352 W,small as the stirring power of weak electromagnetic74LIU Zheng, et al/Trans. Nonferous Met. Soc. China 16(2006) 71-76stirring increases to 352 W (still belonging to the weakstrring category) and the melt poured at differentpouring temperatures. In other words, the morphologynd the grain size of primary phase obtained fromincreasing pouring temperature is comparedfavourably with those obtained from lower pouringtemperature. In this way, it is very significant foroperation in productive practice.3.2 Comparison with samples poured at low super-heat temperature without stirringFig.3 shows the microstructures of semisolid A356Al alloy poured at the same pouring temperature(b)without weak electromagnetic stirring. As 650 Cpouring temperature, the primary phase in A356 stillpresents dendritic-like morphology without stirring,but the primary crystal arms and the second arms ofthe dendritic crystal obviously becomes fine and short,and there is no third arms in the microstructures, asshown in Fig.3(a). And that there is no dendriticcrystal in the microstructure of A356 Al alloy pouredat the same temperaturewiththeelectromagnetic stirring and there is more amount ofine primary a-Al with globular-like or particle-likeF(C)morphology, as shown in Fig.1(a). At pouringtemperature of 630 C, the morphology of primarya-Al in A356 Al alloy gradually changes fromdendritic-like into rosette-like morphology, but there isstill a little primary a-Al with dendritic-likemorphology not changed fully in the microstructure,as shown in Fig.3(b). Comparison with themicrostructure of A356 Al alloy poured at the sametemperature with weak electromagnetic stirring, thereis the more primary 0-A1 in the amount with0.2 mmglobular-like or particle-like morphology and witheven finer size, as shown in Fig.lb. As 615"C pouringFig.3 Morphologics of primary phase in A356 Al alloy obtainedtemperature, the morphology of primary a-A1 in A356at diferent pouring tenperatures without stiring: (a) 650 C;Al alloy is further varied, in which the primary a-A1(b)630 C;(c)615 Cbasically presents the non-regulate globular-like orparticle-like morphology. The grains become finingstirring at the same pouring temperature, and it isand their distribution is more uniform. Compared withthe microstructure obtained at higher pouringexpressed that low superheat pouring and weakelectromagnetic stirring play improvement role on thetemperature without stirring, the microstructurenucleation ratio and the morphology in A356 Al alloy;obtained at 615 C pouring temperature is greatlyon the other hand, as the temperature of the melt isimproved, as shown in Fig.3(c). Compared withraised to 630 C, even to 650 C and meanwhile themicrostructure (as shown in Fig.1(C)) obtained at themelt is stirred by weak electromagnetic stirring, thesame pouring temperature with weak electromagneticmorphology of primary phase corresponds with thatstirring shows that the diference exists on thepoured at the temperature of615 C without stirring,roundness of primary a-Al with globular-like Orparticle-like morphology and with the coarse size ineve中国煤化工than those shown inA356 Al alloy without stirring. It can be seen from theFigjig.2(b) and Fig.3(c).TheMHCNMH Glring lempratre cCanresults of this test that, on one hand, the morphologybe suitably raised about 15一35 C above theand the size of primary phase obtained from the weakelectromagnetic stirring are superior to that withoutliquidus temperature of 615 C under the condition ofLIU Zheng, et al/Trans. Nonferrous Met, Soc. China 16(2006) 71-76weak electromagnetic stirring. It is very practical foras the intensity of electromagnetic stirring increasing.the production to conveniently operate pouring and toThen the effectiveness produced by powerful stiringsimplify working procedure.will happen on the condition of weak electromagneticstirring?4 DiscussionDuring the preparation of the semi-solid slurry inA356 Al alloy using low superheat temperature andCARDOSO et al[7] pointed that the finalweak electromagnetic stiring, the certain extent of themicrostructure was very sensitive to the superheat offorced convection with low intensity in the melt resultsthe melt and low pouring temperatures promotedfrom the weak electromagnetic stirring. The forcedspheroidal morphology of the a-Al phase, when theyconvection, on one hand, puts cooling of the melt onstudied the cvolution of microstructure in A356 Althe mould wall and the melt surface off, on the otheralloy prepared by NRC processing. The results in thehand accelerates temperature decrease in the wholetest also show this view. Decreasing pouringmelt. Though there is stronger cooling effect on thetemperature can obtain the primary phase withmould wall and the melt surface, the forced convectionparticle-like morphology, but it is very difficult towill take the melt from the cooler position into theoperate the low temperature pouring in production,internal area of the melt and carry the melt with higherespecially to operate pouring closing to liquidustemperature to makeup, thereby the melt temperaturetemperature of alloy. It is hoped that the pouringon the mould wall and the melt surface is delayedtemperature is just a little higher and the opcration isdecreasing, so to postpone the formation of the stablejust more convenient in production. To solve the issue,frcczing layer on the surface of mould. The cooler meltthe improvement should be taken in the processing ofenters the internal area of the melt to be heated and thelow temperature pouring, such as suitable weakhotter melt is taken to the surface layer of mould to bestirring in melt poured at low temperature for a shortcooled. The convection effect caused by the circle flowtime to increase flow of the melt during solidification.of the melt can promote heat of the intemal melt toThe researches[12- 15] have indicated that the meltgive off, sequentially the temperature of the wholeflow at initial solidification plays an important role inmelt reaches the freezing temperature at a short time.formation of the primary phase with particle-likeNot only the melt on the surface but also the wholemorphology.melt is kept undercooling, and the melt temperature isAs well known, there are two kinds of flow inrelatively uniform, therefore, a large number of crystalmelt during solidification: one is the naturalnuclei can be formed, which survive to continuouslyconvection caused by liquid alloy wasbing out or thegrow in the intermal flow of the melt.density difference or temperature difference in theSimilar to the common alloy casting, the meltmelt as the melt poured into mould, the other is thesolidification also carries out in the continuous droppingforced convection caused rtificially by the externalof temperature under the condition of stirring. The mainfield or extra physical disturbance as the melt freezing.positions of giving heat off are still the wall of mouldThe two kinds of convection will have an importantand the melt surface, so that more crystal nuclei forms ateffect on the formation of crystal nuclei and the grainthese positions. Due to the forced convection caused bygrowth as well as the grain morphology duringthe stirring, these crystal nuclei enter the inside of thesolidification of the melt[16]. .melt with melt flow and grow up grains during the meltThat the melt is stirred by electromagnetic force asflow and itself whirlpool. It does not bappen under thepreparation of semisolid slurry can promote dendniticcondition of the traditional casting.arms growing to remelt and to draft away. The driftingThe forced convection in the melt caused bygrains flowing in the melt are continuously scoured byelectromagnetic stirring changes the heat transferringhe temperature fluctuation and the concentrationand the mass transferring. The rapid heat transferringfluctuation and repecatedly kept the state of remeltingand mass transferring convection makes an environmentand growing. The broken fragments from primarywith rclative uniformity of temperature and compositiondendritic crystal grow up the new drifting grains at lowin the melt. This environment is not favorable to thetemperature to realize drifting grain multiplication andpreferred growth of the grains, and the grains are forcedto increase the number of the grains. The new dynamic中国煤化二raction. So the grainscondition of nucleation is created by the drasticfinallicro- structure.electromagnetic stirring, namely the low temperatureTYHCNMHGgradient, which promotes second arms of primary phase5 Conclusionsfusing and breaking and primary crystal arms refining.Therefore, the grain size will obviously become smaller1) The semisolid slurry of A356 AI alloy76LIU Zheng, et al/Trans. Nonferrous Met. Soc. China 16(2006) 71-76contained primary a-Al withglobular-like or3] Xia K, Tausig G Liquidus casting of a wrought Al alloy 2618 forthixoforming [J]. Mater Sci Eng A, 1998, A246:1- 10particle-like and with the finer size can be prepared by4] Masao Kikuchi, Osamu Nagasawa. Application of Nano cast method{U]low superheat pouring and weak electromagneticAlutop, 2004, 34(11):21 -26.stirring.5] Flemings M C. Solidifcation process M. New York: Mcgraw-Hill,2) There is an important effect of pouring1974. 223 - 231.temperature (superheat temperature) on the grain6] XING Shu-ming, TAN Jian-bo, ZHANG Li-zhong, ct al Study onmorphology and the grain size of the primary a-A1 inkey problems on nustrializations of secmisolid rhcologic formingprocess[A]. Proc of the 8曲Int Conf on Semi-solid Processing ofA356 Al alloy. In the range of researched temperature,Alloys and Comnposites[C]. Limassol, Cyprus, 2004. 61-69.the morphology of the primary a-A1 presents[7] Cardoso E, Atkinson H V, Joncs H. Microstructural evolution ofrosette-like morphology and the size of grain is coarseA356 during NRC processing[A]. Proc of thc 8由Int Conf onat high pouring temperature, the morphology of theSemi-solid Processing of Alloys and Composites[C]. Limassol,primary a-A1 presents globular-like or particle-likeCyprus, 2004.296- 307.morphology and the size of grain is fine as the high8] Pan Y, Aoyama s, Liu Ch. Spherical structure and formationcondition of semi-solid Al-Mg-Si alloy[A]. Proc of the 5" Asianpouring temperature.Foundry CongressfC]. Nanjing, China, 1997. 443 -451.3) Putting the low superheat pouring at the higher9] Vieira E A, Junior B A 0, Ferrante M. Microstucture and theologypouring temperature in practice and using the suitableof an A356 aly in the scmisolid statc, conditioned by a lowweak electromagnetic stirring, such as stirring powerpouring temperature technique[A]. Proc of the 8m Int Conf onof 136 W and pouring temperature of 630 C orstirring power of 352 W at 630 C, even at pouringCyprus, 2004. 243- 251.[10] ZHAO Jian-xin, ZHU Ming-fang, Kim Jic-min, ct al. Evolution oftemperature of 650 C，can obtain the semi-solidglobular and dendritic structures in sldication of AL-Si aloys [slurry of A356 Al alloy with particle-like primaryPTCA(PartA: Plhysical testing), 2004.40(9): 433 - 438. (in Chinese)phase. It is convenient to operate pouring in[11] WANG Ping, LU Gui-ming CUI Jian-zhong Stucure andproduction.technology of A356 Al alloy second heating in liquidus4) Compared with the samples poured at lowsemi-continuous casting []. Material Review, 2002, 16(5): 72- -74.(in Chinese)superbeat temperature and without weak electro-[12] PAN Ye, ZHANG Chun-yan, YUAN Hao-yang, ect al, Effect of meltmagnetic stirring, the pouring temperature can beflow at initial solifiation on granular primary crystal formation insuitably raised to 15一35 C above the liquidussemi-solid aloylDI. Acta Metall. Sinia 2001, 37(10); 1035 - 1039.temperature of 615 C if the same grain size andmorphology are hoped under low superheat pouring[13] Stefanescu D M, Padhya G Yopdhyay D B. Heatand weak eclectromagnetic stirring. It is of practicaltransfer-solidification kinetics modeling of solidification of casting[J]. Metall Trans, 1990, 21A: 998 - 1007.significance for convenient pouring operation.[14] L Tao, HUANG Wei-dong, LIN Xin. Formation of globular stnuctureduring semi-solid matcrial processing [小. The Chinese Joumal ofReferencesNonferrous Mtals, 2000, 10(5): 635 - 639. (in Chinese)[15] ZHANG Jingxin, ZHANG Kui, LIU Guo-jun, et al. Formation1]Wang H, StJone D H, Davidson C , et al. Controlled nuclcation mcthodmechanism of nom-dendritic stnucture in semi-solid metals producedfor formation of semisolid foedstock[A]. Proc of the 8h Int Conf onby ES process []. The Chinsse Joumal of Noferous Metals, 2000,Semi-solid Processing of Alloys and Composites[C]. Limassol, Cyprus,10(4): 511-515. (in Chinese)2004: 269- 276.[16] HU Han-qi. The Metal Solidification Theory[M]. Beijing: China2] CAO Hong- bo, Pan Y, Zhang C. Compressing defoming chMachine Industy Pess, 200 109 - 1. (in Chinse)of semi-solid AI aly prepared by conrololing crystal method[J](Edited by LONG Huait zhong)Special Casting & Noferous Aloys, 2002(3): 9- 11. (in Chinese)中国煤化工MYHCNMHG