Manufacture technique of semi-solid slurry of hypoeutectic Al-Si alloy by low superheat pouring and

●102.Vol.3 No.2, May 2006CHINA FOUNDRYManufacture technique of semi -solid slurryof hypoeutectic AI- Si alloy by low superheatpouring and weak electromagnetic stirring*LIU Zheng", MAO Wei-ming, ZHAO Zheng -duo2(1. Faculty of Material and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, P. R.China; 2.School of Material of Science and Engineering, University of Science and Technology Bejing, Beijing 100083, P.R. China)Abstract: The semi-solid slurry of hypoeutectic Al-Si alloy was manufactured by low superheat pouring and weakelectromagnetic stiring. The effects of pouring temperature and stiring power on the semi-solid slurry making processwere investigated. The results indicate that the semi-solid slury to satisfy rheocasting requirement can be made by acombination of low supertheat pouring and weak electromagnetic stiring. The pouring temperature (or superheat) and thestiring power significanty afect the morphology and the size of primary a-Al, while there is no obvious effect of the stiringtime on primary a-Al. Compared with the samples made by low superheat pouring without stiring, the nucleation rate,particle morphology and grain size of primary a-AI in A356 AI alloy are markedly improved by a process of applying bothlow superheat pouring and weak electromagnetic stiring. Under the condition of weak electromagnetic stiring applied, thepouring temperature with low superheat can be equivalerttly to reach the efectiveness obtained from the even lowerpouring temperature without stiring.Key words: semi-solid; low superheat pouring; weak electromagnetic stiring; hypoeutectic AI-Si lly; A356 AI alloyCLC number: TG146.2*1Document code: AArticle ID: 1672-6421(2006)02-0102-06Semi-solid metal (SSM) process is a relatively newstrictly controlled, because the morphology of primarytechnology that offers distinct advantages over otherphase in the slury used in thixoforming can be improvednear-net-shape casting processes, such as a moreby the remediation of the subsequent reheating process 0.homogeneous microstructure, less porosity andTo obtain the semi-solid slurry for satisfying thesegregation, and improved mechanical propertislJ. SSMrequirement of rheoforming, the pouring temperature andcan be divided into thixoforming and rheoforming, incooling rate of liquid alloy should be strictly controlledl89which rheoforming is usually advantageous from the pointor alternatively. And the liquid alloy is to hold at theof view of an energy and cost saving when compared withsolid-liquid phase zone to ensure the suitable morphologythixoforming, so rheoforming has been paid attentionof primary phase. Thus, the primary particle with the goodagain by engineers in recent years. As known early, themorphology can be obtained, but the operation is difficultgrain size in SSM microstructure is fined and the grainand the cost will increases in the production. Based onmorphology is improved by the control pouringlong time experience on the SSM research, authors havetemperature or by the low temperature technique用. Adeveloped a new technology- -low superheat pouring andoumber of new technologies have been recently reported,weak electromagnetic stirring, to bring into playsuch as controlled nucleation method I5, liquid casting间.superiority of low temperature pouring and weakAll of the new technologies, based on the control pouringelectromagnetic stirring. In the technology, the liquidtemperature or by the low temperature technique, realizealuminum alloy poured at low superhcat is stirred for ato control the solid morphology. If the semi solid sluryshort time by a simple electromagnetic stirring device tofrom low temperature pouring technique is used forachieve the purposes of reducing energy consumption,rheoforming, the morphology of primary phase must besim中国煤化工s, increasing workingef=quality.YHCNMHG*LIU Zheng: Male, bom in 1958， professor. Rescarch ficld:1 Experimentssemi-solid metal of aluminum alloy.A356 Al alloy, as a kind of hypoeutectic Al-Si alloy, isE-mail: liuzheng@mail.jxust.cnextensively applied in semi-solid process because of itsReceived date; .200S-12-26;Accepted date: 2006-03-29”Vol3 No.2Manufacture technique of semi-solid slurry of hypoeuteetic Al-Si alay●103●wider solid-liquid range and good fluidity. For this reason,metallographic practice, etched with 0.5% aqueousA356 Al alloy was selected to study low superheatsolution of hydrofluoric acid. The microstructures of thepouring and weak electromagnetic stirring in this test. Thesamples were observed under an Axioskop II opticscomposition of A356 was with 7.22 Si, 0.49 Mg andmicroscope and the sizes of grains were measured bbalance AL. The liqudus temperature of A356 Al alloy wasMIAPS sofware.determined as 615.3 C by a differential thermal analysis.A356 Al alloy was melted in an electric resistance2 Results and discussionfurmace. The melted temperature was 700 C. The mould,2.1 Effect of pouring temperaturea cylinder made of stainless steel with 102 mm indiameter and 220 mm in depth, was inserted with anFigure 1 shows the semi- solid structures of A356 Alelectromagnetic stirrer. During electromagnetic stirring of alloy prepared at the variable pouring temperatures withliquid alloy, the siring force is an important factor inthe same stirring power (136 W). The microstructure ofachieving the quality of slurry, but the actual stirring forceA356 Al alloy poured at 650 C is shown in Fig.1 (a). Itis difficult to measure in practice. In this test, the stirringcan be seen from the view that the primary a-Al under theforce is indirectly represented as the stirring power sincecondition gave rosette-like morphology in the majority,variable stiring power values can be available bybut small minor amount of globule-like and particle-likeadjusting the input voltage of the stirrer at the samegrains with relatively coarse size measured as 92.4 μm.current frequency.The microstructure of A356 Al alloy poured at 630 C isWhen liquid A356 Al alloy was prepared and pouredshown in Fig.1 (b), which consists of the majority ofinto the mould at the variable holding temperatures ofprimary a-AI with globule-like and particle-like650 C, 630 C and 615 C, the stirrer was switched onmorphology, but a minor amount of primary a-Al withand stired the liquid at the siring powers of 60 W, 136rosette-like; the mean size of the grain was 88.0 μm.W, and 352 W for 8 s, respectively. After pouring andMoreover, the microstructure of A356 Al alloy poured atstirring, the mould was quenched in order to maintain the615 C as shown in Fig.1 (c) was basically composed ofstructure stirred. To evaluate the effect of weakprimary ax-AI with globule-like and particle-like and withelectromagnetic stirring on low superheat pouring A356fine and small grain size which was measured as 68.1 μum.Al alloy, the samples without stirring were also poured atIt can be concluded from Fig.1 that the morphology of650 C, 630 C and 615 C, respectively.primary Q-AI in A356 Al alloy tranforms from rosette-likeSome sample wafers were cut from the top, the middleinto particle-like while the size of grain graduallyand the bottom of the ingots, respectively. The wafer wasbecomes finer as the superheat (pouring temperature) ofabout 10 mm in thickness. Then, small pieces of samplesthe melt decreasing.were sliced from the wafer and polished using standard回) 650C(b) 630C(C)615CFig.1 Morphologies of primary phases in A356 poured at dfferent temperatureCardoso et a1.10] pointed out that the final microstructureprimary phase with particle-like but it is very difficult tois very sensitive to the superheat of the melt and lowoperate in production, especialy when the pouringpouringtemperature could promote spheroidaltempe中国煤化工aperature of the alloy.morphology of the a-Al phase after they studied thFor tCNMH(:roduction easy, peopleevolution of microstructure in A356 Al alloy prepared bysual, mrupc puunug iciupciaiuic olightly higher. To solvenew rheocasting processing. The results in the currentthe issue, the improvement should be taken to add suitablestudy also give the evidence for the view. Decreasingweak stirring in melt poured at low temperature for a shortpouring temperature could be favorable to obtain thetime，so that, to increase flow of the melt duringCHINA FOUNDRYMay_ 2006solidification. The research!" indicated that the melt flowseen from the preparation that there was the weaker extentt initial solidification plays an important role inof stiring in the melt as a smaller stirring power (e.g. 60formation of the primary phase with partil-like.W), and there was no the obvious liquid concave on theAs well known, there are two kinds of flow in melttop surface of the melt, where the forced convectionduring solidification: one is the natural convection causedproduced by stirring melt was very weak. As the stirringby liquid alloy washing out or the density difference orpower gradually increases, the extent of stiring in thetemperature difference in the melt as the melt poured intomelt gradually rises and the shallower liquid concavemould, the other is the forced convection causedoccured on the top surface of the melt, where there wasartificially by the external field or extra physicalthe more obvious the forced convection. As the stirringdisturbance as the melt freezing. Two kinds of thepower further increased (i.e. 352 W), the stiring in theconvections will have an important effect on the formationmelt became strong, and the deep liquid concave occursof crystal nuclei and the grain growth as well as the grainon the top surface of the melt, where the forcedmorphology during solidification of the melt [2.convection resulted from stirring the melt greatly becameDuring the low superheat pouring and weakstrong. The melt flow would have notable effect on theelectromagnetic stirring, as stirring the melt pouredmicrostructure of alloy[2.(despite weak), the fluctuation occurs in the melt atAs the larger strring power (i.e. 352 W) applied, and thetemperatures close to the liquidus temperature, and theforced convection produced by stirring melt made thecertain extent of the forced convection can be resulted inarms of Q-A1 re-fused and re-fined, forming a great lot ofthe melt, too. In addition, the local energy fuctuationdendritic fragments as nuclei4y. At the same time, theexisted in the melt makes the whole or local temperatureshrewd convection movement could greatly quicken upin the melt fluctuate at the liquidus temperature. Thetransfer of heat from the center of liquid so that thefluctuation of temperature makes the melt have a similarsuperheat in liquid alloy was dissipated rapidly, and theeffect to the repeated melting and freezing, which issolid-liquid phase area was enlarged. All this wasequivalent to increase undercooling 3. The pouringfavorable to evolution of equiaxed grains and thinness oftemperature in the test was chosen in the range of 615-650the grains凹, as a result, the morphology of most primaryC，closing to the liquidus. The more closing to thea-Al gives priority to globule-like and the mean size ofliqudus temperature, the more undercooling of the meltgrain is 78.7 μm, as shown in Fig.2 (a). As the stiringachieved. This is helpful to reduce the critical radius ofpower decreased to 136 W, the extent of "stirring effect"the nuclei formation and the critical power of the nucleiin liquid alloy gradually reduced. The mean grain size offormation.primary a-Al is 93.8 μm, and only the morphology of theTherefore, during the operation of low superheat pouringgrain has a little change, a few rosettes and most globuleunder the weak electronmagnetic stirring, the morphologyor particles, as shown in Fig.2 (b). As the stiring powerof primary x-Al changes into particle-like or globule-likefurther reduced to 60 W, the "tiring effect" in liquidfrom rosette-like and the grain size also becomes furtheralloy became weak. The morphology of primary a-Alfine and small as the pouring temperature decreasing.gives priority to rosette-like and the mean size of grain is2.2 Effect of stiring power96.4 μm. There is no dendritic primary a-Al in themicrostructure, as shown in Fig.2(c).Figure 2 is the semi- solid microstructures of A356 AIThough the mechanism on formation of nuclei andalloy prepared by same pouring temperature (630 C) andnon-dendritic grain under the condition of low superheatsame stiring time (8 s) but different strring power. It ispouring is being explored and researched, it is different中国煤化工CHCNMHG102mmU emm RA.CwnTr 0.2mm(田) 352 w(b) 136 W(C) 60 WFig.2 Morphologies of primary phases in A356 poured at dferent stiring powerVol3 No. 2Manufacture technique of semi- solid slurry of hypoeutectic Al-Si alloy●105.from that under the condition of traditional mechanicalare changed by vigorous blending and convection in thestirring or electromagnetic stiring. The researches 1.1.1518melt caused by electromagnetic stirring to create anof scholars from domestic and foreign countries indicatedenvironment with relative uniformity of temperature andthat the formation of nuclei is heterogeneous nucleationcomposition in the melt, the grains grow relativelywhen liquid alloy at the temperature near to its liquidusuniformly in all directions. This environment is not aswas carried out low superheat pouring, the same as thebeing created by purely low superheat pouring. Thsolidification of other common alloy. The team (19 atprocess similar to equiaxed structure in the grains is takenMassachusetts Institute of Technology developed a uniqueplace and the stirring effect makes the grains change intoand highly efective process to make grain refine slurry atnear globular-like (20.the onset of solidification. They applied a so- calledTherefore, it is provided that the weak electromagnetic"spinning cold finger" to the upper surface of melt in astirring is applied in the melt with low superheat for shortsmall crucible while it cooled to just below liquidus,time, as low superheat pouring, to enhance temperaturenamely the semi-solid slury with particle, the primaryfluctuation just below liquidus in small range, and tophase could be prepared under the condition of stirringmake the melt with the certain extent forced convection,with low strength. The results in the current study alsoso that the radius of nuclei is decreased to form moreshow this finding. But， it is worthy that the grain nucleus. The grain in the environment withelectromagnetic stiring belongs to a type of non-touchingrelatively uniform temperature and composition createdstirring, in which there is no corrosion of stiring blade orby the forced convection grows more uniformly at eachstirring stick as well as no pollution of alloy slurry by thedirection to form the primary phase microstructure withblade and the stick in the mechanical stirring; in addition,non-dendritic morphology.the determination of operating parameter is flexible and2.3 Comparison with the samples poured at lowconvenient to control the production and the quality ofsuperheat temperature without stiringsemi-solid slurry.The melt stirred by electromagnetic force as preparationFigure 3 shows the microstructures of semi solid A356of semi-solid slurry can promote dendritic arms inAl alloy poured at the same pouring temperature withoutgrowing to be remelted and draft away. The drifing grainselectromagnetic stirring. As 650 C pouring temperature,floating in the melt are continuously by the temperaturethe primary phase in A356 still presented dendritic-likefluctuation and the concentration fluctuation andwithout stirring, but the primary crystal arms and therepeatedly kept the state of remelting and growing. Thesecondary arms of the dendritic crystal obviously becamebroken fragments from primary dendritic crystal make thefine and short, and there was no the tertiary arms in thenew drifting grains grow up at low temperature to realizemicrostructures, as shown in Fig.3 (a). The mean size ofdriting grain multiplcation and to increaese the numberof grains was 11.5 um. And that there was no the dendriticthe grains. The new dynamic condition of nucleation iscrystal in the microstructure of A356 Al alloy poured atcreated by the drastic electromagnetic stirring, namely thethe same temperature with the weak electromagneticlow temperature gradient, which promotes second arms ofstirring and as expected, found the more amount of theprimary phase fusing and breaking and primary crystalfine primary ax-A1 with globule-like or particle-like in thearms fining. Therefore, the grain growing up size willmicrostructure, as shown in Fig.1 (a). As 630C pouringobviously become smaller as the intensity oftemperature, the morphology of primary a-A1 in A356 Alelectromagnetic stirring increasing.alloy changed dendritic-like into rosette-like, but thereAs a result that both heat and mass transfer processeswas still a little primary -Al with dendritic-like小心中国煤化工0HCNMHG(a) 650(b) 630C(C) 615CFig.3 Morphologies of primary phases in A356 poured at dfferent temperature without stiring. 106. CHINA FOUNDRYMay 2006remained in the microstructure, as shown in Fig.3(b). Thelower pouring temperature.mean size of grains was 103.5 μm. Comparison with the(3) The effect of stirring power on the grain morphologymicrostructure of A356 Al alloy poured at the sameand the grain size of the primary a-A1 in A356 Al alloy istemperature with weak electromagnetic stirring, there wasanother important factor. As the power of weakmore primary a-Al in the amount with globule-like orelectromagnetic stirring increases, the extent of the forcedparticle-like and with even finer size, as shown in Fig. lb.convection induced in the melt becomes higher, and theAs 615 C pouring temperature, the morphology ofmorphology of primary a-A1 solidified changes fromprimary a-A1 in A356 Al alloy was further varied, inrosette-like to particle-like, and the size of primary a-A1which the primary Q-A1 basically occured as the irregularbecomes much finer.globule-like or particle-like. The grains became about(4) Compared with the results of A356 alloy without101.8 pum in diameter and their distribution was moreweak electromagnetic stirring as A356 poured at lowuniform. Comparison with those obtained at the highersuperheat temperature, there is the obvious effect of weakpouring temperature, the microstructure obtained at 615electromagnetic stiring on raising the nucleation ratio ofC pouring temperature was improved greatly, as shown inprimary a-Al, on improving the grain morphology and onFig.3(c). Furthermore,comparison with thefining the grain size in A356 Al alloy as the highermicrostructure, as shown in Fig.1(c) obtained at the samepouring temperature applied. On the contrary, thispouring temperature with weak electromagnetic stirring,infuence becomes less important as the lower pouringthere was the difference on the roundness of primary a-A1temperature applied.with globule-like or particle-like and with the coarse size(5) Comparison with the samples poured at lowin A356 Al alloy without stirring. It is seen from thesuperheat temperature without weak electromagneticresults of this test that, on the one hand, the morphologystirring, the temperature on low superheat pouring underand the size of primary phase obtained from the weakthe condition of weak electromagnetic stirring can beelectromagnetic stirring are superiority to that withoutsuitably raised as the same grain size and morphology ofstirring at the same pouring temperature, and it isthe primary phase can be assured in quality. This isexpressed that low superheat pouring and weakpractically significant for convenient operation in theelectromagnetic stiring play an important role in theproduction.nucleation ratio and the morphology in A356 Al alloy; onthe other hand, as the temperature of the melt is raised 1Referencesto2 grade (i.e. 630 C, even 650 C) to pour and with[1] SPENCER D B, MEHRABIAN R, FLEMINGS M C. Rheologicalweak electromagnetic stirring, the morphology of primarybehaviour of Sn-15Pct. Pb in the crystalization range. Metallphase corresponds with that poured at the temperatureTrans, 1972, 3A: 1925- 1932.with below one grade (i.e. 615 C) without stiring, as2] FLEMINGS M C. Behavior of metal aloys in the semisolid state.shown in Fig.1(a), Fig.1(b) and Fig.3(c). These mean thatMetall Trans, 1991, 22A: 957. 981.the pouring temperature can be suitably raised under the13] KIRKWOOD D K. Semisolid metal processing. 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