Preparation of semi-solid AlSi7Mg alloy slurry with big capability

RARE METALSVol. 29, No.6, Dec 2010,p. 642DOI: 10.1007/s 12598-010-0185-1Preparation of semi-solid AISi7Mg alloy slurry with big capabillityLI Sha and MAO WeiminSchoo of Materials Science and Enginering, University of Science and Teclnology Beijing, Beijing 10083, ChinaReceived 9 June 2009; received in revised form 28 April 2010; accepted 6 May 2010◎The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2010AbstractSemi-solid AISi7Mg alloy slurry was prepared by low superheat pouring and weak traveling wave electromagnetic stirring. The effects ofpouring temperature and strring power on the microstructure of AISi7Mg alloy slury were studied. The results show that the semi- solidAISi7Mg alloy slurry of 5 kg can be prepared. This new technology can save energy and make the pouring process convenient. When thepouring temperature is decreased at a stirring power of 0.41 kW, the shape of primary 0a-Al grains gradually changes from dendritic-like tospherical. When the alloy melt is poured at the temperature (6309C) with a certain superheat, the pouring process becomes easier, and thespherical microstructure of AISi7Mg alloy slury can be prepared by the weak traveling-wave electromagnetic stiring. When the pouringtemperature is 630°C, increasing the stiring power appropriately can result in better spherical primary a-A1 grains, but if the stirring poweris increased to a certain value (1.72 kW), the shape of primary a-Al grains does not obviously improve when the sirring power is continuallyincreased.Keywords: aluminum alloys; metal forming; semisolid slurry; electromagnetic stirring; microstructure1. Introductionat the near liquidus temperature, it is very difficult to controlthe pouring process during practical production. Mao et al.Semi-solid metal forming technology originated in thedeveloped a low superheat pouring and weak electromag-United States [1]. Because this technology has many specialnetic stiring technology for preparing semi-solid alloy slur-advantages, much attention has been given to it in the de-ries or billets [12-13]. This new technology has much lowerveloped countries and it has become a very exciting field ofenergy exhaustion and is more easily controlled, which canmaterial forming technology researches [2-5]. At present, decrease the preparation cost of semi-solid alloy slurries orthere are many methods for preparing semi-solid metal slur-billets and make the pouring process control simple. Mean-ries, such as mechanical stiring method [6], electromagneticwhile, an ideal microstructure of the semi-solid alloy slurrystirring method [1], strain- induced melt activation process [3can be obtained. More attention has been given to semi-solid7], and ultrasonic vibration method [8]. In practical produc-AISi7Mg alloy slurries with small capabilty during thetion, electromagnetic stirring method and strain-inducedprevious research study [14-17]， and the mass of themelt activation method still hold a dominant position, butprepared slurry is about 1.5-2.6 kg every time, whichthe production cost of these two kinds of preparation meth-satisfies the production requirement of smaller rheo-formingods are much higher, and the widespread application of theparts. In practical application, however, bigger rheo-formingsemi-solid metal thixo-forming technology is restricted. Toparts are needed to some extent. Whether the technology ofmake full use of the advantages of semi-solid metal formingow superheat pouring and weak electromagnetic stirringtechnology, searching for new and low-cost preparation can meet the preparation of semi- solid alloy slurry with bigmethods has been an object for many domestic and interma-capacity and how the rules of preparation change will alsotional experts and manufacturers. In recent years, some new have to be studied significantly. In this article, the semi-solidsemi-solid forming technologies have appeared, such asAISi7Mg alloy slurry with big capacity (about 5 kg) wascontrolled crystal method [9], controlled nucleation methodprepared by low superheat pouring and weak travelling-[10], and liquidus casting method [11]. Although an idealwave electromagnetic stirring, and the pouring temperaturemicrostructure can be gained when the liquid alloy is pouredand stirring power were studied.中国煤化工Corresponding author: LI ShaE-mail: hbylyl@ 126.comfHCNMH G.Li s. et al, Preparation of semi-solid AISi7Mg alloy slurry with big capability6432. ExperimentalMetallographic specimens were cut off from thequenched slurries, then roughly ground, finely polished, andThe experimental material was a commercial AISi7Mgetched by an aqueous solution of 0.5 vol.% HF. The etchedalloy. Its chemical composition is listed in Table 1. Thespecimens were cleaned with an alcohol solution and dried.theoretical alloy liquidus and solidus temperatures tested byThe microstructures were observed and analyzed with andifferential thermal analysis are 615 and 555°C, respec-optical microscope of the Neuphoto 21 model.tively.Table 1. Composition of the AISi7Mg alloywt.%3. Results and discussionSiZn FCu MAI3.1. Effect of pouring temperature on the microstructure6.5-7.1 0.25-0.45 <0.1 <0.2 ≤0.2 <0.1 BalanceFig. 2 shows the microstructures of AISi7Mg alloy slur-ries prepared with weak travelling-wave electromagneticThe AISi7Mg alloy was melted in an electric resistancestirring at different temperatures. When the liquid melt isfurnace and then refined. The melted temperature waspoured at 700°C, the microstructure of the AISi7Mg alloy720°C. When the liquid alloy was cooled to the chosenslurry consists of dendritic-like primary a-Al grains. Thepouring temperature, it was poured into a stainless steel cru-primary dendritic arms are much coarser and longer than thecible of 127 mm in diameter and 255 mm in height. Aftersecondary arms, and the microstructure is not homogeneous,pouring, the melt in the crucible was stirred by an electro-as shown in Fig. 2(a). When the liquid melt is poured atmagnetic stirrer at low power for 8 s. The slurry was then650°C, the microstructure has an obvious change, most ofrapidly quenched into cold water to maintain the microstruc-the primary a-Al grains are rosette-like and the microstruc-ture of AISi7Mg alloy slury at high temperature. Specificture is more homogeneous, as shown in Fig. 2(b). When thepreparation parameters were as follows: the pouring tem-liquid melt is poured at 630°C, the primary a-AI grainschanges from rosett-like to spherical, most of the primaryperature was 630, 650, and 700°C; the stirring power was0.41, 1.72, and 3.90 kW. The preparation process schemat-a-Al grains are spherical, a few of rosette-like primary a-Algrains appear, and the microstructure becomes more homo-ics of low superheat pouring and weak traveling-wave elec-geneous, as shown in Fig. 2(c).tromagnetic strring is shown in Fig. 1.These experimental results show that an ideal micro-structure of the AISi7Mg alloy slurry prepared at low su-perheat pouring and weak electromagnetic stirring for a-(5short time can be obtained, which can satisfy the need fortheo-forming or thixo-forming. When the pouring tempera--(4)ture is decreased, the microstructure of the AISi7Mg alloy(I- -(3)slurry is gradually changed and obviously improved, asshown in Fig. 2. If the liquid alloy melt is poured at the liq-55559uidus temperature, primary a-Al grains appear in sphericalFig. 1. Preparation process schematics of low superheatshape and the microstructure is homogeneous. However, inpouring and weak traveling-wave electromagnetic stirring: (1)practical production, it is very difficult to control the pouringtraveling-wave etromagnetie sirrer; (2) refractory; (3) process at such a low pouring temperature. Therefore, itisstainless steel mould; (4) AISi7Mg alloy melt; (5) meltingoften hoped that the pouring temperature would be appro-crucible.priately promoted to being higher than 615°C. When the200 umFig. 2. Microstructures of AISi7Mg alloy slurries poured at a stirring power o| 中国煤化istemperatures:(a) 700°C; (b) 650°C; (C) 630°C.TYHCNMHG.644R4RE METALS, Vol. 29, No.6, Dec 2010pouring temperature is increased somewhat, it is easier tospherical primary a-Al grains of the AISi7Mg alloy slurrycontrol the pouring process. But the solidified microstruc-can also be obtained. In this experiment, the optimizedture will become worse and primary a-Al grains graduallypouring temperature parameter is 630°C.change from spherical to rosette-like. It is reported that themelt flow motion at the initial solidification has an important3.2. Effect of stirring power on the microstructureeffect on the shape of primary a-Al grains, which helps theFig. 3 shows the microstructures of AISi7Mg alloy slur-formation of spherical primary 0.-Al grains [18]. When the ries prepared at the pouring temperature of 630°C, stiringalloy melt is stired by a weak electromagnetic field for afrequency of 30 Hz, and stirring time of 8 s with differentshort period of time, there will be a certain extent of flowstirring powers. It shows that the stiring power has an ob-motion in the melt. The flow motion makes the temperature vious effect on the microstructure. When the stirring powerfield of the melt homogenous and the heat diffusion and heatis 0.41 kW, most of the primary a-Al grains appear inexchange with the mould will increase. The solidificationspherical shape and a few primary 0.-Al grains are r0-time of the slurry is reduced, the growth of primary a-Alsette-like, as shown in Fig. 3(a). When the stirring power isgrains is restrained and primary a-Al grains are refined inincreased to 1.72 kW, the microstructure is obviously im-the end. Moreover, when the pouring temperature is lower,proved. Most of the primary a-Al grains are spherical andsuch as 630°C, the alloy melt has a lower supercooling de-finer, and the more homogeneous microstructure is obtainedgree during the solidification, and much finer grains will beas shown in Fig. 3(b). When the stirring power is increasedformed. When the pouring temperature is appropriately de- to 3.90 kW, the microstructure is not further improvedcreased, the pouring process becomes easier and idealcompared with that of Fig. 3(b), as shown in Fig.3(c).200μmFig.3. Microstructures of AISi7Mg alloy slurries prepared at a pouring temperature of 630°C with different electromag-netic stirring powers: (a) 0.41 kW; (b) 1.72 kW; (C) 3.90 kW.When the stiring power is 0.41 kW, the alloy melt is notwhile, the flow motion accelerates the mass transfer processeasily stirred. There is no obvious liquid concave on the top and makes the solute field near the solid-liquid interfacesurfacc of the melt in the preparing crucible and there is onlyhomogenous and decreases the constitutional supercooling.a weak flow motion on the liquid surface. Therefore, most ofThe lower constitutional supercooling makes primary den-the primary x-Al grains appear in spherical shape and a few drite separation difficult and most of the primary x-Alprimary a-AI grains are rosette -like. When the strringgrains appear rosette-like. This flow motion also makes thepower is increased to 1.72 kW, a more obvious liquid con-separated fine rosette-like primary a-Al grains move eve-cave is generated on the top surface of the melt, and the meltrywhere, and a strong manmade temperature fluctuation onflow motion is promoted. Therefore, the microstructure ofthe rosette-like primary a-Al grains exists. This phenome-the AlSi7Mg alloy slurry is obviously improved. Most of the non makes the secondary arm roots of rosette-like primaryprimary a-Al grains are spherical and finer, and the micro-0-Al grains fuse together and the rosette-like primary a-Alstructure is more homogeneous. When the stirring power isgrains gradually change to spherical grains [19-20]. The ex-further increased to 3.90 kW, there is a deeper liquid con- perimental results show that the above-mentioned phecave on the top surface of the melt and the flow motion innomenon seems to be true and an appropriate stirring power,the melt is further promoted. Hence, the microstructure ofsuch as 1.72kW, is needed for the preparation of ideal slur-the AISi7Mg alloy slurry is obviously improved and most of ries. Yurko et al. studied the semi-solid alloy slurry preparedthe primary a.-Al grains are spherical, too. This intense flowby low superheat pouring and weak mechanical stirringmotion induced by stirring makes the temperature field oftechnology [21]. Their study indicates that a semi-solid alloythe melt more homogeneous, and increases the region areaslury with a perfect shape of the primary phase can be pre-in which many primary a-Al grains separate at the samepared if the mec中国煤化工60 r/min and ittime and so primary a-AI grains are greatly fined. Mean- is not necessaryYHCN MH ring rates. The.Li S. et al, Preparation of semi-solid AISi7Mg alloy slurry with big capability645experimental results also show the following view. When [6] Midson S.P, Ththe pouring temperature is 630°C, appropriately increasingthe USA, [in] Proceedings of the 4th International Confer-the stirring power can gain better spherical primary a-AIence on Semi-solid Processing of Alloys and Composites,grains, but if the stirring power is increased to a certainSheffield, England, 1996: 251.value, the shape of primary a-Al grains does not improve7] Young K.P., Kyonka C.P, and Courtoris J.A, Fine GrainedMetal Composition, US Patent, Appl. 4415347, 1983.further when the stirring power is continually increased. In8] Dobatkin V.I. and Eskin G.I, Ingots of aluminum alloys withthis experiment, the optimized stirring power parameter isnondendritic structure produced by ultrasonic treatment for1.72 kW.deformation in the semi-solid state, [in] Proceedings of the4th International Conference on Semi-solid Processing of AI-4. Conclusionsloys and Composites, Sheffield, England, 1996: 193.9] Cao H.B., Pan Y, and Zhang C, Compressing deforming be-(1) Semi-solid AISi7Mg alloy slury of 5 kg can be pre-havior of semi-solid Al-Si alloy prepared by crystalliza-pared by low superheat pouring and weak traveling-wavetion-contoling, Spec. Cast. Nonferrous Alloys, 2002, (3): 9.electromagnetic strring technology and this new technology10] Wang H, Stjone D.H, and Davidsion CJ, Controlled nu-can save energy and make the pouring process convenient.cleation method for formation of semisolid feed stock, [i]Proceedings of the 8th International Conference on(2) When the pouring temperature is decreased at a stir-Semi-solid Progress of Alloys and Composites, Limassol, Cy-ring power of 0.41 kW, the shape of primary a-Al grainsprus, 2004: 269.gradually changes from dendritic-like to spherical. When the[11] Liu D. and Cui J.Z., New technique of nonstirring solid slurryalloy melt is poured at a certain superheat, the pouring proc-making and liquidus casting, Foundry Technol, 1998, (6): 44.ess becomes easier, and the spherical microstructure of the[12] Mao W.M., Zhao A.M, and Cui C.L, The Preparing MethodAISi7Mg alloy slurry prepared by weak traveling-waveand Apparatus of the Spherical Primary Grain Semi-Solidelectromagnetic stirring is formed. In this experiment, theSlury or Continuous Slurry, Chinese Patent, Appl. 00109540,optimized pouring temperature parameter is 630°C.2000.(3) When the pouring temperature is 630°C, increasing [13] Mao W.M., Zhao A.M, and Cui C.L, The Direct Preparingthe stiring power appropriately can result in better sphericalMethod and Apparatus of the Spherical Primary Grainprimary a-Al grains, but if the stirring power is increased toSemi-Solid Slurry or Contimuous Slurry, Chinese Patent, Appl.a certain value (1.72 kW), the shape of primary a-Al grains02104349, 2002.does not improve further when the stiring power is continu-[14] Liu Z, Mao W.M, and Zhao Z.D., Research on semi-solidslurry of a hypoeutectic Al-Si alloy prepared by low superheataly increased. In this experiment, the optimized stiringpouring and weak electromagnetic strring, Rare Met, 2006,power parameter is 1.72 kW.25 (2): 177.[15] Mao W.M., Bai Y.L., and Lin H, Effect of pouring height onAcknowledgementsthe soldified microstructure of AISi7Mg alloy,J. Univ. Sci.Technol. Bejing, 2006, 13 (4): 324.The work was financially supported by the National[16] Mao W.M., Bai Y.L, and Tang G.X， Preparation forHigh-Tech Research and Development Program of Chinasemi-solid aluminum alloy slurry under weak electromagnetic(No.2006AA03Z115), the National Basic Research Pro-stiring condition, J. Mater Sci .Technol, 2006, 22 (4): 447.gram of China (No. 2006CB605203), and the National[17] Gao S.F, Mao W.M., and Bai Y.L, Li Q, Filling ability ofNatural Science Foundation of China (No. 50374012).semi-solid A356 alloy in rheo-casting and microstructure dis-tribution, Spec. Cast. 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