Preparation of semi-solid aluminum alloy slurry poured through a water-cooled serpentine channel

International Journal of Minerals, Metallurgy and MaterialsVolume 19, Number 1, Jan 2012, Page 48DOI: 10.1007/s12613-012-0513-6Preparation of semi-solid aluminum alloy slurry poured througha water-cooled serpentine channelZheng zhou Chen, Wei -min Mao, and Zong -chuang WuSchool of Materials Science and Engineeing, University of Science and Technology Bejing, Beijng 10083, China(Received: 10 January 2011; revised: 17 February 201; accepted: 6 March 2011)Abstract: A water-cooled serpentine channel pouring process was invented to produce semi-solid A356 aluminum alloy slurry for rheocast-ing, and the effects of pouring temperature and circulating cooling water flux on the microstructure of the slurry were investigated. The论sults show that at the pouring temperature of 640-6809C and the circulating cooling water flux of 0.9 m'/hn, the semi-solid A356 aluminumalloy slurry with spherical primary a(AI) grains can be obtained, whose shape factors are between 0.78 and 0.86 and the grain diametercanreach 48-68 um.When the pouring temperatures are at 660-680C, only a very thin solidifed shell remains inside the serpentine channel andcan be removed easily. When the serpentine channel is cooled with circulating water, the microstructure of the semi-solid slurry can be im-proved, and the serpentine channel is quickly cooled to room temperature after the completion of one pouring. In terms of the productivity ofthe special equipment, the water-cooled serpentine channel is economical and fficient.Keywords: semi-solid slurry; aluminum alloys; microstructure; rheocastingf China (No.50774007), the National High-Tech Re-search and Development Program of China (No.2006AA03Z115), and the National Basic Research Priorities Program of China(No.2011CB606302-1.)1. Introductioninvented according to the theories of“controlling nucleationand inhibting dendrtes growth" and“promoting the earlyThe theo- diecasting technique of semi -solid metal (SSM)solidification nucleation” [10], which have one commonwas put forward and researched by Flemings in the earlymerit that a special stiring device is cancelled, so these1970s [1], but owing to the difficulties of preservation andpreparation processes are simple, inexpensive, and have atransportation of SSM slurry, its development was very slowbroad prospect for application.over a long period of time. Meanwhile, the studies of SSMforming technology were mainly focused on thixo-diecas-Before this study, the preparation technology of SSMslurry through a serpentine channel has been introducedting or thixo-forging. Fortunately, several novel preparationmethods of SSM slury have been developed in recent dec-[11-12]. In this paper, an innovative preparation techniqueades, so much attention has been focused on rheoformingof SSM slurry, water-cooled serpentine channel pouringtechnologies again. These preparation processes include theprocess, is presented, which can produce satisfactory SSMcooling slope casting process [2-3], damper cooling tubeslurry with a larger volume. At an appropriate pouring tem-method (DCT) [4], cooling slope casting [5-6], vertical pipeperature, no solidified shell can form in the serpentinepouring process [7], vibrating wavelike sloping plate proc-channel after the pouring process. Moreover, the serpentineess [8], and low superheat pouring with a shear fieldchannel can be fast cooled to room temperature, which can(LSPSF) [9]. These preparation processes may have beenpromote the preparation efficiency of the special equipment.Corresponding author: Zheng zhou ChenE-mail: chzz19710425@126 com◎University of Science and Technology Bejing and Springer Verlag Berlin Heidelberg 2012包Springer中国煤化工MHCNMH G50Im. J. Miner. Metall Mater, VoL.19, No.I, Jan 2012(a)(b200 um(d)(e)(fFig. 2. Effects of pouring temperature and cooling water flux on the microstructure of semi-solid A356 alloy quenched slurries: (田)680°C, 0.9 m/h; (b) 680°C, 0 m/h; (C) 660°C, 0.9 m/n; (d) 660°C, 0 m/mn; (e) 640°C, 0.9 m/h; (D 640°C, 0 m2/h.After pouring at 680 or 660°C for 12 s, it was found thatthe channel, which miglno solidified shell remained in the serpentine channel, butThe experimental results show that when the water cooledwhen the pouring temperature was decreased to 640°C, theserpentine channel is used to produce the semi-solid A356solidified shell formed in the serpentine channel, as shownAl-alloy slurry, if the pouring temperature is appropriate, notin Fig.3. The left shell (Fig. 3(a)) is formed under the con-only can the microstructure of the samples be improveddition of no cooling water and the right shell Fig. 3(b)) isduring a longer pouring time, but also can the volume offormed with a cooling water flux of 0.9 m'/h. At the sameTable 1. Characteristics of the samples prepared under vari-pouring temperature and in the same pouring time, the vol-ous conditionsume of the solidified shell with the cooling water flux of0.9Pouring temperaure/9C680 680 660 660 640 640m2/h is larger than that without any cooling water. If the liq-uid A356 alloy was poured at a lower temperature and theCooling water fux/ (m2.hr) 0.9 0 0.9 0 0.9 0channel was cooled by water, the serpentine channel couldOutlet temperaure/°C611 61260860960660Shape factor0.78 0.75 0.82 0.80 0.86 0.84provide high cooling strength on the alloy, and the primarya(AI) nuclei could easily grow and adhere on the surface ofGain diameter/ um70_ 56624856中国煤化工TYHCNMHGz.Z. Chen etal, Preparation of semi-solid aluminum alay slurry poured through a water-cooled serpentine channel51ciency of the serpentine channel is acceptable.The temperatur-time variation curves and the criticaltemperature data of the serpentine channel wall are shown inFig. 4(b) and Table 2 when there is no circulating water inthe cooling-jacket. Pouring at 640°C for 12 s, the tempera-ture of the channel wall is 306°C. Owing to a thicker solidi-fied shell remaining in the serpentine channel, the maximumtemperature of the channel wall can reach 340°C, and itneeds 4980 s for the channel wall to be cooled to room250 [8)+ 6400C00 t-←6609C十680°Ci 100Fig. 3. Photos of the slifned shell in the serpentine channelpoured at 640°C and cooled with different water fuxes: (a) 0m/h; (b) 0.9 m/h.20406080100120Time/sthese semi-solid slurries reach 5 kg before the channel is50 r1)blocked.- + 640°C+ -660°C3.2. Temperature variation of the serpentine channel)250Hwall00 H-←680°CThe temperature timne variation curves and the criticaltemperature data of the serpentine channel wall with thecooling water fux of 0.9 m'/h at different pouring tempera-50tures are shown in Fig. 4(a) and Table 2, respectively, andthe location of temperature measurement is shown in Fig. 1.Pouring at 640°C for 12 s, the temperature of the channelwall rises to 120°C. Owing to a thicker solidified shell reFig. 4. Temperature-time variation curves of the serpentinemaining in the serpentine channel, the temperature of thechannel wall with different cooling water fuxes: (a) 0.9 m'/h;channel wall can continuously rise to 2019C. It needs 105 s(b)0 m/h.for the channel wall to be cooled to room temperature.Pouring at 660 or 680°C for 12 s, the temperature of theTable 2. Critical temperature data of the serpentine channelchannel wall rises to 103 and 112°C, respectively. Owing towall under the conditions of different cooling water fuxes andno solidified shell remaining, the maximum temperatures ofpouring temperaturesthe channel wall are 148 and 162°C, and it needs 90 and 100Water fux/0.0.9.9s for the channel wall to be cooled to room temperature, re-(m'h)spectively. In short, pouring at 640-680*C for 12 s and withTp/°C680680660660.40the circulating cooling water flux of 0.9 m'/h, the tempera-TI2/°C112 198 103 189 120 306ture of the channel wall is between 103 and 120°C. WithinTmx/°C162 220 148 214 201 340 )this narrow temperature range, the chilling effect of the ser-t/s42604080 105 4980pentine channel wall is impossible to vary greatly, so thatNote: Tp is the pouring temperature, Ti2 is the temperature of thethe microstructure of the semi-solid A356 A-alloy slury ischannel wall just at the pouring end, Tmax is the maximum tem-mainly dominated by the pouring temperature. In addition,perature of the channelwall after pouring, and t is the needed timefor the cooling time of only 90-105 s, the preparation eff-for the serpentine channel being cooled to room temperature.中国煤化工MYHCNMH G52Int. J. Miner Metall, Mater, VoL19, No.1, Jan 2012temperature. Pouring at 660 and 680°C for 12 s, the tem-semi-solid slurry prepared by the water- cooled serpentineperatures of the channel wall are 189 and 198°C, respec-channel pouring process was more satisfactory.tively. Owing to no solidified shell remaining, the maximumIf the pouring time is longer, for the cooling slope castingtemperatures of the channel wall are 214 and 220°C, and itprocess, the cooling chute process or the vertical pipe pour-needs 4080 and 4260 s for the channel wall to be cooled toing process, the temperature of the special device must riseroom temperature, respectively. In short, pouring640 -680°C for 12 s and without any water, the temperaturehigher, so the chilling effect of the special device could beof the channel wall can rise very high, and the cilling efectreduced, while it is difficult to produce a larger volume ofsatisfactory SSM slurry in one time. Moreover, it must takeof the channel is weakened obviously, so that the micro-a longer time for the special device being cooled to roomstructure of the semi-solid A356 Al-alloy slurry is deterio-tempcrature, so the preparation efficiency is not high. How-rated, as shown in Figs. 2(b), 2(d), and 2(). At the sameever, for the water-cooled serpentine channel pouring proc-time, it must take longer for the next preparation of theess, it is convenient to produce a dose of satisfactory SSMsemi-solid shurry, so the productivity of the serpentineslurry with a larger volume and it takes a shorter time tochannel is very low.prepare the next dose of SSM slurry. In terms of the produc-tivity of the equipment, the water-cooled serpentine channel4. Discussionis economical and efficient.Since the serpentine channel wall was cooled continu-ously by cold water, the chilled primary a(AI) nuclei could5. Conclusionsbe generated largely in the alloy melt layer contacting di-(1) With the pouring temperature of 640-680°C and therectly with the serpentine channel inner wall. These chilledcirculating cooling water flux of 0.9 m'/h, the semi solidprimary a(A1) nuclei might be of two trends. Most part ofA356 aluminum alloy slurry with spherical primary a(A1)the chilled primary a(AI) nuclei could separate from the in-grains can be obtained, whose shape factors can reachner wall and enter the melt because of the combined effect0.78-0.86 and the grain diameters can reach 48- 68 μm.of alay melt flowing motion and temperature fluctuation. Inthe meantime, the alloy melt was cooled continuously dur-(2) When the serpentine channel is cooled with circulat-ing flowing, whose temperature decreased gradually belowing water, the microstructure of the semi-solid A356the liquidus temperature, and most of the separating nucleiAl-alloy slurry can be improved, and the serpentine channelcould survive and become the main part of the nuclei multi-can be fast cooled to room temperature.plying in the alloy melt. The related research results have(3) During the preparation of semi solid A356 Alalloyshown that if there are sficient primary a(A1) nuclei in anslurry by the water-cooled serpentine channel pouring proc-aluminum alloy melt, they can be spheroidized in the endess, because of the chilling effect of the serpentine channeland prevented from growing too large [13-15]. Therefore,wall, a large number of primary a(AI) nuclei are generatedenough primary a(A1) nuclei in the slurry is the main reasonand then separated from the channel wall into the alloy meltfor these nuclei evolving into spherical grains. Another partand spheroidized finally.of the chilled primary a(AI) nuclei may remain on the innersurface of the serpentine channel and grow, and so a layer ofReferencesthin solidified shell can fom. However, if the pouring tem-[1] M.C. Flemings, Behaviour of metal alloys in the semisolidperature is high enough, the thin solidifed shell could bestate, Metall. Mater. Trans. A, 22(1991), No.5, p.957.melted again or grow slowly, and the channel could be kept[2] M.A. Easton, H. Kaufmann and W. Frangner, The effect ofunobstructed for a longer time; otherwise, the thin solidifiedchemical grain refinement and low supertheat pouring on theshell could grow fast, and only a small quantity of semi-structure of NRC castings of aluminium alloy Al-7Si-0.4Mg,solid slury could be obtained. Based on the analysis, in or-Mater. Sci. Eng. A, 420(2006), No.1-2, p.135.der to obtain a large volume of semi-solid slury, there[3] J. Yurko and R. Boni, SSRTM semi-solid theocasting, Metall.Italiana, 98(2006), No.3, p.35.should be a proper pouring temperature range of 640-680°C.[4] X.IL. Zhang, S.S. Xie, TJ. Li, H.Q. Yang, and J.Z. Jin, A356During flowing along the serpentine channel, the liquid meltaluminum alloy semisolid slurry prepared by damper coolingcould be sirred and mixed strongly, which could be signifi-tube process, Rare Met. Mater. Eng, 36(2007), No.5, p.915.cantly higher than that in the cooling slope casting process [5] LE. Cardoso, H.V. Atkinson, and H. Jones, Cooling slopeor the cooling chute process, so the microstructure of thecasting to obtafarlotnok. T∩heervations with a中国煤化工TYHCNMH GZ.Z. Chen et al, Preparation of semi-solid aluminum alloy slurry poured through a water cooled serpentine channel53transparent analogue, J. Mater. Sci, 43(2008), No.16, p.5448.2004, p.571.6] E.C. Legoretta, H.V. Atkinson, and H Jones, Cooling slope[11] z.z. Chen, W.M. Mao, and z.C. Wu, Mecbanical propertiescasting to obtain thixotropic feedstock II: Observations withand microstructures of Al alloy tensile samples produced byA356 aloy, J. Mater. Sci, 43(2008), No.16, p.5456.serpentine channel pouring rheo diccasting process, Trans.刀] X.R. Yang, W.M. Mao, and s. Pei, Preparation of semisolidNonferrous Met. Soc. China, 21(201 1), No.7, p.1473.A356 alloy feedstock cast through vertical pipe, Mater. Sci[12] X.R. Yang, W.M. Mao, and C. Gao, Senisolid A356 aloy .Technol, 23(2007),No.9, p.1049.feedstock poured through a serpentine channel, Int. J. Miner.8] R.G. Guan, F.R. Cao, L.Q. Chen, JP. Li, and C. Wang, Dy-Metall. Mater, 16(2009), No.5, p.603.namical solidification behaviors and microstructural evolu-[13] Y. Pan, s. Aoyama, C. Liu, G.X. Sun, and H. Yuan, Sphericaltion during vibrating wavelike sloping plate process, J. Mater.structure and formation conditions of semi-solid Al-Si-MgProcess. Technol, 209(2009), No.5, p.2592.alloy, [in] Proceedings of the 5th Asian Foundry Congress,9] H.M. Cuo, XJ. Yang, and B. Hu, Rheocasting of aluminumNanjing, 1997, p.443.alloy A356 by low superheat pouring with a shearing field,[14] W.M. Mao, C.L. Cui, and A.M. Zhao, et al, Effect of pour-Acta Metall. Sin, 19(2006),No.5, p.328.ing process on the microstructures of semi solid AISi7Mg al-[10] H. Wang, D.H. St John, CJ. Davidson, and z. Ning, Con-loy,J. Mater. Sci. Technol, 17(2001), No.6, p.515.trolled nucleation method for formnation of semisolid feed-[15] HM. Guo and XJ. Yang, Formation mechanism of sphericalstock, [im] Proceedings of the 8th Intemnational Conference onparticles in undercooled melt, Chin. J. Nonferrous Met,Semi-Solid Processing of Alloys and Composites, Limassol,18(2008), No.4, p.651.中国煤化工MHCNMH G