Preparation of semisolid AlSi7Mg alloy slurry through weak traveling-wave electromagnetic stirring

International Journal of Minerals, Metallurgy and MaterialsVolume 16, Number 5, October 2009, Page 554MaterialsPreparation of semisolid AISi7Mg alloy slurry through weaktraveling-wave electromagnetic stirringZhen-duo Zhao, Wei-min Mao, Sha Li, and Rong mao ZhongSchool of Materals Science and Engineering, University of Science and Technology Beijing, Beijng 10083, China(Received 2008-10-25)Abstract: The semisolid AISi7Mg alloy slurry with large capacity was prepared by low superbeat pouring and week traveling-waveelectromagnetic stirring. The effects of electromagnetic stirring power and frequency on the shape and distribution of primary a-Algrains in the AISi7Mg alloy slurry were discussed. The experimental results show that the AISi7Mg alloy slurry with fine andspherical primary a-AI grains distributed homogeneously can be obtained. Under the condition of low superheat pouring and weektraveling-wave electromagnetic sirring, when the pouring temperature is 630°C, raising the sirring power or frequency appropriatlycan gain a better shape of primary a-Al grains; but if the strring power or frequency is increased to a certain value (1.72 kW orl0Hz), the shape of primary a-Al grains cannot be obviously improved and spherical primary a-Al grains distributed homogeneouslycan be still obtained.Key words: semisolid; AISi7Mg aloy;, surry; electromagnetic strring; primary a-Al[This work was. financially supported by the National High Tech Research and Development Program of China (No.20064403Z115),the National Basic Research Priorities Program of China (No. 2006CB605203), and the National Natural Science Foundation ofChina (No.50374012).]1. Introductionnucleation method [8], and so on. All these prepara-tion technologies can obtain an ideal microstructureAt present, there are many methods to preparewith finer grains and make the shape of the sphericalsemisolid metal slurries, such as the mechanical stir-primary phase better by controlling the pouring tem-ring method [1], electromagnetic stirring method [2], .perature. An ideal microstructure of the alloy slurystrain-induced melt activation process [3-4], ultrasoniccan be prepared when the liquid alloy is poured at thevibration method [5], and so on. In the practical pro-near liquidus temperature to obtain the semisolidduction of semisolid metal billet, the electromagneticslurry for rheoforning; the pouring temperature andstirring method and strain-induced melt activationcooling rate of the liquid alloy must be strictly con-method are still main semisolid metal forming meth-trolled or the alloy melt must be held at the semisolidods, but the production cost of these two kinds ofstate for some time to obtain an ideal microstructurepreparation methods are much higher, so the wide-[9-10]. Although an ideal microstructure of the alloyspread application of the semisolid metal thixoformingslurry can be prepared when the liquid alloy is pouredtechnology is restricted. In order to exert the advan-at the near liquidus temperature, it is very difficult totage of the semisolid metal forming technology, it iscontrol the pouring process during practical produc-very significant to develop the preparation technologytion. Recently, a low superheat pouring and weakof semisolid metal slurry with a short process and lowelectromagnetic stirring technology for preparingenergy exhaustion.semisolid alloy slurries or billets [11-12] was devel-In recent years, some new semisolid forming tech-oped中国煤化工- echnology Bejing.nologies have appeared, such as the liquidus castingThisE lowerenergy ex-method [6], controlling crystal method [7], controlledhaustMYH. C N M H Gled. Therefore, theCorresponding author: Zhen-duo Zhao, E-mall: zzd. 80@163.comAlo available onlie at ww.cencedlrectcm。2009 University of Science and Technology Bejig All rights reserved.2.D. Zhao et al, Preparation of semisolid AISi7Mg alloy slurry through weak traveling wave electromagnetic stirring55technology can make the pouring process control sim-the electromagnetic power and frequency were stud-ple, decrease the preparation cost of semisolid alloyslurries or billets, and meanwhile, an ideal micro-2. Experimentalstructure of the semisolid alloy slurry can be obtained.However, more attention has been paid to the semi-The experimental material is a commercialsolid AISi7Mg alloy slurry with small capacity duringAISi7Mg alloy. Its chemical composition is listed inprevious studies [13-14].Table 1. The theoretical alloy liquidus and solidusIn this article, the semisolid AISi7Mg alloy slurrytemperatures tested by diferential thermal analysis arewith large capacity was prepared by low superheat615 and 555°C, respectively.pouring and weak travel-electromagnetic stirring, andle 1. Composition of the AISi7Mg alloysiMgZnFeCuMnA16.5-7.10.25-0.45s0.1s0.2≤0.2<0.1The AISi7Mg alloy was melted in an electric resis-middle position (B) of the AISi7Mg alloy slurriestance furmace and then refined. The melted tempera-prepared at the same pouring temperature (630°C), atture was 700°C. When the liquid alloy was cooled tothe same travel-wave electromagnetic frequency (10the chosen pouring temperature, the liquid alloy wasHz), and stired for the same stiring time (8 s) butpoured into a stainless steel crucible with 127 mm inwith different stiring powers. It shows that stiringdiameter and 255 mm in height. After pouring, thepower has an obvious effect on the microstructure.melt in the crucible was stirred by an electromagneticWhen the stiring power is 1.27 kW, most of the pri-stirrer at a low power for 8 s. The slurry was then rap-mary a-Al grains appear rosette-like and a few pri-idly quenched in cold water to maintain the micro-mary a-AI grains are dendrite-like, as shown in Fig.structure of the AISi7Mg alloy slurry at a high tem-2(a). When the strring power is increased to 1.72 kW,perature. Specific preparing parameters were as fol-the microstructure is obviously improved. Most of thelows: pouring temperature, 630°C; stirring power,primary a-Al grains appear spherical, finer in size, and1.27，1.72, and 5.26 kW; pouring temperature,the microstructure is more homogeneous, as shown in630°C; stirring power, 1.27 kW; stirring frequency,Fig. 2(b). When the stiring power is increased to 5.265, 10, and 30 Hz.kW, the microstructure is not further improved com-Metallographic samples were cut off longitudinallypared with that of Fig.2(b). It is found that the micro-from the quenched slurry, schematics of the metal-structure of Fig. 2(c) is very similar to that of Fig.lographic samples intercepted from the slurry in the2(b). .axes-direction is shown in Fig.1. A transverse sectionAs shown in Fig. 2, increasing the electromagneticof each sample was roughly ground, finely polished,stiring power appropriately can gain better sphericaland etched by a aqueous solution of 0.5vol% HF. Theprimary a-Al grains, which is related to the alloy meltsamples etched were cleaned with an alcohol solutionflow induced by traveling-wave electromagnetic stir-and dried. The microstructures were observed anding. During the process of traveling-wave electro-analyzed with an optical microscope of the Neuphotomagnetic sirring, this intense flow occurs upwards ordownwards along the crucible wall in the AISi7Mg21 type.alloy melt, makes the temperature field in the melthomogeneous, and increases the region area in which>Amany primary a-Al grains separate at the same timeand so the primary a-Al grains are greatly fined.BMeanwhile, the flow accelerates the mass transferprocess and makes the solute field near thesolid-liquid interface homogenous and decreases theqocgconstitutional supercooling. The lower constitutionalFig.1. Schematies of the metallographic samples inter-supercooling makes nrimarv dendrite separation dif-cepted from the slurry in the axes-direction.ficult,中国煤化工grains appear ro-3. Results and discussionsette-1Y片C N M H Cette-ike primaryx-Al grans move everywuere, anu an intense tem-3.1. Effect of strring power on the microstructureperature fluctuation on the rosette-like primary a-AlFig. 2 shows the solidifed microstructures from thegrains exists. This phenomenon makes the secondary556International Journal of Minerals, Meallurgy and Materials, VoL.16, No.5, Oct 2009arm roots of the rosett-like primary a-AI grains fuseshape of primary a-Al grains is not obviously im-together and the spherical primary a-Al grains can beproved and most of the primary a-Al grains appeargained advantageously [9, 15-16]. In addition, thespherical and homogeneous, too, which is similar toconvection induced by stirring in the melt makes theYurko's experimental results [17]. Yurko et al. stud-friction and collision between primary a-Al grains oried the semisolid alloy slurry prepared by the low su-between primary a-Al grains and the liquid again andperheat pouring and weak mechanical stirring tech-again. The convection also makes the primary a-Alnology. Their study indicates that a semisolid alloygrains greatly fined. When the electromagnetic strringslurry with a perfect shape of the primary phase can bepower is increased from 1.27 to 1.72 kW, the stiringprepared if the mechanical stirring rate exceeds 60strength of the alloy melt is further increased and ther/min, and it is not necessary to use higher mechanicalfusing possibility of the second arm roots of ro-stirring rates. The ideal microstructure of the semi-sette-like primary a-Al grains is also increscent. Moresolid AISi7Mg alloy slurry can be gained by low su-and more primary a-Al grains appear spherical and theperheat pouring and weak electromagnetic stiringmicrostructure is more homogeneous. But when thewith a short time.stirring power is further increased to 5.26 kW, theFig. 2. Microstructures of the AISi7Mg alloy slurries prepared at different electromagnetic stirring powers of 1.27 kW (a),1.72 kW (b), and 5.26 kW (C) when the pouring temperature is 630°C.3.2. Effect of stirring frequency on the microstruc-shown in Fig. 3(a). When the stiring frequency is in-turecreased to 10 Hz, the microstructure is obviously im-Fig. 3 shows the solidified microstructures from theproved. Most of the primary a-Al grains appearbottom position (C) of the AISi7Mg alloy surriesspherical, finer in size, and the microstructure is moreprepared at the same pouring temperature (630°C), athomogeneous, as shown in Fig. 3(b). When the stir-the same travel-wave electromagnetic stirring powerring frequency is further increased to 30 Hz, the mi-(1.27 kW), and stirred for the same stirring time (8 s)crostructure is not further improved compared withbut with different electromagnetic stiring frequencies.that of Fig. 3(b). Most of the primary a-Al grains alsoWhen the stiring frequency is 5 Hz, most of the pri-appear spherical and the microstructure is still homo-mary a-AI grains appear rosett-like, and sphericalgeneous. It is found that the microstructure of Fig. 3(c)primary a-Al grains distribute in the microstructure, asis very similar to that of Fig.3(b).200um2000Fig, 3. Microstructures of the AISi7Mg alloy slurries prepared at dif中国煤化工quencies ors Hza),10 Hz (b), and 30 Hz () when the pouring temperature is 630°C."TYHCNMHGIt can be observed from Fig. 3 that the strring fre-When the stiring frequency is less than or equal to 10quency has an obvious effect on the microstructure.Hz, with the siring frequency rising, most of the2.D. Zhao et al, Preparation of semisolid AISi7Mg alloy slurry through weak traveling-wave electromagnetic strring557primary a-Al grains appear spherical and the grainprove further. Most of the primary a-Al grains appearsize is smaller and finer. When the stiring frequencyspherical and the microstructure is still homogenous.is continually increased to 30 Hz, the shape of theIn this experiment, the optimized stirring power andprimary a-Al grains in the microstructure of thefrequency are 1.72 kW and 10 Hz, respectively.semisolid AISi7Mg alloy slurries does not improvegreatly; most of the primary a-AI grains also appearReferencesspherical and the microstructure is still homogeneous,[1] S.P. Midson, The commercial status of semi-solid castingbut there are more rosette-like primary a-Al grainsin the USA, [in] D.H. Kirkwood and P. Kapranos eds.than Fig. 3(b).Proceedings of the 4th International Conference onsemi-solid Processing of Alloys and Composites, Sheffield,During the traveling wave electromagnetic stirring1996, p.251.process, the higher the electromagnetic stiring fre-[2] M.C. Flemings, Behavior of metal aloys in the semisolidquency, the faster the linear speed of the moving trav-state, Metall Trans. 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Therefore, when the stiring frequency is in-nucleation method for formation of semisolid feedstock,creased to 30 Hz, the practical stirring power of the[in] Proceedings of the 8th Ilnternatimnal Conference onSemi- solid Progress of Allys and Composites, Limassol,semisolid AISi7Mg alloy melt may be similar to that2004, p.269.of the semisolid AISi7Mg alloy melt with a stiring[9] JX. Zhao, M.F. Zhu, and Z.M. Jin, Evolution of globularfrequency of 10 Hz; the microstructure of the semi-and dendritic structures in solidification of Al-Si alloys,solid AISi7Mg alloy slurries does not improve obvi-Phys. Test. Chem. Anal. Part A (in Chinese), 40(2004),ously, and most of the primary a-Al grains appearNo.9, p.433.spherical and the microstructure is still homogeneous.[10] E.A. Vieira, B.A.O. Junior, and M. 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