Effect of pouring temperature on semi-solid slurry of ZL101 alloy prepared by slightly electromagnetic stirring

Transcription

1 February 2009 Research & Development Effect of pouring temperature on semi-solid slurry of ZL101 alloy prepared by slightly electromagnetic stirring *Liu Zheng 1, 2, Mao Weimin 2 (1. Faculty of Material and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou , China; 2. School of Material Science and Engineering, University of Science & Technology Beijing, Beijing , China) Abstract: The semi-solid slurry of ZL101 alloy is prepared by a combination technology of low superheat pouring and slightly electromagnetic stirring. The effects of pouring temperature on the slurry prepared by the technology are investigated. The results indicate that it is feasible to prepare the slurry with globular primary phases by low superheat pouring and slightly electromagnetic stirring, and that the pouring temperature has an important effect on the morphology and the size of primary α-al in ZL101 alloy. By applying suitable slightly electromagnetic stirring combining with relatively increased pouring temperature, i.e., in a practical way to apply low superheat pouring technology, is capable of obtaining appropriate semi-solid slurry of ZL101 alloy with globular shape of primary phase. Compared with the samples made by low superheat pouring only without stirring, the samples prepared by applying both slightly electromagnetic stirring and low superheat pouring can enable to achieve the same grain size and morphology of the primary phase with that of pouring at higher. Key words: semi-solid slurry; pouring temperature; slightly electromagnetic stirring; ZL101 alloy CLC number: TG Document code: A Article ID: (2009) In rheo-forming process, a special attention, for cost saving, has been paid to the preparation of semi-solid metals. In recent years, some of new technologies have been delivered [1], which is based upon controlling the pouring temperature or low temperature pouring technology to achieve the appropriate morphology of solid particle, to refine grain size or to improve grain morphology [2]. The morphology of primary phase must be strictly under controlled since the semi-solid slurry prepared by low temperature pouring is directly applied to the rheoforming operation, unlike thixo-forming process, in which the morphology of primary phase in a billet can be improved by the additional reheating process [3]. Some researches [4] reported *Liu Zheng Male, born in 1958, Ph.D. Prof., graduated from Nanchang University in 1981 and majored in Foundry. He got his master s degree in 1989 from the Harbin Institute of Technology and Ph. D degree from University of Science and Technology Beijing in His research interests include manufacture technology and microstructure control of metal matrix composite, semi-solid processing theory and technique of nonferrous metal and alloy, solidifi cation theory and control technology of metal materials, application of rare earth in nonferrous metal and alloy. So far he has published more than 150 academic papers in JMST, Rare Metals, Acta Metallurgica Sinica, etc. and about 40 of them are in the list of SCI, EI and ISTP. liukk66@163.com Received: ; Accepted: that the final microstructure was very sensitive to the superheat of the melt and the application of low pouring temperatures promoted the formation of spheroidal morphology of the a-al phase, while studied the evolution of microstructure in ZL101 alloy by the new rheocasting (NRC) process. To obtain the semi-solid slurry to satisfy rheo-forming, the pouring temperature and cooling rate of liquid alloy [5-6] must be strictly controlled, or the melt is helding at the solid-liquid zone [7] to ensure the morphology of primary phase, or the alloy is reheated into the solid-liquid zone and being held for a period of time [8]. In this way, the primary phase particle with round morphology can be obtained but the process is difficult to operate in practice. Under precondition of keeping the economics of low temperature pouring technology, it is very significant that the pouring temperature is suitably raised so that the low temperature pouring is conveniently operated. On the basis of researches on semi-solid metal (SSM) for long time, a new technology was developed low superheat pouring and slightly electromagnetic stirring, i.e., to bring into play the superiority of low temperature pouring with slightly electromagnetic stirring. The liquid aluminum alloy poured at low superheat is stirred for a short time by a simple electromagnetic stirring equipment to realize the purposes, such as reducing energy consumption, simplifying working procedure, increasing working efficiency and ensuring the quality of slurry. The effect of pouring temperature on the 9

2 CHINA FOUNDRY Vol.6 No.1 semi-solid slurry of ZL101 alloy under the condition of slightly electromagnetic stirring is studied in this work. 1 Experimental ZL101 alloy, as a kind of hypoeutectic Al-Si alloy, is extensively used to semisolid processing because of its wide solid-liquid range and good fluidity. In this test it was used to study low superheat pouring and slightly electromagnetic stirring. The composition of ZL101 is 7.46 Si, 0.49 Mg and balance Al. The liquidus temperature of ZL101 alloy was determined as by differential thermal analysis. ZL101 alloy was melted in an electric resistance furnace. The melted temperature was 700. The mould, a cylinder made of stainless steel with 102 mm in diameter and 220 mm in depth, was inserted into an electromagnetic stirrer. During electromagnetic stirring of the liquid alloy, the stirring force is an important factor to influence the quality of slurry, but the actual stirring force was difficult to measure in practice. In this test, the stirring power was indirectly represented as the stirring force because several of stirring powers can be got by adjusting the input voltage of the stirrer at the same current frequency. To study the effect of pouring temperature on ZL101 alloy prepared by slightly electromagnetic stirring, the pouring temperatures applied are 650, 630 and 615. When liquid ZL101 alloy was prepared and poured into the mould at the test temperature, the stirrer was started and slightly stirs the liquid alloy at the same stirring power for a short time, respectively. Then the mould was quenched in order to maintain the structure stirred. To inwestigate the effect of slightly electromagnetic stirring on low superheat pouring ZL101 alloy, the samples without stirring were also poured at 650, 630 and 615, respectively. Some wafers with thickness in 10 mm were cut from the same position of the ingots. The sector samples (through the circle center of the wafer) were sliced from the wafer. The samples were polished according to standard metallographic practice, etched with 0.5% aqueous solution of hydrofluoric acid. The microstructure of the sample was observed under an optical microscope, and MIAPS (Micro-image Analysis & Process) image analyzing software was used to determine the average equal-area-circle grain diameter and shape factor of the primary phase. 2 Results 2.1 The effect of pouring temperature Figure 1 shows the three semi-solid microstructures of ZL101 alloy obtained at the same stirring power (136 W) and stirring time (8 s) with various pouring temperatures. The microstructure of ZL101 poured at 650 is shown in Fig.1. It shows that the morphology of primary a-al gives priority to rosette-like, and a few globular-like and particle-like grains with (c) (c) 615 Fig.1 Morphology of primary phase in ZL101 alloy obtained at various pouring temperatures with weak electromagnetic stirring (136 W) coarse size. The microstructure of ZL101 alloy poured at 630 is shown in Fig.1, in which primary a-al with globularlike and particle-like is in the majority and a few with rosettelike, with the fine grain size. Moreover, the microstructure of ZL101 alloy poured at 615 is shown in Fig.1(c), and basically consists of primary a-al with globular-like and particle-like, with the finer grain size. Figure 1 shows that under the condition of weak electromagnetic stirring (136 W), the morphology of semi-solid primary a-al obtained from ZL101 alloy is changed from rosette-like to particle-like, and the grain size is gradually decreased with the decrease of the superheat temperature of liquid alloy (namely pouring temperature). It 10

3 February 2009 also shows that under the condition of weak electromagnetic stirring with 136 W, there is the better morphology of semisolid structure in ZL101 alloy poured at 615 and 630, and that the stirring power 136 W shows little effect on the semisolid structure of ZL101 alloy poured at 650. In addition, from the point of view in convenient operation, selecting 630 as pouring temperature can ensure semi-solid slurry of ZL101 alloy obtained to satisfy rheo-forming. To further study the effect of pouring temperature on the morphology of particle in ZL101 alloy slurry, the variation tendency of its morphology and the grain size have been investigated with 352 W stirring power applied, against variable pouring temperatures. Figure 2 shows the semisolid microstructures of ZL101 alloy obtained at the 352 W stirring power and 8 seconds of stirring time against variable pouring temperatures. As the pouring temperature is 650, the morphology of primary a-al gives priority to particle-like and some of primary a-al grains, in small amount, still present the rosette-like, the grains are fine, as shown in Fig.2. As the pouring temperature at 630, the morphology of primary a-al gives priority to particle-like and globular-like, and there is basically no primary phase with rosette-like, as shown in Fig.2. The morphology, besides giving priority to particlelike, presents rosette-like in small amount of primary a-al as the pouring temperature decreasing to 615, as shown in Fig.2(c). It is inferred that this phenomenon relates to the pouring operation. Because the pouring temperature of the melt is about 615, closing to the liquidus temperature of ZL101 alloy, a layer of dendritic crystal is formed on the cylindrical wall due to the chilling of the wall as the melt poured into the cylinder. The dendritic crystal is not broken by the stirring force resulted from the weak electromagnetic stirring but part of dendritic crystal falls off as the solute concentrated on the root of the dendritic crystal. There is no enough time for the dendritic crystal to change into the particle-like grain, so that the dendritic crystal is reserved in the form of rosette-like grain. Figure 2 shows that under the condition of weak electromagnetic stirring (stirring power 352 W), satisfied morphology of semi-solid microstructure in ZL101 alloy can be obtained from pouring at 615, 630 and 650, respectively. This indicates that the stirring power applied in the case can ensure to obtain the semi-solid slurry with particle-like primary phase for ZL101 alloy even at the pouring temperature as high as 650. In addition, from the standpoint of convenient operation, the pouring temperature selected as 630 and 650 can ensure to obtain the semisolid slurry of ZL101 alloy satisfied for rheo-forming. Compared with Fig.1, it is known that the morphology of primary a-al obtained under this condition is better than that obtained at the lower stirring power (136 W) and the grain size of primary phase obtained is finer and smaller as the stirring power of weak electromagnetic stirring increases to 352 W (still belonging to the weak stirring category). In other words, when the stirring power is increased to 352 W, the morphology and the grain size of primary phase obtained from the relatively higher pouring temperature are almost as same as those from Research & Development lower pouring temperature. In this way, it is more favorable and feasible for operation in production practice. (c) (c) 615 Fig.2 Morphology of primary phase in ZL101 alloy obtained at various pouring temperatures with 352 W stirring power Figure 3 illustrates the effect of pouring temperature on average equal-area-circle grain diameter and shape factor of semi-solid ZL101 primary phase under certain stirring power. Figure 3 shows that reduction of pouring temperature can obviously fine size of primary a-al grain in semi-solid ZL101 alloy by applying both low superheat pouring and slightly electromagnetic stirring under certain stirring power (136 W), from µm pouring at 650 reducing to 81.6 µm pouring at 615. In addition, the reduction of pouring temperature can 11

4 CHINA FOUNDRY Vol.6 No.1 Fig.3 Effect of pouring temperature on average equal-area-circle grain diameter and shape factor of primary phase in semi-solid ZL101 alloy 12 obviously improve shape factor of primary a-al grain in semisolid ZL101 alloy under the condition with the 136 W stirring power as shown in Fig.3, from 0.63 pouring at 650 increasing to 0.84 pouring at 615. The results in this test confirm that it is feasible to refine grain size and to improve grain morphology through controlling pouring temperature or making use of low superheat pouring [2, 9], and the results are also identical with the experimental results from Easton M A, et al [10]. 2.2 Comparison with the samples poured at low superheat temperature without stirring Figure 4 shows the microstructures of semisolid ZL101 alloy poured at the same pouring temperature without stirring. As pouring at 650, the primary phase in ZL101 still presents dendritic-like without stirring, but the primary crystal arms and the second arms of the dendritic crystal obviously becomes fine and short, and there is no third arms visible in the microstructures, as shown in Fig.4. While, there is no dendritic crystal in the microstructure of ZL101 alloy poured at the same temperature with the weak electromagnetic stirring, and many fine primary a-a1 with globular-like or particle-like appear as shown in Fig.1. When pouring at 630, the morphology of primary a-a1 in ZL101 alloy gradually changes from dendritic-like to rosette-like, but there is still a little primary a-a1 with dendritic-like unchanged fully in the microstructure, as shown in Fig.4. Comparing with the microstructure of ZL101 alloy poured at the same temperature with slightly electromagnetic stirring, there is more primary a-a1 with globular-like or particle-like and with even finer size, as shown in Fig.1. As pouring at 615, the morphology of primary a-a1 in ZL101 alloy is further varied, and basically presents the non-regulate globular-like or particle-like. The grains become finer and their distributions become more uniform. Comparing with those obtained from the higher pouring temperature without stirring, the microstructure poured at 615 is greatly improved, as shown in Fig.4(c). While comparing with the microstructure (as shown in Fig.1(c)) obtained at the same pouring temperature with slightly electromagnetic stirring, there is difference on the roundness of primary a-a1 between with and without stirring. The former is with globular-like or particle-like and (c) (c) 615 Fig.4 Morphology of primary phase in ZL101 alloy obtained at different pouring temperatures without stirring

5 February 2009 the latter is with the coarse size. Results of this test show that, on the one hand, the morphology and the size of primary phase obtained from the slightly electromagnetic stirring are superiority to that without stirring at the same pouring temperature, and it can be concluded that both low superheat pouring and slightly electromagnetic stirring play a positive role in improving the nucleation ratio and the morphology in ZL101 alloy; on the other hand, as the pouring temperature of the melt is raised 1 to 2 grade (630, even 650 ) and stirred by slightly electromagnetic stirring, the morphology of primary phase is equivalent to that poured at the low temperature (615 ) without stirring, and even better, as shown in Fig.1, Fig.1, Fig.2, Fig.2 and Fig.4(c). This supports that, if the pouring temperature is suitably raised about under the condition of slightly electromagnetic stirring, it can still ensure the requirement of semisolid slurry for rheo-forming on the morphology of primary phase. This is practical for the production, which makes the pouring performance convenient and the operation procedure simplified. 3 Discussion Cardoso et al. [4] pointed out that the final microstructure is very sensitive to the superheat of the melt and low pouring temperatures promote the forming of spheroidal morphology of the a-al phase, when they studied the evolution of microstructure in ZL101 alloy prepared by NRC processing. The current results in the test are also agreement with the viewpoint above. Decreasing pouring temperature can obtain the primary phase with particle-like, but it is very difficult to operate the low temperature pouring in production, especially to operate pouring at a temperature closing to liquidus. As a desired, if the pouring temperature is to be slightly higher then the pouring operation will be more convenient in production. To solve the issue, the improvement should be taken in the processing of low temperature pouring, such as suitable weak stirring in melt for a short time to increase flow of the melt during solidification. The research [11] has indicated that the melt flow at initial solidification plays an important role in formation of the primary phase for a particle-like microstructure. As well known, there are two kinds of flow in melt during solidification: one is the natural convection caused by liquid alloy washing out or the density difference or temperature difference in the melt after poured into mould, another is the forced convection caused artificially by an external field or extra physical disturbance as the melt freezing. The two kinds of convection have an important effect on the formation of crystal nuclei and the grain growth as well as the grain morphology during solidification of the melt [12]. That the melt stirred by electromagnetic force, in preparation of semisolid slurry, can promote dendritic arms in growing to remelt and to drift away. The drifting grains in the melt are continuously scoured by the temperature fluctuation and the concentration fluctuation and alternatively kept the state of remelting or growing. One primary dendritic crystal will be broken into several fragments, then these fragments grow up as Research & Development the new drifting grains with the dropping temperature to realize multiplication of drifting grains and to increase the number of the grains. The new dynamic condition of nucleation generated by the drastic electromagnetic stirring, for instance, the low temperature gradient formed, promotes the fusing and breaking of the second arms of primary phase and refines the primary crystal arms. Therefore, the grain size will obviously become smaller as the intensity of electromagnetic stirring increasing. During the preparation of the semi-solid slurry in ZL101 alloy using low superheat temperature and slightly electromagnetic stirring, the certain extent of the forced convection with low intensity in the melt is resulted from the slightly electromagnetic stirring. The forced convection, on the one hand, prolongs the cooling of the melt on the mould wall and the top surface, on the other hand, accelerates temperature decreasing in the bulk melt. Though there is the stronger cooling effect in the melt on the mould wall as well as on the melt surface, the forced convection can enable to take the cooler melt from the wall/surface positions into an internal area and carry the melt with warmer temperature from the internal to the wall/surface, thereby, the dropping of the melt temperature on the mould wall and the melt surface is delayed, so to postpone the formation of the stable freezing layer on the surface of mould. Thus, the cooler melt entering into the internal area is to be heated and the hotter melt taken to the surface layer of mould is to be cooled. The convection effect caused by the circle flow of the melt can promote the emission of the heat in the internal melt, and sequentially the temperature of the whole melt can reach the freezing temperature at a short time. Not only the melt on the surface but also the whole melt is kept undercooling, and the temperature distribution in the melt is relatively uniform, therefore, a large number of crystal nuclei can be formed, and continuously grow up in the internal flow of the melt. Similar to the common alloy casting, the melt solidification also carries out in the continuous dropping of temperature under the condition of the stirring. The main positions of heat emission are still at the mold wall and the melt surface regions, where more crystal nuclei are formed. Due to the forced convection caused by the stirring, these crystal nuclei enter the inside of the melt with melt flowing and grow up during the melt flowing and whirling motion itself. This is not happened under the condition of the traditional casting. The forced convection (whether strong or weak) caused by electromagnetic stirring in the melt changes the heat transfer (single direction conduction) and the mass transportation processes (slow diffusion) of traditional casting. The rapid heat transferring and mass transferring caused by the convection in the melt generates an environment with relative uniformity of temperature and composition in the melt. This environment is not favorable to preferential growth of the grain, or the manner of the preferential growth of the grain is strongly suppressed, and the grain can only grow relatively uniformly in all directions. This finally develops non-dendritic globularlike microstructure. 13

6 CHINA FOUNDRY 4 Conclusions (1) The semisolid slurry of ZL101 alloy contained the primary a-a1 with globular-like or particle-like and with the finer grains can be prepared by a combination technology of low superheat pouring and slightly electromagnetic stirring. (2) The pouring temperature has an important effect on the grain morphology and the grain size of the primary a-a1 in ZL101 alloy. At the range of temperature researched, the morphology of the primary a-a1 presents rosette-like and the size of grain is coarse at high pouring temperature; the morphology of the primary a-a1 presents globular-like or particle-like and the size of grain is fine under the condition of low pouring temperature. (3) Applying low superheat pouring at the higher pouring temperature in practice and using the suitable slightly electromagnetic stirring, such as under 136 W stirring power pouring at 630 or 352 W at 630 and even at 650, can ensure to obtain the semi-solid slurry of ZL101 alloy with particle-like primary phase. It is convenient for pouring operation in production. (4) Comparing with the samples poured at low superheat temperature without stirring, the temperature of low superheat pouring under the condition of slightly electromagnetic stirring can be suitably raised higher while the same grain size and morphology of the primary phase can ensure to be achievable. It has the practical significance for the production in operating the pouring conveniently. References [1] Fan Z. Semisolid metal processing. Int. Mater. Reviews, 2002, 47(2): Vol.6 No.1 [2] Flemings M C. Solidifi cation Process. New York: Mcgraw-Hill, [3] Xing Shuming, Tan Jianbo, Zhang Lizhong, et al. Study on Key Problems on Industrializations of Semisolid Rheologic Forming Processes. Proc. of the 8th Int. Conf. on Semi-solid Processing of Alloys and Composites. Limassol, Cyprus, 2004: [4] Cardoso E, Atkinson H V, Jones H. Microstructural Evolution of A356 during NRC Processing. Proc. of the 8th Int Conf on Semi-solid Processing of Alloys and Composites. Limassol, Cyprus, 2004: [5] Pan Y, Aoyama S, Liu C. Spherical Structure and Formation Condition of Semi-solid Al-Mg-Si Alloy. Proc. of the 5th Asian Foundry Congress. Nanjing, China, 1997: [6] Vieira E A, Junior B A O, Ferrante M. Microstructure and Rheology of an A356 Alloy in the Semi-solid State, Conditioned by a Low Pouring Temperature Technique. Proc. of the 8th Int. Conf. on Semi-solid Processing of Alloys and Composites. Limassol, Cyprus, 2004: [7] Yang Z, Seo P K and Kang C G. Grain size control of semisolid A356 alloy manufactured by electromagnetic stirring. J. Mater. Sci.Technol., 2005, 21(2): [8] Martinez R A and Flemings M C. Evolution of particle morphology in semisolid processing. Metall. and Mater. Trans., 2005, 36A: [9 Wang H, St John D H, Davidson C J. Semisolid microstructural evolution of AlSi7Mg alloy during partial remelting. Mater. Sci. Eng., 2004, A368: [10] Easton M A, Kaufmann H, Fragner W. The effect of chemical grain refi nement and low superheat pouring on the structure of NRC castings of aluminium alloy Al-7Si-0.4Mg. Sci. Eng., 2006, A420: [11] Pan Ye, Zhang Chunyan, Yuan Haoyang, et al. Effect of melt flow at initial solidification on granular primary crystal formation in semi-solid alloy. Acta Metall Sinica, 2001, 37(10): (in Chinese) [12] Hu Hanqi. The Metal Solidification Theory. Beijing: China Machine Press, (in Chinese) This work was supported by the Hi-tech Research and Development Program of China (Authorized No. G2002AA336080), the National Natural Science Foundation of China (Authorized No ) and the Natural Science Foundation of Jiangxi Province (Authorized No ). 14