Modeling of microstructure evolution of magnesium alloy during the high pressure die casting process

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1 IOP Conference Series: Materials Science and Engineering Modeling of microstructure evolution of magnesium alloy during the high pressure die casting process To cite this article: Mengwu Wu and Shoumei Xiong 2012 IOP Conf. Ser.: Mater. Sci. Eng Related content - Determination of the metal/die interfacial heat transfer coefficient of high pressure die cast B390 alloy Yongyou Cao, Zhipeng Guo and Shoumei Xiong - Modeling and simulation of dendrite growth in solidification of Al-Si-Mg ternary alloys Yufeng Shi, Yan Zhang, Qingyan Xu et al. - A cellular automaton model for dendrite growth in magnesium alloy AZ91 Hebi Yin and Sergio D Felicelli View the article online for updates and enhancements. This content was downloaded from IP address on 19/06/2018 at 23:13

2 Modeling of microstructure evolution of magnesium alloy during the high pressure die casting process Mengwu Wu 1,2 and Shoumei Xiong 1,2 1 Department of Mechanical Engineering, Tsinghua University, Beijing , China 2 State Key Laboratory of Automobile Safety and Energy, Tsinghua University, Beijing , China smxiong@tsinghua.edu.cn Abstract. Two important microstructure characteristics of high pressure die cast magnesium alloy are the externally solidified crystals (ESCs) and the fully divorced eutectic which form at the filling stage of the shot sleeve and at the last stage of solidification in the die cavity, respectively. Both of them have a significant influence on the mechanical properties and performance of magnesium alloy die castings. In the present paper, a numerical model based on the cellular automaton (CA) method was developed to simulate the microstructure evolution of magnesium alloy during cold-chamber high pressure die casting (HPDC) process. Modeling of dendritic growth of magnesium alloy with six-fold symmetry was achieved by defining a special neighbourhood configuration and calculating of the growth kinetics from complete solution of the transport equations. Special attention was paid to establish a nucleation model considering both of the nucleation of externally solidified crystals in the shot sleeve and the massive nucleation in the die cavity. Meanwhile, simulation of the formation of fully divorced eutectic was also taken into account in the present CA model. Validation was performed and the capability of the present model was addressed by comparing the simulated results with those obtained by experiments. 1. Introduction Magnesium alloy is the lightest of all metals used for structural applications. With the increasing demand for light weighting, energy saving and environment protection in the automobile industry, magnesium alloy has been widely used by automobile manufacturers to replace steel, cast iron, and even aluminium alloy. Successful applications of magnesium alloy in automobile industry components include steering wheels, gear boxes, instrument panels and air intake systems, etc. [1]. The high pressure die casting (HPDC) process is a net-shape or near net-shape process with the advantages of high efficiency, considerable economic benefit and high precision of the product size, and generally, it is the dominant manufacturing process for magnesium structural applications. The microstructure of die cast magnesium alloy has a great influence on the final performance of castings. One of the key microstructure characteristics of high pressure die cast magnesium alloy is the appearance of relatively coarse dendrites at the central region of castings. Based on experimental studies and numerical simulation of the filling sequence and solidification behaviour during the HPDC process [2-4], these coarse dendrites have been proven to originate from the nucleation and crystal growth in the melt in the shot sleeve, and they are named Externally Solidified Crystals or ESCs for short. The externally solidified crystals have a significant impact on the quality and performance of Published under licence by Ltd 1

3 magnesium alloy die castings. For example, they obviously affect the grain size distribution, which consequently affects the mechanical properties of die castings. Meanwhile, under extremely poor conditions, for instance, the pouring temperature is too low, or the delay time of the melt in the shot sleeve is too long, cold flakes (or externally solidified products, ESP) may form and occur in the microstructure of die castings. This will severely lower the quality of the castings. Another influence of the externally solidified crystals is that they may affect the filling sequence and solidification behaviour of the melt. Based on the ESCs theory, Dahle et al. [5] explained the formation of defect band in the microstructure of die castings properly. Since most commercially available magnesium alloys for the HPDC process contain aluminium as the main alloying element, another microstructure characteristic of die cast magnesium alloy is the formation of fully divorced eutectic in the interdendritic liquid at the last stage of solidification in the die cavity. The constitution, morphology and distribution of the divorced eutectic in the as-cast microstructure also influence the performance of magnesium alloy die castings, especially the ductility, corrosion resistance, high temperature strength and creep resistance [6]. Accordingly, one of the key aspects of die casting technology is to investigate the effect of process parameters on the microstructure of magnesium alloy, consequently, optimize the process parameters and control the microstructure formation during the HPDC process. In the last two decades, numerical simulation has been rapidly developed as a powerful tool for simulating and predicting the time-dependent microstructure evolution during various solidification processes. Among those deterministic and stochastic methods, the cellular automaton (CA) method has been used extensively for simulation of casting processes on both micro and macro scales with a relatively small computational load [7]. However, most of the published works using the CA method focus on the microstructure simulation of cubic metals, whereas the CA method has considerable difficulties in simulating the microstructure of magnesium alloy with hcp crystal structure. Besides, for the simulation of solidification structure of magnesium alloy during the HPDC process, the formation of externally solidified crystals and divorced eutectic ought to be considered. In the present paper, a cellular automaton model was proposed to simulate the crystal growth of magnesium alloy. Based on the nucleation characteristics and solidification behaviour during the coldchamber HPDC process, both of the nucleation and crystal growth in the melt in the shot sleeve and in the die cavity were taken into account. Meanwhile, the present CA model considered the formation of fully divorced eutectic at the last stage of solidification in the die cavity. The model was applied to simulate the microstructure of cover-plate die castings of AM60 magnesium alloy at different process parameters, and the simulated results were then compared with the experimental results. 2. Morphology and distribution of the ESCs and the divorced eutectic During the cold-chamber HPDC process, firstly, a certain amount of melt at a certain temperature is poured into the shot sleeve. After the slow shot and fast shot phases, the melt fills the die cavity with a high velocity and pressure. Experimental and modeling studies demonstrate that when the melt is hold in the shot sleeve, especially at the slow shot phase, the melt s superheat is lost due to the impingement on the relatively cold shot sleeve wall and plunger. In this case, nucleation and crystal growth occur in the melt in the shot sleeve. At the fast shot phase, a mixture of liquid and crystals is injected into the die cavity, and during this phase, the floating crystals migrate to the central region of the die cavity due to the force of the flowing melt. Therefore, the final microstructure of die castings is not the same as commonly observed microstructure of castings with conventional casting processes. Figure 1 shows the typical microstructure at the central region of AM60 magnesium alloy die castings using OM, EBSD and SEM, respectively. It can be seen that the central region comprises a mixture of coarse dendrites and fine grains. The size of the basal fine grains is about 5~30 μm, while the equivalent grain size of the coarse dendrites is up to 100 μm. Based on the fact that the melt in the die cavity solidifies with a high cooling rate and a short solidification time, the coarse dendrites should not nucleate and grow in the melt in the die cavity. This could also help to validate the theory that the coarse dendrites in the microstructure of die castings originate from the nucleation and crystal growth in the melt in the shot sleeve. 2

4 (a) (b) (c) Figure 1. Typical microstructure at the central region of AM60 magnesium alloy die castings using OM (a), EBSD (b) and SEM (c). Figure 2 shows the schematic diagram of a cover-plate die casting associated with the experimental observation and quantitative statistics of the ESCs distribution over cross section A. A detailed description of the dimensions of the cover-plate die casting and the process parameters could be found in reference [8]. During the statistics of the area fraction distribution of the ESCs by using an image analysis software, those crystals which grain size was more or less equal to or larger than twice the average grain size were treated as ESCs. It can be noted from figures 2(b) and 2(c) that since the ESCs migrate to the central region of the castings during the fast shot phase of the melt into the die cavity, the percentage of the ESCs is higher at the central region than the surface layer of the castings. Meanwhile, the area fraction of the ESCs along the thickness of the die casting satisfies a Gaussian distribution, while its value at the central region may be up to 25%. Based on experimental studies [3, 4], the percentage of the ESCs is higher at the area near the gate than that of far from the gate of the castings. Besides, the percentage of the ESCs increases when decreasing the pouring temperature and the slow shot phase plunger velocity. At a lower fast shot phase plunger velocity, coarse ESCs are more easily to be observed at the cross section of the castings. In addition, the percentage of the ESCs in the die castings may also be influenced by the dimensions of the shot sleeve and the filling fraction of the melt in the shot sleeve. (a) (b) (c) Figure 2. Schematic diagram of the cover-plate die casting (a) associated with the experimental observation (b) and quantitative statistics (c) of the ESCs distribution over cross section A. Since the solidification of the melt in the die cavity during the HPDC process is far away from equilibrium state, fully divorced eutectic appears in the as-cast microstructure of Mg-Al alloy as shown in figure 1(c). In other words, the eutectic α-mg nucleates and grows attached to the primary α- Mg, while the eutectic β-mg 17 Al 12 nucleates and grows independently. According to the authors previous work, islands and networks of eutectic are observed at the central region of AM60 magnesium alloy die castings, while the eutectic reveals a more dispersed and granular morphology at the surface layer. 3

5 3. Model description Simulation of the microstructure evolution during the solidification process involves modeling of the nucleation and crystal growth at the grain-size scale and even at the atomic scale. One of the key aspects relates to the establishment of the nucleation model and also the crystal growth model suitable for the solidification process. In the present work, as for the solidification of the primary α-mg, a nucleation model was proposed which was divided into two stages based on the nucleation characteristics and sequence during the cold-chamber HPDC process. When the melt is still in the shot sleeve, it starts to solidify in a manner resembling gravity pour permanent mould casting. In this case, the continuous nucleation model with a Gaussian distribution of nuclei is adopted [9]. When the melt enters the die cavity, enormous nucleation and crystal growth occurs in the bulk liquid metal. As for the surface layer of castings, or castings with thin thickness, the cooling rate during solidification may be as high as 1000 K/s. Such high cooling rate will induce the formation of free chill crystals. Based on experimental statistics and data fitting, the nucleation density and cooling rate satisfy a linear relationship. As for the central region of castings with thick thickness, the cooling rate during solidification is much lower due to the weakening of heat transfer and the release of latent heat. In such case, the nucleation density and cooling rate satisfy an exponential relationship. Based on the fact that Mg has a hcp crystal structure, the dendrites of commercially available magnesium alloys grow with six-fold symmetry. In the present paper, based on the CA method, a growth model was established in which the growth kinetics was calculated from the complete solution of the transport equations. By defining a special neighbourhood configuration with the square CA cell, and using a set of capturing rules which was proposed by Beltran-Sanchez and Stefanescu for the dendritic growth of cubic crystal metals during solidification [10], modeling of dendritic growth of magnesium alloy with six-fold symmetry and different growth orientations was achieved. More details relating the nucleation model and dendritic growth model, as well as assumptions and simplifications for simulation of the solidification of the primary α-mg during the HPDC process can be found in references [8, 11]. As for simulation of the formation of fully divorced eutectic at the last stage of solidification in the interdendritic liquid in the die cavity, firstly, a nucleation model proposed by Charbon and LeSar was adopted to simulate the eutectic nucleation [12]. Each eutectic nucleus is generated by two CA cells, one of which represents a nucleus of eutectic α-mg, and the other represents a nucleus of eutectic β- Mg 17 Al 12. Based on the nucleation and growth mechanism of fully divorced eutectic, the eutectic α- Mg nucleates attached to the primary α-mg, while the eutectic β-mg 17 Al 12 nucleates at a certain distance from the eutectic α-mg in the interdendritic liquid. Once the eutectic α-mg and β-mg 17 Al 12 have nucleated, the cooperative and competitive growth of the two phases begins. A detailed description regarding the growth kinetics and growth algorithms of the two eutectic phases of Mg-Al alloy could be found in the authors previous work [13]. 4. Simulation results and discussion To validate the capability of the present CA model, simulations were carried out to predict the microstructure of cover-plate die castings of AM60 magnesium alloy at different process parameters, and then the simulated results were compared with the experimental results. The standard process parameters during the cover-plate die casting experiments were as follows: pouring temperature (973 K), mould temperature (453 K), casting pressure (77 MPa), slow shot phase plunger velocity (0.15 m/s), fast shot phase plunger velocity (4.0 m/s), and filling fraction of the shot sleeve (18.3%) Modeling of the solidification structure of primary α-mg considering the ESCs in the shot sleeve During simulation of the nucleation and dendritic growth of the primary α-mg, the software Anycasting was used firstly to calculate the temperature field of the melt in the shot sleeve. After the slow shot phase, simulation of the solidification of the melt was switched from in the shot sleeve to in the die cavity, since the time of the fast shot phase was extremely short and generally at the magnitude of dozens of milliseconds. Meanwhile, in the present CA model, remelting and fragmentation of the 4

6 dendrites during the fast shot phase of the melt into the die cavity was not taken into account. The temperature field of the melt in the die cavity was obtained using an inverse heat transfer model [11]. Figure 3 shows the simulated microstructure evolution of AM60 magnesium alloy at the central region of cross section A of the cover-plate die casting under the standard process parameters. The calculation domain consists of cells with a cell size of 1.0 μm. It can be seen that the microstructure with a mixture of coarse ESCs and fine grains at the central region of the die casting was acquired via simulation. With an equivalent grain size of the ESCs up to 100 μm, the area fraction of the ESCs in figure 3(c) is about 24.5%. (a) (b) (c) Figure 3. Simulated microstructure evolution of AM60 magnesium alloy at the central region of cross section A of the cover-plate die casting under the standard process parameters: the melt is still in the shot sleeve in figures 3(a) and 3(b), whereas it is in the die cavity in figure 3(c). (a) (b) (c) (d) (e) (f) Figure 4. Simulated (a-c) and experimentally observed (d-f) microstructure of AM60 magnesium alloy at the surface layer (a, d), one-fourth thickness (b, e) and the central region (c, f) of cross section A of the cover-plate die casting under the standard process parameters. Figure 4 shows the simulated and experimentally observed microstructure of AM60 magnesium alloy at the surface layer, one-fourth thickness and the central region of cross section A of the cover-plate die casting under the standard process parameters. The calculation domain was divided into cells with a cell size of 0.5 μm and 1.0 μm, respectively in figure 4(a) and figures 4(b-c). It can be seen from figure 4(a) and 4(d) that the surface layer comprises uniformly fine grains. The simulated average grain size is about 14.4 μm, that s very close to the experimentally measured value 14.7 μm. Comparing figures 4(b-c) with figures 4(e-f), it also can be illustrated that the simulated results agree well with the experimental results relating the grain size and the area fraction of the ESCs. 5

7 At the one-fourth thickness and the central region of cross section A, the simulated area fractions of the ESCs are 16.1% and 24.5%, while the experimentally measured are 16.8% and 26.7%, respectively. (a1) (a2) (a3) (b1) (b2) (b3) (c1) (c2) (c3) (d1) (d2) (d3) Figure 5. Simulated solute map (a1-d1), grain size distribution (a2-d2) and experimentally observed grain size distribution (a3-d3) of AM60 magnesium alloy at the central region of cross section A of the cover-plate die casting under different pouring temperatures (T p ) and slow shot phase plunger velocities (V s ): (a1-a3) T p = 953 K, V s = 0.15 m/s (b1-b3) T p = 993 K, V s = 0.15 m/s (c1-c3) T p = 973 K, V s = 0.3 m/s (d1-d3) T p = 973 K, V s = 0.4 m/s. Simulations were further conducted to predict the microstructure of AM60 magnesium alloy at the central region of cross section A of the cover-plate die castings at different pouring temperatures (953, 973 and 993 K) and slow shot phase plunger velocities (0.15, 0.3 and 0.4 m/s). As shown in figure 5, and in conjunction with figures 4(c) and 4(f), it can be concluded that the simulated results are consistent with the experimental results. With a decrease of the pouring temperature, the percentage of the ESCs increases. Under the three different pouring temperatures of 993, 973 and 953 K, the simulated area fractions of the ESCs are 19.2%, 24.5% and 33.5%, while the experimentally measured are 19.3%, 26.7% and 32.1%, respectively. Meanwhile, the percentage of the ESCs increases when decreasing the slow shot phase plunger velocity. The reason for this is that the holding 6

8 time of the melt in the shot sleeve would be longer in such case, which results in a more severely loss of the superheat of the melt then. Under the three different slow shot phase plunger velocities of 0.4, 0.3 and 0.15 m/s, the simulated and experimentally measured area fractions of the ESCs are 14.8%, 18.1%, 24.5% and 14.5%, 17.2%, 26.7%, respectively Modeling of the formation of fully divorced eutectic When the temperature of the melt in the die cavity goes below the eutectic temperature, and the solute concentration in the interdendritic liquid is up to the eutectic composition, eutectic nucleation and growth begins. Figure 6 shows the simulated and experimentally observed eutectic morphology, as well as distribution at the central region of AM60 magnesium alloy die castings. The calculation domain consists of cells with a cell size of 1.0 μm. Firstly, it can be seen from the amplified view of the rectangle region in figure 6(c) that the eutectic α-mg indeed nucleates and grows attached to the primary α-mg, while the eutectic β-mg 17 Al 12 nucleates and grows independently at the early stage of eutectic solidification. As the solidification proceeds, the two eutectic phases impinge on each other, and then the cooperative and competitive growth of the two phases begins. Comparing figure 6(c) with 6(d), it can be noted that the simulated result agrees well with the experimental result relating the morphology and distribution of the eutectic, which grows into the form of islands/networks at the grain boundary of the primary α-mg. (a) (c) (c1) (b) (d) Figure 6. Simulated microstructure evolution of the eutectic (a-c) and experimentally observed eutectic morphology (d) at the central region of AM60 magnesium alloy die castings: figure 6(c1) shows the amplified view of the rectangle region in figure 6(c). 5. Conclusions A cellular automaton model was developed to simulate the microstructure evolution of magnesium alloy during the cold-chamber HPDC process. Based on the solidification behaviour and sequence during the HPDC process, both of the nucleation and crystal growth of the primary α-mg in the melt in the shot sleeve and in the die cavity, as well as the formation of the fully divorced eutectic at the last stage of solidification in the interdendritic liquid in the die cavity were taken into account in the present CA model. During simulation of the microstructure of a cover-plate die casting of AM60 magnesium alloy, it can be found that the percentage of the ESCs is higher at the central region than that at the surface layer of the die castings. With a decrease of both of the pouring temperature and 7

9 slow shot phase plunger velocity, the percentage of the ESCs increases in the die castings. Meanwhile, the fully divorced eutectic grows into the form of islands/networks at the central region of the die castings. The simulated results are in accordance with the experimental results. Acknowledgments This work was financially supported by the Ministry of Science and Technology of China under grant Nos. 2011BAE21B00, 2011ZX , 2011ZX and 2010DFA The die casting experiments were conducted with the help of engineers from Toyo Machinery & Metal Co., Ltd. References [1] Friedrich H and Schumann S 2001 J. Mater. Process. Technol [2] Helenius R, Lohne O, Arnberg L and Laukli H I 2005 Mater. Sci. Eng. A [3] Laukli H I, Lohne O, Sannes S, Gjestland H and Arnberg L 2003 Int. J. Cast Met. Res [4] Wu M W and Xiong S M 2011 Acta Metall. Sin [5] Dahle A K, Sannes S, StJohn D H and Westengen H 2001 J. Light Met [6] Cao H and Wessén M 2004 Metall. Mater. Trans. A [7] Boettinger W J, Coriell S R, Greer A L, Karma A, Kurz W, Rappaz M and Trivedi R 2000 Acta Mater [8] Wu M W and Xiong S M 2011 Proc. 11th Asian Foundry Congress (Guangzhou) vol 2 (Li R D: Shenyang/Foundry Institution of Chinese Mechanical Engineering Society) p 139 [9] Thévoz P, Desbiolles J L and Rappaz M 1989 Metall. Trans. A [10] Beltran-Sanchez L and Stefanescu D M 2002 Int. J. Cast Met. Res [11] Wu M W and Xiong S M 2010 Acta Metall. Sin [12] Charbon C and LeSar R 1997 Model. Simu. Mater. Sci. Eng [13] Xiong S M and Wu M W 2011 Experimental and modeling studies of the lamellar eutectic growth of Mg-Al alloy Preprint /s