A s the lightest known structural metals, magnesium alloys have attracted much attention due to their superior properties such as low density, high specific strength, excellent castability, good machinability and recyclability [1,2] . Magnesium alloys have been widely used in automotive, aerospace and 3C (computer, communication and consumer electronics) industries to replace steel, cast iron and even aluminum alloy [3] . The high pressure die casting (HPDC) process is a netshape or near net-shape process with the advantages of high efficiency, considerable economic benefit and high precision of the product size. These remarkable advantages make the HPDC process particularly suitable for various complex productions with magnesium alloys, and generally, magnesium alloy parts are mainly formed by the HPDC process [4] .The performance of magnesium alloy die castings
Abstract:The morphology and content of the divorced eutectic in the microstructure of high pressure die casting (HPDC) magnesium alloy have a great influence on the final performance of castings. Based on the previous work concerning simulation of the nucleation and dendritic growth of primary α-Mg during the solidification of magnesium alloy under HPDC process, an extension was made to the formerly established CA (Cellular Automaton) model with the purpose of modeling the nucleation and growth of Mg-Al eutectic. With a temperature field and solute field obtained during simulation of the primary α-Mg dendrites as the initial condition of the modified CA model, modeling of the Mg-Al eutectic with a divorced morphology was achieved. Moreover, the simulated results were in accordance with the experimental ones regarding the distribution and content of the divorced eutectic. Taking a "cover-plate" die casting with AM60 magnesium alloy as an example, the rapid solidification with a high cooling rate at the surface layer of the casting led to a fine and uniform grain size of primary α-Mg, while the divorced eutectic at the grain boundary revealed a more dispersed and granular morphology. Islands of divorced eutectic were observed at the central region of the casting, due to the existence of ESCs (Externally Solidified Crystals) which contributed to a coarse and non-uniform grain size of primary α-Mg. The volume percentage of the eutectic β-Mg 17 Al 12 phase is about 2%-6% in the die casting as a whole. The numerical model established in this study is of great significance to the study of the divorced eutectic in the microstructure of die cast magnesium alloy.
A s the lightest known structural metals, magnesium alloys have attracted much attention due to their superior properties such as low density, high specific strength, excellent castability, good machinability and recyclability [1-3]. Magnesium alloys have been widely used in automotive, aerospace and 3C (computer, communication and consumer electronics) industries to replace steel, cast iron and even aluminum alloys [4-6]. The high pressure die casting (HPDC) process is a netshape or near net-shape process with the advantages of high efficiency, considerable economic benefit, and high precision of the product size [7,8]. These remarkable advantages make the HPDC process particularly suitable for various complex castings' production with
Directionally solidified samples of Mg-32.3 wt pct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients, and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory.
Melt convection significantly affects the solute distribution and dendritic growth during metal alloy solidification. In this work, a numerical model is developed by coupling the cellular automaton (CA) method and Lattice Boltzmann method (LBM) to simulate the dendritic growth of Al-Cu alloy both in two and three dimensions. An improved decentered square algorithm is proposed to overcome the artificial anisotropy induced by the CA cells and realize simulation of dendritic growth with arbitrary orientations. Based on the established CA-LBM model, the effects of forced convection and gravity-driven natural convection on dendritic growth are studied. The simulation results show that the blocking effect of dendrites on melt flow is advanced with a larger number of seeds. The competitive growth of the converging columnar dendrites is determined by the interaction between heat flow and forced convection. Gravity-driven natural convection leads to highly asymmetric growth of equiaxed dendrites. With sinking downwards of the heavy solute, chimney-like or mushroom-like solute plumes are formed in the melt in front of the columnar dendrites when they grow along the gravitational direction. More details on dendritic growth of Al-Cu alloy under convection are revealed by 3D simulations.
Based on cellular automaton method, a numerical model was developed for the regular eutectic growth of binary alloy. By coupling with the macro-temperature field and considering the solute diffusion, the constitutional undercooling and the curvature undercooling, modeling of the steady-state lamellar eutectic growth was achieved. A systematic investigation on eutectic growth morphology and lamellar spacing of a model alloy was made under unidirectional solidification conditions with different undercoolings, initial lamellar spacings, temperature gradients and solidification rates. The results reproduced the adjustment of lamellar spacing of two eutectic phases under the interaction between solute diffusion and surface energy by mechanisms of nucleation, lamellar branching, lamellar termination and overgrowth. The simulated results were in agreement with those predicted by the Jackson-Hunt model and experimental results by other researchers. Finally, the model was extended to three dimensional systems, which verified its feasibility of modeling the three-dimensional eutectic growth.
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