Acta Metall Sin  2018, Vol. 54 Issue (2): 174-192    DOI: 10.11900/0412.1961.2017.00418
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Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium Alloys
Shoumei XIONG1,2(), Jinglian DU1, Zhipeng GUO1, Manhong YANG1, Mengwu WU1, Cheng BI1, Yongyou CAO1
1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Tsinghua University, Beijing 100084, China
Abstract

Magnesium alloys are widely used in various fields because of their outstanding properties. High-pressure die casting (HPDC) is one of the primary manufacturing methods of magnesium alloys. During the HPDC process, the solidification manner of casting is highly dependent on the heat transfer behavior at metal-die interface, which directly affects the solidified microstructure evolution, defect distribution and mechanical properties of the cast products. As common solidified microstructures of die cast magnesium alloys, the externally solidified crystals (ESCs), divorced eutectics and primary dendrites have important influences on the final performance of castings. Therefore, investigations on the interfacial heat transfer behavior and the solidified microstructures of magnesium alloys have considerable significance on the optimization of die-casting process and the prediction of casting quality. In this paper, recent research progress on theoretical simulation and experimental characterization of the heat transfer behaviors and the solidified microstructures of die cast magnesium alloys was systematically presented. The contents include：(1) A boundary-condition model developed based on the interfacial heat transfer coefficients (IHTCs), which could precisely simulate the boundary condition at the metal-die interface during solidification process. Accordingly, the IHTCs can be divided into four stages, namely the initial increasing stage, the high value maintaining stage, the fast decreasing stage and the low value maintaining stage. (2) A numerical model developed to simulate and predict the flow patterns of the externally solidified crystals (ESCs) in the shot sleeve during mold filling process, together with discussion on the influence of the ESCs distribution on the defect bands of die cast magnesium alloys. (3) Nucleation and growth models of the primary α-Mg phases developed by considering the ESCs in the shot sleeve. (4) Nucleation and growth models of the divorced eutectic phase, which can be used to simulate the microstructure evolution of die cast magnesium alloys. (5) The 3D morphology and orientation selection of magnesium alloy dendrite. It was found that magnesium alloy dendrite exhibits an eighteen-primary branch pattern in 3D, with six growing along $<112?0>$ in the basal plane and the other twelve along $<112?3>$ in non-basal planes. Accordingly, an anisotropy growth function was developed and coupled into the phase field model to achieve the 3D simulation of magnesium alloy dendrite.

 Fig.1  The size, geometry and actual casting of step shape (a, b) and finger shape (c, d) castings (unit: mm)[12] Fig.2  Configuration and boundary conditions of shot sleeve (P—symmetrical plane, q—heat density, F—symmetric line, Δx—spatial step along the x direction, x and z—coordinate axis, respectively)[57](a) cross section (b) longitudinal section (c) heat transfer model Table 1  Thermal physical parameters of the materials[16,17,18,19,20] Fig.3  Variation correlation between solid phase fraction (fs) and h [20] (hmax is the peak value of IHTC, h0 is the initial value of IHTC; while, kh-f, fs0, fsc, fde, hde, hsolid and ε are the fitting parameters; Pc, Ps and P0 are the demarcation points of the four stages, which correspond to the points with the maximum IHTC, the spacing IHTC and the final stable IHTC) Fig.4  The interfacial heat transfer coefficient (h) of A380 alloy (a) and AZ91D alloy (b) in single cycle at the different locations of chamber under normal die casting condition[1] (t—time) Fig.5  Optical micrograph of typical AM50 microstructure at the surface and central regions of the 'step shape' die-casting (a), area fraction distributions of externally solidified crystals (ESCs) over cross section of the five steps (b), and the step 4 (c) on the 'step shape’ die casting [44] (Tp is the casting temperature and Vs is the low speed velocity) Fig.6  Particle distributions on the cross sections of the tensile bars with semi-circle ingate at filling percentage of 37% (a), 45% (b) and 55% (c), and for the tensile bars with circle ingate at filling percentage of 44% (d), 63% (e) and 81% (f) [45] (n—content of ESCs) Table 2  Thermal physical parameters of Mg-Al eutectic alloy[50,53] Fig.7  Comparison of the the microstructure evolution at the central of magnesium alloy die castings (AM60B)[50] (a, b) the modeling results (c) the experimental result Fig.8  Dendrites of AZ91 alloy casting (a, b), microstructures of Mg-30%Gd alloys in as casting state (c, d)(The dendrites are with different growth patterns, such as six-branches, five-branches, four-branches and butterfly shape symbolled as No.1 to No.5, respectively) Fig.9  3D reconstructed dendrites extracted for the Mg-30%Sn (a, b) and Mg-30%Gd (c, d) alloys, and a dendrite of Mg-30%Sn alloy viewed from different perspectives (e, f) (A1~A6 and B1~B4 are used to indicate the preferred growth directions of the dendrite)[33] Fig.10  EBSD analyses on the preferred growth directions of the Mg-30%Gd alloy dendrite[33]: dendritic morphology and the according (0001) orientation (a, b) while those for $(033?1)$ (c, d), and pole distributions with respect to the $<112?3>$ for thirty dendrite (e) Fig.11  Comparison of the dendritic morphologies between phase field simulation (a) and X-ray tomography experiment (b). Dendritic morphologies with respect to four different view directions including $<0001>$, $<112?0>$, $<101?0>$ and $<112?3>$ are shown in Figs.11c~f for simulation and Figs.11g~j for experiment, respectively[29] Fig.12  Atomic densities (a) and inter-planar distances (b) of Mg low index crystallographic planes, and the pyramidal planes for the $<112?0>$ and $<112?3>$ growth directions (c)[33] (The correpsonding a and c, donote the lattice parameters of Mg) Fig.13  The surface energies of low index planes (a) and high index planes (b) of pure Mg by four different potentials[39], the correlation for $<112?x>{112?e}$ (c), the according morphologies in the basal and non-basal plane (d), and the surface energies of low index planes (e) and high index planes (f) of Mg and its alloys by PAW-LDA potentials (PAW, USPP, GGA and LDA are abbreviations of the projector-augmented wave, the ultra-soft pseudo-potential, the generalized gradient approximation and the local density approximation, respectively)[39,40]