Development of a Fluid-Particle Model in Simulating the Motion of External Solidified Crystals and the Evolution of Defe
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HIGH-PRESSURE die cast (HPDC) is one of the most popular casting methods for magnesium alloys, and has attracted more attention in recent years because of the potential benefits of light weight, energy saving, and environment protection.[1] In cold chamber HPDC, solidification starts immediately when the liquid melt is poured into the shot sleeve because of the fast heat transfer. Crystals would then nucleate and grow and subsequently the so-called ESCs are created.[2] During the fast shot stage, ESCs that have been injected into the die cavity could either remelt because of local temperature rise or continue to grow because of the local fast cooling. The microstructure of the HPDC magnesium alloy typically comprises ESCs, porosities, and defect bands.[3] The volume and the distribution of ESCs have a significant effect on the microstructure and subsequent mechanical properties of the components. Cao et al.[4] investigated the influence of pouring temperature and injection speed on the formation of ESCs, and found that more ESCs were formed when pouring temperature was lower, and more broken dendrites and spherical crystals were formed under higher injection speed. Laukli et al.[5] studied the effect CHENG BI and XIAOBO LI, Ph.D. Candidates, are with the School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China. SHOUMEI XIONG, Professor, and ZHIPENG GUO, Assistant Professor, are with the School of Materials Science and Engineering, Tsinghua University, and also with The Key Laboratory for Advanced Materials Processing Technology, Ministry of Education. Contact e-mail: zhipeng_guo@ mail.tsinghua.edu.cn Manuscript submitted on July 11, 2015. Article published online February 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
of grain refiner on the microstructure in HPDC A356 castings and found three different ESCs, including globular crystals, elongated trunks, and branched dendrites. Otarawanna and co-workers[6] continued Laukli’s work and found that the volume fraction of ESCs decreased with distance from the gate and there was a general, slight tendency for ESCs to be aggregated toward the casting center. Wang et al.[7] investigated the effect of the shot speed and biscuit thickness on the ESCs of high-pressure die casting of AM60B alloy, and found that an increase of the biscuit thickness always led to the decrease of ESCs in the cast samples. Li et al.[8] investigated the porosity induced by the ESCs and its effect on the crack initiation and propagation during tensile deformation of HPDC AM60B magnesium alloy, and found that the interdendritic shrinkage during solidification at the ESC boundaries was shown to be the primary reason for the formation of the porosity. For theoretical studies, Otarawanna[6] located the defect bands, defined and counted ESC area, and displayed the distribution of ESCs as a function of sample thickness. In addition, the band thickness was shown to be highly dependent on local solidification conditions, including both cooling rate and gradient of solid
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