考虑压室预结晶的镁合金压铸组织实验及模拟研究

EXPERIMENTAL AND MODELING STUDIES ON THE STRUCTURE FORMATION OF HIGH PRESSURE DIE CAST MAGNESIUM ALLOY CONSIDERING THE EXTERNALLY SOLIDIFIED CRYSTALS IN THE SHOT SLEEVE

针对镁合金压铸件表层晶粒细小均匀而心部晶粒较粗大且不均匀的特点,开展一系列压铸实验,研究镁合金压铸工艺对压室预结晶和压铸件最终凝固组织(特别是对晶粒大小、压室预结晶组织的含量及分布形态)的影响.采用数值模拟方法,以压铸实验统计数据为基础,建立了考虑压室预结晶的镁合金压铸形核模型,以此模拟更接近实际的压铸组织.应用所建立的形核及生长模型,模拟了不同压铸工艺下AM50镁合金"阶梯"压铸件的凝固组织,模拟结果与实验结果吻合较好.

The externally solidified crystals（ESCs）in the shot sleeve have a great influence on the final structure of magnesium alloy during cold-chamber high pressure die casting（HPDC） process.In typical HPDC microstructure,a surface layer with uniformly fine grains and a central region containing a mixture of coarse ESCs and fine grains are commonly observed from the cross section of the castings.In the present work,experiments were conducted to investigate the effects of process parameters on the formation of ESCs in the shot sleeve and the final microstructure of magnesium alloy,especially focusing on the grain size,the morphology and distribution of the ESCs. Based on cellular automaton method,a numerical model was developed to simulate the microstructure evolution of magnesium alloy under HPDC process.According to experimental statistics relating the area fraction of the ESCs,a nucleation model was established in which the ESCs in the shot sleeve were taken into account.Simulations were carried out to predict the microstructure of＂step-shape＂die castings of AM50 magnesium alloy with different process parameters.It was found that the simulated results were in accordance with the experimental ones.