冷却速率对AZ91D镁合金非平衡凝固组织的影响

Effect of cooling rate on non-equilibrium solidified microstructure of AZ91D magnesium alloy

采用铜模喷铸和单辊甩带分别制备AZ91D镁合金非平衡凝固试样,利用ProCAST数值模拟软件及理论公式分别估算相应冷却速率。综合采用光学显微镜、扫描电镜、能谱分析仪、X射线衍射仪及差热分析仪对比研究冷却速率对镁合金非平衡凝固组织的影响规律。结果表明:随冷却速率提高,AZ91D合金的初生相形貌从粗大枝晶向细小等轴晶发生转变,晶粒发生明显细化,平均尺寸从铸态条件下的800μm降低到单辊甩带条件下的5μm以内。共晶相从连续网状向不连续弥散状分布转变,体积分数不断减小,单辊甩带条件下可制备出单相固溶体组织。快速凝固条件下合金基体中Mg和Al元素成分分布均匀,溶质偏析得到有效控制。

Non-equilibrium solidified specimens of AZ91D magnesium alloy were fabricated by copper mould casting and melt-spinning techniques, separately. The corresponding cooling rates were calculated by numerical simulation ProCAST software and theoretical equation. Using optical microscopy, scanning electron microscopy, energy dispersive spectrum, X-ray diffractometry and differential thermal analyzer, the effects of the cooling rate on the non-equilibrium solidified microstructures were investigated. The results show that the morphology of primary phase changes from coarse dendrite to fine equiaxed grain with increasing cooling rate, as indicated by the significant reduction of grain size from 800 μm under as-cast condition to within 5 μm for rapid solidification. Moreover, the distribution of eutectic phase varies from continuous network to discontinuous dispersion and its volume fraction decreases gradually. Single-phase solid solution structure can be fabricated by melt-spinning method. Both the distribution of Mg and Al elements in rapid solidified alloy are homogeneous without the occurrence of solute segregation.