【作者】 刘明伟；

【导师】 陈体军；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2011， 硕士

【摘要】 镁合金具有高的比强度、比刚度,而且资源丰富,占地壳组成的2.7%,在金属元素中仅次于铝和铁,位居第三。但与铝和钢比起来,其强度还比较低,限制了其应用。铸态组织对半固态组织初生相形态、大小和分布有重要影响,探求它们之间的关系进而获得细小圆整的半固态组织具有重要意义。本文利用光学显微镜（OM）、X射线衍射仪（XRD）、扫描电子显微镜（SEM）、电子探针（EPMA）等分析手段对AZ63合金半固态成形性能进行了初步评价,较深入地研究了通过改变MgCO₃的加入量来控制铸态组织,进而达到控制半固态组织的手段,并对细化后的镁合金在部分重熔过程中的组织演变进行了研究。结果表明:AZ63具有较好的铸态力学性能,可以达到198Mpa,并具有合适的加工温度窗口（ΔTTPW=12℃）、固液两相区温度范围适中（ΔT=22℃）、固相率对温度敏感性低（dfs/dT=0.01≤0.015）,很适合半固态加工。细化方面,MgCO₃加入量、加入温度、保温时间、浇注温度及锭料尺寸对其铸态组织都有影响,当添加1.2%的MgCO₃时,加入温度780℃,保温10min,浇注温度720℃时,晶粒尺寸为54μm。MgCO₃细化镁合金的机理主要是生成了Al4C3异质形核质点,使合金在凝固时产生大量的晶核,增加了形核率,使得镁合金晶粒细化。铸态组织对半固态组织有重要影响,铸态铸造越细小,半固态组织越圆整细小。经过细化的AZ63合计加热30min时可以得到良好的半固态浆料,平均晶粒尺寸为72μm,形状系数为1.2。AZ63重熔过程可分为快速粗化阶段（0min-3min）、枝晶分离阶段（3min-7min）、球化阶段（7-20min）和粗化阶段（20min以后）,相变过程为β→α、α+β→L、L与T→L（535℃）、L与T→L（535℃）、α→L与α→L。AZ63合金球化后粗化阶段晶粒长大遵循Ostwald熟化机制。更多还原

【Abstract】 The semi-solid formability of AZ63 magnesium alloy was preliminarily evaluated and the effect of MgCO₃ on the microstructure and properties was also studied. In addition, microstructure evolution of refined alloy during partial remelting was investigated using optical microscopy （OM）, X-ray diffraction （XRD）, scanning electron microscope （SEM） and electron probe micro-analyzer （EPMA）. The results indicate that:AZ63 alloy has good mechanical properties and the UTS can be up to 198MPa. It also has an appropriate processing temperature window（ΔTTPW=12℃）and temperature interval of solid-liquid two-phrase（ΔT=22℃） and low temperature sensitivity to solid fraction. So it is suitable for semi-solid processing.The addition amount of MgCO₃ , addition temperature , holding duration, pouring temperature and mould size all have important effects on the as-casting microstructure of this alloy. The grain size can be decreased to 54μm when 1.2% MgCO₃ is added at 780℃and then held for 10min, cooled to 720℃and poured. The refining mechanism of MgCO₃ is the heterogeneous nucleation taking Al4C3 particles as substrates. A large number of crystal nuclei can be produced and the nucleation rate is increased,which leads the grains to refine.As-cast microstructure has important influence on semi-solid structure: the finer the as-cast microstruture, the smaller and more spheroidal the primary particles in the semi-solid microstructure. A good semi-solid microstructure in which the primary particle is 76μm and the shape factor is 1.2 can be obtained when the refined AZ63 alloy is heated for 30min. The microstructural evolution during partial remelting process of AZ63 includes four steps:the initial rapid coarsening（0min-3min） ,structure separation（3min-7min）,spheroidization（7-20min）and final coarsening（after 20min）. Correspondingly, the phase transformations areβ→α, MgZn→L （ 347℃）、α+β→L、、L and T→L（535℃）、α→L and L→α.更多还原