稀土镁合金近非平衡液相线铸造研究

Investigation on Nonequilibrium Near-Liquidus Casting of Thixotropic Magnesium-Rare Earth Base Alloys

近非平衡液相线铸造方法是测量熔体非平衡凝固过程的冷却曲线,确定其非平衡液相线温度,并在此温度附近进行铸造的工艺方法,为实质意义上的近液相线铸造.采用该方法对ZK60稀土系列镁合金进行了研究,结果表明,近非平衡液相线铸造工艺可获得细小均匀的非枝晶结构,是镁合金半固态制浆的有效方法;不同合金元素对以ZK60为基体的合金的非平衡液相线和近液相线铸造组织都有明显影响,Ag,Y,Nd等元素有利于避免浆料中晶内液相的出现,其中混合稀土最有利于初生晶粒的圆整化,而Nd则造成初生晶粒的不均匀,Ag和Y的添加尽管初生晶粒圆整程度不如Nd,但有较均匀的晶粒和较好的枝晶退化效果.

Nonequilibrium near-liquidus casting （NNLC） is such a process that the casting should be carried out near the nonequilibrium liquidus temperature which was determinated by the cooling curve of melt in a nonequilibrium solidification process. It is substantially a near-liquidus casting. The results of investigation on ZK60 magnesium-rare earth base alloys made with the process indicated that the refined and homogeneous non-dendritic mierostructure can be obtained. Therefore, it is an effective way for thixotropic semi-solid magnesium alloy melt. For the ZK60-base alloys, different alloying additions have different obvious effects on the nonequilibrium liquidus and microstructure of near-liquidus castings. For instance, Ag, Y, Nd and other REs all are beneficial to avoiding the occurrence of intracrystalline liquid. Among them RE-containing addition is most beneficial to the spheroidization of primary grains, but Nd makes them inhomogeneous. Although Ag and Y make primary grains less spheroidized than what Nd offers, they serve the function of homogenization and dendrite regression.