Evaluation Of The Thixo-formability Of AZ63 Magnesium Alloy And Control Of Its Semisolid Microstructure

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.