Fabrication Of Semisolid Nondendritic AZ63Magnesium Alloy And Thixoforming

The semi-solid formability of AZ63magnesium alloy was preliminarilyevaluated and the effects of MgCO₃on the microstructure and properties of theas-cast alloy were also studied. In addition, the refined AZ63alloy by MgCO₃was thixoformed and high mechanical properties were obtained. Finally, therelationship between the microstructure and mechanical properties of the formedalloys were discussed. The results show that:The as-cast AZ63alloy has good mechanical properties and the UTS canbe up to198MPa. More importantly, it also has an appropriate thixoformingtemperature window（ΔTTPW=12℃） and temperature interval of solid-liquidtwo phrases （ΔT=22℃） and low temperature sensitivity to solid fraction. So itis suitable for semi-solid processing.The addition amount of MgCO₃，addition temperature，holding duration,pouring temperature and specimen dimensions all have large effects on theas-cast microstructure of this alloy. The grain size can be decreased from108to54μm when1.2%MgCO₃is added at780℃and thenthe melt was held for10min, cooled to720℃and pouredinto apermanent mould with Φ45mm.Thegrain-refining regime of MgCO₃belongs to heterogeneous nucleation takingAl4C3particles as substrates. A large number of crystal nuclei can be produced,which leads the grains to refine.As-cast microstructure has important influence on semi-solidmicrostructure: the finer the as-cast microstructure, the smaller and morespheroidal the primary particles in the semi-solid microstructure. A goodsemi-solid microstructure with small （size of76μm） and spherical （shapefactor of1.2） can be obtained when the refined AZ63alloy is heated for30minat （605℃）. The microstructural evolution during partial remelting includesfour steps： the initial rapid coarsening, structure separation, spheroidizationand final coarsening. Correspondingly, the phase transformations are β�'α,MgZn�'L（347℃）、 α+β�'L、 L and T�'L（535℃）、 α�'L and L�'α.The thixoformed AZ63alloy has good mechanical properties and itsultimate tensile strength can reach310.4MPa, a sharp increase compared withthat （198MPa） of the permanent mould casting alloy. Its mechanical propertiesare greatly affected by the semisolid microstructures and processing parameters.