Effects of processing parameters on the microstracture of ECAE-formed AZ91D magnesium alloy in the semi-solid state

摘要

The effects of isothermal temperature and isothermal holding time on the semi-solid microstructures of AZ91D alloys prepared by the strain-induced melt activation (SIMA) process were investigated. AZ91D alloys were extruded by equal channel angular extrusion (ECAE) with two passes at 280 deg C. The ECAE-formed alloys were isothermally held at various temperatures for times in the range 0-15 min. The results indicated that recrystallization first occurred in deformed areas and increasing isothermal temperature shortened the time of complete recrystallization. Coalescence and Ostwald ripening were operative simultaneously as soon as liquid was formed. During partial remelting, increasing isothermal temperature, namely increasing liquid fraction was accompanied by semi-solid grain coarsening. Increasing isothermal holding time also led to increasing liquid fraction and grain coarsening. A direct link existed between the volume of liquid fraction and the degree of spheroidization. Increasing the liquid fraction promoted spheroidization.