Microstructural evolution of an ECAE-formed ZK60-RE magnesium alloy in the semi-solid state

AZ91D magnesium alloy semi-solid billets prepared by the strain-induced melt activation (SIMA) process have been proposed or reported in previous literature. However, no detailed investigation has been conducted regarding Mg-Zn-Zr series magnesium alloys. In this paper, the microstructural evolution of ZK60 magnesium alloy with a rare earth (RE) addition during partial remelting has been studied. Equal channel angular extrusion (ECAE) was introduced into the SIMA process to replace conventional upset or extrusion. The results show that as-cast ZK60-RE magnesium alloys are successfully extruded by ECAE. The semi-solid microstructure of ECAE-formed ZK60-RE consists of fine and spheroidal grains surrounded by grain boundary liquid films. The cubic coarsening rate constant, K, increases from 255 to 371 mu m(3) s(-1) when the solid fraction decreases from 0.75 (600 degrees C) to 0.65 (615 degrees C). With increasing numbers of ECAE passes, which means increasing distortion energy, grain sizes are finer and liquid formation rates are enhanced. This kind of research encourages the belief that the combination of ECAE and partial remelting in the semi-solid state can produce magnesium alloy semi-solid billet with fine and spheroidal grains, which is suitable for semi-solid processing. (C) 2008 Elsevier B.V. All rights reserved.