China Foundry (Jul 2013)
Microstructure evolution of an Mg-Zn-Nd-Zr magnesium alloy during recrystallization and partial remelting process
Abstract
To obtain a spheroidal microstructure of a semi-solid Mg-Zn-Nd-Zr alloy, which is favorable for the subsequent thixocasting process, the recrystallization and partial remelting (RAP) method was used in this study, and the microstructure evolution of the alloy and the mechanism of the shape factor change during the RAP process were invesigated. The as-cast Mg-Zn-Nd-Zr alloy was prepared in a metal mold and then extruded into bars with an extrusion ratio of 16:1. Partial remelting was carried out on the extruded samples at 589 ℃ (at a heating rate of 0.5 ℃·s-1) for different holding times ranging from 0 to 90 min. To examine the effect of heating rate, partial remelting of samples at the heating rate of 2 ℃·s-1 was also performed. Results show that the extruded microstructure rapidly evolutes into recrystallized grains in the semi-solid state; the liquid film initially forms at grain boundaries during the partial remelting, and then gradually changes from continuous into discontinuous state with the increase of holding time; this results in the agglomeration of adjacent grains and the decrease in shape factor. The value of shape factor increases continuously with holding time at first and reaches the maximum 0.62 when holding for 60 min, while decreases rapidly after a prolonged holding time. Moreover, local melting, mainly due to the inhomogeneous deformation during extrusion, becomes more significant and less uniformly distributed at a relatively higher reheating rate. The Mg-Zn-Nd-Zr magnesium alloy components have been thixo-cast successfully using the RAP method, which strongly proves the feasibility of RAP process in Mg-Zn-Nd-Zr alloy.
