Rheo-extrusion of AZ61 Mg alloy and its microstructure

The microstructure of AZ61 Mg alloy made by the methods of normal cast and twin-screw rheo-cast was investigated. The grains are coarse dendrite in normal cast billet, while they are tiny ground grains in rheo-cast billet with the average grain size of about 40 μm. The d 22 mm rods were fabricated by hot extrusion and rheo-extrusion separately, the microstructure of these rods and the relationship between the extrusion force and velocity were researched. The result shows that dynamic recrystallization happens in the hot extrusion, the shape and size of the grains are determined by the extrusion temperature and velocity. In the process of rheo-extrusion, slurry solidification occurs and the tiny ground grains form in the extrusion mould with the average grain size of about 8 μm. The reason of grain refinement is that lots of nucleus formed in the secondary nucleation get survived and grow up.