Evolution of grain structure, micro-texture and second phase during porthole die extrusion of Al-Zn-Mg alloy

The porthole die extrusion of high strength Al-Zn-Mg alloy was carried out at 783 K. The reserved materials inside portholes and welding chamber were taken out to investigate the dynamic evolution of grain structure, micro-texture and second phase along streamlines of welding zone and matrix zone. The material of welding zone experienced severe plastic deformation, while the flowing route of matrix zone was relatively smooth. Hence, the welding zone achieved near complete dynamic crystallization after splitting stage, and the grain size kept steady during the subsequent welding and extruding stage. The streamline of matrix zone mainly consisted of elongated grains with small amount of fine equiaxed grains, which indicated the occurrence of dynamic recovery and slight dynamic crystallization. The texture evolution of welding zone was much more complicated due to the combination of compression, shearing and tension stresses. The welding zone of the final extruded profile had the main textures of {112} < 111 > , {101} < 111 > , {101} < 001 > and {111} < 112 > orientations, while the matrix zone of the profile consisted of {112} < 111 >, {101} < 111 > and {101} < 211 > orientations. The extrusion temperature is sufficient for dissolving MgZn2 phase, while it is insufficient for the dissolution of Al23CuFe4. Instead, the coarse Al23CuFe4 was broke into small pieces due to the effects of mechanical deformation.