Dynamic evolution of grain structure and micro-texture along a welding path of aluminum alloy profiles extruded by porthole dies

The solid state welding process during porthole die extrusion of hollow structure aluminum alloy profiles is very important since it directly determines the microstructure and mechanical properties of welding seams in extruded profiles. In this study, the grain structure, special grain boundary and micro-texture along the welding path of an aluminum alloy profile were characterized, and the relation between the microstructure and mechanical properties of the welding seam was revealed. It was found that, along the welding path, the fine equiaxed grains around the welding line are elongated along extrusion direction under the combined action of shearing and compression, then, the elongated grains grow into the strip-shaped coarse grains by means of grain boundary migration, and finally, some strip-shaped coarse grains evolve into the fine equiaxed grains by means of continuous dynamic recrystallization and geometric dynamic recrystallization. Along the welding path, the fraction of special grain boundaries firstly increases and then decreases, while the fraction of low angle grain boundaries increases continually. The strong {111} fiber is formed in the zone near the surface of bridge, and it transforms into the very strong shear-type texture consisting of A*1 and A*2 components at the beginning stage of extrusion welding. Then, these shear-type texture components transform into the strong goss and copper components. Finally, the goss and copper components weaken and the weak cube texture appears in the welding zone of the extruded profile. It was also found that the increase of the low angle grain boundaries and the decrease of the average grain size in welding zone contribute to the improvement of harness, strength and fracture strain, while the existence of the strip-shaped coarse grains and different types of textures in welding zone is adverse for the improvement of welding quality.