Study on solid bonding behavior of AZ31 Mg alloy during porthole die extrusion process

The extrusion experiments were carried out under various temperatures to study the formation of longitudinal weld seams (L-seams) during porthole die extrusion process of AZ31 alloy. Moreover, 3D transient numerical modeling was established to verify the experimental findings. The results indicated that the cross section of profile can be divided into three distinct zones, namely, welding zone, transition zone, and matrix zone, and the fraction of dynamic recrystallization varied in these zones. The average grain size in welding zone increased with increasing temperature, and the profile front had coarser grains in comparison with the other parts. The profile extruded at 380 °C exhibited the highest tensile strength and elongation. Moreover, the profile front always had poor tensile properties in comparison with the other parts. Q criterion is inadequate to describe the welding quality of L-seams, while J criterion shows higher prediction accuracy. Finally, it was demonstrated that the mechanical properties of profile containing L-seams were determined not only by the welding quality but also by the microstructure near L-seams.