Integrated physical and numerical simulations of weld seam formation during extrusion of magnesium alloy

Solid-state bonding takes place during the extrusion process to produce a hollow metal profile through a porthole die, known as extrusion welding. Defective weld seams degrade extruded products in mechanical properties. The present research was aimed to determine the effect of extrusion condition on the longitudinal weld seam quality of a magnesium alloy, Mg-8Al-0.5Zn-0.5RE, using an integrated physical and numerical simulation method. A special die set-up for physical simulation was designed, through which two magnesium alloy rods were welded in the solid-state under high hydrostatic pressure. Extrusion welding experiments under different conditions were performed. It was demonstrated that, with this die set-up, the formation of weld seams during extrusion to produce hollow profiles could be physically simulated. The extrusion welding experiments were then numerically simulated to reveal strains, stresses and hydrostatic pressures that could not be experimentally measured. Finally, the tensile strength and elongation of the extrusion-welded magnesium alloy were determined and its microstructure was examined. The results showed that the bonding strength increased with decreasing extrusion speed and rising extrusion temperature. For well-bonded rods, weld seam was invisible under optical microscope. Attributed to high temperature and large equivalent strain, complete dynamic recrystallization occurred across the interface, leading to a reduced average grain size and disappearance of weld seam. By applying the integrated physical and numerical simulation method, extrusion process parameters for a particular magnesium alloy can be optimized to ensure weld seam quality of extruded hollow profiles.