Numerical and experimental study on the extrusion through a porthole die to produce a hollow magnesium profile with longitudinal weld seams

In this study, the thermomechanical behaviour of a magnesium alloy AZ31B during extrusion through a porthole die to form a square tube was revealed. Three-dimensional numerical simulation demonstrated that metal flow into the portholes, welding chamber and die bearing of the porthole die corresponded to the stepped increases of extrusion pressure at the initial stages of an extrusion cycle. Experimental measurements of extrusion pressure and extrudate temperature were found to be in agreement with the predicted values. Extrusion pressure decreased continuously after the pressure peak while the workpiece temperature increased steadily, leading to gradual increases of the recrystallised grain sizes of the extrudate along its length. Expansion tests showed consistent quality of the longitudinal weld seams along the extrudate length, although the temperature on the welding plane increased and the maximum normal pressure in the welding chamber decreased during extrusion. It is thus the ratio of the maximum normal pressure in the welding chamber to the flow stress of the workpiece material on the welding plane that determines the weld seam quality.