Tube extrusion of AZ31 alloy with Sr additions

Three Mg alloys (AZ31, and AZ31 + 0.4 and 0.8 wt%Sr) were successfully hot extruded into tubes at 400 °C through a porthole die. Strontium (Sr) additions to AZ31 alloy reduced twining and the dynamically recrystallized grain size. However, Sr also increased the surface cracking tendency during extrusion. It was noted that the complex deformation path in the porthole die distributes the Sr-rich precipitates homogeneously across the microstructure and stringer formation is effectively avoided. The material flow in the die resembles that seen in multiple equal channel angular pressing (ECAP) routes and similar and significant effects on the final texture are observed. The material flow curvatures in the extrusion direction rotate the basal poles 25° from the radial direction. Two other texture components (δ1, δ2) are also formed in the welding zone by the circumferential flow of material. The maximum intensity of the orientation distribution function (ODF) is reduced with increasing level of Sr. Noteworthy, the maximum intensity in AZ31 + 0.8Sr occurs at the δ1 component while for AZ31 and AZ31 + 0.4Sr, the maximum intensity is located at the basal components. With Sr, alloying, Al is depleted from Mg solid solution and grains deform more easily in the circumferential direction. As a result, selective workhardening–dynamic recrystallization (SWH–DRX) cycles leave more grains with unfavorable slip orientations (the δ components).