Synergism in pre-upsetting on grain refinement and basal texture weakening of AZ31 Mg alloy by continuous variable cross-section direct extrusion

Pre-upsetting deformation treatment is considered an efficient method to optimize the microstructure of plastic-formed members and realize the depth of modification. This paper is the first attempt at pre-upsetting deformation of AZ31 Magnesium (Mg) alloy billet before continuous variable cross-section direct extrusion molding and compares and analyzes the effect of the presence or absence of pre-upsetting deformation on the microstructure and mechanical properties of extruded Mg alloys. The findings of the study indicate that: The addition of the pre-upsetting deformation process resulted in a significant grain size refinement of the continuous variable cross-section direct extrusion (CVCDE) parts, with an average grain size of only 3.93 μm. The yield/tensile strength and elongation after fracture are significantly improved synergistically, which are increased by 22.4 %, 15.0 %, and 41.6 %, respectively, compared with the extrusion components without pre-upsetting deformation. In addition, the pre-upsetting deformation stores a lot of dislocation and strain energy in the billet, which provides a preferred nucleation site for dynamic recrystallization and promotes the process of dynamic recrystallization in the subsequent extrusion forming process. The proportion of dynamic recrystallization grains in the formed part is as high as 83.5 %, which is 10.8 % higher than that of the conventional CVCDE formed part; The pre-upsetting also increases the non-basal Schmid factor value and GND Density value, weakens the dominant role of the basal texture, promotes the activation of the non-basal slip, and realizes the significant weakening of the basal texture. The maximum pole density is only 7.6. This provides a new idea for the deep modification of the CVCDE Mg alloy.