Numerical Simulation and Microstructural Evolution Investigation on Cyclic Expansion Extrusion of Magnesium Alloys

Magnesium alloys, as the lightest structural metallic materials in engineering applications, serve as critical candidates for equipment lightweighting. Grain refinement strengthening is an effective strategy to simultaneously enhance the strength and toughness. A novel severe plastic deformation process-cyclic expansion extrusion with an asymmetrical extrusion cavity（CEE-AEC） was proposed, which combined compressive and shear stress. By introducing asymmetric cavities into conventional cyclic expansion-extrusion, the process modified metal flow patterns, enabling the fabrication of magnesium alloys with homogeneous refined microstructures through multi-pass deformation. Numerical simulations using Deform-3D finite element software were conducted to investigate stress/strain field evolution under varying die parameters and deformation paths（Route A, B）. The experimental analyses further elucidated the microstructure and texture evolution under different paths and passes. The results show that CEE-AEC regulates material flow through synergistic effects of expansion filling and asymmetric extrusion. In Route A, the gradient asymmetric stress field（with enhanced asymmetry corresponding to increased height difference in the cavity） induces heterogeneous strain distribution across regions, where the strain magnitude follows: asymmetric zone＞symmetric zone＞core region. In contrast, Route B achieves improved stress/strain field uniformity through a multi-directional strain coordination mechanism.Leveraging non-planar strain effects coupled with dynamic recrystallization, Route B exhibits superior grain refinement compared to Route A, along with enhancing fragmentation and homogeneous distribution of long-period stacking ordered（LPSO） phases. Multi-pass deformation processing drives the（0002） basal texture intensity reduction from 3.7 to 1.8, accompanied by significant multi-directional random grain orientation characteristics. A quantifiable process window for regulating anisotropy in magnesium alloys was established.