The development of a W-shaped channel extrusion for fabricating magnesium alloy shells by combining high amplitude shear strain with a shorter process

Abstract

This paper introduced a novel single-pass severe plastic deformation method, W-shaped channel extrusion (WCE), for the direct manufacturing of magnesium alloy shells. Based on a strategically installed mandrel and die with a sloping asymmetric step structure, the WCE constructed a narrow severe plastic deformation gap specially reinforced by a backpressure-shear structure and an asymmetric shear-extrusion section. Therefore, slender bar with large height-to-diameter ratio (i.e. small diameter, large height) could easily be extruded into larger diameter shell in narrow gap through only one pass, and an impressive average plastic strain of over 4.1 was concurrently achieved, evidenced by upper bound theory calculation. The WCE process was further experimentally conducted on AZ80 alloy at 340 °C, demonstrating remarkable improvements in yield strength, ultimate tensile strength, and elongation of the extruded part by factors of 2.4, 2.2, and 4.3 over the as-cast condition. The consecutive multi-stage shear and high hydrostatic pressure catalyzed extreme grain refinement, favored high matrix dislocations and concurrently facilitated the early dynamic conception of granular β-Mg17Al12 particles in subgrains stage, all of which served the superior performance. Moreover, no prominent anisotropy detected in the extruded part due to the development of a special bimodal basal texture under shear stress, where its synergistic effect with grain refinement evidently promoted the balance of strength and plasticity. Finally, the mechanism of bimodal texture formation associated with specific slip activations during WCE and anisotropic weakening was revealed in detail.