Microstructure evolution and tensile behaviour of fine-grained 6082 Al wire with high ultimate strength and high work hardening by friction stir extrusion of bulk Al sheet

Abstract

Driven by the highly enhanced demand for metallic materials with excellent strength-ductility synergy, fine-grained alloy fabricated by severe plastic deformation (SPD) is a long-lasting research interest. In this study, a fine-grained aluminium (Al) alloy wire with high ultimate tensile strength (316.8 MPa) and high work hardening capability (n = 0.34) was developed via a novel bulk-consolidation friction stir extrusion (FSE) enabled by CoreFlow™ process based on FSE processed 6082-T6 Al sheet. Grain refinement of 200 times occurred in CoreFlowed Al alloy, which is rarely observed in FSEed Al alloy counterpart based on milling metal chips. The undesirable feature of coarse and preferred secondary phase particles in the initial 6082-T6 Al billet was well tailored during CoreFlow™ process, which led to an attractive characteristic of dispersed and refined particle phases in CoreFlowed Al alloy. Texture components of S (V = 8.24%) and Goss (V = 6.86%) were observed, however, the overall micro-texture intensity of CoreFlowed Al wire was significantly weakened. Due to the cooperative deformation modes consisting of dislocation slip and grain boundary sliding, as well as the positive compatibility effect of fine-grains on deformation, CoreFlowed Al specimens demonstrated an excellent ductility (EI = 19.3%) and acceptable yield strength (YS = 182 MPa) compared to initial 6082-T6 Al sheet (EL = 10.4%, and YS = 225 MPa). Benefiting from the higher work hardening capability, the final ultimate tensile strength (UTS) of CoreFlowed Al (316.8 MPa) reached almost the same level as the 6082-T6 Al sheet (324.6 MPa). Furthermore, after one simple post-processing one-step heat treatment (175 °C for 10 h), the wire's yield strength was improved to 250 MPa, although only maintaining 9% elongation. This low elongation could be attributed to micro-cracks induced by coarsening of the second phases, while high yield strength resulted from fine grain size and precipitate strengthening. The wide range of variations in the mechanical properties of CoreFlowed Al wire under different conditions provides significant freedom in tailoring the mechanical properties of alloy wire in applications.