Multi-directional forging of AZ61Mg alloy under decreasing temperature conditions and improvement of its mechanical properties

In this study, AZ61Mg alloy was multi-directionally forged (MDFed) up to a maximum cumulative strain of ∑Δɛ = 4.0 at a true strain rate of 3 × 10−3 s−1. The MDF temperature was decreased pass-by-pass from 623 to 503 K. The average grain size decreased with increasing cumulative strain. After MDF was carried out until ∑Δɛ = 4.0, i.e., after five passes, ultrafine grains (UFGs) with an average grain size of 0.8 μm were uniformly evolved. Results revealed that UFG evolution was induced by the combined mechanisms of mechanical twinning, kinking, and continuous dynamic recrystallization. The MDFed AZ61Mg alloy exhibited an excellent balance of strength and ductility at room temperature, i.e., an ultimate tensile strength (UTS) of 440 MPa and plastic strain to fracture of over 20%. In addition, it exhibited superplasticity at elevated temperatures. Because of its superior deformability, the alloy was further made to undergo a cold rolling procedure to achieve up to 20% reduction. The additional thermo-mechanical processes of cold rolling and/or aging successively raised the UTS of the alloy to 550 MPa without much spoiling ductility (plastic strain to fracture of 14%).