B22-O-01Microstructure and Mechanical Properties of AZ61Mg Alloy Multi-Directionally Forged Using Die under Decreasing Temperature Conditions

Abstract

A rectangular shaped AZ61Mg alloy was multi-directionally forged (MDFed) with pass strains of Δε = 0.8 under decreasing temperature conditions form 603 K to 393 K using a die designed for MDF. The coarse initial grains with an average grain size of 21.6 µιη were gradually fragmented pass by pass of MDFing and homogeneous microstructure with an average grain size of 0.3 μm was obtained at cumulative strain of ΣΔε = 7.2 (Fig. 1 ). The ultrafine grained (UFGed) AZ61Mg alloy exhibited superior balance of mechanical properties of hardness of 958 MPa, tensile strength of 465 MPa and ductility of 15 %. The tensile behavior of the UF Ged AZ61 Mg alloy showed large temperature and strain rate dependencies. The strain rate dependency tended more significant with decreasing grain size. Superplasticity of 680% elongation could be achieved at 423 K and at strain rate of 1.0 × 10 −4 s −1 . Because the temperature is rather lower than the temperature range where the conventional superplasticity takes place, it can be called as low-temperature superplasticity. Such mechanical properties of the UFGed AZ61Mg alloy strongly depending on temperature and strain rate were reasonably understood by considering the effect of grain boundary sliding. The tensile strength of the MDFed AZ61Mg alloy was further raised up to 490 MPa by additional cold rolling due to texture and plastic hardening.