Abstract:AZ61Mg alloy was multi directionally forged (MDFed) during decreasing temperature condition from 643 K to 483 K at a true strain rate of 3×10⁻³ s⁻¹ up to cumulative strain of ∑∆ε=4.0 at maximum. A pass strain of ∆ε=0.8 was employed. While average grain size decreased gradually with increasing cumulative strain, the evolution of fine-grained structure strongly depended on the MDF temperature. Under the condition where the temperature was higher than the most adequate one, grain coarsening partially took place during MDF. In contrast, at lower temperature, inhomogeneous microstructure composed of the initial coarse and newly appeared fine grains was evolved. After straining over ∑∆ε=3.2 (i.e., over 4 passes of MDF), equiaxed ultrafine grains (UFGs) having average size of about and lower than 1 μm were uniformly evolved. While the MDFed alloy to ∑∆ε=4.0 possessed relatively high hardness of HV 99, and it accepted further about 20% cold rolling almost without cracking. Because of the superior formability of the UFGed AZ61Mg alloy, the hardness was further easily raised to HV 120 by following cold rolling.

Key words: AZ61 magnesium alloy; ultrafine grain; plastic deformation; multi directional forging