Effect of pass strain on grain refinement in 7475 Al alloy during hot multidirectional forging

Abstract

Effect of pass strain (Δε) on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490°C under a strain rate of 3 × 10-4 s-1. Samples of rectangular shape were deformed up to accumulated strains of around 6 with subsequent changes in loading direction 90° from pass to pass. The pass strains in each compression (Δε) were 0.4 and 0.7. The cumulative flow curves integrated by each compression exhibit significant work softening just after yielding, followed by apparent steady state plastic flow at high strains. Structural changes were characterized by grain fragmentation due to frequent development of deformation and/or microshear bands followed by full evolution of new fine grains in the original grains. Increasing Δε accelerates significantly the kinetics of grain refinement, leading to more clear reduction of flow stresses at moderate to high strains. MDF of Δε = 0.7 results finally in formation of a finer grained structure with an average size of around 7.5 μm at strains of above 3.5, while, the processing with Δε = 0.4 develops a slightly coarser grain structure at higher strain of about 6. The effect of MDF on new grain evolution and the mechanisms of grain refinement are discussed in details.