Recrystallization Behavior of 7175 Al Alloy during Modified Strain-Induced Melt-Activated (SIMA) Process

A modified strain-induced, melt-activated (SIMA) process for semi-solid processing of alloys was proposed. In order to examine the applicability and effectiveness of the modified SIMA process, the recrystallized microstructures of a high strength 7175 Al alloy prepared by the modified SIMA processes were macroscopically compared to that of conventional process. The modified SIMA process employed casting, two stage homogenization, warm multi-forging, and recrystallization and partial melting (RAP) instead of the conventional process consisted of casting, hot working, cold working and RAP. For the alloy processed by the conventional SIMA process, the recystallized grain size decreased with increasing the cold rolling reduction ratio up to 20% and then almost unchanged. RAP treatments of the 20% cold rolled alloy at above 600°C and for longer than 30 min resulted in significant grain growth. In case of the modified SIMA process, the alloy multi-forged with the accumulated strain of about 7 and RAP at 575°C for 10 min exhibited the uniform equiaxed recrystallized grain structure similar to that of the conventional SIMA process under the optimum conditions. The improved processing efficiency of the modified SIMA process over the conventional one, i.e. removal of hot working and saving of RAP time, is attributed to the enhanced recrystallization kinetics due to a high density of Mg(Zn,Al,Cu)2 precipitates which are formed by two stage homogenization, acting as the preferential recrystallization sites and due to the driving force imposed by relatively large amount of uniform deformation by multi-forging.