Study on the effects of the compression ratio and mushy zone heating on the thixotropic microstructure of AA 7075 aluminum alloy via SIMA process

The effects of compression ratios on the microstructure evolution of semisolid Al7075 alloy produced by the strain induced melt activation (SIMA) process were investigated. The samples were cold deformed by compression into the different heights up to 40% reduction. The isothermal holding treatments within mushy zone of the alloy were carried out at 610, 620 and 625 °C for predetermined time intervals.The results revealed that the average grain size gradually reduced with the increase in the compressive ratio. While the compressive ratio surpassed 30%, the descending trend above was not as evident as that of below 30% reduction. The optimum condition presenting the minimum grain size and the maximum shape factor with the highest uniformity is discussed. During the subsequent mushy zone heating, the recrystallization was induced in the deformed samples by the increasingly accumulated strain energy.

Research highlights▶ The average grain size gradually reduced with the increase in the compressive ratio. ▶ The descending trend was not as evident as that of below 30% reduction while the compressive ratio surpassed 30%. ▶ The optimum condition for thixoforming process presenting the highest uniformity is 30% compression ratio.