Effect of the Two-Step Solution Heat Treatment on the Microstructure Of Semisolid Cast 7075 Aluminum Alloy

摘要

Effect of the two-step solution heat treatment on the microstructure of semisolid cast 7075 aluminium alloy has been studied. The microstructure of the as-cast specimens mainly consisted of matrix-α (Al) and grain boundary (GB)-eutectic phase (α-Al + Mg(Zn,Cu,Al)2). After solution treating, coarse black particles were found to form in the single-step solution treated specimens at the condition of 450 oC for 8 h and 480 oC for 1 h, respectively. Two-step solution heat treatment resulted in the reduction of coarse black particle formation while maintaining the same amount of eutectic MgZn2 phase dissolution as the high temperature single-step solution treatment. Therefore, the two-step solution heat treatment enables alloying elements dissolved into the matrix without overheating and hence decreases coarse black particles. The optimum two-step solution heat treatment condition derived from this study was 400 oC for 8 h + 450 oC for 4 h.