Microstructure and homogenization process of semi-continuous casting 7136 ultra high-strength aluminum alloy

The 7×××series aluminum alloy is mainly produced using semi-continuous water-cooled casting with this method, the solidification speed of the alloy is fast, leading to different degrees of dendrite segregation and non-equilibrium eutectic structure in the ingot. This also results in non-uniformity of composition and structure. Further, it adversely affects the subsequent cutting process and comprehensive performance of the alloy. Therefore, ingot homogenization becomes an indispensable and very critical process for the elimination of segregation. There is much research on Al-Zn-Mg-Cu alloys at home and abroad at the present. This research mainly concentrates on alloys such as 7075, 7050, 7150 and 7055. The content of the main alloying elements of these alloys is mostly around 10%. However, at present, there is not much research on alloy materials with a strength and main alloying element content exceeding 12.5%. The main alloy element content of 7136 aluminum alloy is about 13.5%. In 7136 aluminum alloy, the main alloy element content is high and the cast microstructure characteristics and homogenization treatment conditions are very different from other 7×××series aluminum alloys. In this paper, semi-continuous casting 7136 aluminum alloy was taken as the research object, based on the as-cast microstructure analysis, using two-stage homogenization. The results show that, compared with the other 7×××series aluminum alloys, the cast microstructure of 7136 aluminum alloy has no obvious lamellar Al(Al)+T eutectic phase characteristics, and no S phase exists. The dispersed phase in the matrix is a micron-sized round or rod-shaped MgZn₂ phase. The Mg and Zn were precipitated as MgZn₂ phase in the Al matrix as the liquid alloy solidified. In order to balance the partition coefficients of the Mg and Zn, the two metals converted from the liquid to the solid state, which explained why the Zn and Mg crystal contents were high. After a single stage of homogenization at 462℃, 24 h, the residual phase was substantially eliminated. As the homogenization time increased, the residual phase tended to decrease, but the effect was relatively small by this method. After the 7136 aluminum alloy was homogenized for two stages at 450℃, 24 h and 470℃, 24 h. The peak values obtained by differential scanning calorimetry were very small, except for a small amount of high-melting Al₇Cu₂Fe phase remaining between the crystals. Al₂Cu and other phases were basically eliminated and the homogenization effect was significant.