Influence of Morphological Change of Eutectic Si during Homogenization Heat Treatment on the Microstructure and Strength at Elevated Temperatures in Wrought Al-Si-Cu-Mg-Ni Alloy

The evolution of eutectic Si morphology and microsegregation during homogenization heat treatment at 753 K was examined for Al-Si-Cu-Mg-Ni alloy casting billets. Subsequently, casting billets homogenized at 753 K for 0 s, 7.2 ks, and 28.8 ks were forged, and microstructures and mechanical properties at 473 and 573 K were investigated for the forgings with different sizes of eutectic Si. Spheroidization of eutectic Si and elimination of microsegregation were achieved during the heating process of homogenization heat treatment. The spheroidized eutectic Si then coarsened with the homogenization heat treatment time. The interparticle spacing of the eutectic Si and constituent particles after the forging process increased with the coarsening of Si particles during homogenization heat treatment. In the tensile test at 473 K, the 0.2% proof stress, ultimate tensile strength (UTS), and elongation of the forgings were almost the same regardless of the homogenization heat treatment time, i.e., the size of eutectic Si. On the other hand, forgings made of a billet with a shorter homogenization heat treatment time showed more pronounced work-hardening behavior and higher 0.2% proof stress and UTS at 573 K. The increase in strength was explained by the enhancement of work-hardening associated with the decrease in interparticle spacing, which depends on the Si particle size after homogenization heat treatment.