Direct Aging of B319 Al Alloy: Microstructure, Hardness, and Electrical Conductivity

Direct aging a casting to a T5 temper may be a suitable, cost-effective alternative to two-stage precipitation heat treatment schedules for some industrial applications. However, its successful implementation requires an in-depth understanding of the associated phase transformations and their influence on material properties. Hence, this study established the relationships between microstructure, hardness, and electrical conductivity, realized by the direct aging of industrial B319 Al alloy. Samples with various solidification rates and Sr contents were treated at 250 °C for durations up to 24 h, followed by Rockwell hardness and eddy-current electrical conductivity measurements. These data were complemented with novel, in situ analysis of the elevated-temperature microstructural transformations, using high-resolution electron microscopy and x-ray diffraction techniques. Even without prior dissolution and homogenization, direct aging produced extensive precipitation and coarsening of fine θ′ and θ-Al₂Cu phases, yet the products of transformations were localized in the solute-enriched regions near the interdendritic phases. The associated alleviation of as-cast lattice strains substantially enhanced alloy conductivity by up to 5% International Annealed Copper Standard (IACS), yet reduced hardness by up to 15 HRB, independent of the casting prehistory. Therefore, T5 heat treatments at 250 °C can be advantageous for high-conductivity Al components designed for low-stress applications.