Effects of solution heat treatment time on the as-quenched microstructure, hardness and electrical conductivity of B319 aluminum alloy

Abstract In Al-Si-Cu alloys, solution heat treatment promotes several microstructural transformations that influence both hardness and conductivity. Dissolution, spheroidization and coarsening processes can occur concurrently with heat treatment time, directly impacting material properties. However, the absence of time-dependent as-quenched data in the literature impedes distinction of their individual effects. This research aimed to clarify the solution heat treatment kinetics and identify the competing mechanisms controlling electron flow in industrial Al alloys. Samples of automotive B319 alloy were cast in the unmodified and strontium-modified conditions with two solidification rates. After solution treatment at 500 °C for up to 48 h, the Rockwell hardness and electrical conductivity of the samples were measured in the as-quenched condition. Quantitative image analysis revealed a co-dependence between casting prehistory and heat treatment time on the microstructure. Gradual decreases in as-quenched hardness were observed after long treatment times. Yet, clear electrical conductivity trends suggested that there were two distinctive stages in the progression of solution heat treatment of B319 alloy. Initially, Al2Cu dissolution into the supersaturated aluminum solid solution caused transient conductivity reductions. Nonetheless, the eutectic Si spheroidization and coarsening processes promoted subsequent conductivity increases by up to 10%, relative to the as-cast condition. This research elucidates the role of Si morphology in enhancing electron transfer, and it supports the optimization of component performance, manufacturing costs and lead times.