Study on aging and overaging behavior of the AlSi9Cu3 alloy processed by PBF-LB

Al–Si–Cu alloys processed by laser-based powder bed fusion (PBF-LB) are promising candidates for lightweight high-performance automotive components, provided that post-processing heat treatments are developed to tailor their mechanical response. This study investigates the heat-treatment response and high-temperature stability of PBF-LB AlSi9Cu3 alloy, focusing on short-term direct aging treatments and a tailored T6 condition. Direct aging treatments (T5) were designed either to enhance strengthening (T5LowT) or to promote stress relieving (T5HighT), whereas the T6 (solutioning, quenching, and aging) was optimized by avoiding microstructure coarsening. The experimental approach included detailed microstructural characterization, residual stress measurements, and hardness tests before and after long-term exposure at 200–290 °C. The asbuilt (AB) alloy exhibited a high hardness of about 150 HB10, due to its extremely fine cellular solidification structure and supersaturated Al matrix, together with significant tensile residual stresses. The fine cellular structure was only marginally affected by T5 treatments, whereas it was significantly modified after T6. All treatments succeed in reducing the tensile residual stress (by 40% for T5 and completely after T6) and induced the precipitation of second phases. Approximately 160 HB10 was achieved after the T5LowT, while T5highT and tailored T6 treatments resulted in approximately 140 HB10. High-temperature exposure caused an overall hardness decrease in all investigated conditions, with a more pronounced and accelerated drop for T6 samples. The superior thermal stability of the AB and T5 conditions is attributed to the retention of the fine cellular structure, which acts as the most effective strengthening feature during high-temperature exposure.