Effect of post-ageing cooling media on microstructure, mechanical properties, and surface roughness of heat-treated AA6063 alloy

Purpose This study aims to investigate the influence of post-ageing cooling media on the microstructure, mechanical properties, surface roughness after turning and fracture behaviour of AA6063 aluminium alloy subjected to a two-stage heat treatment cycle consisting of solutionising at 500 °C for 2 h followed by water quenching and artificial ageing at 200 °C for 5 h. Design/methodology/approach Six cylindrical AA6063 specimens, including one as-received sample, were heat treated under identical conditions and subsequently cooled using five different post-ageing media: air, oil, furnace, sand and water. Optical microscopy was used for microstructural observation, while hardness testing, tensile testing, surface roughness measurements after turning and field emission scanning electron microscopy-based fractographic analysis were conducted to evaluate the effect of different cooling paths. Findings Distinct differences in mechanical response and surface finish were observed among the cooling conditions. Faster post-ageing cooling media (water and oil) exhibited higher hardness values (≈ 88–89 HRB), whereas furnace cooling resulted in lower hardness (≈ 75 HRB) and increased ductility (elongation ≈ 23.4%). Tensile strength was higher for water and sand cooled specimens (UTS ≈ 206–207 MPa) compared to furnace-cooled specimens (≈ 153 MPa). Surface roughness after turning varied significantly with cooling medium, with sand-cooled samples producing the smoothest surface (Ra ≈ 2.9 µm) and air-cooled samples exhibiting the highest roughness (Ra ≈ 5.6 µm) under identical machining conditions. Optical microstructural features and mechanical trends were consistent with literature-reported cooling rate-dependent precipitation behaviour in Al–Mg–Si alloys. Fractographic analysis revealed ductile fracture dominated by microvoid coalescence across all conditions. Originality/value Unlike conventional studies focusing on T5/T6 temper comparisons or ageing parameters, this work isolates post-ageing cooling medium as the primary variable and demonstrates its significant role in tailoring the strength–ductility balance and surface finish of AA6063 alloy. The findings highlight post-ageing cooling as a simple yet effective heat treatment parameter for optimising mechanical performance and machinability in practical engineering applications.