Investigation on the impact of T6 and direct ageing heat treatment on the microstructure and mechanical behaviour of additively manufactured AA7075 alloy

• Studied the effect of T6 and direct ageing on L-PBF fabricated AA7075 alloy components. • Optical, FESEM, and EBSD analyses revealed microstructural evolution of post heat treatments. • Direct ageing promoted higher intermetallic phase precipitation and grain texture intensity. • Direct-aged samples exhibited enhanced hardness, yield strength, and ultimate tensile strength. The traditional methods employed to produce most of the components for AA7075 alloy possess issues such as material wastage, increased production costs, and the requirement for supplementary post-processing procedures. This study focuses on the effect of two heat treatments on AA7075 alloy samples fabricated via laser powder bed fusion (L-PBF): HT1 (T6 solution treatment at 470°C for 2 hours followed by aging at 150°C for 6 hours) and HT2 (direct aging at 150°C for 6 hours). Furthermore, the mechanical properties were measured using Vickers microhardness testing and a computerised universal testing machine (UTM). As-fabricated (AF) samples exhibited arc-shaped melt pools and columnar grains (average size ∼12.4 µm) with a firm <001> texture (intensity 3.217) and solidification cracks. After HT1, recrystallisation produced equiaxed grains (∼13.1 µm), whereas HT2 preserved columnar features but enhanced precipitation hardening through a higher density of η-Mg(Zn,Cu,Al)₂ and θ-Al₂Cu phases. EBSD analysis confirmed that HT2 produced the strongest crystallographic texture (intensity 3.697 along <001>), compared to HT1 (3.606). Mechanical characterisation revealed that AF specimens exhibited a hardness of 115.5 HV, Yield strength (YS) of 60.36 MPa, Ultimate Tensile strength (UTS) of 101.47 MPa, and elongation of 0.54%. HT1 improved hardness to 130.8 HV and UTS to 200.70 MPa, while HT2 provided the best performance, achieving 157.8 HV, YS of 137.02 MPa, UTS of 230.11 MPa, and elongation of 2.11%. These results demonstrate that HT2 enhances precipitation strengthening, grain texture, and overall mechanical performance more effectively than T6, despite the persistence of solidification-related cracks.