Microstructural evolution and mechanical property degradation of Al–Cu heat-resistant alloy fabricated by wire-arc directed energy deposition after 300°C thermal exposure

High-temperature thermal exposure of Al alloys is used to characterize the real-world performance of components in high-temperature environments. In this study, thin-walled Al–Cu heat-resistant aluminum alloy structures were additively manufactured using a wire-arc directed energy deposition (WA-DED) process. A systematic analysis was conducted to investigate the microstructural changes and determine the cause of mechanical degradation under 300°C thermal exposure. Following T6 heat treatment (solution treatment: 520°C /6 h + 535°C /8 h; aging: 175°C/2 h + 150°C/6 h), the WA-DED 205C (Al–Cu–Mn–Ni–Zr) alloy exhibited a dense distribution of nano-sized \textθ'' , \textθ' , Al₃Zr, and T (Al₂₀Cu₂Mn₃) phases within the grains, whereas the γ (Al₇Cu₄Ni) phase formed during solidification remained at the grain boundaries. Following thermal exposure at 300°C for 24 h, the majority of \textθ'' within the grains was supplanted by \textθ' ; a subset of these \textθ' subsequently transitioned to \theta , while σ (Al₅Cu₆Mg₂) phases concurrently precipitated. As thermal exposure reached 100 h, the \textθ' phase exhibited increased particle size alongside reduced number density, whereas the θ phase showed enhanced density and the σ phase was no longer present. The Al₃Zr and T phases within the grains, and the γ phase located at the grain boundaries, showed no significant changes in morphology or size during the thermal exposure process. Furthermore, after thermal exposure at 300°C for 24 h, the microhardness of the T6 condition specimen experienced a sharp decline from HV 149.6 to HV 98.2, and the yield strength (YS) dropped from 353 to 213 MPa. After 100 h of thermal exposure, the microhardness decreased to HV 91.8 and the YS dropped to 196 MPa. The tensile properties of the alloy were stabilized at a YS retention rate of 56%.