Effects of Cr Addition on Microstructure and Mechanical Properties of Heat-Resistant Al-Cu-Mg Alloys

Al-7Cu-0.3Mg alloys with different Cr contents were prepared by melting and casting. The microstructure and tensile mechanical properties of the as-cast and T6 heat-treated alloys were investigated at room temperature and 350 °C. The results show that Cr addition not only significantly refines the primary α-Al grains and the α-Al + θ-Al2Cu eutectic structure in the as-cast alloy, but also promotes the precipitation and refinement of the θ′-Al2Cu phase in the matrix after T6 heat treatment. However, the refinement effect does not increase monotonically with increasing Cr content; both the as-cast and heat-treated alloys exhibit optimal refinement at a Cr content of 0.36 wt.%. After heat treatment, the average size of the θ′-Al2Cu precipitates in the alloy with optimal refinement is approximately 185 nm, which is 24.8% smaller than that in the base alloy without Cr. Mechanical testing indicates that Cr significantly enhances the tensile strength of both the as-cast and heat-treated alloys at both room and elevated temperatures, and the relationship between Cr content and strength exhibits a Gaussian-type trend, with the maximum tensile strength achieved at a Cr content of 0.36 wt.%. After heat treatment, the highest high-temperature tensile strength reaches 173.43 MPa, representing a 47.2% improvement over the base alloy without Cr. Thermal exposure tests at 350 °C demonstrate that the addition of Cr significantly improves the coarsening resistance of the θ′-Al2Cu phase. Therefore, the enhanced high-temperature tensile strength of the Cr-containing alloy is attributed to an increased number density, refined size, and improved thermal stability of the θ′-Al2Cu precipitates.