Influence of Cu/Mg ratio and content on heat-resistance of Al–Cu–Mg alloys

Heat-resistant Al alloys used in such as aerospace, transportation fields are attracting more and more attention in recent years. Within Al alloy families, Al–Cu–Mg alloys have shown promising heat resistance properties. This work aims to investigate the influence of Cu/Mg ratio and content on the heat resistance of Al–Cu–Mg alloys, based on alloys of Al–4.5Cu–2.5 Mg (referred to as alloy A), Al–4.0Cu–2.2 Mg (alloy B) and Al–4.5Cu–1.6 Mg (alloy C). The alloys A and B possessed approximate Cu/Mg ratio, and they also exhibited nearly identical hardness retention rate during exposure at 200 °C. After 200 h, the rate is ∼75 %, though alloy A showed higher hardness (105 vs. 102 HBW) due to higher Cu, Mg content. In contrast, alloy C with a higher Cu/Mg ratio was less heat-resistant, with hardness retention rate of ∼70.5 % after 200 h exposure. Nano-sized S′(Al2CuMg) precipitate was main strengthening phase for the three alloys. Also, micron and submicron Al2CuMg particles could be formed with increase of Cu and Mg contents, which contributed a lot to yield strength for T6 heat-treated alloys, but slight contribution after exposure at 200 °C for 200 h. The degradation of mechanical properties during heat exposure can be attributed to the transformation and coarsening of S′ precipitates. In alloys with lower Cu/Mg ratio, there was excess Mg dissolved in Al matrix, which reduced Cu solubility in α-Al, and then slowed diffusion flux of Cu element, thus inhibited coarsening of Al2CuMg phase