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-degrees. After 200 h, the rate is similar to 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 similar to 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 heattreated alloys, but slight contribution after exposure at 200 C-degrees 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 alpha-Al, and then slowed diffusion flux of Cu element, thus inhibited coarsening of Al2CuMg phase.