The effects of Cu and Mg contents on the thermal stability of 6XXX-series aluminum alloy

Abstract

The effects of Cu and Mg contents on the thermal stability of 6XXX-series aluminum alloys was investigated using tensile testing, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. Results showed that the thermal stability of 6XXX-series aluminum alloys improved with increasing Cu and Mg contents, and that the precipitate was the most important factor for thermal stability. In the alloy without Cu, the strengthening precipitates are mainly β″ phases. After heat holding at 150°C for 1000 hours, part of the β″ phases transformed into β′ phases and the number density of the β″/β′ phases decreased, leading to a major strength reduction. In addition to β″ phases, Q′ and L phases are the main strengthening precipitates for the alloys containing Cu. Q′ and L phases have higher thermal stability than the β″ phase, and the number density of the Q′ phases decreased slightly after heat holding, while that of the L phases barely changed. This resulted in Cu-containing alloys with higher thermal stability than Cu-free alloys. The L phase number density increases with increasing Mg content. Compared with the Q′ phase, the L phase has higher thermal stability, resulting in higher thermal stability for alloys with higher Mg content.