Precipitation behavior and mechanical properties of Al–1.0Mg–0.6Si–Cu (wt.%) alloy controlled by Cu content

The precipitation and intergranular corrosion (IGC) behavior of Al–1.0Mg–0.6Si–Cu (wt.%) alloys with the addition of Cu ranged from 0.3 to 0.6 wt.% were investigated by optical microscope and transmission electron microscopy. The results indicated that the Cu addition significantly improves the hardness of the alloys during under-aging progress by accelerating the precipitation of GP zones. The addition of Cu has changed the composition of precipitation and facilitated the formation of β′′-Mg5Si6 precipitates with high density and fine distribution. When the Cu content is added to 0.6 wt.%, the type of precipitation has changed. In addition to the needle-shaped β′′ phase, a new-plate Q′ phase precipitated during peak-aging conditions. The 0.6 wt.% Cu alloy shows superior strength with tensile yield strength of 328 MPa, ultimate tensile strength of 368 MPa, and elongation of 16.0%. However, the IGC resistance decreased with the Cu content increasing from 0.3 to 0.6 wt.%, which mainly resulted from the segregation of Cu elements and the formation of continuous Q′ precipitates at the grain boundaries.