Microstructural characteristics and paint-bake response of Al-Mg-Si-Cu alloy

The microstructural characteristics and paint-bake response of 6022 alloy with 0.3% Cu (mass fraction) were studied using optical microscope, scanning electron microscope(SEM), transmission electron microscope(TEM) and tensile tester. The results indicate that the phase constituents in the as-cast microstructure are Mg2Si, Si, Al5Cu2Mg8Si6, Al5FeSi, α-Al(MnCrFe)Si and CuAl2. During the following homogenization, CuAl2, Al5Cu2Mg8Si6 and Mg2Si phases are almost completely dissolved, and Al5FeSi transforms to α-Al(MnCrFe)Si particles. After rolling, the phase constituents in the alloy change less except the precipitation of Mg2Si particles, and the precipitation behavior of Mg2Si strongly depends on the thermomechanical conditions. Cu addition significantly increases the paint-bake response of 6022 alloy by facilitating the formation of β″ phase. Therefore, the tensile strength of 6022 alloy with 0.3% Cu is higher than that of 6022 alloy without Cu after paint-bake cycle.