Improved Distribution and Uniformity of α-Al(Mn,Cr)Si Dispersoids in Al-Mg-Si-Cu-Mn (6xxx) Alloys by Two-Step Homogenization

The recrystallization behaviors of deformed 6xxx aluminum alloys can be effectively controlled with α-Al(Mn,Cr)Si dispersoids by pinning or slowing the movement of dislocations. However, abnormal grain growth still often occurs due to the nonuniform distribution of α-Al(Mn,Cr)Si dispersoids. In the present study, the uniformity of dispersoids was significantly improved by applying two-step homogenization heat treatments. The effect of different homogenization conditions on the distribution of the α-Al(Mn,Cr)Si dispersoid was evaluated. By applying traditional one-step homogenization at 550 °C for 10 hours, the dispersoids exhibited sparse and nonuniform precipitation at the center of grains or dendritic arms. These areas with a nonuniform distribution of dispersoids were regarded as coarse dispersoid zones (CDZs). Relative to one-step homogenization, two-step homogenization led to a considerable reduction in the percentage of CDZ area from approximately 7 pct to less than 2.5 pct. This result was due to the diffusion of solute elements and the precipitation of metastable Mg2Si and Q phases because of isothermal holding during the first step. Metastable Mg2Si and Q phases could act as nucleation sites to promote the precipitation of α-Al(Mn,Cr)Si dispersoids. As a result, α-Al(Mn,Cr)Si dispersoids exhibited uniform precipitation, and the formation of CDZs was avoided. Two-step homogenization also improved the recrystallization resistance of the alloy because of the decrease in the percentage of CDZ area.