The effect of different preaging conditions on mechanical properties and clustering behavior of AA6016 sheets for automotive outer panels

Preaging (PA) is an important process to enhance the performance of automotive aluminum outer panels through regulating solute clustering. In this work, we investigated the effect of PA temperatures in the range of 60°C-225 °C on the hardness before and after paint baking (PB) with different holding times. Tensile properties of partial alloys were checked to help explore the optimum PA condition. Before PB, the PA hardness of alloys preaged in the low-temperature regime (60°C-100 °C) was relatively stable with increasing PA time, but it increased significantly in the high-temperature regime (120°C-200 °C). After PB, the hardness of alloys preaged at temperatures up to 80 °C was low. Alloys preaged at 100 °C for 4 h could reach the same PB hardness as the solution-treated (ST) alloy. However, after reaching saturation of the hardness of preaged alloys in the low-temperature regime, extending the PA time did not induce a higher PB hardness. By contrast, the hardness decreased after long-term PA. The saturation PB hardness of the alloy further increased when preaged at a higher PA temperature and reached a maximum at 170 °C. The optimum PA condition for this alloy is 150 °C for 10 min as it produced better elongation before PB and the highest BHR after PB. The related mechanisms, including clustering and precipitation behaviors, have been discussed in detail.