采用拉伸性能测试和晶间腐蚀浸泡实验, 研究了高温预时效+低温再时效对Al-Mg-Si-Cu合金拉伸性能和晶间腐蚀敏感性的影响, 并通过TEM观察基体和晶界析出相特征. 与常规T6时效(180 ℃, 8 h)相比, 优化双级时效(180 ℃, 2 h+160 ℃, 120 h)能在不降低6061铝合金拉伸性能的基础上彻底消除晶间腐蚀敏感性, 此时铝合金析出特征为基体分布着高密度b ″相兼有少量Q' 相, 而晶界析出相呈现球状、断续分布. 这种特征组织的形成源于降低再时效温度造成基体和晶界扩散速率的下降幅度不同, 导致再时效过程中基体预析出相因长大速度较慢而保持较好的强化效果; 晶界预析出相因粗化速度较快而呈现球形、断续分布.

It is well known that in peak-aged conditions age-hardenable aluminum alloys usually have high strength but low corrosion resistance. Low corrosion resistance of peak-aged Al alloys limits their applications in some corrosive conditions. In order to enhance the corrosion resistance, over-ageing treatments are often carried out but at the expense of strength. Therefore, it is of great industrial value to improve both strength and corrosion resistance of Al alloys simultaneously. In the present work, a novel two-step ageing treatment consisted of high-temperature pre-ageing and low-temperature re-ageing was proposed to improve both the tensile properties and intergranular corrosion (IGC) resistance of Al-Mg-Si-Cu alloys simultaneously. Furthermore, the effects of pre-ageing time at 180 ℃ and re-ageing time at 160 ℃ on the mechanical property and IGC susceptibility of the 6061 Al alloy were investigated by tensile testing and immersion corrosion testing. It was shown that after the optimized two-step ageing treatment of 180 ℃, 2 h+160 ℃, 120 h, the 6061 Al alloy had slightly higher strength than that of the conventional peak-aged samples and no susceptibility to intergranular corrosion. TEM observation revealed that the microstructures of the two-step treated 6061 Al alloy were consisted of high density of b″ phase along with small amount of Q' phase in the matrix and discontinuously distributed, spherical grain boundary precipitates, which led to high strength and IGC resistance of the 6061 Al alloy, respectively. The formation of the characteristic microstructures were attributed to the different decreased level of atomic diffusion rate between the matrix and grain boundary when decreasing from relatively high pre-ageing temperature to low re-ageing temperature, which resulted in the relatively slow growth of the matrix pre-precipitates and rapid coarsening of the grain boundary pre-precipitates, simultaneously.