EFFECT OF HIGH TEMPERATURE PRE-AGEING AND LOW-TEMPERATURE RE-AGEING ON MECHANICAL PROPERTIES AND INTERGRANULAR CORROSION SUSCEPTIBILITY OF Al-Mg-Si-Cu ALLOYS

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 degrees C and re-ageing time at 160 C 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 degrees C, 2h+160 degrees C, 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 beta '' 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.