喷射成形7075合金回归再时效处理的组织和抗应力腐蚀性能

Microstructure and Stress Corrosion Cracking of Spray Forming 7075 Alloy after Retrogression and Re-ageing Treatment

喷射成形7075铝合金是典型的时效强化合金,通过T6处理可得高强度,但其抗应力腐蚀开裂(Stress corrosion cracking,SCC)等性能较差;T7处理虽可改善抗SCC性能,但合金的强度却损失严重。为此,采用常温拉伸测试、电导率测试和透射电镜观察等手段,研究4种回归再时效(Retrogression and re-ageing,RRA)处理对喷射成形7075铝合金显微组织及抗SCC性能变化的影响,分析探讨RRA处理与合金性能变化之间的关系。结果表明,合金经200℃/10 min回归和再时效处理,抗拉强度和电导率均较好,分别是748 MPa和22.8 MS/m;随着回归处理的进行,基体中的GP区和η′相逐渐回溶,当200℃回归处理超过10 min,合金晶粒内再次析出η′相,使合金强度小幅上升;回归过程中η′相先回溶再析出,因此不同回归状态下RRA处理后的合金晶内组织形貌差异较大。RRA处理后晶内大量再析出的η′相与晶界处粗化并断开的η相可以在保持合金高强度的基础上显著改善其抗SCC性能。

A typical age-strengthening 7075 aluminum alloy with high tensile properties can be spray-formed through T6 treatment, but its stress corrosion cracking （SCC） property appears very poor after this treatment. On the other hand, the SCC property may be improved by T7 treatment but the tensile strength of the alloy then be seriously harmed. Therefore, the effects of four kinds of retrogression and re-ageing （RRA） treatments on the microstructure and mechanical properties of a spray forming 7075 alloy are investigated by using mechanical, electric and transmission electron microscope observation measures in this research. It is obtained that the tensile strength and electric conductivity of the alloy reaches up to 748 MPa and 22.8 MS/m respectively under the condition of retrogressed at 200 C for 10 min, and re-aged at 120 ℃for 24 h. During retrogression treatment, GP zone and η＇ phase are dissolved in matrix, η＇ phase is then re-precipitated at 200 ℃ retrogression treatment over 10 min, which leads to the slight increase of tensile strength of the alloy. Because of the precipitation of shaped-q＇ phases during retrogression and the corresponding RRA treatment a lots ofη＇ phases re-precipitate and η phases at grain boundary may improve the SCC resistance of the alloy.