铝锂合金超声增强搅拌摩擦焊空位诱导沉淀相析出机制及接头性能调控

航天结构材料铝锂合金在搅拌摩擦焊时，存在溶解沉淀相难以再析出的固有不足，导致接头发生软化，力学性能降低。现有的增加焊接热输入或焊后热处理等促进沉淀相析出的方法，存在热软化效应显著、效率低、难以应用于大型结构件等技术局限。为此，本项目提出采用超声增强搅拌摩擦焊提高焊缝空位浓度、诱导沉淀相快速析出的新途径，突破现有方法的技术瓶颈，一步实现铝锂合金接头的组织优化和性能提升，获得优质高强的铝锂合金接头。为深入揭示这一新途径的微观机制及其对接头性能的调控机理，本项目采用试验表征与第一性原理计算相结合的方法，研究超声作用下沉淀相的演变特征，阐明空位对溶质原子跃迁能垒和沉淀相界面应变能的影响，揭示空位诱导沉淀相析出机制；研究超声与焊接热循环协同作用对沉淀相界面结构的调控规律，通过控制沉淀相的析出行为实现对铝锂合金接头性能的优化调控。研究成果为铝锂合金大型结构件的高质量焊接提供理论依据。

For the friction stir welding of Al-Li alloy employed in the aerospace industry, an inherent technical problem, namely joint softening and its resultant reduction of mechanical properties, is urgent to be overcome. The present approaches, such as increasing the heat input and using post-weld heat treatment, have prominent disadvantages involving serious heat softening phenomenon, low production efficiency, and application difficulty in large components. This project proposes an innovative approach to improve the mechanical properties of the Al-Li alloy joint, i.e., increasing the vacancy concentration in the weld by ultrasonic enhanced friction stir welding to induce the rapid precipitation of solid solute alloying elements. This single-step welding technology can break through the bottleneck that confines the improvement of joint performance, and efficiently achieve the optimized microstructure with excellent properties. Experimental analyses and first-principles calculations are both employed in this project. The precipitate evolution with the effect of the ultrasonic is investigated. The mechanism for the vacancy-induced precipitation in ultrasonic enhanced friction stir welding is revealed by investigating the effect of vacancies on the energy barrier for atomic migration and interface strain energy of precipitates. Besides, with the effect of the ultrasonic, the control principles of the thermal cycle on the precipitate interface structure evolution are systematically discussed and revealed, guiding the further improvement of Al-Li alloy joint mechanical properties. The results in this project provide the theoretical basis for the high-quality welding of the Al-Li alloy large components.