搅拌摩擦焊作为一种新型的固态连接技术，在焊接过程中材料不熔化，因此可以避免诸如气孔、热裂纹等凝固缺陷，在铝合金等轻合金焊接领域有着巨大的应用前景。因此，搅拌摩擦焊自一发明就受到了国内外各大公司、高校和研究院所的重视，其焊接过程机理的研究更是受到了极大关注。阐明搅拌摩擦焊过程中组织演化的规律和动态再结晶的影响因素及其微观机理，对于理解搅拌摩擦焊的本质，为工程应用提供理论依据，具有重要的意义。本文进行了2024和7050两种铝合金不同旋转速度和焊接速度下的搅拌摩擦焊试验，分析了不同焊接参数对其接头不同区域晶粒大小的影响规律。并进行了不同应变速率和变形温度下的等温压缩试验，研究了变形条件对再结晶晶粒尺寸的影响，揭示了焊接参数对焊核区晶粒尺寸的影响机理。结果表明，焊核区晶粒尺寸随转动速度的变化不大，随焊接速度的增加而减小，焊接参数对Zenner-Hollomon参数的不同影响导致了焊核区晶粒尺寸随焊接参数的不同变化趋势。采用“急停快冷”的方法对2024铝合金搅拌摩擦焊(FSW)的组织演化过程进行了研究，对搅拌头前、后方不同区域进行了EBSD测试，分析了FSW过程中组织演化的规律和机理。结果表明：在FSW前期发生了不连续动态再结晶，在FSW过程中与亚晶界吸收位错而取向差增加相关的连续动态再结晶是其动态形核的主导机制，在随后的冷却过程中晶粒长大成为终态组织，期间发生了部分的静态再结晶。研究了不同热处理方案对7050铝合金搅拌摩擦焊接头焊核区组织的影响。结果表明，7050铝合金FSW后第二相粒子主要包括含铁Al-Cu-Fe杂质相、沿晶界析出的长条状不含铁相以及细小的η相(MgZn2)。退火使得晶界氧化严重。固溶+时效后轴肩影响区个别晶粒异常长大，形成超大晶粒；焊核区晶粒发生部分长大，且尺寸分布不均匀；轴肩影响区粒子密度较小，焊核区密度较大；晶界是第二相优先析出的位置。
Friction stir welding (FSW) is an advanced solid-state welding technology. The materials do not melt during the FSW process, and the defects associated with solidification such as porosity and hot cracking can be avoided, therefore, the FSW has obtained extensive attention in the welding of light alloys such as aluminum alloys. During FSW, the joint is affected by the drastic mechanical stirring of the tool and consequent friction heat; and the grains will experience complex processes including deformation, broken, and crystallization. The final microstructure has a decisive influence on service properties of the joint. At present, the studies on the microstructure evolution in FSW are still not thorough and systematic. In this thesis, the processes of microstructure evolution, and the affecting factors and mechanism of dynamic recrystallization during FSW are studied of two types of aluminum alloys used in aeronautical manufacturing. Firstly, the influences of rotation rates and tool traverse speeds on the grain size of nugget zone during FSW of 2024 and 7050 aluminum alloys were studied. Meanwhile, isothermal compression experiments of 7050 aluminum alloy were performed by using the thermal-mechanical simulation system, and the influences of different strain rate and deformation temperature on the recrystallization grain size were analyzed. Based on previous work, the relationships between the welding parameters and deformation parameters were established; influence mechanism of welding parameters on the grain size in nugget zone was revealed. Results show that grain size in shoulder affected zone and nugget zone changes little with increasing rotation rate, while it decreases with increasing tool traverse speed. The welding parameters have different influences on the Zenner-Hollomon parameter, and hence affect the grain size in the nugget zone differently.Furthermore, the “abrupt stop and rapid cooling” technique was used to study the microstructure evolution during friction stir welding of 2024 aluminum alloy. By EBSD tests in different regions ahead of and behind the tool keyhole, the characteristics and mechanism of microstructure evolution were revealed. The results show that discontinuous recrystallization occurs during early stage of FSW; the continuous recrystallization is the dominating mechanism of dynamic nucleation during FSW; grains grow to the final structures during the subsequent cooling cycle, in which partial static recrystallization has occurred.Finally, the effects of two post-weld heat treatment processes on the microstructure of nugget zone were studied, with emphasis on the grain size and precipitates in the nugget zone before and after welding. The results show that the annealing process reduced the amount of η phase in the grain boundaries. After the processes of solid solution plus ageing, some oversized grains can be found in the shoulder zone; the grains in the nugget zone grow to different extents and make the microstructure inhomogeneous; the precipitates in shoulder zone have a high density, while those in the nugget zone have a low density; grain boundaries are the preferred sites for precipitation.The work in this thesis has the scientific significance for understanding the characteristics of microstructure transformation in FSW, and it can provide useful guidelines for practical engineering application.
