The precipitate evolution of friction stir welding (FSW) 2195-O Al–Li alloy was analyzed by back scattered electron (BSE), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Efforts were made to explore the correlation between precipitates and hardness, tensile properties of the FSW joints and discuss the reasons for the generation of a serration shape pattern. It can be found that the base material (BM) consists of large coarse equilibrium precipitates, such as θ, TB, T2, T and Al7Cu2Fe, which change a little in the heat affection zone (HAZ) and the thermo-mechanically affected zone during FSW. Whereas, these precipitates significantly decrease in the nugget zone (NZ) with many δʹ/βʹ, TB and GP zones generating. The FSW joints are fractured in the HAZ/BM, developing a joint coefficient of 99% and the decrease of elongation from 20.6% to 15%. Within the NZ, these coarse precipitates completely disappear in the advancing side (AS-NZ), while remain a lot in the retreating side (RS-NZ) with evidently decrease in content and size. Therefore, the increment of hardness decreases from the AS- NZ to RS-NZ. The develop of serration shape pattern is attributed to distribution difference of coarse equilibrium precipitates in content and size.
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