Affecting mechanism of deep cryogenic treatment on nugget zone softening in dissimilar aluminum alloys friction stir welded joint

As far as the 2219 Al-ceramic Al dissimilar friction stir welded joint is concerned, the weld nugget zone surprisingly shows a notable softening phenomenon after the deep cryogenic treatment (DCT). The effect of DCT on microstructure evolution of nugget zone in 2219 Al was investigated in this paper in order to explore the reasons for the unique softening phenomenon. The results indicate that the DCT further promotes recrystallization and causes the grain growth slightly. The diffusion of Ti element from ceramic Al to 2219 Al occurs during the dissimilar friction stir welding, leading to the formation of Ti-rich secondary phases in the nugget zone of 2219 Al. The DCT can facilitate the precipitate dissolution to a certain extent, and thus the size and distribution density of the secondary phase particles are both reduced. Essentially, DCT produces internal stress in the dissimilar friction stir weld, and the rotation of grain boundaries from dislocation movement and the dissolution of secondary phases are able to release the internal stress, which are fundamental reasons for the softening of nugget zone after DCT.