Microstructure characterization and evaluation of mechanical properties in 2A97 aluminum-lithium alloys welded by stationary shoulder friction stir welding

Abstract

2A97 Al–Li alloy joints with the thickness of 1.5 mm were fabricated by stationary shoulder friction stir welding (SSFSW) under rotational speeds of 600–1000 rpm and a fixed welding speed of 200 mm/min. As the rotational speeds increased, the grain sizes of the nugget zone (NZ) kept almost constant with the presence of zigzag defects. The electron backscattered diffraction (EBSD) analysis suggested that the initial crystal orientation with near ‖TD and ‖WD textural components exhibited weaker anisotropy induced by the severe shearing deformation. The dynamic recrystallization (DRX) mechanisms were clarified in the transition from low-angle grain boundaries (LAGBs) to high-angle grain boundaries (HAGBs). In contrast to the conventional FSW joints of Al–Li alloys, the dispersions were indexed as Al11Cu5Mn3, Al2Cu within the grains, and Al–Cu–Mn–Zn phase at the grain boundaries in the SSFSW joints. The T1, δ′/β' and θ′ precipitates were detected in the base metal (BM), and the θ′ precipitates were dissolved in the heat affected zone (HAZ). However, those strengthening precipitates were absent in the NZ. The variation of the ultimate tensile strength first increased and then decreased with the increasing rotational speeds. Moreover, the joint obtained at a rotational speed of 1000 rpm exhibited the best combination of strength and ductility. The defect-free joints failed in the thermo-mechanically affected zone (TMAZ), which corresponded to the hardness minima positions (HMP). However, an abnormal fracture occurred in the NZ due to the presence of weak-bonding areas and curved band structures.

Keywords

• Aluminum-lithium alloys
• SSFSW
• Microstructure characterization
• Mechanical properties