Microstructure evolution of 7050 Al alloy fasteners during cold upsetting after equal channel angular pressing

The innovative combination of equal channel angular pressing (ECAP) and cold upsetting (CU) is significant for producing high-strength Al alloy fasteners with ultrafine grains. However, the relationship between microstructure evolution and mechanical strength during composite deformation remains unclear. In this study, using transmission electron microscopy, electron backscatter diffraction, and mechanical property tests, it is found that the mechanical strength increases with each step of composite forming operations due to the accumulation of dislocations. Forest dislocations also contribute to the generation of ultrafine grains within deformed grains, particularly in ECAP-CU processed grains. Moreover, we observe the formation of lamellar structures within the shear bands in the head of the ultrafine grain fastener after final composite deformation, which is full of dense dislocation walls and dislocation cells. The rearrangement of these fine grains and lamellar structures yield a strong (011) <211> (brass) texture during composite deformation under the effect of shear force and accumulation of plastic strain. This study provides a theoretical reference for the manufacture of high-strength aluminum alloy fasteners through composite deformation and helps improve processing technology.