Improving the hydro-bulging formability of anisotropic aluminum alloy tubes by the flow forming process

To address the problem that aluminum alloy tubes are prone to cracking during hydroforming due to inferior circumferential performance and significant anisotropy, 5A02 aluminum alloy tubes with high hydro-bulging formability were fabricated by flow forming and rapid annealing. EBSD, axial tensile, segment-type ring expansion, and hydro-bulging tests were conducted to investigate the anisotropy and hydro-bulging performance of tubes. The results show that tubes with thinning ratios of 20% and 50% exhibit a grain size gradient along the thickness direction. The anisotropy of gradient-structured tubes is significantly reduced due to the dispersed and weakened textures. The outer layer features predominantly recrystallization textures, whereas the tube with a 20% thickness reduction ratio exhibits a Goss texture in the middle layer and a P texture in the inner layer. The r_z and r_θ values for tubes with a 20% thickness reduction ratio in 8 passes are 0.79 and 0.86, respectively, representing improvements of 32% and 28% over the as-received tube blank. Furthermore, the uniform hydro-bulging ratios of gradient-structured tubes range from 15.81% to 18.08%, corresponding to 66 ~ 88% improvements relative to the as-received tube blanks, with their hydro-bulging formability being superior to that of fine-grained tubes. This work provides a practical technical approach for hydroforming aluminum alloy tubes with a large bulging ratio.