Improved deformation uniformity of an aluminum alloy tube at cryogenic temperatures

Poor deformation uniformity limits the forming of complex aluminum alloy tubular components. In this study, the uniaxial and biaxial cryogenic deformation behaviors of an Al–Mg–Si alloy tube were investigated. A tube cryogenic gas bulging test and the corresponding device were developed to evaluate biaxial deformation characteristics from 25 °C to −196 °C. The mechanism underlying cryogenic deformation uniformity was analyzed through theoretical modeling and microstructural characterization. The results show that −170 °C to −196 °C is a promising cryogenic forming temperature range for the Al–Mg–Si tubes, in which both plasticity and deformation uniformity are substantially improved. Compared with room temperature, the elongation and expansion ratio in this temperature range increase by more than 49.6% and 40.2%, respectively. Deformation uniformity also improves markedly, whereas thinning sensitivity decreases substantially. The increased cryogenic strain-hardening exponent n is confirmed theoretically to enhance deformation uniformity under both uniaxial and biaxial stress states. A hardening effect–softening effect ratio κ is proposed to quantify uniform deformation capacity by simultaneously considering material hardening and geometric softening. Microscopically, decreasing temperature changes the slip morphology from coarse planar slip to finer, denser and multi-system slip, indicating improved slip coordination and a more homogeneous strain distribution. Hindered dislocation motion in the grain interiors, enhanced dislocation storage, and formation of more developed dislocation networks increase cryogenic strain-hardening capacity and improve deformation uniformity. This research provides insight into the uniform deformation behavior of aluminum alloy tubes at cryogenic temperatures.