Cryogenic Failure Behaviors of Al–Mg–Si Alloy Tubes in Bulging Process

Cryogenic medium pressure forming has been developed to form the complex-shaped tubular components, in which the needed shape and tube diameter directly determine the complex evolution of biaxial stress in bulging process. The superposition of biaxial stress and cryogenic temperature complicates the deformation behaviors, especially for the final fracture and bulging limit, which determine the forming quality of components. Therefore, the effects of tube geometry on failure orientation and fracture strain of Al–Mg–Si alloy tubes under cryogenic biaxial stress were elucidated, by utilizing cryogenic free bulging with different length–diameter ratios. The failure orientations and corresponding damage modes under different bulging geometric conditions were revealed. The influence mechanism of tube geometry and temperature on the failure mode was analyzed theoretically. A fracture model was established to predict the fracture strain in cryogenic bulging. The failure mode changes from circumferential cracking to axial cracking with the decreasing length–diameter ratio, owing to the stress sequence reversal induced by the significant nonlinearity of the stress path under a small length–diameter ratio. The failure mode can inverse under a larger length–diameter ratio of 1.0 at -196 °C because of the enhanced nonlinearity, which is promoted by the improved plasticity at cryogenic temperature. The established model based on the more accurate assessment of hardening ability during deformation can accurately predict the fracture strain with an average deviation of 10.6% at different temperatures. The study can guide deformation analysis and failure prediction in the cryogenic forming of aluminum alloy tubular parts.