Mechanical Properties and Microstructure Evolution of Aluminum Alloy Tubes with Normal Gradient Grains under Biaxial Stress

Grain size gradient materials are a type of new structural material with the advantages of both coarse and fine grains. To study the effect of normal gradient grain on the mechanical properties and microstructure of aluminum alloy tubes during hydroforming, the normal gradient grain distribution of the outer fine and inner coarse grains was obtained using spinning and annealing methods, and the biaxial stress was determined using hydraulic bulging experiments. Gradient grain tubes with thicknesses of 300, 475, and 575 μm were obtained through spinning and annealing at different temperatures; the tensile strengths of the tubes were 79, 89, and 109 MPa; the maximum expansion rates were 18%, 17%, and 10%; and the work-hardening indices were 0.19, 0.20, and 0.17, respectively, under biaxial stress. With an increase in the refined grain area, the density of the low-angular grain boundaries (LAGBs) increased and the chance of stitching dislocation increased in the process of intragranular deformation, causing an increase in strength. However, the increase in the refined thickness weakened the compatible deformation due to a reduction in the number of large grains, leading to a decrease in plasticity. The obtained quantitative relationship between gradient grain and strength/ductility can be applied to guide production of aluminum alloy tubular parts in the aerospace and automotive industries.