Wrinkling mechanism and elimination strategy during necking and thickening hot extrusion of Al alloy tube

Surface wrinkle defects have been restricting the high-quality forming of hot extrusion for variable cross-section and unequal thickness hollow structures (VCUTHS). To reveal the formation mechanism and suppression strategy of inner surface wrinkles during the hot extrusion necking-thickening process of aluminum alloy tubes, this work integrated numerical simulations with extrusion experiments. The evolution of inner surface wrinkles during the hot extrusion of an Al–Cu–Mg alloy tie rod (initial diameter: Ø65 mm, wall thickness: 5 mm) was analyzed, and the influence of grain size on wrinkle formation was investigated. Results revealed that the necked and thickened zones underwent triaxial compressive stress, approximating biaxial equal compression, which tended to induce radial wrinkles. Minor wrinkles initiated during necking and worsened in the thickening stage. Coarser grains caused more prominent radial wrinkles, while finer grains led to finer, milder wrinkling with an orange-peel morphology. For the cost-effective, high-quality hot extrusion of tie rods, the adoption of fine-grained tubes is recommended to improve surface quality.