Severely weakened extrusion strengthening effect in porthole die extrusion of Al-Mg-Si alloy miniature complex hollow profile under an ultra-large extrusion ratio

Ultra-large extrusion ratio is hardly inevitable during porthole die extrusion of miniature complex hollow profile on industrial horizontal extruders. Although batch extrusion of miniature complex hollow profile has been achieved at ultra-large extrusion ratio, their microstructure and mechanical properties are still unclear. To this end, under the extrusion temperature ranging from 400 ℃ to 480 ℃, a 4×4 mm Al-Mg-Si alloy micro heat pipe profile was extruded at an ultra-large extrusion ratio of 137. The grain size and crystal orientation of the profile were obtained by metallographic and electron backscattered diffraction (EBSD) techniques, and further exploration of the microhardness, tensile properties and pressure-resisting performance were conducted. Interestingly, severely weakened extrusion strengthening effect and low sensitivity of microstructure and mechanical properties to extrusion temperature were observed. Compared to the as-cast extrusion billet, the profile exhibits slight grain refinement and improvement in mechanical performance, and the extrusion temperature does not significantly affect the microstructure and mechanical properties. Instead, as the extrusion temperature increases, the recrystallization fraction generally decreases. These abnormal phenomena are inferred from the fact that even at lower extrusion temperatures, an ultra-large extrusion ratio leads to premature dynamic recrystallization (DRX), and more abnormal growth grains are formed in the profile, weakening the mechanical properties. The higher the extrusion temperature, the earlier DRX occurs, generally forming more abnormal growth and dynamic recovery grains.