Investigation of interface evolution, microstructure and mechanical properties of solid-state bonding seams in hot extrusion process of aluminum alloy profiles

Solid-state bonding is an important problem in hot extrusion process of aluminum alloy profiles, especially for hollow section profiles. Predicting and decreasing the length of transverse weld seam (T-seam), improving the solid-state bonding degree and controlling microstructure of longitudinal weld seam (L-seam) are major concerns. In this work, a set of porthole extrusion dies with different shapes of legs (pointed and square) and depths of welding chambers were designed and manufactured. A series of extrusion experiments for different die structure parameters were performed, and microstructure observations, tensile tests and fracture feature analyses for the extruded profiles were conducted. The influences of the shapes of legs and the depths of welding chambers on the evolution and the length of T-seams and on the formation, microstructure and mechanical properties of L-seams were respectively investigated by means of experiment and numerical simulation. The distribution law of the ratio of pressure to material flow stress, p/σ, and the material flow behavior in welding plane were studied by using numerical simulation method. The results showed that the profiles extruded by the dies with pointed legs have single-convex T-seams and straight L-seams on their cross sections, and their T-seams are relatively shorter but their ductility is relatively inferior. While, the profiles extruded by the dies with square legs have double-convex T-seams and cross-shaped L-seams, their ductility is relatively superior but their T-seams are longer. With the increase of the depth of welding chamber, the ductility of the extruded profiles is increased, but the lengths of T-seams are almost invariable. In addition, it was found from numerical simulation results that, in the welding plane, the deforming material with a higher flow velocity has a lower value of p/σ, and vice versa. It was concluded that an effective welding zone should exist in the welding plane, and the deforming material only in this effective welding zone can flow into the extrusion die bearing to form an L-seam in extruded profile.