Strength and ductility evaluation of cold-welded seams in aluminum tubes extruded through porthole dies

The mechanical behavior (strength and ductility) of the cold-welded seams that are inevitable in hollow profiles and tubes extruded through porthole dies are investigated with the aid of the Ring Hoop Tension Test (RHTT) and Digital Image Correlation (DIC). In the RHTT, a dogbone-shaped gage-section machined on a ring extracted from the parent tube is placed over two rigid, D-shaped mandrels and it is stretched as the mandrels are parted in a testing machine. RHTT experiments were performed on specimens extracted from Al-6061-T4 extruded tubes, with the cold-welded seams placed at the center of the gage-section. Using DIC to probe the full strain fields and assess their evolution, the local apparent stress-strain responses along the specimen's gage-section were determined. It was found that the cold-welded regions are somewhat stiffer than the remainder of the specimen, the latter approaching the apparent response of the base material. Furthermore, each of the three cold-welded seams in the tubes had a different apparent stress-strain response. In every case failure occurred away from the cold-welded seam. The work described here allows the direct probing of the mechanical behavior of a cold-welded seam in its as-received state.