Structure–properties relations in spot friction welded (also known as friction stir spot welded) 6111 aluminum

Abstract

This work details some aspects of the microstructure–properties–processing relations in spot friction welded (also known as friction stir spot welded) 6111 Al sheets, joined in a lap configuration. We have shown that the tool pin penetration depth has a strong effect on the failure mode of the joined samples and a lesser effect on the joint shear strength. With increasing tool pin penetration depth, and consequently with increasing depth of the tool shoulder pressing into the top sample, the failure mode in a lap-shear test changes from brittle and concentrated near the pin hole, to ductile and away from the weld towards the base metal. The sheet interface under the tool shoulder consists four regions; a region where there is no contact at all between the two surfaces, a region where only a mechanical bond (“kissing bond”) exists, a region where there is partial metallurgical bond, and a region with full metallurgical bonding. There is evidence that during welding the Fe–Si–Mn–Cu inclusions present in the as-received material are swept towards the joint interface, thus degrading the joint quality. The aluminum in the partially metallurgically bonded region, in the fully metallurgically bonded region and under the tool pin is fully recrystallized. In these regions, the presence of low angle grain boundaries indicates that additional deformation has occurred after recrystallized grains were formed. It is thus likely that recrystallization has occurred dynamically during the welding process. The material under the shoulder (a portion of the non-contacting region, the “kissing bond”, the partial metallurgical and the full metallurgical regions) has a significantly larger grain size than under the pin, as well as a different texture.