Characteristics and joint microstructure-property analysis of bobbin tool friction stir welding of 2A14-T6 aluminum alloy

A 2A14-T6 aluminum alloy was successfully bobbin tool friction stir welded. The defect-free sound joint with good surface and inner formation was produced. It was found that a converging zone of material plastic flow, composed of upper- and lower-shoulder-driven flows as well as pin-driven flow, was formed at the lower part of weld nugget zone. Microstructural analysis indicated that the grain size of the upper part was smaller than that of the middle and lower parts in the weld nugget zone. A softening region, whose width was approximately twice that of the shoulder diameter, was formed in the joint due to the coarsening and dissolution of the block-shaped second phases. The hardness distributions of the three layers of the joint were nearly the same, indicating that the heterogeneity of joint properties commonly occurred in conventional friction stir welding was not present in bobbin tool friction stir welding. The tensile test results showed that the strength efficiency of the bobbin tool friction stir welded joint can reach 71%. The joint was fractured through the converging zone of material flow in the weld nugget zone.