Quality improvement of bobbin tool friction stir welds in Mg-Zn-Zr alloy by adjusting tool geometry

Abstract

The bobbin tool friction stir welding (BTFSW) technique was applied to a Mg -Zn- Zr alloy, ZK60-T5. The effects of shoulder diameter and stir pin geometry on the macrostructure, microstructure and mechanical properties of the BTFSW joints were studied. Weld quality was sensitive to the interaction of bobbin tool geometries and processing parameters. The reduction in the shoulder diameter led to less frictional contact and resistance and hence lowered the mechanical load imposed on the tool. The employment of the tapered stir pin reduced the traversing force, which displaced substantially less plasticized metal during the BTFSW than the cylindrical stir pin. The welding speed for the BTFSW was increased benefiting from these mechanical load alleviations. The visual aspects of the BTFSW joints, characterized by limited tunneling and voiding phenomena occurring were improved with the increase in welding speed. The BTFSW resulted in extensive softening in the stir zones of the joints due to dissolution of the β 2 ´precipitates. The tensile properties of the joints were inferior to those of the base metal due to the crystallographic isotropy in the stir zone. The improvements in tensile properties of the BTFSW joint were achieved through texture tailoring by adding threads on the stir pin. The complicated plasticized metal flow introduced by the threaded stir pin provided a randomized crystallographic texture in the stir zone which contributed to the preferable tensile properties.