Elimination of root kissing bond in friction stir welding of thick plates

Root kissing bond in thick-plate friction stir welded joints limits the application of plates in fuel tanks of heavy launch vehicles. Here a root-enhanced friction stir welding from aspects of the on-demand distribution of heat input and plastic deformation through an enlarged-tip pin was proposed. The pin was designed with an enlarged tip for enhanced plasticizing and forging on root materials. Root tolerance, defined by the distance between the pin tip and the lower surface of the plate without a root kissing bond, was proposed to evaluate the feasibility of the designed pin. This value increased from 0.1 mm to 0.6 mm compared to the conventional conical pin. The ultimate tensile strength and elongation reached 345 ± 4 MPa and 11.8 ± 0.7 %, which were 11.3 % and 87.3 % higher than the joint by the conical pin.