Novel technique of friction extrusion self-refilling for repairing keyhole of flat clinched joint

Flat clinching (FC) technique has been extensively employed in industry for joining lightweight materials. However, its mechanical properties and industrial application are severely restricted by the keyhole defect. In this study, a novel friction extrusion self-refilling (FES) method is proposed to repair the keyhole of FCed joint and improve its mechanical properties. The innovation of this method is the friction extrusion self-refilling of the keyhole and the metallurgical bond strengthening of mechanical properties. The microstructure, mechanical properties and fracture failure behavior of the FCed and FESed joints are comparatively investigated to verify the feasibility and effectiveness of this method. Results reveal that the keyhole-free joint is successfully obtained without additional material. The smooth and flat weld surface of the FESed joint is conducive to expanding its industrial application. The shear tension strength and failure energy of the FESed joints are improved with the increase of refilling parameters. It is important to note that excessive welding heat input will result in severe grain growth and a decrease in mechanical strength. The FESed joint shows 363.37% higher maximum shear tension strength and 1385.48% higher maximum failure energy than the initial FCed joint. The strengthening of mechanical properties benefits from metallurgical bonding and refilling material. In addition, the FESed joints exhibit three fracture modes, which also demonstrate that they are much stronger than the initial FCed joints. Therefore, this method is proved to be feasible and effective for repairing the keyhole of the FCed joint.