A Comparative Study of Local Heat Treatment for Enhancing Overall Mechanical Properties of Clinched Joints

Clinching technology is widely used for joining lightweight materials. It is well known that the fatigue life of clinched joints is outstanding, but its static strength is inferior to other conventional joints such as resistance spot welding joints. In this study, a post-processing method was proposed to enhance the overall mechanical performances of clinched joints in material of steel plate cold commercial steel. Based on the failure mechanism analysis, we performed quenching, a local heat treatment process, on the joining zone to improve the static strength of clinched joints. The static strength and fatigue life of clinched joints before and after quenching were compared via experimental study. The experimental results showed that the tensile–shearing strength of clinched joint increased by 60.65% and the fatigue life was extended when the joint was under high fatigue load levels after local heat treatment. Besides, metallographic test and fracture analysis were conducted to analyze the mechanism of the change of mechanical behaviors. Lath martensite was observed in the cross section of clinched joint after local heat treatment, which is the main reason for the tensile–shearing strength increase as martensite has higher strength than ferrite and pearlite. The fracture analysis showed that the fatigue fracture mode became transcrystalline rupture after local heat treatment when the clinched joint was under low fatigue load levels, which lead to the fatigue behavior of clinched joints weakened. Nonetheless, the overall mechanical properties of clinched joints were improved.