Influence of Hydrogen for Crack Formation during Mechanical Clinching

Abstract

Hydrogen intrudes into the steel during pickling process which is a pre-processing before a joining process, promoting crack formation. In a mechanical clinching which is one of joining method in the automotive industry, cracks due to large strain sometimes forms. In order to guarantee reliability, it is important to clarify the influence of hydrogen on crack formation of the joint. In this study, we clarified the influence of hydrogen for the crack formation on the mechanical clinching. Hydrogen charge was carried out using an electrolytic cathode charge. After the charging, mechanical clinching was performed. Mechanical clinching was carried out with steel plate and aluminium alloy plate. To clarify the influence of hydrogen, mechanical clinching was conducted without hydrogen charring. To investigate the crack formation, the test piece was cut and the cut surface was observed. When the joint was broken during the clinching, the fracture surface was observed using an optical microscope and an electron microscope. The load-displacement diagram showed that without hydrogen charging, the compressive load increased as the displacement increased. On the other hand, the compressive load temporarily decreased with high hydrogen charging, suggesting that cracks formed at the time. The cut surface observation showed that interlock was formed in both cases with low hydrogen charging and without hydrogen charging. With low hydrogen charging, no cracks were formed in the joint. When high hydrogen charging was performed, cracks were formed at the joining point. Fracture analysis showed brittle-like fracture surface. These results indicate that hydrogen induces crack formation in the mechanical clinching.