Detection Method for Liquid Metal Embrittlement Cracks Inside the Intermediate Sheet Zone of Dissimilar Resistance Spot Welds

Liquid metal embrittlement (LME) is a phenomenon where a liquid metal damages a solid bulk metal. Notwithstanding previous investigations, LME remains an actual as well as a still not fully understood topic. A so far not yet investigated area can be defined with ultra‐high strength austenitic stainless steels for passenger cars. The most commonly used joining procedure in car body engineering is still resistance spot welding. During this vehicle assembly step, the uncoated surface of an austenitic stainless steel is because of the process‐related lap joint configuration in direct contact with zinc‐coated surface‐finished steels as a dissimilar material combination. During welding, liquid zinc could penetrate inside the intermediate sheet zone in the grain boundaries of the austenitic steel and is therefore able to initiate cracks in the heat‐affected welding zone. Herein, the radiographic inspection is introduced, which is a process‐reliable, nondestructive detection method for the inaccessible intermediate sheet zone area, which is usable in automotive big‐industrial scale. With the implemented detection method, liquid metal‐induced cracks can be detected in the intermediate sheet zone down to a crack length of 50 μm. Subsequently, the radiographic inspection helps to analyze the crack characteristic depending on surrounding conditions and welding parameters. The radiographic inspection is applied as a testing method to detect cracks because of liquid metal embrittlement in the inaccessible heat‐affected intermediate sheet zone of dissimilar resistance spot welds down to a crack length of 50 μm. The radiographic inspection leads to a understanding of the crack characteristic, following in the hypothesis of a crack‐enabling “hem” surrounding the spot welds.