Liquid Metal Embrittlement Cracking in Uncoated Transformation-Induced Plasticity Steel during Consecutive Resistance Spot Welding

Abstract

In the automotive production line, a single pair of electrodes is employed to produce hundreds of consecutive welds before undergoing dressing or replacement. In consecutive resistance spot welding (RSW) involving Zn-coated steels, the electrodes undergo metallurgical degradation, characterized by Cu-Zn alloying, which impacts the susceptibility to liquid metal embrittlement (LME) cracking. In the present investigation, the possibility of LME crack formation in uncoated TRIP steel joints during consecutive RSW (involving 400 welds in galvannealed and uncoated TRIP steels) was investigated. The results have shown that different Cu-Zn phases were formed on the electrode surface because of its contamination with Zn from the galvannealed coating. Therefore, during the welding of the uncoated TRIP steel, the heat generated at the electrode/sheet interface would result in the melting of the Cu-Zn phases, thereby exposing the uncoated steel surface to molten Zn and Cu, leading to LME cracking. The cracks exhibited a maximum length of approximately 30 µm at Location A (weld center) and 50 µm at Location B (shoulder of the weld). The occurrence and characteristics of the cracks differed depending on the location as the number of welds increased due to the variation in Zn content. Type A cracks did not form when the number of welds was less than 280. Several cracks with a total length of approximately 30 μm were suddenly formed between 280 and 400 welds. On the other hand, type B cracks began to appear after 40 welds. However, the number and size of these exhibited inconsistency as the number of welds increased. Overall, the results have shown that small LME cracks can form even in uncoated steels during consecutive welding of Zn-coated and uncoated steel joints.