Liquid Metal Embrittlement Cracking During Resistance Spot Welding of Galvanized Q&P980 Steel
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CURRENTLY, automobile manufacturers are working to reduce the weight of cars to meet increasingly stringent environmental legislation because the lighter the car body is, the lower the fuel consumption and gas emissions.[1] The need for lighter car bodies
ZHANXIANG LING and TING CHEN are with the Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai, 200240, China and also with the Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China. LIANG KONG and MIN WANG are with the Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University and also with the Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration and also with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Room 407, F building, No. 800 Dongchuan Road, Minhang District, Shanghai 200240, China. Contact e-mails: [email protected] and [email protected] HUA PAN and MING LEI are with the Baosteel Central Research Institute, Baosteel Group Corporation, Shanghai, 201900, China. Manuscript submitted October 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
promotes the replacement of traditional carbon steels by ultrahigh-strength steels (UHSSs) in the car bodies due to their superior strength-to-weight ratios.[2] Resistance spot welding (RSW) has been the dominant method used to join car bodies for decades. Thus, the resistance spot weldability of UHSSs is especially important. To provide corrosion resistance, automotive steels are always coated with zinc. However, cracks may occur on the surface of the spot welds of galvanized UHSSs, which have been identified as liquid metal embrittlement (LME)-induced cracks because of their absence in uncoated steels.[3] LME is the loss of ductility of a normally ductile metal when it is under tensile stress and in contact with a liquid metal.[4] LME occurs in specific solid metal-liquid metal couples,[5] and hot tensile testing on galvanized steels confirmed that liquid zinc could embrittle the steel.[6–9] LME of galvanized steels was found during some hot working processes, such as hot stamping.[10–12] During the RSW of galvanized steels, the zinc coating layer melts due to its low melting point, and the complex stress situation in the spot weld resulting from the mechanical stress, thermal stress, and transformation
stress makes the existence of a local tensile stress on the surface of the spot weld highly probable. Thus, the prerequisite for LME can be met, and LMEinduced cracks occur. Although LME cracking during other welding processes were also found, such as arc welding[13] and laser welding,[14] the occurrence of these cracks requires additional stress, which can be avoided in industry. Thus, LME cracks in resistance spot-welded joints are a major concern and have been studied by many researchers. Tolf et al.[15] observed surface cracks in resistance spot-welded joints of galvanized DP1000 steels and f
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