Suppression of liquid metal embrittlement in resistance spot welding of TRIP steel

Liquid metal embrittlement (LME) in Zn-coated steels is a serious issue in automotive design. The risk of rising LME surface cracks in resistance spot welding (RSW) of Zn-coated high strength steels has triggered significant research activities across the globe. This paper presents a state-of-the-art review of the various phenomena and issues related to LME during RSW. Various aspects of LME surface cracks have been described in this review, focusing on the macro- and microscopic features of LME, spot weld cracks, the sensitivity of the LME cracks towards surface locations, welding conditions, and susceptibility to high strength and galvanized steels. We also focus on the effects of various processing factors, such as temperature, stress, microstructure, and the nature of the galvanized layer, related to studies with actual spot welds LME cracks. Finally, we summarize the possible mechanisms of embrittlement and the remedies for minimizing LME cracks, with suitable guidelines to suppress surface cracks during RSW.

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Effect of Multiple Pulse Resistance Spot Welding Schedules on Liquid Metal Embrittlement Severity

Journal of Manufacturing Science and Engineering ◽

10.1115/1.4044099 ◽

2019 ◽

Vol 141 (10) ◽

Cited By ~ 7

Author(s):

E. Wintjes ◽

C. DiGiovanni ◽

L. He ◽

S. Bag ◽

F. Goodwin ◽

...

Keyword(s):

Liquid Metal ◽

Resistance Spot Welding ◽

Spot Welding ◽

Welding Current ◽

Liquid Metal Embrittlement ◽

Additional Heat ◽

Multiple Pulse ◽

Resistance Spot ◽

Pulse Welding ◽

The Impact

Zinc-coated advanced high strength steels (AHSS) used in automotive applications are susceptible to liquid metal embrittlement (LME) during resistance spot welding (RSW). This study examines the impact of multiple pulse welding schedules on LME severity in welds of TRIP1100. Two different types of pulsing methodologies have been proposed to reduce LME severity: applying a pre-pulse before the welding current to remove the zinc coating and pulsing during the welding current to manage heat generation. However, the mechanisms by which these methods affect LME severity have not been fully explored. This work showed that a welding schedule consisting of two equal length pulses resulted in the least severe LME because it reduced the amount of free zinc available for LME without creating too much tensile stress. The majority of pre-pulse welding schedules caused an increase in LME cracking due to the additional heat introduced into the weld. However, a 4 kA (low current) pre-pulse applied for 3 cy (low time) reduced LME cracking by almost 30%. The pre-pulse allowed zinc to diffuse into the coating and stabilize the zinc, without introducing too much additional heat into the weld. These results indicate that multiple pulse welding schedules may be successfully used to reduce LME cracking, although the mechanisms by which they impact LME are more complicated than previously thought.

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