Experimental and Numerical Analysis of Liquid Metal Embrittlement Crack Location
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JMEPEG https://doi.org/10.1007/s11665-019-04005-2
Experimental and Numerical Analysis of Liquid Metal Embrittlement Crack Location C. DiGiovanni, X. Han, A. Powell, E. Biro, and N.Y. Zhou (Submitted September 19, 2018; in revised form January 16, 2019) Advanced high-strength steels used in automotive structural components are commonly protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement during welding. Although liquid metal embrittlement cracks are known to form, the current literature does not include crack location when assessing crack severity. In this work, TRIP1100 joints showed that LME cracks decreased strength from 7.8 to 42.2%, depending on location, between the coated (cracked) and uncoated (non-cracked) condition. Liquid metal embrittlement cracks in critical locations were observed to propagate until fracture from lap shear testing. However, cracks in non-critically loaded areas were not part of the fracture path and did not result in a significant loss in strength. This shows LME crack location can be controlled to improve joint performance and vehicle safety. In addition, a model of lap shear testing in a cracked sample showed how the presence of a crack can affect the internal stress field depending on its location. Keywords
advanced high-strength steel, cracking, finite element analysis, lap shear test, liquid metal embrittlement, resistance spot welding, weld strength
1. Introduction Automotive manufacturers are required to improve vehicle fuel consumption and efficiency due to rising regulations related to the environmental concerns surrounding carbon emissions and fossil fuel usage (Ref 1-4). The industry is moving toward the reduction in vehicle weight to increase fuel efficiency; however, the structures and materials must retain their mechanical performance. This has led to the development of advanced high-strength steels (AHSS) which have high strength and ductility, allowing parts to be made from thinner sheets, and have been increasingly implemented in automotive structures (Ref 5-7). Furthermore, a zinc coating is used to protect AHSS from the corrosive environment. A thin layer of zinc provides cathodic protection to the underlying AHSS substrate. The most common joining process associated with automotive assembly is resistance spot welding (RSW). RSW functions by passing a large current through a lap joint to generate heat. Heat is concentrated at the sheet-to-sheet interface, due to its high contact resistance, and a volume of metal is melted and solidified to form a weld nugget (Ref 8). However, the addition of the zinc coating adds complexities to the welding process, such as liquid metal embrittlement (LME) (Ref 9-11). Zinc melts at 419 C, far below the melting point of the steel substrate ( 1500 C). When the Zn melts on the solid steel C. DiGiovanni, X. Han, A. Powell, E. Biro, and N.Y. Zhou, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Canada. Contact e-mail: ctdigiov@uwaterloo.
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