Liquid Metal Embrittlement of Advanced High Strength Steels during Resistance Spot Welding
S.C. Simon (TU Delft - Mechanical Engineering)
M.J.M. Hermans – Mentor (TU Delft - Team Marcel Hermans)
G. Mahadevan – Mentor (TU Delft - Team Marcel Hermans)
Yan Ma – Graduation committee member (TU Delft - Team Maria Santofimia Navarro)
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Abstract
Twinning-induced plasticity (TWIP) steels are emerging candidates for the automotive industry, prized for their outstanding combination of strength and formability. However, their susceptibility to corrosion necessitates a protective zinc coating, which in turn initiates liquid metal embrittlement (LME) during resistance spot welding (RSW) — the industry's primary joining technique. This thesis examines the effect of hold time, a welding parameter, on the presence of liquid metal embrittlement in TWIP steels during resistance spot welding. The effect of hold time is not yet discussed in depth in the relevant literature. Several microscopic and other evaluation methods are utilised to evaluate the LME presence for a series of samples. This series of samples, differentiated solely by their hold times, are subjected to a comprehensive set of microscopic and analytical evaluations. Optical microscopy is used to quantify and measure crack dimensions, while energy-dispersive X-ray spectroscopy (EDS) is employed to corroborate the zinc-induced nature of the cracks. Further analysis includes scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to achieve higher magnification imaging, determine average grain size, and analyse high-angle grain boundaries. With the help of these methods, it is found that hold time does significantly affect the presence of LME, mainly by altering the zinc availability and distribution due to the cooling effect of the electrodes, while concurrently altering local microstructure and stress state of the samples.