Abstract Scope |
Liquid metal embrittlement (LME) cracking in zinc coated steels during resistance spot welding has received considerable attention in the last five years because finding a solution to this problem has become the key to the implementation of several recently developed advanced high strength steels (AHSS), including some Generation 3 (GEN3) steels. Studies have shown that Transformation-Induced Plasticity (TRIP) steels and Gen3 steels were found to be highly prone to LME cracking. These two steel grades possess high elongation, hence attractive for automotive use. Large amount of silicon is typically added to these grades to prevent carbide precipitation so as to stabilize austenite at room temperature, and enrich it with carbon. Cracking during resistance spot welding (RSW) has received considerable attention because RSW is the predominant mode of fabrication in body-in-white applications. Studies were undertaken to determine the causative factors that contribute to cracking so that steps could be taken to eliminate LME cracks from occurring. These studies utilized resistance spot welded samples as well as samples prepared using a GleebleŽ thermo-mechanical simulator. Variables studied included welding heat input, type of coating, coating thickness, effect of silicon content in the steel and the alignment of work pieces to the electrodes. Both optical and electron microscopy techniques were used to characterize cracks found. Electron back scattered diffraction was used to examine grain boundaries in the weld fusion zone and heat affected zone. Observed LME cracks in weld samples were classified into three types based on the location of occurrence. The results indicated that, of the three types of cracks observed, two of them were caused by the welding set up and heat input used. Presence of LME cracks in the heat affected zone at the shoulder of the electrode indentation was influenced by the silicon content in the steel. Based on these studies suggested welding practices to mitigate/eliminate LME cracking in steels are provided. Role of the nature of grain boundaries is briefly discussed.
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