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Effect of liquid metal embrittlement cracks on mechanical behavior in advanced high strength steel
Molnar, Kayla M.
Molnar, Kayla M.
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2020
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2021-06-04
Abstract
Liquid metal embrittlement (LME) is a serious and prevalent issue during resistance spot welding with the new generation of galvanized advanced high strength steels. Extensive studies focus on the elimination of LME cracking during the welding process; however, little research has been performed on the mechanical performance of spot welds if LME is present. Additionally, mechanical performance at low temperatures is of interest due to the range of operating conditions automobiles are exposed to. An experimental matrix was established using three ranges of crack sizes, three testing configurations (cross-tension, coach peel and flange-tensile) and three temperatures (room temperature down to -40 °C). Small cracks (551 ±169 µm) were found to have no effect on the mechanical performance of spot welds compared to uncracked samples. There was no degradation in peak load, energy absorption or change in failure mode. Large cracks (1194 ±246 µm) showed significant degradation in all three of these categories, while medium cracks (910 ±202 µm) had mixed results, due to the fact that the crack length range spans between small and large cracks. In all testing configurations large LME cracks changed the failure mode compared to uncracked welds, e.g. button pull-out failure in large and some medium uncracked welds protruded into the base metal, whereas cracked weld failure was fully contained in the weld nugget, with LME present on the fracture surface. Low temperatures did not have a significant effect on the results. Cracked samples followed the same trends with decreasing temperature as their uncracked counterparts. The failure mode was independent of temperature. Finite Element modeling (FEM) was also used to examine von Mises stresses at the LME crack tip and weld edge for a 3D cross-tension test. The objective was to identify a crack length and geometry at which the crack tip stress is higher than the weld edge stress, implying the crack degrades mechanical properties and changes the failure mode. Four parametric studies were conducted for each of the crack variables: center angle, arc angle, crack angle and crack length. For cracks larger than about 1000 μm, the crack tip stress is larger than the weld edge stress. The results from these studies support findings from previous literature and the experimental testing performed in this study.
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