Significance of cutting plane in liquid metal embrittlement severity quantification

Abstract

The automotive industry is turning to advanced high strength steels (AHSS) to reduce vehicle weight and increase fuel efficiency. However, the zinc coating on AHSS can cause liquid metal embrittlement (LME) cracking during resistance spot welding. To understand the problem, the severity of the cracking must be measured. Typically, this is done from the weld cross-section. Currently, there is no standard procedure to determine which plane through the weld must be examined to gauge cracking severity, leading to a variety of practices for choosing a cutting plane. This work compares the magnitude and variability of LME severity measured from the plane of exhibiting the most severe surface cracking to arbitrarily chosen planes. The plane exhibiting the most severe cracks had more and longer cracks on the cross-section than the arbitrarily chosen plane, resulting in a higher crack severity measurement. This higher absolute measurement increased the relative accuracy of the examination, allowing for fewer welds to be examined to precisely determine the effect of LME mitigation methods on cracking severity, how welding parameters affect LME cracking severity and the predicted LME affected strength of a particular weld.

Using short hold times to mitigate LME cracking in electrode indent of resistant spot welded third generation advanced high strength steel

“…To observe cracks, cross sections through to the diameter of the weld with largest visible surface cracks were obtained according to the best practice described in [30]. The cross sections were mounted and prepared using standard metallography techniques up to a 1-µm diamond nish.…”

“…Only cracks longer that 10 µm were taken in consideration. Five replicate welds were used to measure cracking severity, as previous work showed that this was su cient characterize LME severity with low variability [30] and showed a good correlation between cracking severity and decrease in tensile…”

3D characterization of the propagation of liquid metal embrittlement inner cracks during tensile shear testing of resistance spot welds

“…Nowadays, LME cracks characterization is often made by (destructive) metallographic cross sections in the welds (Benlatreche et al [19]), providing only a partial view of the 3D crack morphology. Because the crack depth is usually not uniform through the weld, the observed severity of the crack depends on the selected cutting plane (DiGiovanni et al [20]). Improper choice of the plane of observation might lead to a misunderstanding of LME severity.…”