The Influence of Electrode Force on Liquid Metal Embrittlement of Third Generation Advanced High Strength Steels during Resistance Spot Welding

Abstract

Zinc coatings are generally applied on the surface of advanced high-strength steels (AHSS) for corrosion resistance. However, the elevated temperature during resistance spot welding process melts zinc coating and the intact contact of liquid zinc on the underlying steel substrate with the presence of tensile stress may induce liquid metal embrittlement (LME) cracking, potentially degrade the weld strength. It is evident that the alteration of welding parameters, one of which being electrode force, affects the LME response of steels. However, the mechanisms of how electrode force influences LME cracking is not fully understood. By assessing how electrode force variation affects LME cracking severity for welds in two newly designed 3G-AHSS, with low-LME and high-LME sensitivity, this work illustrated that electrode force had a two-fold effect on LME development. When welds did not exhibit expulsion, the increase of electrode force promoted heat extraction from the joint, which in-turn reduced LME cracking severity. However, when welds experienced expulsion, the increase of electrode force, accentuated electrode collapse, thus increased rapid cooling on the weld shoulder with its associated thermal stresses, increased LME cracking severity. The welding work was performed on three heat input levels to relate the electrode force and the LME cracking severity experimentally. At each heat input level, the welding current was adjusted to compensate the influence of electrode force variation on the nugget formation and growth. Therefore, comparable nugget diameter (a measure of weld strength) was achieved regardless of various electrode force at each heat input level. The LME cracking severity of the welds made in low-LME and high-LME sensitive materials was quantified by the number of cracks, the absolute maximum crack length and the potential weld strength loss using crack index. The LME cracks were classified according to their locations on the weld where Type A cracks located at the center of the weld surface; Type B cracks located at the weld shoulder extending to the edge of the heat affected zone; Type C cracks located at the weld notch. For both low and high LME sensitive materials, Type B cracks were observed at all levels of heat input while Type C cracks were not, and Type A cracks were only observed at moderate and high heat inputs. Welds made in the low-LME sensitive material displayed an overall less susceptibility to LME cracking (crack index below 0.15). For the high-LME sensitive material, at low heat input, the LME severity decreased from a total crack index of 0.14 to 0 when the electrode force was increased from 4.4 kN to 5.4 kN. At moderate heat input, the LME severity decreased from a total crack index of 0.67 to 0.11 when the electrode force was increased from 4.4 kN to 4.9 kN, then with a further increase of electrode force to 5.4 kN, the Total crack index increased to 0.37. At high heat input, the increase of electrode force from 4.4 kN to 5.4 kN resulted in the increase of LME cracking severity on Total crack index from 0.36 to 0.77. The observation that the increase of electrode force could result in both increased and decreased LME severity, depending on situation, contrasted with the published literature where it was seen that LME cracking severity universally decreased with increasing electrode force. To understand the mechanisms resulting in different LME response seen in welds made in the two investigated materials and why the present results diverged from the literature, experimental analysis was first conducted on results from the tests on the low-LME sensitive material. From this analysis, it was seen that increasing electrode force promoted heat loss and the expulsion event increased with the increase of applied welding heat. However, there was not a linear relationship observed between the change of heat input, heat loss, expulsion condition and the LME cracking severity that related to variations in electrode force for welds made in the low-LME sensitive material. With the assistance of ANOVA analysis and linear regression modelling, it was seen that whether the weld exhibited expulsion had a significant effect on its LME behaviour. The results indicated that analysing both LME-free (20 out of 45 welds made) and LME-containing welds in a single dataset led to inaccurate prediction of LME crack length when LME cracking was modeled as a function of heat input, heat loss, and expulsion count. The majority of LME-free cracks were found at low heat input and the probability of these welds experiencing expulsion was low. In welds made in the low-LME sensitive material that contained LME, the LME cracking severity of LME-cracking welds was significantly influenced by the heat input, heat loss and the occurrence of expulsion from the statistical analysis. Conversely to the LME-free welds, the welds exhibiting LME cracking predominately experienced expulsion during welding. Welds made in the high-LME sensitive material exhibited much more LME cracking than seen in the low-LME sensitive material. From all on the welds made, there was only 5 LME-free welds out of 45 welds produced. The tested factors (i.e. heat input, heat loss and expulsion condition), which had a significant impact on LME cracking behavior on low-LME sensitive material, were then experimentally correlated with the LME cracking severity for the high-LME sensitive material. The results indicated that the influence of electrode force on LME cracking depended on whether or not welds experienced expulsion. When welding with low heat input, without expulsion, LME cracking severity decreased as electrode force increased. In such cases, the increase of electrode force aided with heat extraction during welding, relieved the critical stresses required by LME cracking. In contrast, when welding with high heat input, resulting in expulsion, the increase of electrode force elevated LME cracking. It was shown that high electrode force increased the sudden indentation of the electrode into the steel substrate (electrode collapse), leading to rapid cooling of the weld shoulder. The rapid cooling increased the thermal stresses associated with the electrode collapse event, promoting LME. The results from this study show that expulsion itself (excluding its association with increased heat input) is a factor contributing to LME cracking, which highlights the importance of considering the expulsion phenomenon in designing LME resistant welding schedules.