Effect of a variable electrode force on the LME crack formation during resistance spot welding of 3G AHSS

Abstract

In the pursuit of lightweight vehicles, third-generation advanced high-strength steels (3G AHSS) with increased mechanical properties are desired to be used for critical components. However, the exposure of these zinc-coated AHSS to the manufacturing conditions during resistance spot welding can trigger liquid metal embrittlement (LME), possibly compromising the mechanical properties. As the reproducibility of LME cracks in resistance spot welding is a challenge, the effect on the static and dynamic mechanical properties of the welds is not yet fully clarified and therefore a distinction between critical and non-critical cracks is not implemented in current standards. To achieve this, it is necessary to provoke LME cracks of a given size, for example by increasing the welding current, reducing the electrode force and hold time, or using manufacturing discontinuities. Due to its significant effect on the heat input and the tensile stresses during the resistance spot welding process, which impacts the LME crack propagation, the focus of this paper is on the electrode force. An expulsion-free decreasing force profile, which consists of a force run-in, force decrease, and force run-out time, has been derived in a two-stage Face-Centered-Central-Composite design of experiment for an electrogalvanized third-generation advanced high-strength steel (3G AHSS) DP1200 HD. The crack location, length, depth, and nugget geometries were investigated for each weld. With the decreasing force profile, it was possible to generate type A, B, and C cracks by parameter adaption, with type B and C cracks being the most dominant. The type C crack formation was investigated by aborting the welding process in defined time steps and the LME cracking mechanism was confirmed by welding dezincified samples. Based on the investigations carried out, the force profile was found suitable for generating different LME crack sizes to further investigate the mechanical joint properties as it was able to reproducibly generate defined cracks without expulsion and excessive electrode indentation while maintaining a minimum nugget diameter.