In the automotive industry, corrosion protected galvanized advanced high strength steels with high ductility (AHSS-HD) gain importance due to their good formability and their lightweight potential. Unfortunately, under specific thermomechanical loading conditions such as during resistance spot welding galvanized, AHSS-HD sheets tend to show liquid metal embrittlement (LME). LME is an intergranular decohesion phenomenon leading to a drastic loss of ductility of up to 95%. The occurrence of LME for a given galvanized material mainly depends on thermal and mechanical loading. These influences are investigated for a dual phase steel with an ultimate tensile strength of 1200 MPa, a fracture strain of 14% and high ductility (DP1200HD) by means of systematic isothermal hot tensile testing on a Gleeble^® 3800 thermomechanical simulator. Based on the experimental findings, a machine learning procedure using symbolic regression is applied to calibrate an LME damage model that accounts for the governing quantities of temperature, plastic strain and strain rate. The finite element (FE) implementation of the damage model is validated based on the local damage distribution in the hot tensile tested samples and in an exemplary 2-sheet resistance spot weld. The developed LME damage model predicts the local position and the local intensity of liquid metal induced cracking in both cases very well.

In the present work, the influence of the cooling time on the mechanical performance, hardness, and microstructural features of a double pulse resistance spot welded medium-Mn steel are investigated. Curves of the electrical resistance throughout the welding revealed that the cooling time strongly influences the heat generation during the second pulse. A second pulse after a short cooling time re-melts the center, and heat treats the edge of the primary fusion zone. This desired in-process heat treatment leads to a modification of the cast-like martensitic structure by recrystallization illustrated by electron backscatter diffraction measurements and to a homogenization of manganese segregations, visualized by energy-dispersive X-ray spectroscopy, which results in an enhanced mechanical performance during the cross tension strength test. In contrast, during excessively long cooling times, the resistance drops to a level where the heat generation due to the second pulse is too low to sufficiently re-heat the edge of the primary FZ. As a consequence, the signs of recrystallization disappear, and the manganese segregations are still present at the edge of the fusion zone, which leads to a deterioration of the mechanical properties.

The properties of the heat-affected zone (HAZ) are reported to have a great influence on the mechanical performance of resistance spot welded advanced high strength steels. Therefore, in the present work, the HAZ of a medium-Mn steel is characterized regarding its microstructure and its mechanical properties depending on the distance to the fusion zone (FZ). In order to obtain the local mechanical properties of the HAZ, samples were heat-treated in a joule-heating thermal simulator using different peak temperatures to physically simulate the microstructure of the HAZ. By comparing the microstructure and the hardness of these heat-treated samples and the HAZ, the local peak temperatures within the HAZ could be determined. Subsequently, tensile tests were conducted, and the austenite phase fraction was measured magnetically on the physically simulated HAZ samples in order to determine the local mechanical properties of the HAZ. As verified by energy-dispersive X-ray spectroscopy, peak temperatures above 1200 °C led to a uniform distribution of manganese, resulting in a predominantly martensitic microstructure with high strength and low total elongation after quenching. Below 1100 °C, the diffusion of manganese is restricted, and considerable fractions of austenite remain stable. The austenite fraction increases almost linearly with decreasing peak temperature, which leads to an increase of the total elongation and to a slight decrease in the strength, depending on the distance to the FZ. Temperatures below 700 °C exhibit hardly any effect on the initial microstructure and mechanical properties.

Liquid metal embrittlement (LME) poses a challenge in resistance spot welding (RSW) of high-strength steels. The diffusion of zinc along the steel’s grain boundaries leads to a loss of ductility and premature material fractures. To test steel grades for LME susceptibility, RSW tests and hot tensile tests (HTT) have been developed. However, these methods sometimes fail to differentiate accurately. This study introduces a constant load test (CLT) and evaluates its feasibility as alternative test method for assessing LME susceptibility. The current work is conducted using a thermomechanical simulator, with specific settings developed for testing. Three steel grades are examined within a balanced preload range. The test speed being a dynamic parameter is measured at up to 180 mm/s during fracture. Temperature and elongation are correlated dependent on the preload, allowing better differentiation of steels compared to the standard HTT. Additionally, key values are derived to estimate LME susceptibility as relation of uncoated and Zn-coated samples.

The automotive industry’s push for lightweight, high-strength materials has led to the advancement of third-generation advanced high-strength steels (AHSS). Known for their blend of ultra-high tensile strength and ductility, these steels are ideal for structural applications. However, their adoption has faced obstacles, notably due to Zinc (Zn)-assisted liquid metal embrittlement (LME), particularly in cases where Zn-based corrosion-resistant coatings are used during resistance spot welding (RSW). This study explores testing methodologies for evaluating LME susceptibility and examines the impact of silicon (Si) concentration, specifically between 0.5 and 1.4 wt.-%, on Zn-LME susceptibility in AHSS. This research introduces two primary testing approaches: spot welding and hot tensile tests, each designed to quantify LME behavior. Both methods show that higher Si levels correlate with increased LME sensibility, evidenced by greater ductility loss and the formation of longer critical cracks during welding. Thermodynamic modeling further demonstrates that Si affects phase stability in the Fe-Zn system, broadening the stability of liquid Zn. The findings highlight Si’s significant role in Zn-LME susceptibility and underscore the importance of robust testing methods to facilitate the safer application of AHSS in automotive manufacturing.

The impact of Si on Zn-induced liquid metal embrittlement (LME) in 3 ^rd generation advanced high strength steels (AHSS) during resistance spot welding has been widely studied, but the effect of Al is rather unknown. This study investigates the substitution of Al for Si by analyzing two steels with the fixed C and Mn-contents of 0.2 and 3 wt.-% respectively. Si and Al-contents are both set to 1.4 wt.-%. To minimize microstructural effects, all steels were quenched and tempered before electro-galvanizing. The effects of Si and Al were examined using hot tensile testing (600 – 900 °C, in 50 K steps) on a Gleeble 3800, resistance spot welding with prolonged welding times, thermodynamic calculations with Thermo-calc® and dilatometry. Results indicate that the use of either Si or Al increases the LME-susceptibility but substituting Al for Si significantly reduces Zn-induced LME-cracking. In hot tensile testing, higher testing temperatures generally increase the steel’s vulnerability to LME. But comparing both alloying elements to one another, Si causes a higher LME-susceptibility.