Kim, JiUng2026-03-022026-03-022026-03-022026-02-26https://hdl.handle.net/10012/22958Resistance spot welding (RSW) is a predominant joining technique for sheet metals in the automotive sector due to its efficiency and rapid processing capabilities. However, when applying RSW to zinc-coated advanced-high-strength steel (AHSS), surface cracking near the weldment is frequently observed. These cracks are primarily attributed to zinc infiltration along the grain boundaries of the substrate, which significantly decreases ductility. This type of cracking is classified as liquid metal embrittlement (LME) cracking. The presence of such cracks undermines weld integrity, prompting ongoing research efforts to develop effective prevention methods. Most studies exploring the underlying mechanism of LME, such as critical stress levels, LME-sensitivity temperature range, and zinc transportation method, used physical or numerical simulation to recreate conduction during welding using high-temperature tensile testing (HTTT) or finite element method (FEM) simulation that do not necessarily completely replicate the welding environment. Using research methods to simulate welding process is preferred because in-situ monitoring of LME cracking during the RSW process is challenging due to the cracking location in welding being hidden from the exterior. LME cracks appear as circumferential surface cracks with narrow widths that extend internally within the steel sheet. Although the half-sectioned RSW (H-RSW) process has been introduced for real-time monitoring of LME crack analysis, no substantial findings have been reported concerning quantitative crack behavior or the underlying mechanisms. One of the limitations of the alternative method is its inability to determine the mechanism of zinc transport into grain boundaries. The literature presents conflicting views on whether liquid zinc first infiltrates the grain boundaries or whether the crack initially forms due to weakened grain boundaries caused by stress-assisted zinc diffusion, followed by zinc infill. Additionally, recent FEM-based studies have identified thermal stress induced by thermal shock, as a consequence of an instantaneous temperature drop (ITD) due to mechanical collapse between electrodes and the steel sheet surface, as a key factor contributing to LME crack formation. However, it remains unclear whether the findings attained from the HTTT are applicable or relevant to the RSW process, as the stresses and constraints applied to the joint differ from those in the HTTT. Furthermore, the effect of thermal stress induced by ITD on LME crack initiation must be validated through experimental observation from an actual RSW process. The first stage of this research program consisted of parameter optimization for the H-RSW process of third-generation AHSS (3G-AHSS) through the development of a process map. Thermal cycles of the H-RSW and RSW processes were compared by examining nugget size and indentation depth. It was determined that the H-RSW process enables the identification of LME cracking, which is linked to the temperature gradient in the weld shoulder area. An issue was identified where the emissivity value fluctuates within the LME-sensitive temperature range from 700 °C to 900 °C, leading to inaccurate temperature measurements. To address this, a temperature correction model for an infrared (IR) camera was developed, resulting in an 85% improvement in measurement accuracy over non-calibrated data, particularly within the LME-sensitive temperature range. The optimized H-RSW setup enables a comprehensive understanding of the kinetics involved in LME cracking behavior. It has been observed that the crack growth rate diminishes as the crack length increases. Additionally, the cracking process can be divided into two stages based on the crack growth rate: an initiation stage characterized by rapid crack propagation, followed by a propagation stage with slower crack growth. In-situ monitoring analysis revealed the timing of crack propagation and zinc infiltration, showing that cracking preceded liquid zinc infiltration, providing experimental evidence that stress-assisted zinc diffusion facilitates LME crack formation. The impact of thermal stress induced by ITD on LME cracking behavior has been validated, and the comprehensive underlying mechanism of LME cracking has been identified. During the initiation stage, thermal stress is the primary factor influencing crack formation, with higher thermal stress resulting in longer cracks. In the propagation stage, LME crack growth rate is affected by four factors: the momentum of the crack aiding its propagation, thermal stress at the crack tip acting as a driving force, damping effects that oppose crack growth, and the reduction of thermal stress due to temperature increases at the crack tip during extension, which causes a ferrite to austenite phase transformation. A damage map has been developed to predict the initiation of LME cracks at sensitive temperatures and thermal stress levels. Additionally, a crack and crack growth rate prediction model has been established to elucidate the influencing factors during the welding process. These findings can be effectively utilized to develop in-process mitigation strategies and to manufacture LME-resistance AHSS.enliquid metal embrittlementresistance spot weldingadvanced high-strength steelin-situ monitoringcrack propagationcrack initiationDoctoral Thesis