Towards an explanation of liquid metal embrittlement cracking in resistance spot welding of dissimilar steels

The effect of silicon content on liquid-metal-embrittlement susceptibility in resistance spot welding of galvanized dual-phase steel

Giga-grade martensitic advanced high-strength steels are prone to sub-critical heat-affected zone (SCHAZ) softening during resistance spot welding. The article aims at understanding the role of HAZ softening on the fracture mode, load-bearing capacity, and energy absorption capability of MS1400 resistance spot welds during the cross-tension test. The highest load-bearing capacity was obtained when pullout failure was initiated from the martensitic coarse-grained HAZ. However, more severe HAZ softening and formation of a wider softened zone, promoted at high heat input conditions, encourages strain localization in SCHAZ, promoting transition in failure location to sub-critical HAZ. This change in pullout failure location is responsible for the observed reduction in the weld peak load at high welding currents. Therefore, control of martensite tempering in the HAZ is critical to obtain strong and reliable resistance spot welds in martensitic advanced high-strength steel sheets. To preclude the detrimental effect of the martensite tempering on the weld strength, the minimum welding current, which enables pullout failure mode, should be used for resistance spot welding of MS1400 advanced martensitic steel.

The Role of HAZ Softening on Cross-Tension Mechanical Performance of Martensitic Advanced High Strength Steel Resistance Spot Welds

High temperature tension tests were performed to investigate the influence of variations in the starting microstructure on zinc (Zn)-assisted liquid metal embrittlement (LME) susceptibility of an advanced high strength C–Mn–Si steel. Using a single alloy, the starting microstructure was modified by heat-treatment prior to zinc electroplating. The hot tension test data revealed comparable Zn-LME susceptibility for different martensite and bainitic-ferrite based microstructures generated through full austenitization, such as as-quenched martensitic, quenched and partitioned (Q&P), and transformation-induced-plasticity (TRIP)-assisted bainitic ferrite (TBF). LME cracks propagated through these microstructures via intergranular fracture along prior austenite grain boundaries, with no apparent involvement of microstructural components like retained austenite or carbide-free bainite in the LME cracking behavior. In contrast, a dual phase (DP) microstructure variant of the same steel, generated through intercritical annealing (partial austenitization), exhibited suppressed LME, revealing that the starting microstructure can influence Zn-LME susceptibility. The lower LME susceptibility of the DP steel compared to the martensitic, Q&P, or TBF steels is explained by the presence of ultrafine ferrite grains and discrete martensite islands, and a smaller area fraction of prior austenite grain boundaries in the DP microstructure.

Influence of the starting microstructure of an advanced high strength steel on the characteristics of Zn-Assisted liquid metal embrittlement

One of the most effective ways to improve reactive wetting of dual phase (DP) steels during galvanizing process is annealing under relatively high oxygen partial pressure. This annealing process can lead to internal oxide formation rather than external ones. Since these oxides are mostly observed in grain boundaries (GBs), their presence could influence GB-related phenomena such as liquid metal embrittlement (LME) cracking during further manufacturing processes such as resistance spot welding (RSW). The present work has shown that internal oxides located at GBs assisted LME crack formation during RSW of DP steel. Two types of LME cracks were observed in the shoulder and center of the weld. It has been shown that at the initial stages of welding time, LME cracks initiate in the shoulder of the nugget by the Zn diffusion into oxide-bearing GBs. As welding time continues, higher temperatures and tensile stresses are applied to the steel sheets which allow center cracks to be formed more easily due to the presence of oxides which degrade the GBs.

The Role of Internal Oxides on the Liquid Metal Embrittlement Cracking During Resistance Spot Welding of the Dual Phase Steel

Resistance spot-welded galvanized ultrahigh-strength steels are sensitive to liquid metal embrittlement (LME), which is manifested by surface cracks on the joints. LME occurs when a solid metal contacts a liquid metal under tensile stress, and the phenomenon has not been fully understood until now, especially for resistance spot welding. In this study, the susceptibility of hot-dipped galvanized Q&P980 steel to LME cracking during resistance spot welding was systematically investigated by an orthogonal experiment. Cracks were detected by fluorescent magnetic particle testing and cross-sectional microscopic observation. Cracks were mostly located at the indentation edge and slope, and a few cracks were also located at the indentation center and slope periphery. The severity of the cracking increased with the increasing welding current and welding time, and the decreasing electrode force. The sequence of influence degree from high to low was welding current > electrode force > welding time. Holding time had no obvious effect. Microstructural analysis revealed that the content of martensite in areas with cracking increased, indicating that a high temperature was experienced at these locations. Zinc accumulated inside the cracks, and the cracks were intergranular, which coincides with the characteristics of LME. The LME cracking was provoked by the simultaneous occurrence of a tensile stress, an appropriate temperature and liquid metal, and the cracks were influenced by the welding parameters. Under suitable conditions, Zn diffused along the grain boundaries and weakened them, resulting in LME cracking.

Hua Pan

Papers

Liquid Metal Embrittlement Cracking During Resistance Spot Welding of Galvanized Q&P980 Steel