Welding Metallurgy of

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

This study aims at understanding the metallurgical and mechanical response of Hastelloy X nickel-based superalloy to the thermal cycle of gas tungsten arc welding (GTAW), post welding solution treatment (ST), and aging. In the as-weld condition, the weldment was characterized by the formation of the M6C carbide, sigma phase, and micro-solidification cracks in the interdendritic zones of the weld metal, and constitutional liquation of M6C carbide in the heat-affected zone. It is demonstrated that neither the weld metal nor the heat-affected zone could adversely affect mechanical properties of the welded joint in as-weld condition. However, to alleviate the microstructural heterogeneity, tensile residual stresses, and non-equilibrium microstructure developed due to the welding thermal cycle, the weldment was subjected to the solution treatment cycle (1177 °C/5 min) as post-weld heat treatment. The post-weld heat treatment enabled complete elimination of sigma phase. The tensile and creep properties of the weldment after the solution treatment cycle were compared to that of the as-weld condition. Despite the 8% decrease in the strength of the weldment after the ST cycle, the sample not only still met the minimum mechanical requirements but also benefited from a homogenized and nearly equilibrium microstructure. Microstructural observations of the weldment after the aging cycle (800 °C/8h), as well as its mechanical response, revealed that the formation of continuous Cr-rich carbides at the grain boundaries of the base metal deteriorates the room temperature ductility and creep life of the joints.

Microstructure-performance relationships in gas tungsten arc welded Hastelloy X nickel-based superalloy

This paper addresses the factors affecting the mechanical properties of dissimilar resistance spot welds between Nimonic 263 and Hastelloy X nickel-based superalloys. The fusion zone (FZ) of the dissimilar weld displayed higher hardness values compared to the base metals which can be attributed to (i) the formation of Mo-rich carbide in the interdendritic zone due to positive segregation behavior of Mo and C during non-equilibrium solidification of the weld, (ii) the enhanced solid solution strengthening due to mixing of the base metals, and (iii) the formation of ultra-fine dendritic structure in the FZ due to ultra-fast cooling rate of the resistance spot welding. Formation of interdendritic carbides did not induce any brittleness in the FZ. The variation of weld heat input did not significantly influence the weld metallurgical attributes (e.g., chemical composition, dendrite arm spacing) and the hardness values of the FZ and heat affected zone. It is identified that controlling the weld physical attributes (i.e., fusion zone size and electrode indentation) is the key to achieve a dissimilar Nimonic 263/Hastelloy X resistance spot weld with adequate peak load and energy absorption and desirable failure mode (i.e., pullout mode).

Microstructure and mechanical properties of dissimilar nickel-based superalloys resistance spot welds

Resistance spot welding for industrial applications is a fully automated process which however lacks this aspect when it comes to quality assessment (QA). The current study introduces an online QA approach that utilizes machine learning methods, on data captured from an infrared (IR) camera, mounted on an RSW-robotized system. The models’ development was carried out in the context of two experimental approaches which included a different number of process parameters and weld-quality criteria. Results indicated that the quality-prediction uncertainty of the models depends on the proximity of the points of the process parameter space. The maximum IR intensity and the temporal features of the IR cooldown profile offered the greatest class separability and marked the corresponding classifiers as the most successful ones. In addition, the advantage of IR monitoring was highlighted, and it was justified that in an industrial scenario where the time between welds is short, the IR data from such a monitoring system may include limited temporal information, which depending on the model used could potentially compromise the model’s prediction performance.

Infrared (IR) quality assessment of robotized resistance spot welding based on machine learning

This paper addresses the metallurgical and mechanical response of Nimonic 263 nickel-based superalloy to resistance spot welding. Solidification structure of the fusion zone is described in terms o...

M. Bemani

Papers

Microstructure and mechanical properties of dissimilar nickel-based superalloys resistance spot welds

Resistance spot welding of Nimonic 263 nickel-based superalloy: microstructure and mechanical properties