Effect of pulsed current frequency on alloy chemistry, microstructure, and mechanical responses of Hastelloy B-2 superalloy in gas tungsten arc welding process

This study examined the effect of pulse frequency on weld quality in a nickel-molybdenum alloy (Hastelloy B-2) using the Pulsed Current Gas Tungsten Arc Welding (PC-GTAW) technique. ERNiCrMo-2 filler metal was utilized to weld 1 mm thick sheets of the superalloy. The welding operation was performed with a constant background current (Ib) of 40 A and a peak current (Ip) of 60 A, while the pulse frequency was adjusted to three levels: 8 Hz, 12 Hz, and 16 Hz. The influence of frequency on weld properties was analyzed using various characterization methods. Tensile strength evaluations and Vickers hardness measurements were performed to assess the mechanical properties. At the same time, detailed microstructural analysis, chemical composition evaluation, and scanning electron microscopy (SEM) of fracture surfaces were performed to uncover the underlying mechanisms. The findings revealed that higher pulse frequencies enhanced arc stability and heat distribution, resulting in narrower weld widths without compromising weld depth. Increased pulse overlap reduced the duration of high heat input into the weld pool. Additionally, higher frequencies created more turbulence and vibration in the molten pool, encouraging dendrite fragmentation and enabling the transformation from columnar grains to finer equiaxed dendrites. This microstructural refinement enhanced weld ductility, microhardness, and tensile strength. Simultaneously, the study identified decreased molybdenum carbide formation within the interdendritic regions and a more uniform dispersion of alloying elements across the weld metal at elevated frequencies. Fractographic analysis consistently showed ductile failures in the welds, with all fractures occurring in the base metal.