Full-domain power modulation laser welding induced columnar-to-equiaxed transition for enhanced molten salt corrosion resistance in Inconel 625 joints

The premature failure of Inconel 625 welded joints in molten nitrate environments presents a critical bottleneck for the longevity of concentrating solar power components. This study identifies that conventional Gaussian heat source laser welding (GHS-LW) promotes an epitaxial columnar grain structure, which induces severe sub-grain elemental segregation (Nb enrichment >3 wt.%) and establishes a pronounced Volta potential gradient (>50 mV) between dendrite cores and boundaries. Such microstructural heterogeneity inhibits the formation of a protective, continuous Cr-rich passive film, accelerating localized corrosion. To circumvent these limitations, a full-domain power modulation laser welding (FDPM-LW) strategy is proposed, integrating high-frequency beam oscillation with phase-dependent, point-by-point power gradients. This approach homogenizes the spatial energy input and diminishes the transverse temperature gradient by two orders of magnitude, from 106 to 104 K/m. The resulting thermal-mechanical-solute synergy drives a complete columnar-to-equiaxed transition (CET), yielding a refined microstructure defined by fine equiaxed grains (~10 μm), a uniform Cr spatial distribution, and a reduced Volta potential discrepancy (<20 mV). Consequently, following 1200 h of immersion in nitrate molten salt, the FDPM-LW joints exhibit a corrosion weight loss one-third that of GHS-LW counterparts. Furthermore, a continuous and dense bilayer oxide scale, composed of an inner Cr2O3 barrier and an outer NiO layer, forms on the FDPM-LW surface, effectively shifting the degradation mode from localized lamellar exfoliation to stable, uniform corrosion. These findings validate a "three-field synergistic regulation" framework for solidification control and establish FDPM-LW as a robust, practical route for fabricating high-reliability welded structures in molten-salt energy storage systems.