Characterization of Microstructure and Mechanical Properties in Incoloy 925 and Inconel 625 Dissimilar Friction Stir Welds

This study investigated the feasibility of joining Incoloy 925 and Inconel 625, two high-performance nickel-based alloys, using Friction Stir Welding (FSW). Various process parameters were examined, with tool rotation speeds between 300 and 500 rpm and welding speeds ranging from 20 to 100 mm/min. Satisfactory welds were consistently achieved at welding speeds of 20, 25, and 30 mm/min while maintaining a constant tool rotation speed of 400 rpm. Microstructural analysis identified four distinct zones within the weld: the base metal, Heat-Affected Zone (HAZ), Thermo-Mechanically Affected Zone (TMAZ), and Stir Zone (SZ). Interestingly, a well-defined HAZ was absent, likely due to rapid cooling during FSW and the inherent thermal properties of the alloys. The SZ exhibited dynamic recrystallization, producing a fine-grained microstructure, while minimal changes were noted in the TMAZ. X-ray diffraction confirmed no phase transformations or significant oxidation in the welded alloys. Microhardness measurements showed an asymmetric distribution, with hardness increasing from the base metal to the SZ. This was attributed to grain refinement and strengthening mechanisms within the SZ. Tensile testing revealed that Inconel 625 had inherently higher yield and tensile strengths than Incoloy 925. However, at a welding speed of 30 mm/min, the weld metal exhibited tensile strength comparable to the base metal of Incoloy 925. A slight reduction in elongation was observed, likely due to the differing deformation behaviours of the alloys. Major Findings: This study demonstrated the feasibility of joining Incoloy 925 and Inconel 625 using Friction Stir Welding (FSW), achieving optimal welds at speeds of 20–30 mm/min with a rotation speed of 400 rpm. The stir zone exhibited dynamic recrystallization, fine-grained microstructures, and increased hardness due to grain refinement. Tensile strengths comparable to the base metals and improved mechanical properties highlight the potential of FSW for high-temperature applications.