Comparative evaluation of Nb, Nb-Cu, and Ta-Cu interlayers on microstructural and mechanical behavior of dissimilar welds of grade 6 Ti and stainless steel

Dissimilar metal welding between titanium alloys and stainless steels presents significant challenges due to the formation of brittle intermetallic compounds (IMCs) at the interface, which compromise mechanical integrity. This study addresses these issues by investigating the interfacial microstructural evolution and mechanical behavior of gas tungsten arc welded (GTAW) Grade 6 Ti alloy and Stainless Steel 304 joints using different interlayers. Three interlayer configurations, including Nb, Nb-Cu, and Ta-Cu were evaluated to suppress IMC formation and enhance joint performance. The results revealed that interlayer composition critically affects microstructural and mechanical properties. The Nb interlayer causes the formation of brittle TiFe and TiFe₂ phases as confirmed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), which resulted in interfacial cracking and reduced ultimate tensile strength (UTS) of 173 MPa, and elevated microhardness to 380 HV. In contrast, the Nb-Cu interlayer encourages the development of a Cu-rich solution, which increases UTS to 293 MPa and reduces microhardness to 106 HV. The Ta-Cu interlayer yields the most significant improvement in joint strength by attaining a UTS of 450 MPa, i.e., increases of 61.55% and 34.88% compared to the Nb and Nb-Cu interlayers, respectively. This enhancement is attributed to the formation of ductile microstructures containing Ta dendrites and Cu-enriched regions along with Fe₅Ta₃ and Ta₂Cu IMCs having a microhardness of 107 HV. X-ray diffraction further confirmed the suppression of brittle TiFe and TiFe₂ phases in Nb-Cu and Ta-Cu interlayered joints, corroborating the SEM/EDS findings. Fractographic analysis indicated improved metallurgical bonding and ductile fracture characteristics in Ta-Cu joints, highlighting their potential for enabling high-performance dissimilar welding of titanium and stainless steel.