Copper contribution on microstructure and mechanical properties of dissimilar gas tungsten arc welded Ti–6Al–4V to Inconel® X-750

Dissimilar metal welding (DMW) between titanium alloys and nickel-based superalloys can contribute to acquiring light, heat-resistant frameworks of enhanced efficiency. The current study constitutes an attempt to obtain joints between a Ni-superalloy and a Ti-alloy, investigating the beneficial impact of copper on the microstructure of the weld metal (WM), through precipitation of ternary Ti–Ni–Cu intermetallic compounds (IMCs), instead of Ni–Ti ones, previously reported in weldments between such alloys. Gas tungsten arc welding (GTAW) was performed between Ti–6Al–4V and Inconel® X-750, varying the welding current and using different copper filler wires (pure Cu and NiCu). The microstructural characterization of weldments was conducted by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and a further nanoscale examination was performed using a transmission electron microscope (TEM). Identification of IMCs and precipitates was achieved by X-ray diffraction (XRD), while the mechanical properties were investigated through microhardness Vickers measurements. Crack-free joints were obtained, albeit with the presence of IMCs comprising Ti–Ni, Ti–Cu, and Ti–Ni–Cu, as well as titanium carbides (TiCs). Cu exhibited a positive influence on the joints’ microstructure. Elevating the welding current led to accelerated cooling and solidification rates, resulting in enhanced Vickers microhardness values. This phenomenon can be attributed to either the formation of finer microstructures or the precipitation of brittle IMCs, which were observed in specimens welded with higher currents. Especially near the Ti-alloy interface, it was found that a brazing-type bond took place instead of welding, while high hardness values (up to 800 HV) were also detected.