Use of pure vanadium and niobium/copper inserts for laser welding of titanium to stainless steel

Abstract

Niobium and vanadium have high metallurgical compatibility with titanium and therefore can be used as inserts to avoid the accumulation of brittle intermetallic phases such as Fe2Ti during the fusion welding of titanium alloys with steels. In the present study, the continuous double pass welding of 1 mm thick Ti-6Al-4V alloy and 316 L stainless steel plates through several mm wide pure vanadium or niobium insert was studied. In case of a vanadium insert, a beam offset on the vanadium was found to produce cold crack formation in vanadium/316 L melted zones containing more than 40 wt.% V despite the absence of σ phase. Whereas a centered beam position and offset on the steel side produced crack-free melted zones still composed of (Fe, V) solid solutions, and exhibiting a ductile fracture in the unmelted vanadium insert at UTS of 493 ± 25 MPa. The niobium insert produced brittle niobium/316 L melted zones with UTS of 160 ± 10 MPa, because of the formation of Fe2Nb and Fe7Nb6 layers at the niobium/316 L interface. The addition of a copper insert between niobium and steel allowed avoiding the development of Fe-Nb brittle intermetallics thanks to the absence of brittle phases in Cu-Nb system. For optimal welding conditions, a ductile fracture occurred in the copper interlayer at maximal UTS of 255 ± 10 MPa.