Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding

Nickel-titanium shape memory alloys (NiTi) as well as titanium alloys (Ti) are essential materials in various modern medical technology applications. Combining them in functionally-graded components would allow the fabrication of highly innovative products with major economic and technical advantages. While dissimilar fusion welding of these materials is not feasible due to the formation of brittle intermetallic compounds, recent studies have shown that niobium (Nb) is a very promising filler material to overcome this limitation while simultaneously maintaining the biocompatibility of welded components. The present study seeks to expand the current knowledge regarding dissimilar fusion welding of the material combination NiTi / Nb / Ti by investigating micro electron beam welding in a butt-joint configuration. In addition to adapted power modulation, a novel approach of utilizing the process-inherent fast beam oscillation is applied to optimize the melting and intermixing behavior of the comparatively high-melting Nb. Furthermore, two different dimensions of the filler material measuring 0.2 and 0.4 mm in thickness are implemented and compared with regard to the microstructural evolution in the weld metal. It is demonstrated that the welding experiments are associated with major challenges due to the considerable differences in melting temperature and thermal conductivity of the base and filler materials. Nevertheless, the welded joints exhibit excellent mechanical properties under quasi-static tensile load, which can be attributed to a reduced formation of Ti ₂ Ni intermetallic compounds. Ultimate tensile strengths of up to 673 MPa can be achieved, proving that micro electron beam welding is a suitable process to produce high-quality dissimilar NiTi / Nb / Ti joints.