Microstructural characterization of laser micro-welded Nitinol wires

Highlights: • FIB technique was used in the preparation of TEM foils of laser micro-welded joint. • Phase content in various zones of laser micro-welded Nitinol wires was revealed. • Welding induces the R-phase formation in HAZ and intermetallics precipitation in FZ. - Abstract: Laser micro-welding has been considered one of the most promising joining methods of manufacturing Nitinol biomedical devices. However, there is still a lack of understanding about how laser micro-welding influences the microstructure. This work attempts to reveal the phase content within various microstructural zones of laser micro-welded crossed Nitinol wires by transmission electron microscopy (TEM) with the assistance of the focused ion beam (FIB) technique. The base metal is composed of a single austenite phase (B2). The fusion zone exhibits the coexistence of austenite matrix and several intermetallics (T₂Ni, TiNi₃ and Ti₃Ni₄). The precipitation of R-phase was observed in HAZ with B2 matrix due to thermally induced stresses during welding.