Multiscale characterization of NiTi shape memory alloy to Ti6Al4V dissimilar laser welded joints: Reasons for inherent brittleness

Dissimilar joining of advanced engineering alloys is of major importance for several applications to takeadvantage of the properties of each one of the base materials. Due to the formation of Ti2Ni and to the thermalexpansion coefficient mismatch, dissimilar laser welding between Ti6Al4V and NiTi is particularly challenging.Nevertheless, a joint without pores or cracks was obtained using a Nd:YAG laser system with a wavelength of1064 nm and a spot size of 0.45 mm. Full-penetration occurred and the weld pool is asymmetric with a key-holeshape. Typical dendritic structure is observed throughout the welded zone, although without apparent orientationon the middle. Close to the Ti6Al4V side a zone with a different morphology is observed along the entirethickness with high hardness corresponding to NiTi2 . Multiscale microstructure and mechanical characterizationencompassing electron microscopy, synchrotron X-ray diffraction, electron probe microanalysis and instrumentednanoindentation were used to reveal the microstructure/properties relationships correlating theextremely high hardness and brittleness of the welded joint to its microstructural constituents