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The biomedical industry shows increasing interest in the joining of dissimilar metals, especially with the aim of developing devices that combine different mechanical and corrosive properties. As an example, nickel–titanium shape memory alloys joined to stainless steel are very promising for new invasive surgery devices, such as guidewires. A fracture mechanics study of such joined wires was carried out using in situ tensile testing and scanning electron microscopy imaging combined with chemical analysis, and revealed an unusual fracture behaviour at superelastic stress. Nanoindentation was performed to determine the mechanical properties of the welded area, which were used as an input for mechanical computation in order to understand this unexpected behaviour. Automated image correlation allowed verification of the mechanical modelling and a reduced stress–strain model is proposed to explain the special fracture mechanism. This study reveals the fact that tremendous property changes at the interface between the NiTi base wire and the weld area have more impact on the ultimate tensile strength than the chemical composition variation across the welded area.

Nickel-Titanium (NiTi) shape memory alloys are often used in medical component devices, for instance as guide wires for neurological surgery applications. The manufacture of such devices becomes more and more challenging, especially considering the need to join them with other metals. Laser welding is a promising technique to realize and to guaranty the mechanical stability of dissimilar metal welds, although inherent differences in chemical compositions, absorption, physical and thermo-mechanical properties can lead to severe problems, in particular fracture of the weld due to the possible formation of brittle intermediate phases. Laser welds of NiTi - stainless steel (SS) pieces have been made with a Nd:YAG laser and the weld microstructure have been studied by Scanning Electron Microscopy (SEM). The phases and defects in these welds have been compared with those observed in autogenous NiTi welds and SS welds. They have been put into relation with the Fe-Ni-Ti ternary phase diagram. In addition, Differential Thermal Analyses (DTA) of NiTi-SS alloys and NiTi-SS diffusion couple experiments have been performed in order to gain a better understanding of the phases and reactions occurring during laser welding. This diffusive couple experiments have been analyzed by Energy Dispersive X-ray Spectroscopy (EDX) It appeared that although experiments were performed under controlled atmosphere, oxide layers have restrained the chemical diffusion of concerned elements. Further diffusion couple experiments will be realized with stainless steel welded caps to avoid oxygen contamination during heating.

The biomedical industry has an increasing demand for processes to join dissimilar metals, such as laser welding of NiTi and stainless steel wires. A region of the weld close to the NiTi interface, which previously was shown to be prone to cracking, was further analyzed by energy dispersive spectrometry (EDS) extended in the third dimension using a focused ion beam. As the spatial resolution of EDS analysis is not precise enough to resolve the finest parts of the microstructure, a new segmentation method that uses in addition secondary-electron images of higher spatial resolution was developed. Applying these tools, it is shown that this region of the weld close to the NiTi interface does not comprise a homogeneous intermetallic layer, but is rather constituted by a succession of different intermetallics, the composition of which can be directly correlated with the solidification path in the ternary Fe-Ni-Ti Gibbs simplex.