Laser Welding of Nickel-Titanium and Stainless Steel Wires:Processing, Metallurgy and Properties

The biomedical industry has an increasing demand for dissimilar metal joining processes, which are used for complex configuration designs, such as guidewires and other intravascular interventional devices. Their production becomes more and more challenging as they decrease in size to reach components at the micron range. Nickel-titanium alloys are commonly used for their shape memory and biocompatibility properties, but are difficult to combine with other biocompatible metals, especially ferrous alloys such as stainless steels. Laser welding is a promising technique to achieve such small and complex shape joints. Indeed the laser high energy density reduces the size of the heat affected zone and the high cooling rate can avoid unwanted phase formation, especially in the particular case of dissimilar joining. Moreover, the high versatility of the technique allows to change the dilution factor in the weld pool in order to carefully select the joint microstructure. In this thesis, the laser welding process has been applied to superelastic nickel-titanium (NiTi) joining to stainless steel (SS) in the case of submillimetric diameter wires. The welded couple strength and microstructure have been optimized by investigating the influence of the laser parameters of both pulsed and continuous laser welding modes, to achieve sound welds. First, the NiTi-SS system has been studied using controlled speed solidification experiments that were performed to characterize the solidification path and its resulting microstructure according to the dilution factor of the base materials. Bridgman and infrared furnace experiments were correlated to the ternary Ni-Ti-Fe phase diagram to identify the possible phases that might form during laser dissimilar welding. Then, the laser welding process was optimized according to the previous results using several parameters to modify the solidification interval, dilution factor and cooling rate in particular. The weld quality was characterized by tensile testing and fracture surface analyses, in order to select the welding parameters leading to repeatable sound welded couples. Finally, the fracture behaviour of the welded couples was carefully investigated to understand the limitation of the tensile strength by the NiTi superelastic stress. In situ tensile experiments, mechanical property characterization and modelling were performed to determine the fracture mechanism occurring at the NiTi-weld interface during testing. Based on these observations, a simple composite model was designed to explain this precise fracture location and the upper limit, which is equal to the superelastic stress. Moreover, perspectives were detailed in order to possibly avoid this mechanical strength issue. This thesis has also emphasized the need to connect several complementary techniques, such as mechanical properties investigations, solidification path characterization and modelling to tackle complex materials science issues, such as dissimilar laser welding

Mechanical and microstructural integrity of nickel–titanium and stainless steel laser joined wires

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