Laser Micro Welding Of Nitinol for Cardiovascular Applications

Abstract

Nitinol (NiTi), an equiatomic nickel-titanium alloy, is widely used in the medical field due to its unique properties such as superelasticity and shape memory effect. Nitinol alloy is sensitive to thermo-mechanical processing which leads to reduction in the superelasticity and shape memory effect at the joint. These thermo-mechanical processes are necessary in the manufacturing procedure which are involved in the construction of cardiovascular devices and implants. Most medical devices are currently obtained from lasercutting of nitinol tubes, or from assemblies of nitinol elements (e.g. wires) joined at specific locations by crimping. However, these approaches reduce the strength of the structure at the joints. The main objective of this study is to analyse the most common joining techniques adopted for nitinol-nitinol and optimise the joint. Preliminary experiments were conducted on superelastic nitinol wires of 0.44 mm diameter, employing resistance discharge welding and percussive arc welding processes. Successively, laser micro welding was tested, and resulted to provide superior joining properties, due to increased mechanical strength. Examination of the microstructure and microhardness of the welded specimens was carried out. The mechanical strength of the welded specimens was evaluated using tensile testing. An elemental study was performed using energy dispersive X-ray spectroscopy (EDS) to assess nickel and titanium concentration at the fusion zone. Differential scanning calorimeter (DSC) investigations was carried out to determine the phase transformation temperatures. The results suggest that the laser micro welding procedure preserves the pseudoelastic properties of laser welded specimens in comparison to the reference material. The proposed joining approach may enable further expansion in the use of the properties of nitinol in the medical area, and result in improvements in the safety and durability of cardiovascular implants.