Microstructure And Properties Of Laser Spot-welded Joint Of Superelastic NiTi Alloy Wire

A combination of shape memory effect, superelasticity and good biocompatibility makes NiTi alloy a desirable candidate material for certain biomedical device applications. There is a growing demand for smaller and thinner medical devices that can be surgically implanted or delivered via catheter, into small diameter, highly tortuous parts of human body. However, present technique, laser cutting or hand made technique, usually takes high costs and it is difficult to fabricate finer or more complicated medical devices. Micro laser welding is considered to be one of the available and promising methods for the fabrication of micro medical devices.Process, microstructures and properties of pulse laser spot welding NiTi shape memory alloy have been studied in this paper in order to reveal the applicability of laser welding to the fabrication of small medical devices. Ti-50.6at.%Ni wires were welded using pulsed YAG laser. Theoretic analysis and experimental results show that welding parameters possessing lower current, higher pulse duration, defocusing distance and Ar protecting atmosphere are advantageous to getting better joint. The laser spot-welded joints were tested for investigation the microstructure, transformation behavior, composition, micro-hardness and stress-strain curves in comparison with the base metal. It can be concluded that laser welding NiTi alloy is advisable when NiTi alloy is used as functional material. The fusion zone features dendrite structure, the microstructures of heat-affected zone can be divided into two parts: coarse equiaxial crystals near the fusion zone and fine equiaxial crystals near the base metal. The transformation behavior of the joint will be affected by the vaporization of Ni-content during the laser welding process. Carbide in laser spot-welded joint is lower than that in base metal. The ultimate tensile strength of the joint is up to 70 per cent of the base metal, the maximum recoverable strain is up to 92 per cent of the base metal. When unloading at the maximum plateau strain, the residual strain is 4.3% for laser spot-welded joint and 1.8% for base metal, respectively.