Study Of The Properties Of The Laser-welded NiTi Alloy

A combination of shape memory effect, superelasticity and good biocompatibility makes NiTi alloy a desirable candidate material for certain biomedical device applications, successfully used in the field of interventional therapy to fabricate different stents. There is a growing demand for smaller and thinner endocascular stents. However, present technique, laser cutting or hand made technique, usually takes high costs and it is difficult to fabricate these stents. In addition, it is one of the difficulties to find better joining method for hand made peripheral endoluminal stent, which is confined to mechanical fastening by tube at the present time. Micro laser welding is considered to be one of the available and promising methods for resolving these difficulties.Process, microstructures and properties of pulse laser spot-welded NiTi shape memory alloy have been studied in this paper in order to reveal the applicability of laser welding to the fabrication of medical devices. Superelastic Ti-50.6at %Ni wires were welded using pulsed YAG laser. The primary conclusions are listed as follows: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 fusion zone features dendrite structure, the microstructures of heat-afected zone can be divided into two parts: coarse equiaxial crystals near the fusion zone and fine equiaxial crystals near the base metal. Laser welding affects the transformation behavior of NiTi alloy wire, which varies from B2→R→B19' to B2→B19'. Carbide in laser spot-welded joint is lower than that in base metal. The ultimate tensile strength and elongation of the joint are lower than that of the base metal. When unloading at the 7% strain, the residual strain is 1.06% for laser spot-welded joint and 0.45% for base metal, respectively. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints.The corrosion resistance and hemocompatibility of laser-welded NiTi are better than that of base metal. The improvement of the corrosion resistance and hemocompatibility of laser-welded NiTi is due to the sharp decrease of surface defects. At the same condition, laser-welded NiTi exhibits a higher breakdown potentials,wider passive range,longer cruor time.The transformation behavior of joint changes with thermal cycling and annealing, varying from B2→B19' to B2→R→B19'. With increasing thermal cycles, the R phase transformation start temperature increases and the temperature region of the R phase is broadened. But the martensitic transformation start temperature decreases with increasing thermal cycles. It is thought that dislocations play important role in the transfer from one-step to the two-step transformation and in the shifts of transformation temperature. R phase can also appear in the laser-welded NiTi after annealing. R phase and martensite transformation temperature move to the low temperature side with increasing annealing temperature. The variations in transformation behavior are attributable to the presence of Ti₃Ni₄ precipitates after annealing.The laser spot-welded NiTi has good superelastic fatigue property. Although it has a larger residual strain than base metal at the first cycling, the residual strain decreases with the increase of the cycle number. The rotation-bending fatigue test results show significantly reduced numbers of cycles to failure when strain amplitude remains above 0.4%. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints. Both the welded microstructure and the unwelded one is able to be loaded up to 10⁶ cycles without failure if the strain amplitude remains below 0.4%. The fatigue life of laser spot-welded joint and base metal has no significant changes with increasing of Cl concentration and pH value.The properties of laser-welded NiTi are affected by annealing temperature because of the presence of Ti₃Ni₄ precipitates during annealing. Smaller cohe, rent Ti₃Ni₄ precipitates (10nm) produced at 400℃annealing have a dispersive distribution and can strongly strengthen the joint. Therefore, it can improve the properties of the joint; When annealing at 500℃, the Ti₃Ni₄ precipitates become coarse, so the properties of joint decreases.