Investigations On Microstructure And Properties Of Laser Micro-Welded Medical NiTi Shape Memory Alloy Joint

Although human have overcome most diseases,it is still an indisputable fact that human health is facing enormous challenges.Cardiovascular and cerebrovascular disease has the highest mortality rate among all kinds of diseases,which have become the first killer for human health.Interventional stent has been proven to be the most effective treatment for cardiovascular and cerebrovascular diseases,and nickel-titanium shape memory alloy?NiTi SMA?is considered to be the most ideal material for stent due to its excellent shape memory effect and biocompatibility.At present,vascular stents are mostly made by laser engraving.However,due to the limited size of the extruded tubes,manufacturing of small and complicated stents cannot be realized.As a result,direct interventional treatment of cerebral vessels is not possible in clinical practice.In addition,the stent made by laser engraving has poor flexibility,which makes the implantation process difficult.In view of this,we are proposing to make fine,flexible and diversified stents through the laser micro-welding of NiTi SMA wires.In this study,NiTi SMA wires were successfully crossly joined by Nd:YAG?Neodymium Doped Yttrium Aluminum Garnet?pulse laser welding.The macro morphology of welded joint was examed and its phase compositions were analyzed by optical microscope?OM?,scanning electron microscope?SEM?and transmission electron microscope?TEM?.The phase transition temperatures in various zones of the joint were measured by DSC?differential scanning calorimetry?.Micro mechanical testing machine and nanoindentation were used to study macro and micro mechanical response,and shape memory effects of the NiTi SMA after laser welding were measured through SEM and atomic force microscope?AFM?.Finally,the adaptivity of the joint in simulated body fluid and artificial blood environment was studied,and the biological compatibility of laser welding to NiTi SMA was revealed.Major research findings are listed below.Nd:YAG pulse laser welding was used to join NiTi SMA wires.Good crossed joint was found free of obvious weld defects,and the surface of joint was smooth satisfying the implantation requirement in the body.The grain morphology of various zone in joint are found different by metallographic observations:Base metal consists of uniform equiaxed grain.Heat affected zone near the fusion boundary contains with equiaxed grains and partial dendrite,and showed a typical epitaxial solidification.The fusion zone consists mainly of columnar grains and a small amount of equiaxed grains was observed in the center of fusion zone.The microstructures of base metal and joint were studied:Base metal consists of single austenite phase.Heat affected zone consists of mainly austenite phase and partially pre martensite phase?R-phase?.Fusion zone consists of austenite phase and the second phases distuibuted in the austenite phase.These second phases were found to be Ti₂Ni phase,Ti₃Ni₄ phase and TiNi₃ phase by HRTEM observations.Nanoindentation tests were conducted at various locations in joint and the load-displacement curves were studied.Self-elastic recovery rates in various zones were analyzed using the hemispherical cavity model.It was found that the self-elastic recovery rate of the base metal is batter than that of the heat affected zone and fusion zone.The shape memory effect was quantified by analyzing the nanoindentation size changes in various areas in the welded joint before and after heating by SEM and AFM.The results showed that the shape recovery rate of the base metal was the highest?96.23%?,followed that of the heat-affected zone?93.53%?,and the shape recovery rate of the fusion zone was the lowest?78.82%?.This difference is due to the appearance of the second phases in the weld microstructure which destroyed the ordered lattice structure.The corrosion behaviors of base metal and joint in simulated body fluid were studied.The results show that the corrosion resistance of base metal is better than that of the heat affected zone and fusion zone.In simulated body fluid,the corrosion products of nickel-titanium alloy were TiO₂ and Ni?OH?₂.It is due to this passivation behavior which makes the nickel-titanium alloy has posses good corrosion resistance.The hemocompatibilities of base metal and joint in human blood were measured.The results show that base metal and joint have approximately the same dynamic clotting time which meets the anticoagulant condition in human body;the hemolysis ratio of base metal and joint both are 3%,meeting the hemolysis standard of medical material;morphologies of the platelet in base metal and joint were observed.The morphologies of platelet were found all normal.Compared to base metal,joint can effectively reduce the platelet adhesiveness to restrain platelet activation.From above studies,Nd:YAG pulse laser welding is found to be an optimal method that can be used to join NiTi SMA well.It can be used not only to obtain good forming joint,but also to enhance mechanical property and biocompatibility of the base metal.The results of this study can not only provide theoretical foundation for design and manufacture of intravascular stents,but also enrich the basic theory of laser welding in microscale.Hence,it has great theoretical and practical values.