Study On The Process And Properties Of NiTi Shape Memory Alloys For Vascular Stents Manufactured By Metal Injection Moulding

NiTi shape memory alloys with the highest transformation strain are currently attracting attention.Due to its low thermal conductivity and cutting elasticity,it poses difficulties in the machining process.Metal Injection Molding（MIM）,as a one-piece batch forming and processing technology,is suitable as a manufacturing technology for complex components such as engineering parts and medical devices.However,the research on NiTi alloy MIM,which started early,has developed slowly.There is no representative innovative application.On the one hand,the effect of carbon and oxygen impurities is not well understood and impurity control is difficult;on the other hand,the supply chain progress is slow and the cost is too high.Therefore,the selected topic of this paper focuses on NiTi alloy vascular stent,an implantable medical device,and attempts to develop metal injection molding technology as a new manufacturing technology for vascular stents.In this paper,the high-vacuum sintering properties of different starting powder,MIM process and properties,mechanism of precipitation formation are investigated.Finally,the long-term fatigue property,cytocompatibility,and hemocompatibility of MIM NiTi alloy vascular stents were evaluated in vitro.The main results are as follows:（1）The vacuum sintering of two raw materials,elemental mixed（EP）and pre-alloyed（PP）NiTi powders,was investigated.It was found that the EP NiTi powder suffers from previously unreported deflagration reactions in the low temperature region and loss of liquid phase in the high temperature region under high vacuum,resulting in macroscopic defects.The macroscopic defects can be avoided to some extent by process optimization,and high vacuum sintering at 1250°C at 10^-4 Pa was achieved,resulting in a significant improvement in the tensile properties of the EP NiTi alloy（UTS 701-763 MPa,EL 16-22%）compared to literature data.1242 MPa,EL 11.5%).At 8%pre-strain,over 90%shape recovery was achieved for both EP and PP NiTi alloys.（2）A complete process study of metal injection moulding of pre-alloyed NiTi powders was carried out.By evaluating the rheological stability of the feedstocks and the variation of impurities and density of the various feedstocks during injection,degreasing and sintering,a relatively suitable powder loading was determined where 65%loading of NiTi alloy after sintering achieved 0.16 wt.%oxygen,0.03 wt.%carbon,58°C thermal hysteresis,812 MPa tensile strength and 5.66%elongation with tensile 100%complete recovery at 5%pre-strain.A low impurity,high performance,wide hysteresis MIM NiTi alloy was produced.（3）It was found that there was precipitation of Ni₄Ti₃ phase without aging spontaneous formation and multi-step martensitic phase transformation in MIM NiTi.Through experimental design,it was concluded that carbon and oxygen impurities and the diffusion effect of the sintered liquid phase were the main influencing factors in determining whether the precipitation formed or not.The loss of Ti atoms by carbon-oxygen impurities and the solid phase sintering process of pre-alloyed NiTi powders are conducive to the formation of Ni-rich clusters and have a precursor effect on the spontaneous formation of precipitates.（4）The possibility of the existence of interstitial solid solution oxygen in the NiTi phase is proposed for the first time.The stability of the interstitial NiTi[O]system is demonstrated by experimental observations using DSC,APT and DFT simulations.The research concluded that the powder metallurgical process is prone to the introduction of oxygen impurities to NiTi in a way that,in addition to the formation of Ti₄Ni₂O,may also enter the NiTi phase lattice and will promote austenite stabilization during the heating process and martensite stabilization during the cooling process.（5）Prototype open and closed vascular stents based on MIM NiTi alloy were fabricated,and the mechanical fatigue properties and in vitro biocompatibility of the stents was evaluated.Research shows that the MIM NiTi alloy vascular scaffold always maintains good super elasticity and stable support diameter in 380 million in vitro accelerated fatigue tests.It has no toxic effect on human umbilical vein endothelial cells,no significant inhibition on cell migration ability and adhesion ability,and has good affinity;the average hemolysis rate is 0.4%,which is far below the national standard of 5%,and has no significant destructive effect on human hemoglobin.