Study On The Microstructural Evolution,phase Transformation Characteristics And Property Regulation Of Laser Powder Bed Melted NiTi Shape Memory Alloys

Nickel-titanium(NiTi)shape memory alloy has attracted much attention in the development and application of biomedical devices and intelligent bionic devices because of its excellent functional properties,high damping,high specific strength,non-magnetism,good biocompatibility,and so on.However,as one kind of intermetallic compound with high machining difficulty and poor weldability,it is difficultly to directly fabricate NiTi functional devices and bionic components with complex shapes and structures,such as porous,reticular,medium channel,biological model,human simulated skeleton,Y-shaped vascular stent,to name a few.Nowadays,a developed additive manufacturing technology,known as the laser powder bed melting(LPBF)technology,is expected to solve the fabrication problem of NiTi devices with complex shapes.The LPBF technology employs CAD data to selectively melt the metal powder point-by-point,line-by-line,and layer-by-layer by means of a laser beam.This technology has the advantages of high machining precision,wide range of forming size and high surface finish,which is especially suitable for the preparation of fine and complex functional devices of NiTi alloy in aerospace,medical,precision control,and other fields.The development of the LPBF technology can significantly reduce the design cost and shorten the development cycle of bionic configurations and smart devices.After years of research,scholars have been able to use LPBF technology to prepare NiTi alloys and functional devices without obvious forming defects.However,the performance of LPBF-fabricated NiTi alloy has not been able to meet the requirements of practical application.Moreover,due to the unique forming characteristics of LPBF technology,microstructural characteristics,and phase transformation behaviors of LPBF-fabricated NiTi alloys and devices are significantly different from those of traditional commercial NiTi alloys.Up to now,there are few basic studies on the microstructural evolution,phase transformation behavior and property regulation of LPBF-fabricated NiTi alloys,and the related theoretical knowledge framework has not been formed yet.The lack of these key information undoubtedly impedes the processing and production of such alloy,as well as the design,optimization,and application of such functional devices.In order to solve these problems,this paper conducted a systematic study on the characteristics of microstructure and thermoelastic martensite transformation behavior and of the LPBF-fabricated NiTi alloy and its functional devices and analyzes the formation mechanism of these characteristics.This paper also studied the regulation mechanism of microstructure and phase transformation behavior by different heat treatment processes.In addition,this paper studied the effect of size on the microstructure and property of the LPBF-fabricated NiTi biomimetic implants.And then based on the research results of transformation regulation,one heat treatment process is developed to eliminate such reverse defect.Finally,this work introduced a new type NiTi biomimetic composites that fabricates by considering the unique characteristics of solidification process and thermal effect of LPBF technology,and the improvement concept of bionic multi-factor coupling.The hard phase of nano Ni₄Ti₃ceramic particles and the soft NiTi matrix constitute a biphasic coupling microstructure that endows high strength,self-repairing feature and wear-resistant to the composite materials.This novel composite materials shows a better wear-resistant than that of commercial20CrMnTi gear steel in dry wearing test.The information obtained in this work provides a favorable reference for the study of microstructure,phase transformation and performance of LPBF-fabricated NiTi alloy and also builds a theoretical basis for the optimization and regulation of microstructure and phase transformation behavior,so as to accelerate the practical application of NiTi alloy prepared by LPBF and its functional devices.The main conclusions obtained by present work are shown as follows:(1)LPBF-fabricated NiTi alloys in present work have good forming quality and high density but few internal defects,also exhibit a large tensile strain up to 15%,and an excellent shape memory that show a recovery rate of?90%when pre-deformed by 6%,which have better performance than other reported results.(2)The microstructure and formation mechanism of LPBF-fabricated NiTi shape memory alloys are deeply explored.It is found that the special thermal characteristics of non-equilibrium solidification,directional temperature gradient and rapid cooling and heating cycle of LPBF process endow unique microstructure characteristics to NiTi alloys that it fabricated.In particular,it is found that LPBF-induced dislocations have unique characteristics and a significant impact on the precipitation process in the subsequent heat accumulation stage.(3)The unique thermoelastic martensitic transformation characteristics of LPBF-fabricated NiTi alloy are systematically studied,the microstructure evolution law of its transformation process and cycle process is explained,and the regulation mechanism of heat treatment process on transformation behavior is revealed.These results provide a theoretical basis for constructing the knowledge framework of intelligent metal bionic additive manufacturing.The in-situ TEM observation results show that the characteristics of"small-scale diversity and large-scale homology"of the microstructure are the main reason for its unique phase transformation characteristics.In the cycle process,LPBF-induced dislocations react with twins to form stair-rod dislocations and dislocation-lock structure,both of which limit the generation and proliferation of cyclic dislocations.As a result,LPBF-fabricated NiTi alloys have better cycle stability than that of common NiTi alloys.Heat treatment can regulate the microstructure and phase transformation characteristics in effcient,and the residual stress generated during LPBF processing will significantly affect the precipitation process during aging treatment,but the cycle stability of the alloy becomes worse after heat treatment.(4)The influence mechanism of forming size on the microstructure and functional properties of NiTi bionic implants during LPBF processing is revealed,and the influence law of forming size on microstructure evolution,phase transformation characteristics and shape memory effect is summarized.It is found that the composite NiTi alloys are treated by aging for 30 min at 773 K to have microstructure homogenization can effectively eliminate the adverse effect caused by the size dependence of shape memory effect.(5)Based on the solidification characteristics and thermal history law in the manufacturing process of lpbf additive,a new high-strength hyperelastic NiTi wear-resistant alloy with "rigid-flexible"two-phase coupling structure was developed.The alloy shows an ultimate compressive stress of up to 2 GPa during compression,achieves almost complete recovery after 6%deformation,and its wear resistance under high load is twice that of commercial gear steel 20CrMnTi.