| In many engineering fields such as aerospace,biomedical,and deep-sea machinery,frictional wear of working surfaces and moving interfaces is a major scientific and technical problem that limits the life of components,endangers operational safety,reduces production efficiency,and hinders economic development,so it is of great practical significance to explore multifunctional wear-resistant materials that can adapt to complex environments,These advantageous properties can be well adapted to a variety of complex working conditions,which broaden the application scope of NiTi alloy as a wear-resistant material for critical components.As an intelligent material that can change its properties and respond to changes in the environment or external forces,it has another unique super-elastic property,which provides a new idea to realize the bionic wear-resistant idea of "rigidity and flexibility".However,the design,research,and application of NiTi alloys are limited due to the high ductility and work-hardening effect in the traditional preparation process.In recent years,the rise of additive manufacturing technology has provided a new means for the preparation of difficultto-machine alloys.The use of laser melting deposition additive manufacturing technology to prepare NiTi alloys has the advantages of high automation,adjustable and controllable material properties,and easy molding of complex structures.However,there are relatively few studies on the preparation of NiTi alloys by laser melting deposition at home and abroad,and the mechanism of the influence of process parameters on the molding properties and functional characteristics of the materials is still unclear.In this thesis,the influence law of process parameters for good forming performance is investigated through laser melting deposition tests,so as to further analyze the influence of additive manufacturing process parameters on material microstructure,phase composition,phase transformation behavior,and mechanical properties,and to study the functional and wear resistance characteristics of the formed samples.The results of this paper are expected to provide a valuable reference for the study of NiTi alloy additive manufacturing and frictional wear properties.Laser melting deposition process parameters have significant effects on the forming effect and material properties of NiTi alloy.It was found that increasing the laser power and decreasing the scanning speed would increase the height and width of the melting channel and slightly decrease the densities and hardness of the molded samples,and the martensite phase transition onset temperature of the molded samples would also increase,resulting in the appearance and increase of the martensite phase in the sample phase.Too large or too small a feeding rate is not conducive to symmetry,aspect ratio,and other properties of the molded channel.By evaluating the surface morphology,cross-sectional characteristics,and aspect ratio,the following process parameters were determined: laser power range of 490-550 W,scanning speed range of250-310mm/min,powder feeding rate range of 1.2-1.8r/min,lap rate of 60%,protective gas flow rate of 8L/min and powder carrier gas flow rate of 9L/min./The compressive strengths of the samples prepared under this process were above 2500 MPa and the compressive strains were in the range of 32%~40%,indicating that the samples had high strength and toughness.In the multi-strain cyclic compression test,when the strain rate reached 9% unloading,the minimum residual strain of the sample was 1.28%;in the fixed-strain cyclic compression test,the minimum residual strain was 3.49% after10 cycles of compression to 9% strain,which showed stable and good superelastic properties.Engineering equipment is often used in complex working conditions with heavy loads,wide temperature variations,and reciprocal friction.Therefore,the study of the friction reliability and wear mechanism of NiTi alloy under dynamic changes provides an important reference for the development of intelligent wear-resistant materials and the design of bionic wear-resistant structures.The reciprocating friction test shows that the samples with better superelasticity exhibit better wear resistance in both low and high-speed environments with similar material hardness.When the friction speed increased from 8mm/s to 4Hz(48mm/s),the friction temperature increased,resulting in an increase in the critical stress of martensite phase transformation and a significant decrease in wear.Upon observing the characteristics of the wear marks,it was found that the samples with good superelastic properties exhibited shallow plowing and peeling wear.When the same load is applied,the "pseudo-yielding" of NiTi alloy leads to the increase of deformation area on the surface of the material,which reduces the stress and relieves the permanent deformation caused by the excessive local stress,and the large recoverable deformation caused by the superelastic property delays the dislocation slip and plastic strain accumulation inside the material so that NiTi alloy shows stronger wear resistance.The larger recoverable deformation caused by the hyperelasticity slows down the dislocation slip and plastic strain accumulation inside the material so that NiTi alloy shows stronger wear resistance. |
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