The Study On Microstructure And Properties Of NiTi Alloy Fabricated By Wire Arc Additive Manufacturing

Shape memory alloy is a new type of functional material with shape memory effect and superelasticity,NiTi shape memory alloy with nearly equal atomic ratio has been widely used in aerospace,automotive engineering,biomedicine and other fields for its excellent shape memory effect and superelasticity,good biocompatibility and corrosion resistance.It has become the shape memory alloy material with the most significant application potential at present.However,due to the high ductility and high work hardening of NiTi alloy,tool wear,poor surface finish and burr is a challenge to mechanical NiTi alloy materials.In recent years,additive manufacturing has acquired extensive attention in the field of advanced manufacturing and industrial applications,but powder-based additive manufacturing has high powder cost and low material utilization rate.In contrast,wire arc additive manufacturing are also becoming more and more obvious for reducing the material cost,high material utilization rate,high deposition rate and suitable for depositing large-size components.At present,there are few researches on fabricating NiTi alloys by wire arc additive manufacturing at home and abroad.The research on theory and technology of WAAM-NiTi alloys is of great significance to its wider application.In this paper,commercial alloy wire was deposited onto NiTi baseplate by cold metal transfer technology for carry out the research of arc additive manufacturing of NiTi alloy,the influence of traveling speed,wire feeding speed,deposition strategy and welding torch swing trajectory on forming quality was deeply explored.The effect of deposition height and solution aging treatment on the microstructure and properties of NiTi alloy by wire arc additive manufacturing was also analyzed,which laid the foundation for the application of wire arc additive manufacturing technology in the field of shape memory material NiTi alloy manufacturing.Among many process parameters,wire feeding speed and traveling speed mainly affect the forming quality through the weld width,residual height and wetting angle of single pass.The optimal combination of parameters is traveling speed of 30cm/min and wire feeding speed of 7m/min.With the increase of wire feeding speed,the weld bead width keeps increasing,the height increases slightly and the growth rate is much smaller than that of the weld width,the wetting angle generally presents a downward trend.With the increase of traveling speed,both the weld width and the height of the weld bead show a downward trend,the former has a more obvious downward trend and the wetting angle also basically shows a downward trend.The deposition strategy has a great influence on the forming height of arc initiation and extinguishing point,the selection of reciprocating deposition reduces the influence of arc initiation and arc extinguishing.The rectangular scanning strategy promotes droplet transition and reduces channel defects while increasing weld width.The deposited upper samples showed high strength and elongation,the strength increases with the increase of forming height.During the cyclic stretching process,there is obvious stress hysteresis in the first tensile curve,which decreases with the increase of the number of cycles and the decrease of the deposition height.After 10 cycles,the recovery rate of the upper and middle samples still remains above 60%,showing good shape memory performance and application prospect.The low temperature solution and aging treatment have a significant effect on the microstructure and properties of NiTi alloy.Solution and aging treatment promoted the composition homogenization,the phase transformation peak became narrower and higher.The two-step phase transformation appeared in the sample treated by aging treatment,which was related to the precipitation of Ni-rich precipitated phase.The two heat treatment processes improved the tensile properties,hardness and wear resistance of the deposited alloy to varying degrees,indicating that the post heat treatment is necessary to optimize the properties of NiTi alloy fabricated by wire arc additive manufacturing.