Study On The Additive Manufacturing Of NiTi Alloy By Selective Laser Melting

Ni Ti shape memory alloy is a typical representative of smart materials.It exhibits unique shape memory effect and superelasticity under the stimulation of temperature or force.It has a wide range of applications in aerospace,medicine,consumer goods and engineering fields.With the progressing of science and technology and developing of society,new-type Ni Ti alloys with complex geometries are in high demand.However,the poor machinability of Ni Ti alloys makes it difficult to realize the forming process of complex components,which greatly limits the structure design and development of Ni Ti alloys.In recent years,additive manufacturing(3D printing technology)has been extensively developed,which makes the fabrication of complex metallic components and related research possible.Laser selective melting(SLM),which is a powder bed fusion technology,is one of the mainstream 3D printing technologies for metallic parts.It slices the 3D models of components into layers using built-in software,obtains the profile data,and then uses high-energy laser to selectively melt the metal powder layer by layer.It is a rapid prototyping manufacturing method for near net forming of metallic products.In this thesis,we use SLM technology to fabricate the Ni Ti parts in both bulk form and porous form with a triply periodic minimal surface(TPMS)structure.The effect of printing parameters on the relative density,microstructure,crystal structure,phase transformation behavior and mechanical properties of Ni Ti parts was analyzed in details.The effect of heat treatment on the phase transformation behaviour of the printed Ni Ti was explored.The mechanical properties and differences in various TPMS structures were compared in details.The effect of wall thickness on strength,ductility and superelasticity of TPMS structure was investigated,and the failure mechanism of the structure was explained.The results show that the laser scanning rate has a significant effect on the formation of defects in Ni Ti parts while maintaining the laser power(60 w and 95 w),hatch spacing(110?m)and layer thickness(25?m)constant.The larger the scanning rate resulted in the lower porocity in general.When the laser power is 60 w,with the increase of scanning rate to 360 mm/s,the relative density of the part reached 99%.At the same time,the phase transition temperature decreased continuously with increasing the scanning rate,and the peak temperature of forward phase transformation(M_P)decreased from 278 K at 300 mm/s to 243 K at 480 mm/s.When the laser power is 95 w,with the increase of scanning rate to 700 mm/s,the relative density of the printed part reached 99%,and the M_P decreased from 253 K at 475mm/s to 233 K at 850 mm/s.The mechanical experimental results show that the tensile strength and ductility of the as-printed bulk Ni Ti were 500 MPa and 25%,while the compressive strength and ductility were 1900 MPa and 25%at room temperature.The compression experiment conducted at-50?showed that,when the energy density is the same,the critical stress for martensite reorientation was higher when printed at 60 w than that preinted at 95 w.The compression experiments conducted 40?and 60?showed that,the critical stress for stress-induced martensitic transformation was higher when printed at 60 w than that printed at 95 w.The cyclic compression experiment exhibited partial room temperature superelasticity in both 60 w and 95 w printed parts,showing the initial superelasticity of 5%and 6.5%.The superelasticity then decreased with the increase of cycling number,and stablised at 4.5%and 6%after 10 cycles.The energy consumption of each cycle also decreased with the increase of cycles,and finally stablised at 2 J/cm~3.The heat treatment experiment shows that heat treating at1000?for 2 hours followed by quenching was the best among all conditions.After heat treatment,the endothermic and exothermic peaks of the phase transformation were sharper,the phase transformation temperature was reduced to-35?,and the latent heat of phase change was increased to 1.5 J/g.The critical stress for stress-induced martensitic transformation of the heat treated sample was about 100MPa higher than that of the as-printed sample.The flag shaped superelastic characteristic of the stress-strain curve was best shown at-5?,and the superelasticity reached 6.8%.The mechanical test results of the printed TPMS Ni Ti parts show that the failure modes of both P surface and G surface were brittle fracture,and the ductility of parts increased with higher relative density.When the relative density of P surface reached36.73%,the ductility was 14%,and when the relative density of G surface reached42.77%,the ductility was 19%.During deformation,the plastic deformation always starts in the unit cell in the top or bottom corner of the structure,and gradually extended to the inside of the part with the increase of stress.When the stress level was further increased,the crack initiation occurred at surface defect sites of the unit cell in the top or bottom corner,and then more cracks formed and expanded along the 45°shear band direction of the structure,eventually leading to the fracture failure.The critical stress of stress-induced martensitic transformation and yield strength of P and G surfaces all increased with the increase of relative density,and their superelasticity were 6.97%and 5.68%at relative densities of 37%,respectively.When the relative densities are similar,the P surface showed higher critical stress of stress-induced martensitic transformation by 24%,higher yield strength by 33%and higher resilience by 23%than those of the G surface.