Study On The Two-way Shape Memory Effect And Application Of Ni_50.6Ti_49.4 Alloy Fabricated By L-PBF

Ni Ti shape memory alloys（SMA）have been widely used in biomedical and aerospace industries owing to its unique shape memory effect,pseudo-elasticity and good biocompatibility.However,due to the high strength and poor machinability of Ni Ti alloys,their applications are often limited to wires,strips,springs,and other simple shapes,which greatly limits the further promotion and application of Ni Ti alloys.Additive manufacturing uses computer CAD to slice the model to realize the layer-by-layer accumulation of materials to manufacture parts,which solves the problem of poor machinability of Ni Ti alloys.In this thesis,a Ni_50.6Ti_49.4 alloy was prepared by the laser powder bed fusion（L-PBF）technology,and its shape memory effect was trained by the constrained aging method.Finally,the intelligent structure and rigid connecting parts were designed,and various deformations were achieved by training the memory effect of the intelligent structure.The results show that the higher the energy density,the higher the density of the sample.Laser power has the most significant effect on the density,and the density increases greatly with the increase of laser power.A honeycomb-like morphology can be observed on the cross-section perpendicular to the growth direction,and clear molten pool lines arranged in parallel along a certain direction can be observed;on the cross-section parallel to the growth direction,the sample can be observed exhibiting a fish scale shape.Two different sections have obvious<110>||X and<111>||X preferred orientations.The tensile strength of the as-printed sample is positively related to its density,and the increase in the density can effectively reduce the number of defects inside the sample.The smaller the number of defects,the more crack initiation of the sample when it is under the tensile stress,thereby its mechanical properties are improvedThe aging-treated samples of Ni Ti alloy prepared by L-PBF technology are dominated by austenite phases,accompanied by Ti₂Ni and Ni₄Ti₃ precipitation phases.The samples treated at 450℃for 1h had obvious<110>||X and<100>||X textures in the austenite phase.Under the same aging time,the increase of the aging temperature did not change the phase composition of the sample,but it changed the martensitic transformation temperature.At the same aging temperature,the increase of aging time changed its phase composition,and at the same time,the characteristic temperature of each phase transition of the sample was reduced.The two-way memory effect can be obtained by constraining the aging of the sample after solution treatment.At different aging temperatures,the phase composition of each sample is similar.The Unix phase consists of austenite and martensite phases,accompanied by Ni₃Ti precipitates on phase.All Ni Ti alloy samples trained by this process have an obvious two-way shape memory effect,and the shape memory recovery rate of each sample increases with the prolongation of aging time.The aging time is 7 hours,and the maximum can reach～54%.Based on the above research,three different deformation structures were designed.Among them,the intelligent deformation structure can generate a deformation force of 620N when it is deformed by heat,and the overall structure can generate elongation,bending,and other actions when it is deformed.The actual bending displacement can reach 15%of the total length.Based on the intelligent deformation structure of the torque tube intelligent deformation unit,it is realized that the deformation structure unit is driven by the temperature of the heating rod,and the output shaft is driven to rotate through the external planetary gear of the torque tube.The rotation angle is 25.8°,and the estimated torque can reach 80Nm.Tubes based on topology structure can greatly reduce the mass of the deformed structure and realize a lightweight and miniaturized design.At the same time,the method of using cold and hot fluid medium to drive the nickel-titanium intelligent component can greatly improve its response speed.