Microstructure And Martensitic Transformation Behavior Of NiTi Alloy Manufactured By Selective Laser Melting

Ni Ti alloys have been widely used due to its excellent shape memory properties and superelasticity.Additive manufacturing technology provides an effective way to prepare Ni Ti alloy components with complex shapes.Selective laser melting(SLM),as a common additive manufacturing technology for preparing Ni Ti alloys,has attracted wide attention.In this paper,Ni_50.7Ti_49.3 alloy was selected as the target material.Firstly,different SLM process parameters were used to prepare the alloy,and the optimized process was obtained.Then,different aging treatments were carried out on the alloy.The microstructure and phase composition of the alloy were studied by optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffractometer;The martensitic transformation of the alloy was studied by differential scanning calorimetry and the mechanical properties were studied by mechanical testing machine and microhardness tester.The results show that the energy density is the main factor affecting the density of SLM Ni Ti alloy.When the energy density is in the range of 53?83 J/mm³,the density of the sample can reach more than 99%.The SLM Ni Ti alloy has a checkered microstructure in the cross section perpendicular to the growth direction,and a columnar microstructure in the cross section along the growth direction.The microstructure of the aged sample is different from that of the deposited sample.There are more irregular small grains at the big grain boundary,and the size of the small grains increases with the aging time.The as-deposited samples show a one-step phase transition of B2?B19'during heating and cooling,and the phase transition temperatures are higher than those of the original powder.The phase transition temperatures of the as-deposited sample increase with the increase of the energy density.The complex thermal process in the SLM process leads to the evaporation of Ni in the alloy and the precipitation of Ti₃Ni₄ phase,resulting in an increase in the Ti content of the matrix and the phase transition temperature.The aged sample shows a two-step phase transition B2?R?B19'during the cooling process.The phase transition temperatures of aged samples increase sharply at first and then tend to be stable with the increase of aging time.The energy density has an important influence on the microhardness of the deposited sample.In the optimized energy density range,the microhardness of the sample is the highest among all samples.The results show that the microhardness of the alloy increases obviously after aging treatment,and increases sharply at first and then tends to be stable with the prolongation of aging time.