| Additive manufacturing offers a viable solution for the fabrication of NiTi smart components due to its unique advantages,Yet the fabrication of NiTi shape memory alloys is extremely challenging due to the high complexity of the L-PBF.In this thesis,the effects of L-PBF process and heat treatment on the microstructure,phase composition,phase transition temperature,mechanical properties and residual stress of NiTi parts are studied.In order to obtain a more profound understanding of the basic principle and key technology of NiTi alloy produced by L-PBF.First,the influence of process variables on microstructure is investigated.The sample density is in the range of 92.5-99.5%.The samples with poor density have keyholes and partial fusion of powder."S"-shaped microstructure is found in the sections perpendicular to the deposition direction.fish-scale molten pools are found in the sections parallel to the deposition direction.Samples have two-phase composition of B2 and B19’.The tensile stress-strain curve at room temperature can be divided into four stages.The sample tensile strength is in the range of 546-883.6MPa.The sample critical stress of martensite is in the range of 129.5-265MPa.The sample elongation is in the range of 2.9-9.6%.The fracture morphologies of samples show obvious ductile fracture morphologies.Then the influence rules of the microstructure and various properties of printed samples and heat treated samples are compared.The printed samples have band,paste-like morphology and Moiréfringes.Moreover,unidentified tiny precipitates,Ti2Ni and acicular precipitation are observed.The contents of B19’and Ti2Ni increase with the increase of secondary aging temperature.Phase transition temperatures of B2 and B19’both decrease after high-temperature aging and the trend is opposite after low-temperature aging.The tensile strength of printed samples at room temperature increases after single aging,and decreases after secondary aging.The elongation decreases after aging.Heat-treatment sample breaks as soon as it finished elastic deformation.Tensile strength of samples at 50℃is generally higher than that at room temperature,and elongation is lower than that at room temperature.The fracture morphology of samples shows obvious brittle fracture morphology at both 50℃and room temperature.The influence of process variables on the residual stress of 316L and NiTi is studied.As for NiTi the residual tensile stress is up to 232.2 MPa,and compressive stress is up to 266.6MPa.Residual stress and energy density or phase volume fraction don’t have an obvious linear relationship.Subsequently,the influence of heat treatment is explored.The residual stress is compressive after single aging,and secondary aging produces the tensile residual stress.The residual compressive stress can be up to 229MPa,and tensile stress can be up to 268MPa.The samples with residual compressive stress have higher tensile strength,while with tensile stress have lower tensile strength. |
