Research On Mathematical Modeling Of 4D Printing NiTi Shape Memory Alloy Forming And Service Process

4D printing technology is a new intelligent component forming technology,which is to form intelligent materials by 3D printing.The formed components have excellent deformability,denaturation and functional intelligence characteristics.As a typical shape memory alloy,NiTi alloy will undergo the transformation between austenite and martensite under the influence of stress and temperature,resulting in shape memory effect.This kind of intelligent performance is very sensitive to the temperature and external load in the forming and service process.The traditional test method for forming intelligent components has long cycle and high cost,and it is difficult to quickly and accurately predict the effect of various factors.Therefore,this paper uses numerical simulation technology to study the 4D printing forming and service process,taking laser selective melting forming of NiTi alloy as the research object,establishing an accurate thermal stress mathematical model at the local scale during the forming process,and calculating the macroscopic stress and deformation based on the mapping algorithm.Based on the above algorithm,the characteristics of temperature,molten pool morphology and stress during the forming process of NiTi alloy components are simulated.The prediction model of service performance of shape memory alloy components considering residual stress is established,which provides a theoretical basis for the prediction of superelastic behavior of 4D printed NiTi alloy components with residual stress in service.The main work and conclusions are as follows:1)A CFD model considering the characteristics of powder particles and the flow behavior of molten pool is established.Based on the law of conservation of energy,a CFD-FEM heat source model is established.The sequential coupled thermalelastoplastic model algorithm is used to accurately predict the local model temperature,surface morphology,and stress during the 4D printing process.The error of the molten pool size co MPare with the actual measurement is about 5%,which verifies the correctness of the heat source model.Based on the theory of plastic strain mapping,the local model stress and strain are mapped to the overall structure to realize the accurate prediction of the stress evolution behavior during the standard block forming process.2)The temperature-stress-tissue incremental constitutive model of 4D printing component with residual stress as initial stress is established,and the solution algorithm based on incremental format are proposed.The corresponding solution code is written based on C++ language.A test case of superelastic behavior is established,and the simulation results are basically consistent with the literature,which verifies the correctness of the model.And it provides technical support for predicting the superelastic behavior of 4D printed NiTi shape memory alloy components with residual stress during service.3)The mechanical response behavior model of porous components of 4D printed NiTi shape memory alloy during forming and service is established.During the forming process,as the forming thickness increases,the equivalent stress of the component gradually increases,and stress concentration occurs inside.During the service process,when the compressive load stroke reaches 75% of the original length,the porous component is in complete martensite elastic or plastic state,and for components that are subjected to compressive loads,the existence of tensile residual stress slows down the martensite transformation process.