Investigation On Machined Surface Integrity And Zero Phase Transformation Cutting Optimization For Ni50.8Ti Shape Memory Alloy

In many metal materials,due to the superior shape memory and superelastic properties,NiTi shape memory alloys（SMA）are widely used in biomedical,automotive,aerospace,and other fields.However,the characteristics of martensitic transformation,high specific heat,and low thermal conductivity also bring difficulties to the fabrication of NiTi SMA products.During the cutting process of NiTi SMA,the occurrence of martensitic transformation and work hardening lead to poor machining accuracy and surface quality.This seriously restricts the quality and service performance of NiTi SMA products.This study takes Ni50.8Ti（at%）SMA as the research object.The machined surface integrity and zero phase transformation process of Ni50.8Ti SMA are studied.To control the "shape" and "property" of NiTi SMA during cutting process,the nonlinear deformation mechanism,the effect of martensitic transformation on surface integrity,and other scientific problems are studied by combination of theoretical analysis and experimental study.The main research contents are as follows:According to the physical and mechanical properties of Ni50.8Ti SMA,Hopkinson pressure bar tests are carried out at different temperatures and strain rates.Based on the analysis of the dynamic mechanical behaviors and microstructure changes,the influence of temperature,stress,and strain rate on the nonlinear deformation of Ni50.8Ti SMA is studied.The dynamic mechanical behaviors are analyzed in detail,and the nonlinear deformation constitutive model of Ni50.8Ti SMA is established by the critical temperature.According to the physical properties and phase transition characteristics of NiTi SMA.The influence mechanism of thermal/mechanical loadings on phase characteristics of NiTi SMA under different initial phases during cutting is explained theoretically.Combined by the modeling of thermal/mechanical loadings,the critical condition of martensitic transformation during cutting Ni50.8Ti SMA is proposed.Based on the critical condition of martensitic transformation,turning experiment is designed.The cutting temperature and microstructure are analyzed to study the change of phase characteristics during machining and the machined surface.The influence mechanism of cutting process on surface roughness and surface hardness in cutting Ni50.8Ti SMA is analyzed by theory and experiment.The influence of the martensitic transformation on work hardening degree and surface morphology during cutting are compared and analyzed.The mechanism of cutting process on the functional characteristics is expored by study the phase transformation behavior and functional characteristics of machined surface materials.Aiming at the influence of martensitic transformation on surface quality in cutting,the zero transformation cutting strategy of Ni50.8Ti shape memory alloy is proposed,and the boundary conditions of zero transformation cutting are determined;The zero phase change cutting experiment is designed by response surface method,the mapping relationship model between machining parameters and surface roughness and recovery degree is established,and the effects of machining parameters on surface morphology and functional characteristics are studied Based on the genetic algorithm,the optimal zero phase transformation cutting parameters are obtained by multi-objective optimization of process parameters.The corresponding finite element simulation modeling of zero phase transformation cutting is established to study the thermal/mechanical loadings characteristics of Ni50.8Ti shape memory alloy.