| Selective melting metal additive manufacturing is the most advanced and promising additive manufacturing technology,which has the advantages of high degree of freedom,efficient molding,flexible molding and short machining cycle.Because of the forming process characteristics of 316 L stainless steel additive manufacturing parts,there are fatal forming problems such as porosity,cracks and delamination,which seriously affect the mechanical and structural properties of the parts.In order to effectively control the forming quality of 316 L stainless steel and ensure the efficiency of finished products,it is of great significance to the testing of 316 L additive manufacturing parts.At present,laser ultrasound,as a nondestructive testing tool to detect and characterize the quality of parts,has gradually become a popular research direction.Laser ultrasonic detection technology has the advantages of non-contact,large frequency band range and high detection accuracy,becoming a powerful tool to characterize the physical parameters of 316 L stainless steel additive manufacturing parts.Therefore,this thesis is based on laser ultrasonic measurement study of 316 L stainless steel material manufacturing physical parameters,establish relationship between elastic constants,density and porosity of 316 L stainless steel material manufacturing performance and ultrasonic longitudinal wave velocity and shear velocity,the relationship between study of316 L stainless steel material manufacturing transverse isotropic,judge processing and manufacturing capabilities.The main contents are as follows:Firstly,the thermoelastic mechanism and ablation mechanism were described respectively based on the thermoelastic coupling theoretical model of laser ultrasonic,and the propagation characteristics of laser ultrasonic on the thermoelastic mechanism and ablation mechanism were analyzed,providing a basic reference for the subsequent work in signal measurement.In addition,the receiving characteristics of laser ultrasonic testing tools were discussed,and a completely contactless laser ultrasonic measuring system was constructed.Secondly,because of the different laser printing power not only to the 316 L stainless steel material to make the elastic constants of increasing make a huge difference,but also for 316 L stainless steel material manufacturing a porosity and density show significant difference,therefore,to carry out the laser ultrasonic constituency of laser melting technology of 316 L stainless steel material manufacturing sample density and porosity test research,The ultrasonic wave was induced in 316 L stainless steel additive manufacturing samples by focusing a pulsed laser as a line source.A method combining longitudinal wave and surface wave was proposed to evaluate the density and porosity of 316 L stainless steel additive manufacturing parts.The effects of porosity and density in different forming directions on longitudinal wave and surface wave velocity were analyzed respectively.Longitudinal wave velocity and surface wave velocity and 316 L stainless steel material manufacturing model between the porosity and density respectively,comparing binarization method for evaluating the pore and Archimedes drainage method for characterization of porosity and density effect and the longitudinal wave and surface wave velocity on the density and porosity of effect,providing a quantitative characterization of 316 L stainless steel material manufacturing the density and porosity of the effective methods.Finally,the longitudinal wave and shear wave velocities of 316 L stainless steel additive manufactured parts with different processing parameters were measured by laser ultrasound,and the elastic constants of 316 L stainless steel additive manufactured parts with transverse isotropic structure characteristics were estimated by isotropic theory.The finite element simulation using COMSOL under different elastic constants of laser ultrasonic transmission were built,and the error of laser ultrasonic experiment and finite element simulation were analyzed.In addition,the young’s modulus and Poisson’s ratio of transverse isotropic 316 L stainless steel additive manufactured parts in different forming directions were calculated,The inversion results were good agreement with the actual Young’s modulus and Poisson’s ratio.It provided an effective application basis for measuring elastic constants of 316 L stainless steel additive manufactured parts by laser ultrasonic.In this thesis,the application of laser ultrasonic detection technology to the quantitative characterization of physical characteristic parameters of 316 L stainless steel additive manufacturing parts was studied,and some effective and practical detection methods were proposed.The research results have good reference value for further engineering practice. |
