Research On Grain Size Measurement Of Polycrystalline Materials Based On Ultrasonic Transmission Method

Laser additive manufacturing technology can quickly transform design ideas.It is a revolutionary development and an important part of the material processing technology field.It is easy to form coarse crystal structure with non-uniform distribution during its processing,so crack source will appear under fatigue load,and it is difficult to effectively suppress the propagation of cracks.As an important engineering parameter that affects material fatigue,creep and yield strength,grain size is an important means of evaluating structural mechanical properties.With the development of new technologies for ultrasonic nondestructive testing,the use of ultrasonic waves to evaluate the grain size of materials has become an effective method.The result of ultrasonic measurement is the overall effect of the material grain size on the sound wave propagation path,which can make the measurement more accurate.This study is based on the ultrasonic propagation characteristics at the interface of non-uniform polycrystalline materials,and explores the changes in the ultrasonic transmission coefficient spectrum of laser additive parts materials affected by the grain size.It lays a theoretical foundation for characterizing mechanical properties and evaluating grain size by ultrasonic method.The main contents are as follows:(1)To explore the propagation characteristics of sound waves in multi-layer media with discontinuous material properties,the transfer matrix method was used to calculate the ultrasonic transmission coefficient spectrum at the heterogeneous interface of the isotropic plate structure,and the theoretical solution was obtained.The simulation model of ultrasonic transmission measurement of plate structure under different thickness is established,and the ultrasonic transmission signal of plate structure is obtained by finite element analysis.For the energy attenuation problem in the transmission coefficient spectrum solution process,the reference signal is introduced to compensate,and the simulation results are compared with the theoretical results to verify the feasibility and accuracy of the finite element method for solving the ultrasonic transmission coefficient.(2)Using Voronoi diagram to build simulation models of equiaxed crystal structures with different average grain sizes,and calculate the transmission coefficient.Acquire ultrasonic transmission signals of polycrystalline materials and process multiple sets of signal data.Establish the correspondence between the grain size and the characteristic parameters of the transmission coefficient spectrum.Then,explore the influence of the grain refinement of polycrystalline structure on the mechanical properties of materials.(3)Based on the existing research foundation of the microstructure in laser additive parts,a gradient simulation model with columnar crystals of different average grain sizes is established.The correspondence between the grain size and the characteristic parameters of the transmission coefficient spectrum is obtained.Comparing the fitting results of the transmission characteristic parameters and the grain size of the two crystal structures,the effect of the change of the grain size of the polycrystalline structure on the mechanical properties of the material is explored.It provides the possibility to characterize the mechanical properties of materials through the measurement of transmission coefficient.