Research On Laser Ultrasonic Detecting And Imaging Technology Of Metal 3D Printing

Metal 3D printing,as a new manufacturing technology,enables free-form fabrication of complex structural components,and has broad application prospects in the industrial fields of automobile,nuclear power,aerospace,etc.However,due to the rapid and repeated melting and solidification,and complex heat transfer and phase transition during the 3D-printing process,leading to large size columnar crystals and various types of defects in its formed parts.In fact,lack of effective quality monitoring methods is a technical bottleneck restricting the promotion and application of metal 3D printing technology.Laser ultrasonic testing technology has shown great potential in the 3D printing quality monitoring and control due to its high sensitivity,strong penetrability,wide applicability and for materials.In this thesis,we conducted a series of research works on the characterization of microstructure characteristics and imaging detection of defect by laser ultrasonic technology of metal 3D printing.The presence of large microtextured clusters（MTC）composed of smallα-phase crystallites with preferred crystallographic orientations in 3D printed near-αtitanium alloys leads to poor mechanical and fatigue properties.It is therefore crucial to characterize the size of MTCs nondestructively.Ti6Al4V/B₄C composite materials are manufactured using Laser Melting Deposition（LMD）technology by adding an amount of nano-sized B₄C particles to the original Ti6Al4V powder.Ti B and Ti C reinforcements precipitating at grain boundaries stimulate the elongatedαcrystallites and coarse columnar MTCs to equiaxed transition,and microstructures composed of approximately equiaxed MTCs with different mean sizes of 11–50μm are obtained.Theoretical models for scattering-induced attenuation and centroid frequency downshift of ultrasonic waves propagating in such a polycrystalline medium are presented.It is indicated that,the studied composite material has an extremely narrow crystallographic orientation distribution width,i.e.,a strong degree of anisotropy in MTCs.Therefore,MTCs make a dominant contribution to the total scattering-induced attenuation and spectral centroid frequency downshift,while the contribution of fineα-phase crystallites is insignificant.Laser ultrasonic inspection is performed,and the correlation between laser-generated ultrasonic wave properties and microstructural properties of the Ti6Al4V/B₄C composites is analyzed.Results have shown that the deviation between the experimentally measured ultrasonic velocity and the theoretical result determined by the Voigt-averaged velocity in each crystallite is no more than 2.23%,which is in good agreement with the degree of macroscopically anisotropy in the composite specimens.The ultrasonic velocity seems to be insensitive to the size of MTCs,while the spectral centroid frequency downshift is approximately linear to the mean size of MTCs with a goodness-of-fit（R²）up to 0.99.To meet the urgent need of metallurgical quality inspection and evaluation of metal 3D printing process,we designed and built a metal 3D printing online inspection system based on laser ultrasound and vibrating mirror synergy,which realizes the two-dimensional scanning of the excitation spot in any trajectory in the inspection plane and the cooperative motion control with the inspection probe.The system is used to conduct laser ultrasonic two-dimensional scanning experiments on rolled and LMD aluminum alloy experimental samples with pre-buried surface defects.Due to the high surface roughness of metal 3D printing materials,and the influence of various factors in the molding cavity during the printing process,the signal-to-noise ratio of the laser ultrasonic signal is low,which affects the final imaging effect.To address this problem,a noise reduction algorithm based on improved wavelet thresholding and variational modal decomposition（VMD）is proposed in this thesis,and the noise reduction results show that the noise reduction algorithm improves the signal-to-noise ratio of the original signal to about 17 d B,while controlling the root mean square error to 3×10^-4.This algorithm can be used for noise reduction of laser ultrasound signals on rough surfaces.The visualization study of Rayleigh wave propagation using the noise-reduced ultrasonic signal shows that the amplitude of Rayleigh wave does not change significantly when it propagates on the surface of a homogeneous medium.However,when the propagation of Rayleigh wave is reflected by the surface defect boundary,it leads to a significant decrease in the amplitude of Rayleigh wave.Based on this,an ultrasonic imaging detection algorithm based on the surface transmission wave energy method is proposed.The imaging results show that this algorithm can initially detect sub-millimeter defects on the surface of rough LMD aluminum alloy experimental samples.