Research On Nondestructive Testing Of Stainless Steel Additive Manufacturing Parts Based On Laser Ultrasonic Surface Wave

Additive manufacturing,as an emerging advanced manufacturing technology,has achieved rapid development in recent years.It has been more and more widely used in aviation parts manufacturing and medical applications.The reliability and safety of parts is very important.It will be affected by the quality of the additive manufacturing parts and also relates to the prospect of the additive manufacturing technology.Therefore,quality inspection of additive manufacturing parts is an important means to ensure the safety and reliability of parts.Compared with traditional detection methods,laser ultrasonic detection technology has the advantages of non-contact,wide frequency domain,high detection accuracy,etc.Therefore,it has important theoretical and application values by using laser ultrasound technology for nondestructive quality inspection of additive manufacturing parts.This topic is to conduct research on laser ultrasonic testing for additive manufacturing parts,and to explore the correlation between surface wave sound velocity and prat forming quality.The thesis first proposed the mechanism of laser-excited ultrasonic waves,established an ultrasonic model based on the laser thermoelastic effect,and simulated the propagation characteristics of ultrasonic waves generated by laser pulses in additive manufacturing materials through comsol software.Therefore,laser ultrasonic testing has a theoretical basis.The laser ultrasonic non-destructive testing system is built,it can set the appropriate parameters according to the testing needs.A special fixture is designed for testing,and the experimental system is also calibrated.On the other hand,by setting the parameters of the metal 3D printer,a batch of metal samples of the same specifications and different forming qualities were printed.Non-destructive testing was performed on the printed samples using laser ultrasonic testing equipment to collect surface wave signals.In order to obtain a more accurate surface wave propagation speed,the obtained ultrasonic signals are filtered to remove noise.Then under the same excitation source,do cross-correlation analysis on multiple sets of signals at different displacements to obtain discrete sets of surface wave time shift points.The RANSAC algorithm is used to linearly fit the surface wave time-shift point set,analyze the linear models of the surface wave time shift,and obtain the optimal surface wave sound velocity solution.In order to achieve the non-destructive quality inspection of additive manufacturing parts by laser ultrasound,it is necessary to establish the correlation between the surface wave sound velocity and the quality of the sample forming.By observing the surface mirror structure of the additive manufacturing parts,it was found that there were differences in the size and number of surface defects between samples of different forming qualities.Based on the analysis of the sample surface defect images,the image processing methods such as gray-scale transformation and threshold segmentation were used to extract the defect regions,and the proportion of the defect regions in the image was calculated,which was expressed as the porosity of the sample.The experimental results show that there is consistency between the surface acoustic velocity and the porosity of the sample.This thesis mainly applies laser ultrasonic surface wave detection to the nondestructive testing of additive manufacturing parts,and provides an effective and fast testing method for the quality testing of additive manufacturing parts.