Simulation And Experimental Research On Laser Ultrasonic Detection Of Defects In Stainless Steel Additive Manufacturing Parts

Metal additive manufacturing is the most advanced and most promising additive manufacturing technology,and it's an important direction for future technological and industrial development.The forming process of the additive manufacturing parts make it face fatal problems such as holes,cracks and delamination,which seriously affects the structural performance.Nowadays,the use of NDT tools such as ultrasound to locate and quantify the status of defects has gradually become a hot research direction.Laser ultrasonic testing technology has the advantages of non-contact,wide frequency bandwidth,high testing accuracy,etc.,and it has become a powerful tool for detecting small defects in additive manufacturing parts.Therefore,this paper conducts laser ultrasonic testing simulation and experimental research on the surface defects and internal defects of stainless steel additive manufacturing parts.The finite element method was used to establish a numerical model of laser-excited ultrasound,and the thermoelastic coupling theory of laser ultrasound was explained systematically.The distribution of transient temperature field,stress field and displacement field were obtained,and the excitation and propagation characteristics of laser ultrasound were analyzed.Then,in view of the low signal-to-noise ratio to the collected laser ultrasonic signal,a CEEMD combined with wavelet semi-soft threshold denoising algorithm was proposed.The simulation signal and experimental signal were used to compare the denoising effect with different denoising algorithms,which proves the effectiveness and advantages of the algorithm in practical applications.In order to realize the positioning and quantitative detection of surface defects in additive manufactured parts by laser ultrasonic technology,a simulation model with defects was used to analyze the complex process of the interaction between surface waves and defects,and then in the time domain and frequency domain,the effects of defects with different depths and widths on reflected surface wave and transmitted surface wave signals were analyzed.The laser ultrasonic testing experiment on 316 L stainless steel additive manufacturing parts shows that the numerical simulation is basically consistent with the experimental results.The arrival time difference of RS and RR waves generated by the interaction of surface waves and defects can detect the depth of the defect,and the width of the defect has almost no influence to the results.Due to the serious attenuation of the bottom surface reflection wave and weak diffraction echo signal when detecting internal defects,it may be difficult to detect the small defects inside the additive manufactured parts by using laser ultrasound.In this paper,a detection method of transmitted transverse wave based on moving scanning method was proposed.This method adopts the scanning mode of synchronously moving the excitation source and the receiving source twice in opposite directions,and uses the cross-correlation algorithm to calculate the time delay value of the transmitted transverse wave at each scan point.Finally,according to the thickness of the sample and the distance between the two largest signal delay points in the left and right scans,the location of the internal defect is estimated.Numerical simulation and experimental results show that this method can successfully identify internal defects with a diameter of 1 mm in 316 L stainless steel additive manufacturing parts.In addition,aiming at traditional ultrasonic inspection technology,the positioning method in the process of mobile scanning defect was discussed,and a positioning system based on the data fusion of optical mouse and inertial measurement was proposed.This method has good stability and high short-term accuracy,and it is expected to be able to meet real-time position marking of ultrasonic probes during non-destructive testing.The research in this paper applies laser ultrasonic testing technology to the actual surface defect and internal defect detection of additive manufacturing parts,and proposes some effective and practical detection methods.The research results have good reference value for further engineering practice applications.