Study On Ultrasonic Phased Array Surface Wave Total Focusing Method Imaging Of Defects On Laser Cladding Layer

Laser cladding metal surface has high strength,high hardness and good wear resistance,and is widely used in industrial fields.The laser cladding metal components have obvious anisotropy and high attenuation,and the laser cladding layer is very thin.Ultrasonic longitudinal wave or shear wave is used to detect the laser cladding layer defects,the interface echo of the cladding layer will overlap with the defect echo,and the defect cannot be quantitatively detected.However,the ultrasonic phased array surface wave total focus imaging method can achieve high-quality imaging detection of laser cladding defects.Therefore,the ultrasonic phased array surface wave total focus imaging study of laser cladding defects is carried out in this thesis.The main work of the study is as follows:(1)Using finite element simulation method to establish single probe ultrasonic surface wave detection finite element model of laser cladding layer.The ultrasonic surface wave propagation characteristics of the laser cladding layer are explored.The influence of frequency and cladding layer thickness on the ultrasonic surface wave propagation is discussed.The simulation results show that as the excitation frequency increases,the sensitivity to laser cladding defect increases and then decreases.When the thickness of single layer cladding is reduced,the energy of ultrasonic surface wave decreases and the attenuation increases.When the excitation frequency of ultrasonic surface wave is 3 MHz,it is most sensitive to the laser cladding layer defects.(2)The ultrasonic phased array surface wave total focus model of laser cladding is established.Firstly,combined with Fermat principle,the propagation path of ultrasonic surface wave in two layer medium is calculated.The signal is delayed stack calculation,through the ultrasonic phased array surface wave total focusing algorithm,and the total focus imaging of the phased array surface wave on the laser cladding layer defects is realized.Ultrasonic phased array surface wave detection finite element model of laser cladding layer is established,and the effect of array element numbers on ultrasonic phased array surface wave detection of laser cladding defects is analyzed.The simulation results show that as the number of array elements increases,the defect detection accuracy is improved.These contents establish a theoretical basis for the ultrasonic phased array surface wave imaging detection of laser cladding defects.(3)The ultrasonic phased array surface wave detection and total focus imaging experiments were carried out.Firstly,the simulation results are verified by experiments,and the most suitable excitation frequency of ultrasonic phased array surface wave detection of laser cladding layer defects is determined to be 3 MHz.Then,the time-frequency analysis is carried out on the defect signal of the 45 steel and the 316L stainless steel laser cladding specimen,which was used to discuss the effect of anisotropy on ultrasound propagation.The detect imaging effect in the parallel cladding direction is worse than that in the perpendicular direction.Finally,the wavelet packet decomposition method was used to reduce the noise.After processing,the average SNR of defects perpendicular to the laser cladding direction is increased by 1.48 dB,the average SNR of defects parallel to the laser cladding direction increased by 1.65 dB.This imaging resolution is better and the dimensional errors is reduced in defect imaging.