Ultrasonic Phased Array Detection And Recognition Of Image Features For Defects In Friction Stir Welded Aluminum Alloy Joints

In this article,aiming at the different morphology of lack of penetration and tunnel holes in 8mm thick friction stir welded 2A14 aluminum alloy,the study was carried out.With proper craft of ultrasonic phased array detection,detectability of ultrasonic C-scan,X-ray testing and ultrasonic phased array technology for bending lack of penetration interface were compared.In this article,ultrasonic phased array S-scan is used to study the effects of interface orientation and curvature on the detection,and different morphology of defect interface are identified by combining with the image and signal of detection results and texture analysis of the detected image.Conclusions could be drawn as followed:Ultrasonic phased array probe of 5MHz frequency performs the best detection effect by S-scan from the backward side;The detectability of X-ray and ultrasonic C-scan is very poor for the lack of penetration interface with complex orientation,the different morphology of the same kind of defects can not be identified.When the length of vertical lack of penetration is larger than the ultrasonic detection sensitivity(?/2),the amplitude of the signal is proportional to the length of the defect;And the detection of the flat lack of penetration interface is affected by the interface orientation and the incident angle,the signal intensity is inversely proportional to the vertical deviation of the ultrasonic beam direction and the interface orientation;The signal intensity is inversely proportional to the curvature of the interface when the lack of penetration interface is curving;The interface morphology of the tunnel hole can be divided into two types: complex or stead,the A-scan waveform of the former includes a main peak and multiple secondary peaks,which is obviously different from the A-scan waveform of lack of penetration that only have single wave peak,the latter can be distinguished from the lack of penetration by the feature and position of the ribbon in the S-scan image.Multi-angle detection signal can be used to analyse whether the interface is curved or not,the angle-amplitude curve of the detection signal of the bending interface is reduced-platform-reduced,the angle-amplitude curve of the detection signal of the flat interface shows that long range slowly rise-peak value-short range slowly decrease-sharp decrease.Based on the analysis of the gray level co-occurrence matrix parameters of different morphology of each type of defect,it is found that angular second moment is positively correlated with the length of the lack of penetration,and the entropy has obvious feedback on the longer lack of penetration,contrast and correlation have obvious feedback on shorter lack of penetration,and comprehensive analysis can be used to characterize the length of the lack of penetration and difference degree;Bending lack of penetration can be distinguished according to the degree of bending,when bending degree is serious,the angular second moment and correlation are bigger,and entropy and contrast are smaller than them when bending degree is light;Two kind of interface morphology of the tunnel holes can be distinguished by taking angular second moment and contrast as the main factors and taking entropy and correlation as the secondary factor,.Based on the analysis of the gray level co-occurrence matrix parameters of total morphology of defect,it is found that tunnel hole interface of the 3-direction can be distinguished by angular second moment and contrast;The curved interface with serious bending degree is obviously different from other defect interfaces in all parameters;The vertical lack of penetration of small size can be distinguished from other defect by angular second moment,contrast and correlation.