Study On Phased Array Ultrasonic Testing Technology For Arc Plug Welded Joints Of Stainless Steel

The vacuum lyophilizer serves a vital role in the biopharmaceutical industry for the lyophilization of preparations,and the stainless-steel shelf,a critical component of the lyophilizer,facilitates refrigeration.This shelf,a sealed cavity structure,cools via heat exchange between the heat-conductive silicone oil circulating within and the external environment.The silicone oil within the shelf operates under pressure and vacuum conditions,necessitating effective sealing of the shelves to prevent leakage.Due to the unique structural design and process conditions,a significant number of TIG plug welded joints are completed manually,leading to potential welding quality issues and defects,such as lack of fusion and incomplete penetration,which could result in shelf leakage.Therefore,besides strict control over the welding process,it is crucial to explore more effective non-destructive methods and technologies for testing the quality of arc plug welds.Currently,the prevalent method for assessing the welding quality of the shelf is the double-vacuum leak detection technique.However,this method only identifies leaked weld joints and fails to detect potential welding defects that may eventually cause leakage.Consequently,this paper employs phased array ultrasonic testing technology known for its high sensitivity,efficiency,and easy visualization of test results to investigate an effective and highly reliable method for evaluating the welding quality of stainless steel shelf plug welded joints.This study aims to provide a theoretical foundation and practical technology for quality assessment of such joints based on ultrasonic testing.The finite element numerical analysis model of phased array ultrasonic testing of the stainless steel shelf plug welded joint was established.The impacts of ultrasonic testing parameters including emitting aperture,focusing depth,and scanning angle on the focusing performance of acoustic beam were systematically investigated.The effective coverage range of ultrasonic beam on the plug welded joint;The propagation law of ultrasonic wave in the plug welded joint as well as the influence of various types of joint defects on ultrasonic echo signal.Based on the simulation analysis,the phased array ultrasonic scanning testing method and device of the plug welded joint were designed as well as developed independently,and the sector scanning and circumferential scanning testing were creatively realized for the purpose of adapting to the complex defect state existing in plug welded joints and to realizing the automatic detection of plug welded joints with full coverage.The echo feature analysis and image characterization of the joint zone and defect zone in the plug welded joints are examined.Echoes from the fusion zone,incomplete penetration defects and lack of inter-run fusion defects are analyzed using a 0° wedge for vertical incidence in accordance with the joint state and defect characteristics of the plug welded joint.The echo of the lack of side wall fusion defect of the plug welded joint is examined by employing a 36°wedge for oblique incidence.The analysis results of the S-scan image reveal that the joint zone of the plug welded joint can be characterized accurately on the basis of the -6dB method.The segmented -6dB method put forward in this paper can accurately calculate the effective joint zone of the plug welded joint when there are incomplete penetration defects in the plug welded joints.When there are lack of inter-run fusion defects,the background subtraction algorithm based on linear acoustics can be utilized for extracting the defect echo more precisely.Furthermore,the influence of delay and residual delay on the background subtraction algorithm can be eliminated by delay correction,with the testing accuracy of the lack of interrun fusion defects being further improved.When there are lack of side wall fusion defects in the plug welded joint,the ultrasonic echo signal obtained by oblique incidence has obvious defect echo characteristics,allowing for the qualitative identification of a lack of side wall fusion defect.Based on the analysis findings of S-scan images,the echo amplitude at the bottom surface depth of 3.5 mm of the upper layer steel plate is chosen as the characteristic value to form ultrasonic C-scan images of the joint zone and the incomplete penetration zone.With the depth range of 0-3.5 mm and the return amplitude of each 0.5 mm section(0-0.5 mm,0.5-1.0 mm,1.0-1.5 mm,1.5-2.0 mm,2.0-2.5 mm,2.5-3.0 mm,3.0-3.5 mm)as the characteristic value,ultrasonic C-scan images show no inter-run fusion defects.The ultrasonic C-scan image of the lack of side wall fusion defect is created by taking the echo amplitude at the bottom depth of 3.5 mm of the upper steel plate as the characteristic value.According to the -6dB method,the boundary detection of the fusion zone and the lack of inter-run fusion zone in the ultrasonic C-scan image is realized.Wavelet filtering in combination with a second-order derivative algorithm is used to identify the incomplete penetration echo features.By polynomial fitting and integration algorithm for the extracted multi-segment edge information,accurate analysis as well as the calculation of the effective joint zone of the plug welded joint are made possible.The equivalent arc length of the lack of side wall fusion defect is calculated by using the defect center algorithm.The joint area of the plug welded joint along with the area-type defect characteristics such as incomplete penetration and lack of side wall fusion can be accurately detected and identified using an ultrasonic C-scan image.However,for the typical volume defect of the lack of interrun fusion,the ultrasonic C-scan image can merely identify the area information of the defect,not the height information.In this paper,a 3D visualization imaging technique based on Cscan image segmentation is proposed for the ultrasonic testing of the lack of inter-run fusion of plug welded joints.By combining median filtering with the K-means clustering algorithm,the defect echo of the C-scan image is accurately segmented,and the segmentation findings are then applied to the three-dimensional visualization of the marching cubes method,which addresses the issue that it is difficult to accurately characterize the spatial morphology of defects by traditional three-dimensional imaging methods of threshold extraction of isosurface.The three-dimensional spatial morphological details of the lack of inter-run fusion defects are visualized.In contrast to the metal-cut plug welded joint,the 3D visualization imaging method can be employed to restore the morphology of volumetric defects more correctly.In conclusion,the phased array ultrasonic testing technology developed in this paper meets the quality testing requirements for plug welded joints with complex joint and defect states.It enables comprehensive testing of the joint state and defects,providing a solid theoretical foundation and experimental approach for the quality assessment of arc plug welded joints in practical engineering.