Magneto-optical Imaging Analysis And Contour Reconstruction Of Welding Defects In Alternating Magnetic Field

Because of the complex welding process,the welding quality is easily affected by various welding parameters.Different kinds of defects,such as surface cracks,pits,holes,slag inclusion,weld seams not fused and penetrated will be formed,which will greatly harm the quality of welding products.Although some non-destructive testing methods can explain and judge the defects of the welded parts through the physical characteristics of light,electricity,sound,heat and magnetism without damaging the welded parts,and realize the non-destructive testing of the welded defects,they still have some limitations.For example,visual inspection has high technical requirements for inspectors;there is a large amount of radiation in the process of X-ray detection,which is very harmful to human body;coupling agent will be smeared before ultrasonic testing,which will not change the detection;before penetration and magnetic particle testing,the workpiece to be measured needs to be roughened;and the skin effect of eddy current testing is obvious,and the detection ability is limited.This research is a new method to detect welding defects by changing law of magnetic field.Combining Faraday magneto-optical effect to explain the magnetic field distortion and field energy accumulation at the defect,the defect can be judged and analyzed.Firstly,different kinds of welding defects are produced by different welding methods and welding process parameters as test samples.At the same time,different types of magnetic field generators have been tested,such as DC constant magnetic field and AC variable magnetic field generators.The magneto-optic image(MOI)of the defect is obtained by magneto-optic sensor.The rule of MOI is studied based on MOI processing data and electromagnetic field theory.And the magnetic flux leakage(MFL)characteristics of MOI of welding defects under constant magnetic field and alternating magnetic field were compared.Secondly,Maxwell electromagnetic simulation software is used to simulate the MFL detection of welding defects under alternating magnetic field,and the relationship between the signal of MFL and the length,depth,width and detachment of defects is explored.The simulation model is validated and analyzed by experiments,and the reliability of the model is determined.The results of simulation and experiment are consistent.The eddy current effect exists under the action of alternating magnetic field,which affects the detection ability of MOI of welding defects.The simulation model of MFL is established to study the magnetic distribution law of welding defects,and to explore the skin effect and the actual detection ability of MFL detection/MOI detection in the detection process.Thirdly,through the analysis of simulation and experiment of MOI of welding defects under alternating magnetic field,we can understand the law of distribution and imaging of MFL.By acquiring different kinds of MOI of welding defects,we can reconstruct the two-dimensional surface contour of welding defects according to image processing method.At the same time,the component of MFL signal at the defect is obtained by simulation method,and GRNN neural network is trained to determine the mapping model between MFL signal and defect parameters.The conclusion that two kinds of MFL signals B_z and B_y can reconstruct the two-dimensional contour of welding defects through neural network is obtained,which provides a method and basis for the actual two-dimensional contour reconstruction of defects.Finally,the MOI of the defect is obtained by experiment,and the Bz signal of the MFL component is represented by the gray information of the image,which is input into the neural network model to reconstruct the two-dimensional contour of the welding defect.