Study Of Magneto-optical Imaging Characteristics Of Weld Defects Under Magnetic Field Excitation

Welding is a widely used processing technology in modern industry.Due to the extremely complicated welding process,it is easily affected by welding equipment and welding parameters,resulting in a series of tiny weld defects such as crack,sag,nonpenetration and so on,which affect the quality of welding products and reduce productivity.The traditional non-destructive testing methods for weld defects has limitations.For example,the source of radiation detection is harmful to human body,ultrasonic detection requires coupling agent,and magnetic flux leakage detection is only suitable to the detection of ferromagnetic materials.In this paper,a new method of magneto-optical imaging non-destructive testing based on Faraday magneto-optical effect is studied.Under the excitation of different magnetic fields,the leakage magnetic field around the defect is obtained,and the magneto-optical image of the defect is formed to judge the quality of welding products directly and improve the effect of non-destructive testing.The current magneto-optical imaging detection of weld defects is mostly based on constant magnetic field and alternating magnetic field excitation,but the method is still difficult to detect multi-directional weld defects.In order to realize the detection of multi-directional defects and subsurface defects and improve the non-destructive testing efficiency of defects,this paper studies magneto-optical imaging based on alternating magnetic field and rotating magnetic field excitation,and explores the magneto-optical images of weld defects under different magnetic fields excitation.The characteristics of magneto-optical imaging detection for weld defects under different magnetic fields excitation lay the foundation for visual inspection of complex defects and subsurface defects,and improve the nondestructive testing efficiency of welding products.At first,the subject uses TIG welding or laser welding to produce numbers of small defects that can be used for detection,such as crack,sag,and non-penetration.A magnetooptical imaging test platform under alternating magnetic field and rotating magnetic field excitation was built.Based on the platform,different parameters(including excitation voltage,lift-off value)and magneto-optical imaging experiments of different defects were applied.Secondly,in the condition of magneto-optical imaging test system under rotating magnetic field excitation,by changing the excitation voltage of rotating magnetic field generator and the lift-off value of the magneto-optical sensor,the detection effect of weld defects under different rotating magnetic field excitation is explored,and the magnetooptical image under different parameters is compared and analyzed.The imaging condition is used to determine the optimal excitation parameters of the rotating magnetic field.At the same time,the magneto-optical imaging law of weld defects under the excitation of the rotating magnetic field is analyzed,and the influence of hysteresis effect on magneto-optical imaging is explored,which provides a experimental basis for further research on the rotating magnetic field imaging.Besides,a set of defect samples with different directionality are selected as surface defects,and subsurface defects are simulated by adding the plate of same material on surface defects.The surface defects and subsurface defects under the alternating magnetic field and the rotating magnetic field excitation are compared.The difference in magneto-optical images indicates the ability of alternating magnetic fields and rotating magnetic fields to detect multidirectional defects and subsurface defects.Finally,the magneto-optical image of weld defects is processed in the frequency domain,such as two-dimensional discrete Fourier transform,Butterworth high-pass filtering.Through a series of frequency domain filtering and transformation,the frequency domain characteristics of different weld defects and the imaging mechanism of the features are analyzed,and the corresponding relationship between the frequency domain features and the spatial characteristics is obtained.The method improves the nondestructive testing effect of weld defects.