Study On Magneto-optical Imaging Of Weld Microstructure And Defect Detection

With the development of industrial technology,laser welding is applied in ship,pipeline transportation and other fields.However,there are different welding defects during the welding process.It is harmful to the working process.Hence,it is crucial to detect the weld defects and avoid accidents for human security.And magneto-optical imaging detection method is used in academic research frequently.This paper studied the characteristic of magneto-optical images of weld joint microstructure based on Faraday's magnetic rotation principle from a micro perspective.And the relationship between the weld microstructures and the magneto-optical imaging was obtained.On this basis,the detection of defects with different plate thickness by magneto-optical imaging was researched.It supplied the theoretical foundation for raising the detection quality of industrial.Firstly,the butt welding of low carbon steel plate was used to compare the weld formation differences under laser welding parameters.This research compared the magneto-optical images and the scanning electron microscope images of weld microstructures.and the magneto-optical image differences of the weld microstructure in different regions were analyzed,including the grain composition and characteristics of the weld microstructure.The results show that the welding seam can be divided into three regions in the magneto-optical image under the residual magnetic field,which are the fusion region,the heat affected region and the base metal region,and the grain composition,size and morphology of the three zones are obvious.The color,brightness and gray value of the magneto-optical images of the weld under constant magnetic field reflect the characteristics of the weld.I₁,I₂,I₃?Br₁,Br₂,Br₃?S₁,S₂,S₃represent the light intensity,residual magnetic field strength and grain size of fusion zone,heat affected zone and base metal zone respectively.And it can be expressed as I₁<I₂<I₃,Br₁>Br₂>Br₃,S₁<S₂<S₃.With the decrease of grain size of weld,the brightness of magneto-optical image is darker.According to the microstructure characteristics of the weld,the above results show that the weld can be divided into fusion zone,heat affected zone and base metal zone,and different regions had different effects on magneto-optical imaging.Based on the correlation between the weld microstructure and magneto-optical imaging,the magneto-optical imaging detection of defects with different plate thickness was studied.The low-carbon steel with different plate thickness were used for butt laser welding,and acquired the magneto-optical images of defects under the excitation of alternating excitation.The characteristics of magneto-optical image of welding defects with different plate thickness were studied.The second magneto-optical image of each group is transformed and denoised,and extracted the gray curves or three-dimensional gray surface.The amplitude and phase spectrum were obtained to analyze the frequency-domain characteristics of magneto-optical image after image equalization.The results show that there are three regions in the magneto-optical image of weld defects,and the magneto-optical images of weld defects are unclear with the increase of the steel plates thickness,and the brightness is weak.