Study On Ultrasonic Testing For Connection State Of Resistance Spot Welding Of Multilayer Stainless Steel Sheets

With the improvement of people's living standard,traffic becomes more efficient and convenient,and the “high-speed running” and “safety” of rail transit become hot topics.In the production of rail vehicles,resistance spot welding of stainless steel is widely used due to its advantages such as small influence of production environment and beautiful surface of product,and part of the roof and side wall of rail vehicles should be accomplished by the spot welding of multilayer stainless steel sheets.Due to the complex distribution of welding current and high requirements for welding assembly,small fluctuation of welding parameters during multilayer spot welding can cause problems such as nugget migration,small size of welding nuclear,dry joints,over welding,defect of welding nuclear and so on.Especially in the welding for multiple unequal thickness plates,nugget migration will directly affect the connection strength.Therefore,in order to ensure the safety of rail vehicles,it is particularly important to study the quality control and detection of multilayer spot welded joints.At present,the common quality evaluation methods of resistance spot welding of multilayer stainless steel sheets mainly include the monitoring mechanism of parameters in the process of spot welding and the joint damage detection after welding.The parameter monitoring mechanism is to control the welding joint quality by adjusting the heat generated in the process of spot welding.Because the secondary impedance varies greatly in the process of spot welding,the establishment of the welding monitoring mechanism is comparatively complex.The traditional destructive inspection has high cost requirement,low efficiency and it is unable to detect the quality of solder joints in service.For other nondestructive testing methods,magnetic flux leakage testing method is only suitable for the surface or near surface defect detection of the work piece.The effect of thermal infrared image testing method for the surface gloss metal is not ideal and it is not suitable for spot welding joints internal quality detection.Ultrasonic testing has the advantages of low cost,no ionizing radiation and high degree of automation.Ultrasonic testing is suitable for spot welding joint quality testing in industrial production line.Therefore,the research on ultrasonic testing for resistance spot welding state of multilayer stainless steel sheets is of great significance for improving industrial production efficiency and ensuring vehicle safety.In this paper,a time-sharing and same-position ultrasonic scanning method for the upper and lower surface of multilayer spot welding joints was presented,and based on this method,an automatic ultrasonic testing device for multilayer spot welding joints has been developed independently.The device can carry out ultrasonic automatic detection from the upper and lower surfaces of the workpiece,and can realize one-to-one correspondence for the scanning points of double-side detection of the welding workpiece through the precise positioning device,and then the connection area of the upper and lower interfaces can be comprehensively analyze.At the same time,the device has the functions of ultrasonic A-scan detection,C-scan imaging and data storage,which provide support for the further analysis of ultrasonic echo of multilayer spot welded joints.Through the analysis of ultrasonic echo signals from the upper and lower surface of multilayer spot welded joints,it was found that the amplitude of the primary echo in the ultrasonic time domain signal is sensitive to the joints connection interface and can be used to distinguish the connected area from the unconnected area on interfaces.However,due to the influence of surface indentation and other factors,the anti-interference performance for the amplitude of the primary echo of the time-domain signal is poor,and there is a certain detection error.Through frequency domain analysis of ultrasonic A-scan echo signal,it was found that the characteristic frequencies of 10 MHz and 14 MHz in the ultrasonic waveform are sensitive to joints connection state,and their anti-interference ability are higher than the amplitude of the primary echo in the time domain.The dimensional deviation of the interface of the multilayer spot welding joint formed by using eigenvalues of the frequency domain signal is small.Ultrasonic C-scan images of multilayer spot welded joints can be formed by using both time-domain and frequency-domain eigenvalues,and three-dimensional views of welded joints can also be formed.By means of image processing such as Gaussian filtering and edge recognition,the boundary of ultrasonic C-scan image can be extracted effectively.According to the boundary of ultrasonic C-scan image,the equivalent connection diameter can be processed on the upper and lower connection interfaces of the spot welding joints of multi-laminates.The welding joint shear strength test results showed that the failure behavior of the pull-shear of the multilayer spot welded joints basically occurs on the side with the smaller connection area,in other words,for the upper and lower interfaces of welded joints,the interface with smaller equivalent connection diameter is prone to fracture.therefore,based on the equivalent diameters of the connection interfaces in the ultrasonic C-scan images,the connection strength of multilayer spot welding joints can be predicted.