| The progress of technology and individual's increasing travel needs have promoted the rapid development of urban rail vehicle industry.Among many models,stainless steel urban rail buses are widely used for their green,lightweight and corrosion resistance characteristics.Among various welding methods,resistance spot welding is an important one in the manufacture of stainless steel car bodies.For ensuring the quality of welded joints and improve the stability and durability of welded structures,improving the level of stainless steel resistance spot welding quality monitoring technology continuously is of great significance.In this paper,for SUS301 L stainless steel,which is commonly used in stainless steel rail bus body manufacturing,the changing characteristics of its real-time ultrasonic signal during the resistance spot welding process are studied,and the visualization technology of resistance spot welding process based on real-time ultrasonic signal and the spot welding quality assessment technology based on M-type image are studied.A real-time ultrasonic detection signal acquisition system is established for the resistance spot welding process.During the spot welding process,the ultrasonic probe placed inside the electrode emits ultrasonic pulses to the inside of the workpiece,and the ultrasonic detection system acquires and stores ultrasonic echo signals through software automatically and continuously.The transient distributions of ultrasonic field inside the welded workpiece at different stages of resistance spot welding are simulated by COMSOL Multiphysics.The sound field characteristic and propagation law are studied during the pre-pressure stage,the initial stage of energization and the stage of nugget formation and growth,which provides a certain theoretical basis for the development of relevant tests.Ultrasonic M-type images are generated by real time A-type ultrasonic signals acquired during the welding process.The ultrasonic M-type image visualize the resistance spot welding process,especially the expansion and solidification of the solidliquid interface of the nugget in a dynamic and intuitive way,enabling the visualization of the nugget formation process under closed space.In order to improve the clarity of the M-type ultrasonic image,the A-type signals were filtered based on the moving average filtering and wavelet denoising techniques.The results show that the image quality can be improved well after processing by wavelet denoising based on db5 basis and enhancing process by relative technique.In this paper,M-type ultrasonic image characteristics of spot welding defects that are difficult to identify by conventional ultrasonic inspection,such as false joints and internal splash joints,are studied.The relationship between T(time of ultrasonic flight between two interfaces of the nugget)and D(diameter of nuggets)and the relationship between T and H(height of nuggets)have been fitted by the linear fitting method.Fitted equations are D=39.078T+39.917 and H=8.986T+304.950 respectively and the value of r-square are 0.98946 and 0.99077,respectively.Compared with traditional methods for identifying false joints,internal splash joints,and measuring nugget size,the quality assessment method based on M-type images features saves both of time and efforts,eliminates the need for complex signal processing techniques,and achieves 100%detection of resistance spot welding joints.Additionally,the in-situ acquisition of ultrasonic signals during the welding process also avoids the impact of the surface indentation of welded joints on the coupling effect. |
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