Study On Inversion Imaging Of Nugget In RSW Based On Magnetic Field

Resistance spot welding is one of the important parts in the field of welding technologies. The spot welding process is a complicated phenomenon, which involves the mechanical, thermal, electrical, magnetic and metallurgical factors. Because the process is ephemeral and welding joint region is close, it is difficult to monitor the weld quality effectively. Although the current quality detection techniques have made certain effect, they still have some shortcomings. This paper introduces the inverse problem theory, combines inverse problem theory with electromagnetism and temperature field, and utilizes the magnetic field during spot welding to calculate the current density distribution in nugget cross-section. The inversion image can be used to evaluate the spot weld intuitively, which provides new criterion to judge the weld quality.Firstly, this paper establishes the forward model, which describes the relationship between the magnetic field and current distributions of nugget cross-section, based on the Biot-Savart law, and its reliability is verified by experiments. Secondly, inversion model is established according to electromagnetism, and the ill-posedness of the model is analyzed. In order to solve the ill-posedness, this paper simulates the temperature field in the nugget cross-section by finite difference method, and then combines the temperature field with the inversion model. Thirdly, regularization method is used to solve the current density distribution. Finally, different welding currents and sensor arrangements is chose for the simulation experiments resistance spot welding experiments. Matlab is used for data processing and the magnetic signal is transformed into current distribution, which is shown in the form of image. Thereby, we can analyze the characteristics of the inversion image in different current, the effect of sensor arrangements on the inversion and the relationship between inversion image and the nugget quality.The results show that the inversion image reflects the characteristics of nugget cross-section current distribution to a certain extent, and the weld quality can be evaluated by it. In the simulation experiments, the sensor arrangement has no obvious effect on inversion, while in the spot welding experiments the sensor arrangement has large effect on inversion, especially in the region of large error, the inversion result is poor.To apply inverse problem theory and regularization method to resistance spot welding is still in the beginning stage, the algorithms and models are not complete, and hence need further research.