Study On Inversion Imaging Of Nugget In Al Alloy Rsw Based On Thermoacoustic

Resistance spot welding (RSW) is an important component of resistance welding technology, and this process has the advantages of energy concentration, efficiency and easy to automate, which is an indispensable material processing method. It is widely used in various fields. RSW is a process with many factors coupling impact, for example, electric, heat, power, and so on. This process is very short, and the nugget cannot be directly observed, these factors bring more difficult to its quality control effectively. Currently, ultrasound and other testing techniques have done a good job; however, there are still many deficiencies of them. Therefore, this paper introduces the Thermoacoustic theory, combined with the near-field acoustic holography theory and inversion theory, using the periodic heat generated in the workpieces during the ac spot welding and the resulting thermoacoustic effects to stimulate the nugget size information, with which to determine the quality of solder joints. This paper provides a new way for the quality inspection of resistance spot welding.Firstly, with the thermoacoustic theory, according to the similarity of workpieces and flat thermoacoustic devices, a model between nugget size and thermoacoustic emission in the aluminum RSW is established. Taking into account the difficulties of direct measurement on sound pressure of thermoacoustic source, the near-field acoustic holography based on equivalent source method is introduced. An inversion model of thermoacoustic pressure field is established. Then, using the spectral truncation method and the Tikhonov regularization method to calculate the thermoacoustic pressure amplitude of the source was simulated. Finally, the Al alloy ac resistance spot welding experiments are carried out, collecting the acoustic signal around spot welding nugget. According to the collected acoustic signal, the feathers of thermoacoustic emission of RSW and inversion model are considered. Besides, the inversion results are compared with the actual nugget for analysis. The results showed that the thermoacoustic emission intensity of nugget would change with the nugget size, and it will be impacted by the thickness of workpiece. Tikhonov regularization is more suitable for this model calculation than the spectral truncation regularization. The inversion nugget images are seemed to reflect the trends and morphology of nugget to some extent. When the workpiece thickness is 0.5mm, the inversion results are more serious distortion compared with the actual nugget; while the inversion images of workpieces with thicknesses of 1.0mm and 1.5mm do a good job, which can reflect the basic information of nugget, but cannot accurately describe the absolute size of nugget yet.