Study on the nugget growth in single-phase AC resistance spot welding based on the calculation of dynamic resistance

The process of nugget growth is invisible and difficult to test nondestructively in the process of resistance spot welding. In this study, the change of welding current and electrode voltage in the secondary circuit was monitored in real-time. Accordingly, the variation of the dynamic resistance was obtained across electrodes and the dynamic resistance in welding process was analyzed to characterize the nugget growth. The results show that the whole process of nugget growth is composed of initial stage, growing stage and stable stage according to the curve of dynamic resistance. The curve variations of three stages indicate the characteristic of nugget growth. Different types of nugget growth show diverse curves of dynamic resistance, which is a useful means to estimate the nugget growth and nugget quality. But only the forming process of adequate nugget is composed of three stages (initial stage, growing stage and stable stage) completely. The welding parameters such as welding current, electrode force, etc affect the process of nugget growth. Larger welding current and larger electrode force improve the rate of nugget growth.