Experimental study of nugget formation in resistance spot welding

The nugget formation mechanism in resistance spot welding and its effect on the welding process parameters are examined by using a digital highspeed camera to visually monitor and observe the process of nugget formation. A modified half-section truncated-dome electrode and illumination system are introduced for the study, and the dynamic resistance, the main process parameter containing information on the nugget formation, is monitored and compared to the captured faying interface. In addition, the mechanism of nugget formation and the pulsing phenomenon, including heat generation and transfer, are examined based on the color changes of the faying interface and the development of a yellow-red heat zone. Variations of the lengths of the current path and the contact area of the faying interface are discussed together with equivalent resistivity across electrodes to consider the effect of the factors, which contribute to the increase and decrease of the dynamic resistance. Furthermore, the dominant factors contributing to the dynamic resistance change are also presented with high-speed photography.