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A high-speed video imaging system was employed to directly measure the electrode displacement of the small scale resistance spot welding (SSRSW) process. This measurement technique was chosen because it eliminates a number of potential error sources inherent in other electrode displacement measuring techniques. Careful observation of the heating and cooling portions of the electrode displacement curves revealed that each is comprised of two identifiable segments. Distinct high-velocity segments of the displacement curves were thought to correspond to solid-liquid phase transitions in the weld nugget, while lower-velocity portions corresponded to thermal expansion or contraction of solid material. It was found that the magnitudes of the high speed portions of the electrode displacement were more closely correlated with the weld nugget thickness than was the overall magnitude of the electrode displacement. Furthermore, all measures of electrode displacement were more closely correlated to weld nugget thickness than to nugget diameter.

Although there have been many investigations into monitoring and control of resistance spot welding (RSW) of sheet metal having a thickness greater than 0 . 5 mm, that of thinner components has rarely been investigated. Monitoring of expulsion in a small scale RSW process was carried out via measurement of voltage, electrode displacement, and force change during the welding current pulse. It was found that electrode displacement increased steadily during the current pulse for an expulsion free weld. For welds with visible expulsion, the electrode voltage had a small but readily observed spike; the electrode displacement showed a dip or a decrease in total amplitude; the force change during a welding cycle was of greater magnitude. Since the system uses a constant current power supply, the voltage increase corresponds to an increase in dynamic resistance associated with the expulsion event. It was also observed that the magnitude of the electrode displacement dip was directly related to the volume of expelled material. The force change is a relatively sensitive indicator for use in expulsion detection. Of the three signals, detection of the voltage spike is the most readily implemented method for monitoring expulsion in small scale RSW. It was also shown that, owing to the small magnitudes of the forces and displacements, careful design of the welding system is necessary to ensure that unwanted vibrations do not interfere with the process signals.STWJ/367Dr Farson and Mr Chen are in the

Electrode displacement is generally regarded as a variable that can provide real-time information useful for monitoring and controlling resistance spot welding (RSW) process quality. However, in small-scale RSW production, it is difficult to measure the displacement because its magnitude is very small. By contrast, force signals are relatively large and thus are less susceptible to measurement noise. In this article, an empirical model is proposed to simulate the dynamics of an SSRSW head with the objective of calculating electrode displacement from the variation of electrode clamping force measured during welding. The parameters in the model were determined by fitting experimental force and displacement signals with polynomials and then performing an optimization search for parameters of first-order dynamic models. To verify the models' accuracy, they were subsequently applied to simulate the electrode displacement curves of welds with expulsion and without expulsion. The calculated displacement curves agreed well with experimental measurements, and the occurrence of expulsion was clearly indicated by the model predictions. A more comprehensive model is under construction with an objective to eliminate displacement sensor in the monitoring and control of SSRSW process.

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