Monitoring of expulsion in small scale resistance spot welding

Abstract

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

Modeling small-scale resistance spot welding machine dynamics for process control

“…After frame-by-frame processing of images, the electrode displacement was obtained. Previous reports [8,9] provide detailed discussions of the accuracy of this technique and comparisons to alternative methods for displacement measurement in small-scale RSW. The welding current and time were varied so as to produce welds over a range of heat inputs that had different nugget sizes and extents of expulsion of molten metal during welding, as shown in Table 1.…”

Analytical modeling of heat conduction for small scale resistance spot welding process

“…A more rigorous and accurate process model is preferred for a real-time control that give accurate predictions under such conditions. Conduction heat transfer in the dominant process of a small scale RSW (SSRSW) [9] process is analyzed in the present work, first with a simplified one-dimensional model, then with a two-dimensional axis-symmetric model that is closer to the real process. Nugget size is obtained by locating the melting isothermal lines in the calculation domain and the predicted nugget shapes and sizes are compared to experimental measurements.…”

Expulsion analysis of resistance spot welding on zinc-coated steel by detection of structure-borne acoustic emission signals

“…Normally, engineers pay more attention to the first two factors but neglect the last one. Expulsion occurs when the weld nugget grows to such an extent that electrode pressure is insufficient to contain the high pressure caused by volumetric expansion of the liquid metal [6]. So, expulsion is a common phenomenon with molten metal ejection during RSW process, which leads to surface indentations and poor working conditions [7].…”

“…Some previously successful detect schemes are still appropriate to monitor expulsion. Farson et al introduced the measurement of voltage, electrode displacement, and force change during the welding current pulse to monitor expulsion in a small-scale RSW process [6]. Primoz et al pointed to the electrode pressure signal as the most important indicator of the expulsion occurrence.…”