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Process consistency and long electrode-life are essential requirements for users of resistance spot welding (RSW) in the automotive industry. RSW is the dominant joining process for manufacturing automotive body structures from sheet materials. The technique is cost effective (particularly in high-volume production), makes joints rapidly, is easy to automate, and it has no per-joint consumables. These beneficial attributes apply equally to RSW of aluminium automotive structures. However, there has been some reluctance in the industry to embrace spot welding for aluminium. This is because the electrode-life is much shorter than that experienced when welding traditional uncoated, plain-carbon steels, and there is a general lack of confidence in the consistency of the process. This paper describes a potentially non-intrusive method that addresses these concerns.

Nugget expulsion during resistance spot welding and weldbonding has a detrimental effect on weld nugget integrity and adhesive bond line. This study has concentrated on the detection of expulsion from electrical signals because these are easily accessible on-line. A variety of expulsion events has been studied for which a number of detection methods have been developed. A prototype on-line expulsion monitoring system has been developed based on these techniques. The system does not require the measurement of electrode displacement or acceleration. Laboratory experiments have shown that the system can reliably detect expulsions under a wide range of conditions. The techniques developed provide a practicable method for industry application.