Wavelet analysis-based expulsion identification in electrode force sensing of resistance spot welding

Resistance spot welding is one of the most widely used processes in metal fabrication, but the expulsion affects the spot quality. To develop a precise and reliable weld quality assessment method for resistance spot welding, the welding force signal was measured and a novel wavelet transform based on multi indexes is proposed. The impulse and resultant damping vibration signal, which is the most obvious feature of expulsion, was extracted from the welding force waveform by 7-level wavelet transform with Daubechies5 wavelet. The detail signal in level 6 was chosen as the target signal, as it covers the dominant frequency of expulsion. To obtain the local characteristics of the expulsion, a multi indexes were calculated which includes the peak-to-peak value, kurtosis index, and pulse index. The peak-to-peak measures the range of a signal value, and kurtosis index and pulse index are sensitive to the impulse signal. According to experimental analysis, the peak-to-peak value of the expulsion signal is significantly higher than that of other conditions, while other two indexes are not that obvious but with same trend. The experimental data results show that the multi expulsion indexes are effective indicators in evaluating expulsion in resistance spot welding.