Wavelet analysis of plasma optical signals at pool penetration in laser welding

The spectra of the optical signals emitted by plasma during laser welding were studied using wavelet analysis. In the presence of the wavelet analysis, the detection of the pool penetration defects in welding process was realized. Comparing optical signals in welding process with and without joint penetration problem, the significant differences can be observed. By continuous wavelet transform, the coefficient lines of some scales have rapid fluctuation at the positions where pool penetration defect occurred, and the maximal coefficient values at the signal breakdown correspond to the length of the defect. In discrete wavelet analysis, the amplitude of low-frequency component of optical signals is fallen down remarkably when the welding defects occur. Correspondingly, the high-frequency component of the signals is almost decline to zero simultaneously. Considering that wavelet analysis can decompose the optical signals, extract the characteristic information of the signals and define the defects location accurately, it can be used to implement process-control of laser welding.