Non-linear dynamics signal analysis for quality monitoring

There is considerable interest in techniques for ensuring the quality of the laser joining process. One approach to quality monitoring is through analysis of acoustic and optical emission signals acquired during welding. Preliminary analyses confirm that these signals fluctuate chaotically, suggesting that there may be benefits to a nonlinear signal processing approach. Lyapunov exponents of cleaned signals were calculated to quantify the nonlinear nature of the process. Inspection of Lyapunov exponents calculated from optical and acoustic emission signals corresponding to a variety of weld penetration conditions revealed that no sharp distinction could be found. The Lyapunov exponents were found to depend strongly on the power and travel speed settings of the process. This variation masked any variations due solely to weld penetration. There was some distinction in Lyapunov exponents noted between non-gap and gap butt joint welds. The Lyapunov exponent for butt welds with significant joint gap was higher than for non-gapped welds. It was concluded that monitoring of weld gap might be possible using the Lyapunov exponent of the optical and/or acoustic signals.