# Analysis of a Sound Signal for Quality Monitoring in Laser Microlap Welding
## 4 Results and Discussion  
To verify the joint strength of each welded sample, a peeling test was conducted. Moreover, the surface condition of a broken joint was examined using an optical microscope. Surface conditions pertaining to the top and bottom surfaces of both metal-sheet layers after the peeling test are shown in Figure 4. The surface condition presented in Figure 4a was obtained from the case in which the peeling force was higher than 15 N, and that involved proper contact between layers. The surface condition presented in Figure 4b was obtained from the case with very low peeling force that involved improper contact between layers. The clear tearing of the material on the bottom layer is shown in Figure 4a, and the separated part of the material was observed to be joined together on the bottom surface of the top layer of the sample. Conversely, no material separation can be observed in Figure 4b from the bottom layer of the sample. Only heat-effect zoom was observed at the bottom surface of the top layer and the top surface of the bottom layer. This finding suggests that no significant joint was created between the two metal-sheet layers. Peeling forces in the proper contact cases were in the range of 12-17 N in this study. No peeling force was available for low-strength-bonding cases because complete peeling occurred abruptly when the peeling test was implemented. Figure 4: Surface condition of samples after peeling test: (**a**) high joint strength; (**b**) low joint strength. Figure 3: Schematic of determining unknown quality condition by using developed model [17]. Because the AE signal was generated immediately when the laser beam and samples came into contact, AE signals were collected simultaneously with audible sound (Figure 5) to verify the starting point of the laser application to the metal sheets. Both AE and sound signals presented different characteristics when welding was conducted. Sound signals collected from cases with normal and low-joint-bonding strengths are shown in Figure 6. Different patterns were observed between the two cases (Figure 6a,c), and signal energy seemed to be a good feature to identify the case with low-bonding-strength joint. However, by reviewing the sound-signal pattern obtained from 20 samples in the case with normal-joint-bonding strength, and 20 samples in the case with low-joint-bonding strength, the variation of signal energy in the same case was observed. Low signal energy was observed for some samples with high-joint-bonding strength (Figure 6b). On the other hand, high signal energy was observed for some samples with low-joint-bonding strength (Figure 6d). After calculating the root-mean-square (RMS) values of full-length sound signals for 20 samples in the normal-joint-bonding strength case, and 20 samples in the low-joint-bonding strength case (Figure 7), a mixture zone of RMS values of full-length sound signals for both cases was observed. Therefore, by only adopting RMS values of full-length sound signals as the feature in monitoring-system development, this may lead to failure in identifying joint-bonding strength. Based on above discussions, other signal features should be studied and extracted to improve system robustness in monitoring joint-bonding strength in laser microlap welding. Figure 5: Signals collected during laser microwelding: (**a**) acoustic-emission (AE) signal; (**b**) audible sound. Figure 6: Sound signals collected during laser microlap welding: (**a**) normal joint strength—Nsample 1; (**b**) normal joint strength—Nsample 2; (**c**) low joint strength—Lsample 1; (**d**) low joint strength—Lsample 2. To analyze time-domain features and extract features closely related to the change of bonding strength, the signal correlated to the entire welding process was divided into eight sections (Figure 8) for feature analysis. The time-domain signals for each selected section are shown in Figure 9. The mixture of obtained signals from both cases can be observed in each section.