# Analysis of Acoustic Emission (AE) Signals for Quality Monitoring of Laser Lap Microwelding
## 4 Results and Discussions  
The AE signals studied in this work were collected simultaneously with the audible sound signals in the same experiments as in previous work [22]. Two experimental setups were used to obtain different contact conditions between the two layers of metal sheets. The peeling test was performed to verify the joint bonding strength, and the surface condition of each broken joint was investigated using an optical microscope after the peeling test. Figure 5 illustrates the conditions of the top and bottom surfaces of both layers of metal sheets after peeling [22]. The surface condition presented in Figure 5a was obtained from a case with a peeling force of >15 N and considered to be a normal joint. The surface condition presented in Figure 5b was obtained for a case with low peeling force and considered to be a loss of contact between layers and an unqualified joint. Figure 5a reveals clear tearing of the material on the bottom layer, and the part of material that was removed appears on the bottom surface of the top layer of the sample. By contrast, no material was removed from the bottom layer of the sample (Figure 5b), and only a magnified view of the heat effect observed on the bottom surface of the top layer and the top surface of the bottom layer is presented. This result suggests that no relevant joint was created between the two layers of the welded sample when the gap was generated. The range of peeling forces used for the proper contact cases was 12-17 N. No peeling force was obtained for the low-strength bonding because complete peeling occurred abruptly upon initiation of the peeling test. The AE signal was generated immediately after the contact of the laser beam with the sample surface. Figure 6 presents the AE signals collected from cases with normal or low joint strength. Different patterns for the two cases were obtained in the first 1 ms of welding, possibly because various thermal transmission patterns were caused by the different contact conditions between the two layers of metal sheets. Moreover, for the first 4 ms of welding, the energy of AE signals from joints with normal bonding strength was higher than that of the signals from the joints with low bonding strength. The observation matches well the fundamental AE signal generation mechanism in that the higher energy of the AE signal could be generated before the weld pool is created completely because of the higher strain rate generated by thermal expansion on solid material in that period. Figure 5: Surface condition of samples after peeling: (**a**) high and (**b**) low joint strength [22]. Since the signal in the initial 3 ms might contain more information regarding the weld pool generation, the signal in the initial 3 ms was used to extract the features of the frequency domain that are closely related to joint bonding strength. Figure 7 presents the frequency domain of the first 1 ms of AE signals for both cases. The results indicate no considerable difference between the signal energy for both cases at frequencies of <150 kHz. However, the energy level of signals of >150 kHz for normal bonding strength was higher than that for the case with low joint bonding strength. This indicates that the energy level of the time domain signal in the first 1 ms of welding and frequency domain features are promising features for identifying low joint bonding strength. Figure 8 presents the frequency domain signals for the first three (of eight) 1 ms welding intervals. Figure 6: AE signals collected during laser microwelding for joints with (**a**) low and (**b**) normal bonding strength. Figure 7: Frequency AE signals collected during laser microwelding for normal (without gap) and low bonding strength (with gap). For the case with the low bonding strength (Figure 8a), the energy level was the same for the first two 1 ms intervals, and that for the third interval was not much lower.