# Analysis of a Sound Signal for Quality Monitoring in Laser Microlap Welding
## 1 Introduction  
Laser welding is a key technology that has been used for decades to fuse various components, ensuring a low heat-affected zone on components during manufacturing. Due to the continuous development of the fiber laser system and the reduction of its operational costs, laser microwelding has been utilized in various applications throughout the past decade. However, the lack of a quality-monitoring system that confirms joint quality can limit the application of laser microwelding to some products that require high reliability. In laser microlap welding, the gap between two metal-sheet layers increases the energy required for joint formation, thus reducing joint quality. Therefore, a gap or quality-monitoring system is crucial for verifying welding quality, and for extending its industrial applications to more fields in the industry.  
Some studies have reported the monitoring of welding conditions during welding processes. However, most studies focused on keyhole-mode laser welding with the conventional macroscale of welding. [PERSON] et al. [1] reviewed various types of sensors adopted for keyhole-mode laser welding,such as audible-sound, acoustic-emission (AE), and optical sensors. An audible sound sensor has the advantage of easy installation, and sound-signal generation is closely correlated to weld-pool generation. [PERSON] [2] proposed an analytical model to develop a relationship between audible sound and weld-pool oscillation. [PERSON] [3] studied a correlation between sound-signal features and defects generated during keyhole-mode welding. [PERSON] et al. [4] confirmed that the energy level of a sound signal generated through welding has a close relationship with laser-power density and welding-penetration condition. Some studies [5; 6; 7; 8] considered frequency-domain sound signals as potential features to identify defects in the arc-welding quality. [PERSON] et al. used audible sound signals for conduction-mode welding to identify the differences between keyhole and conduction-mode welding [9]. This study also discusses the effect of the gap between two metal sheets and plate orientation on sound-signal features. To study systems that monitor the quality of keyhole-mode welding, we developed a sound-based quality-monitoring system based on a neural network [10; 11; 12]. Moreover, [PERSON] [3] and [PERSON] et al. [13] developed sensor-fusion monitoring systems by integrating sound signals with other AE or optical signals. Noise is a problem encountered while applying a sound signal to monitor welded-joint quality. A noise-reduction method was proposed by [PERSON] et al. [14] to reduce the noise effect and to ensure that sound signals can be used in monitoring joint quality during laser welding.  
Few studies conducted welded-joint quality monitoring in conduction-mode welding. [PERSON] et al. [15] developed a quality-monitoring system on the basis of audible-sound signals for thin-plate butt welding. The present study focused on analysis of the correlation between the low bonding strength of welding and sound-signal features. A hidden Markov model (HMM)-based monitoring system was applied to evaluate the quality-monitoring performance of the selected sound features.