Study on monitoring of plasma emission signal in lap welding of Zn coated steel sheet using CO2 laser

In this study, induced plasma that is created during the CO 2 laser lap welding of 6-mm-thick Zn coated steel for shipbuilding were measured through photo-diode and microphone. Then, we compared and analyzed the RMS value of plasma emission signal and spatter generation amount as a function of lap clearances with Zn coated thickness. The results indicated that as RMS values increased depending on the amount of Zn vaporization, and RMS also changed abruptly when defects occurred depending on lap joint conditions. Also, as a result of obtaining FFT value of raw signal by welding conditions, the obtained frequency signal was about 30 Hz at coating thickness 15 μm and about 50 Hz at coating thickness 30 μm in light signal, and in acoustic signal, peak value appeared at about 1 kHz. In addition, it was found that when the frequency value was set as the bandwidth of bandpass-filter, and the RMS values which were obtained after filtering raw signals were matched to weld bead. Thus we have considered that the signals and defects were very closely related.