Monitoring and phenomena observation during YAG laser lap welding of Zn-coated steel sheets

A study was performed with the objectives of understanding lap welding phenomena of Zn-coated steels with a Nd:YAG laser as well as obtaining a fundamental knowledge of monitoring signals for the formation judgment of sound or bad weld beads. The behavior of a molten pool and a reflected beam was simultaneously observed through a high-speed video together with the monitoring of reflected beam intensity.The effect of a gap between sheets on porosity formation and bead appearances was confirmed,and characteristic monitoring signals were obtained according to the gaps.In the case of no gap,spatters were frequently generated,and a reflected beam was fluctuated intensively at low frequencies.On the other hand,in welding sheets with a wide gap,lap welds were not produced and the high frequency signals of a reflected beam were detected.Moreover,sound welds were produced in the sheets with a proper gap,and a moderate reflected beam was monitored.From these results,it was found that monitoring of a reflected beam was beneficial to the judgment of sound,under-filled or incomplete lap welds.