The Study On The Threshold Characterization For Laser Penetration Welding

There are two fundamental modes of laser welding, namely, conduction welding and penetration welding, on the base of producing keyhole or not. When the mode of the laser welding is changed from conduction welding to penetration welding, the absorptivity of metal to the laser radiation will sharply increase and the depth of the molten pool goes up correspondently. In this paper, a high-power CO₂ laser and a Nd:YAG laser were adopted and the changing characters of laser welding modes were studied with a high speed camera and a optical spectrometer. The materials used in the experiments were steel A3, aluminum alloys 2024 and 5083.Firstly, testes were carried out to determine the expression of the threshold for laser penetration welding. The experiment data show that the threshold laser power density for penetration welding decreases with the increase of the beam spot size. However, the ratio of the laser power to the beam spot diameter (P/d) has no relation with the beam spot size when the welding mode changes from conduction welding to penetration welding both in CO₂ and YAG laser welding. So it is reasonable to applying the ratio of the laser power to the beam spot diameter (P/d) to characterize the mode transition threshold.Secondly, taking P/d as the expression parameter of the threshold for laser penetration welding, the welding mode transition characterization was explored. The results show that materials properties, sample's surface roughness and assistant gas have greater influences on the threshold in YAG laser welding than in CO₂ laser welding.Finally, the spectra of the vapor plume were investigated and the role of laser-induced plasma on the generation of the penetration welding was analyzed. It is found that the metal vaporizes obviously even in the period of conduction welding and the spectra of the vapor plume remain the same between the conduction and penetration welding modes in YAG laser welding. As for CO₂ laser welding, the metal vaporizes weakly just before the mode transition occurs from conduction welding to penetration welding, and the metal vaporizes violently and the vapor ionizes partly in penetration welding. Further analysis shows that the formation of a vapor plasma enhances the absorption of metal to laser beam and thus promotes the establishment of the penetration welding process in CO₂ laser welding.