Investigation On Characterization Of Deep Penetration Welding With High Power Fiber Laser Welding

High power fiber laser has been receiving attention in world wide owing to theirexcelent advantages such as higher beam quality, higher operating flexibility andlower operation cost, etc. In the field of welding, lots of researches have found thatthe deep penetration welding characterization of high power fiber laser is obviousdifferent from that of CO2laser. The deep penetration welding characterization ofhigh power fiber laser is thus investigated by studying the deep penetration weldingthreshold, the formation mechanism of plume, the effect of plume on welding processand the energy coupling mechanism of laser in the keyhole for more comprehensiveunderstanding it in this paper.The bead-on-plate welding is carried out with an IPG YLS-6000fiber laser, andthe physical process of welding mode transition and the deep penetration weldingthreshold are also measured. The self-focusing of molten pool to incident fiber laserbeam, which results in the jump of recoil pressure evaporation, is considered to be thereason why the welding mode changes. Then, together with the―self-focusing‖model,the force balance mechanics in the surface of molten pool and the molten pool surfacetemperature field, the characterization form of deep penetration welding threshold isderived and is P/d. Then the characterization form is proved by numerical calculationand comparing with the experimental result.The spectrum emitted by plume is recored by using a spectrometer during highpower fiber laser welding. The plume temperatures at different heights are alsodiagnosed by using the spectral informations. In order to overcome the problem oflack line spectra for diagnosing plume temperatures at different heights, the newtemperature diagnosis method is developed, based on continuous spectrum and Wiensdisplacement law. The new method used in the temperature diagnose is evaluated bycomparing the laser-arc hybrid welding plasma temperatures, which are diagnosed byusing the new method and Boltzmann plot method.The plume behavior at different welding velocity, different welding surroundingsor before and after the laser beam shifted away from the welded sample are recordedby using a high speed camera. The collected nanoparticles and spatters in weldingspace are also measured by applying a TEM and SEM, respectively. Together with theplume behavior at different conditions, plume temperatures at different hights, and the plume behavior as the welding is carried in cacuum, the formation mechanism ofplume is analysed. The results show lots of spatters with1~20um in diameter areejected from the keyhole during welding, and the high-speed increasing andvaporation of a few spatters in laser beam is the formation mechanism of plumesimilar to the focused laser beam. The plume similar to the focused laser beam is themixture of nanoparticles, a few spatters and hot gas.The severe effect of plume on weld width, penetration and the welding processstability is confirmed by using the supersonic cross jet to blow away plume atdifferent heights, changing the welding velocity or transforming shieldingenvironment. Together with the experimental results and the calculation in theory, themain mechanism of the plume effect on the welding process is revealed to be Miescattering of spatters and Rayleigh scattering of nanoparticles in the laser beam. Thehigh-speed increasing of a few spatters in laser beam is the main reason why thewelding process is affected by plume.The energy coupling way during high power fiber laser welding is studied bycomparing the change of weld width, penetration and molten melt efficiency withwelding velocity during the high power fiber laser welding and CO2laser welding atalmost same weld condition. The changing of front wall of keyhole angle and theabsorption in front wall of keyhole with the welding velocity are calculated in theory.It is proposed that the fiber laser energy coupling way in keyhole is the multiplereflections absorption as the welding velocity is low, and the welding process isunstable. When the welding speed is increased, the main energy coupling way inkeyhole is the first time absorption in front wall of keyhole due to the angle ofkeyhole front wall closed the Brewster and the absorption with about80%, and thestability of welding process is increased. When the welding velocity is furtherimproved, the first time absorption in front wall of keyhole is small due to the angleof keyhole front wall less than the Brewster, the stability of welding process is poorowing to the interference of the reflected light in back wall of keyhole.