Study On The Behavior Of Metallic Vapor Plume And Defects Control During Deep Penetration Laser Welding Of Thick Plate Using10-kW Level High Power Fiber Laser

Compared to the traditional welding processes, deep penetration laser welding has many advantages, such as the larger aspect ratio (bead depth/bead width), the higher welding speed, the lower heat affected zone (HAZ) and the lower weld distortion. The advent of10-kW level fiber laser with high beam quality should now make it possible to join thick plate with single pass deep penetration laser welding. In this case, the10-kW level fiber laser can be used for joining of thick plate in the energy, ship-building, aerospace, equipment manufacturing and railway industries. However, these major engineering projects have strict requirements on the stability and reliability of the welded joint. Only a comprehensive investigation of processing mechanism of deep penetration high power laser welding can fundamentally guarantee reliability and stability of the welded joint. Under this background, supported by the National Natural Science Foundation of China, the dynamic behavior of metallic vapor plume, keyhole and welding pool, the formation mechanisms and control procedures of welding defects during deep penetration laser welding of thick plate with10-kW level high power fiber laser have been investigated experimentally and theoretically. As a result, the following studies are carried out in this dissertation:(1) The metallic vapor plume dynamics and spectra were observed directly using high-speed photography and spectrometer, and then the characteristic parameters of the plasma plume and the scattering and absorption effects of the metallic vapor were calculated. The results show that there are two parts of laser induced vapor plume outside the keyhole with different dynamics and geometries. The lower part is so-called laser induced plasma with blue emission and sharp boarders. The upper and outer part reddish emission, namely metallic vapor, has a changeful geometry wi th a height of more than20mm. The direction of vapor plume ejection is closely related to the position of the keyhole wall. The results of spectral analysis indicate that the vapor plume is weakly ionized plasma. The total attenuation (including scattering and absorption) of the incident laser beam by condensed metallic particles in the vapor plume is several times of the inverse bremsstrahlung absorption by free electrons in the vapor plume.(2) Based on a modified “sandwich” novel method, the inner keyhole wall with “gauffers” and “shelf” moving downwards into the keyhole depth and micro-droplets tearing-off from the peak of gauffer wave are directly observed for the first time via high speed camera. Significantly, the liquid shelf on the front keyhole wall moves downwards fast with flow velocity up to8m/s-15m/s.On the base of direct observation of the keyhole, vapor plume and welding pool dynamics with formation of welding defects and many technological experiments, the formation mechanisms of the welding defects, such as spatter, undercut and root humping, and nail-head-shaped weld cross section, are revealed. The main driving forces of spatter formation are evaporation recoil pressure inside the kehole and friction drag associated with the high-speed ejection of the energized vapor plume. Spatter generation can always be reduced to a fundamental sequence of phenomena: local boiling-acceleration of melt flow-accumulation of vertical momentum overcoming surface tension-droplet ejection to form spatter. Furthermore, the expanding stream of the directed vapor plume outside the keyhole could also exert a strong shear force on the melt column, inducing an “acceleration process” of the spatter through the directed vapor plume outside the keyhole. The formation of the undercut and root humping can be attributied to the downward melt flow caused by evaporation-induced recoil pressure acting on the liquid hump or shelf on the front keyhole wall. As a consequence, a substantial melt accumulates in the bottom melt pool, or ejects out of the melt pool directly. As the melt accumulated to form a droplet with a certain volume (and mass), it begins to sag below the bottom surface of the weld and from root humping under the action of gravity. At the same time, undercut is generated on the top surface of the weld without supplement of the melt. The evaporation recoil pressure inside the kehole and upward friction drag associated with the high-speed ejection of the energized vapor plume could also drive the melt around the keyhole to flow upwards and gather around the keyhole mouth, and then Marangoni convection occurs under the action of surface tension. As a consequence, the molten width of the top welding pool increases, resulting in a distinct nail-head shape of the weld cross section.(3) The suppression procedures of the above-mentioned welding defects during deep penetration laser welding of thick plate with10-kW level high power fiber laser are investigated. The results reveal that defocus is a key parameter in10-kW level high power fiber laser welding of thick plates. At a negative defocus, the match optimization of defocus and welding speed could achieve full penetration joint with a good appearance without defects. The application of a bottom shielding gas improves the weld appearances at both the top and bottom surfaces, resulting in an “I”-shaped cross section. The gas jet assisted deep penetration laser welding could suppress the formation of nail-head-shaped weld and top spatter. Inclining the laser beam forward with a proper angle is effective for the reduction of top spatter. The undercut and root humping could be inhibited by using welding position PC.(4) The microstructure and mechanical properties of the joint are investigated. The results demonstrate that the microstructures in the fusion zone are columnar dendrites growing from the interface to the center symmetrically and equiaxed dendrites in the center. Furthermore, the weld joint contains γ-Fe as the major phase and δ-Fe as the minor phase. The maximum tensile stress of the joint is809MPa at strain of65%. The joint fails at the base metal far from the weld seam with a typical cup–cone-shaped fracture surface, which consists of typical fibrous and shear lip regions. The fibrous region is invariably composed of fine, deep equiaxed dimples, indicating that the specimen fails in a ductile manner under the action of tensile loading. Based on the above research, the thick plate of the reactor core shroud is autogenously welded by15kW high power fiber laser with a small distortion and no subsequent machining. The technological achievements are pioneering in the worldwide. The excellent welding appearance and mechanical properties indicate that10-kW level high power fiber laser welding of stainless steel thick plate is feasible in industrial manufacture, and has an important practical value and economic benefit.