【作者】 张明军；

【导师】 陈根余；

【作者基本信息】 湖南大学 ， 机械工程， 2013， 博士

【摘要】 激光深熔焊接具有深宽比大、焊接速度快、热影响区小、焊接变形小等优点。随着高光束质量万瓦级光纤激光器的出现，厚板激光深熔焊接一次成形成为可能，可广泛应用到能源、造船、航空航天、装备制造、铁路等大厚度大结构件焊接制造中。然而，这些重大工程项目对焊接质量的稳定性和可靠性要求极高，只有对万瓦级激光深熔焊接过程机理进行深入而系统的研究，才能从根本上保证稳定可靠的焊接接头质量。基于上述应用背景，在国家自然科学基金项目的资助下，本文对万瓦级光纤激光深熔焊接厚板过程中孔外金属蒸汽/等离子体、小孔和熔池动态行为，相关焊接缺陷产生机理和控制方法进行试验和理论研究。本论文开展和完成了如下研究工作：(1)借助高速摄影和光谱分析手段，直观观测了万瓦级光纤激光深熔焊接厚板过程中孔外金属蒸汽/等离子体动态行为和光谱信号，并计算了孔外金属蒸汽/等离子体特征参数及对入射激光的散射吸收作用。结果表明，孔外金属蒸汽/等离子体由靠近小孔开口蓝光等离子体和位于上方的微红色的金属蒸汽组成，前者边缘形状锋利光滑，而后者边缘形状变化无常，高度高达20mm。发现孔外金属蒸汽喷射方向与小孔壁方位密切相关。光谱分析结果表明，孔外金属蒸汽汽/等离子体是弱电离等离子体。孔外金属蒸汽中凝结的微细金属颗粒对入射激光束的吸收和散射衰减强度是孔外等离子体中自由电子对激光逆轫致辐射吸收的数倍。(2)基于改进的“三明治”新方法，采用高速摄影手段，首次从侧面直接观测了万瓦级光纤激光深熔焊接超细长小孔内壁，小孔内壁充满皱褶，并沿小孔壁向下流动，同时熔融金属微滴直接从皱褶波峰脱离穿过小孔向后沿飞去。特别地，在小孔前沿壁面存在液体台阶，向下高速流动，速度可达8m/s-15m/s。在直观观测万瓦级光纤激光深熔焊接厚板缺陷产生时的超大金属蒸汽/等离子体团、超细长小孔和熔池动态变化过程的基础上，结合大量工艺试验研究，揭示了飞溅、表面塌陷与底部驼峰和钉子头焊缝缺陷的产生机理。焊接飞溅产生的主要驱动力是小孔内蒸发蒸汽反冲压力和高速喷射的金属蒸汽/等离子体对熔融金属的摩擦力。飞溅产生的基本过程为：局部蒸发——熔融金属加速向上(下)流动——垂直动量积累超出表面张力——熔融金属脱离熔池形成飞溅。孔外金属蒸汽/等离子体对熔融金属的剪切作用使得飞溅在穿过孔外金属蒸汽/等离子体云时有一个“加速过程”。焊缝塌陷和底部驼峰的产生是由于小孔前沿壁流动的液体台阶处局部蒸发导致的熔融金属高速向下流动，大量熔融金属聚集在底部熔池或直接喷出熔池，底部熔融金属不断积累形成熔滴，在重力作用下下掉而形成底部驼峰，焊缝上表面缺少熔融金属的补充而塌陷。钉子头焊缝的产生机理是小孔内部剧烈的蒸汽反冲压力和金属蒸汽/等离子体高速向上喷射对小孔壁的摩擦力驱动底部和周围熔融金属沿小孔壁向上流动，并聚集到小孔开口，然后在表面张力作用下形成由中心向外的Marangoni对流，从而增大熔池上部的熔化宽度，最终形成钉子头焊缝。(3)对万瓦级光纤激光深熔焊接厚板焊接缺陷的控制方法进行了研究。结果表明，离焦量是万瓦级光纤激光深熔焊接厚板过程中一个关键工艺参数。负离焦时，匹配优化离焦量和焊接速度可以获得稳定的金属蒸汽/等离子体、小孔和熔池的耦合效果，从而有效抑制缺陷获得成形良好的全熔透焊缝。背保护气体不仅改善了焊缝下表面成形，同时改善了焊缝上表面成形使整个焊缝截面呈“I”型。侧吹气体射流辅助激光深熔焊接，可以较好抑制钉子头焊缝和上表面飞溅的产生。当激光束沿焊接方向向前倾斜适当角度时，可以有效抑制上表面飞溅的产生。采用横焊焊接方位时，可以有效抑制焊缝塌陷与底部驼峰的产生。(4)对万瓦级光纤激光深熔焊接厚板焊缝接头组织性能进行了研究。结果表明，熔合区两侧边缘是柱状晶，且对称向焊缝中心生长。焊缝中心区主要是等轴树枝晶。焊缝组织仍为奥氏体组织，并在奥氏体基体上存在少量铁素体。焊缝接头拉伸断裂在母材，最大拉伸应力高达809MPa，最大应变为65%。断口呈典型的杯锥形，焊缝断口由纤维区和剪切唇组成。纤维区由细小而均匀一致的深蜂窝状韧窝组成，表明拉伸试件是承受拉伸载荷而韧性断裂。基于本文的研究成果，成功实现了15kW光纤激光自熔焊接核反应堆堆芯围筒厚板，焊接变形小，无需后续机加工，技术成果处于世界领先水平。良好的焊缝成形和力学性能表明万瓦级光纤激光深熔焊接不锈钢厚板可以进一步应用到工业生产，具有重要的实用价值和经济效益。更多还原

【Abstract】 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.更多还原