| The physical mechanism of deep penetration laser welding process are very complex, and the internal mechanism involves many subjects and fields. The internal mechanism and formation of defects of the keyhole and weld pool in the deep penetration laser welding process are still not very clear. Based on this, a numerical model of deep penetration laser welding based on Level Set method is established in the paper, The model is perfectly verified by the experiment. Then, the dynamic morphology, temperature field and velocity field of the keyhole and weld pool are thoroughly and deeply studied by combining the advantages of experimental observation and numerical simulation.Firstly, the interaction between the material and the laser in the process of deep penetration laser welding is analyzed systematically, The numerical model of deep penetration laser welding is established by considering the physical factors such a s multiple reflection absorption, weld pool dynamics and heat and mass transfer. In the model, the Level Set method is used to track the gas-liquid interface, and the mixed model is used to deal with the solid-liquid interface, also the narrow band adaptive grid is adopted to track the gas-liquid interface. In order to verify the numerical model, four experimental platforms are built, which are used to obtain the surface morphology, the profile, the temperature field of the surface and the internal temperature of the molten pool and the keyhole, The accuracy of simulation results is thoroughly and systematically verified by experimental results.Subsequently, the influence of welding parameters on the morphology and temperature field is studied. The results indicate that the length, the width and the temperature of the weld pool, as well as the depth and the temperature of the keyhole, increase with the increase of laser power; the le ngth of weld pool increases, while the width of weld pool as well as the temperature of keyhole and weld pool decrease, as welding speed enhances. Three stages of the temperature field distribution in the formation of molten pool was simulated and studied, the results show that high temperature zone concentrated at the bottom of the keyhole and temperature field distribution has a certain symmetry in the early formation period of the molten pool; the keyhole region is tilted to the welding direction, temper ature distribution is more decentralized and changes fiercely in the middle period formation of weld pool; the high temperature zone of the molten pool are slender and the depth fluctuates in a certain range in the end period formation of weld pool. According to the simulation of the internal temperature field, the mechanism for bubbles formation and high temperature zone at the front wall and the bottom of keyhole are explained.Then, a layer of solidified thin layer near t he tail of weld pool is observed in the experiment. The thickness of the thin layer is large and distribution is dense near the rear, while thin and scattered close to the front of the weld pool, By the simulation, we understand the regional temperature range of the thin layer is 1600–1800 K, which is lower than the solidification temperature 1809 K of the material. The lower temperature zone form by contacting with the air is bigger at the tail of the weld pool than the part near the keyhole, which explain the formation and distribution of solidified thin layers. In addition, the formation mechanism of bubbles is well explained by the internal temperature field. local evaporation generate s on the front keyhole wall when the liquid shelf irradiated by the las er energy, it increases the fluctuation of keyhole, and considering slender shape of the bottom keyhole which is extremely easy to appear local contraction when suffer the squeeze of the surrounding weld pool, so bubbles formed.Finally, the surface velocity and flow of the weld pool are obtained by adding the powder particles. The closer to the keyhole, the faster velocity is and the more intense fluctuation is. The velocity of spatter in the front and rear of keyhole is 10.07 m/s and 2.99 m/s, respectively. Spatter in the front wall of the keyhole is mainly caused by imbalance of the recoil pressure, the f rictional shear forces, the gravity and the surface tension. Spatter in the front wall of the keyhole is mainly caused by the upward movement of the steam wave. The shear friction force and the vapor pressure of the metal vapor/plasma in the keyhole is the main driving force of the steam wave motion. For instant explosive splash, it is produced in the moment when keyhole from closing to opening, the vapor pressure in the keyhole is much larger than that in the outer atmosphere, accordingly, the metal vapor/plasma eject violently that leads to the surrounding weld pool eject all around. In addition, the periodic movement of the keyhole may generate collapse at the bottom of the keyhole, which cause metal vapor/plasma turbulence, a serious collapse will generate spatters. |
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