The Coupling Mechanism And Behaviors Of Arc-molten Pool Under Multifactors During P-GMAW Process For Aluminum Alloy

Pulsed current gas metal arc welding(P-GMAW)has been widely used in jointing of metal for heat sensitive or thin sectioned materials due to lower heat input, smaller distortion and higher productivity than continuous gas metal arc welding(GMAW), especially for aluminum alloy and high-strength steel. However, the advantages of benefit of employing P-GMAW process depend on using appropriate pulsing parameters. And P-GMAW process involves in interactions of the dynamic behaviors of arc plasma, molten pool and droplet transfer, coupling electric, magnetic, temperature and velocity field. Also the dynamic behaviors of arc plasma and molten pool are largely affected by the quickly varied welding current. The complicated process involving multiphysics and multifactors result in the poor understanding of physical mechanism of the dynamic behaviors during P-GMAW. To better understand the influence mechanism of thermal inertial on arc plasma and stability, the dynamic behaviors of arc plasma coupled with multifactors during P-GMAW process are analyzed by numerical simulation combined with experiment. Meanwhile, based on the observation and analysis of the dynamic variation of the arc plasma and molten pool, the oscillation mechanism of the P-GMAW is investigated. Furthermore, by considering the diffusion of metal vapor in arc plasma, the three dimensional(3-D) numerical model is optimized to determine the influence mechanism of metal vapor on arc characteristics and molten pool dynamics for various currents. A 3-D self-consistent model is established by coupling the dynamic behaviors of electrode, arc plasma, molten pool, surface deformation, evaporation and metal droplet to describe the dynamic behaviors of arc plasma and molten pool during P-GMAW process. The model is validated by comparing the predicted arc profile, temperature, voltage and weld profile with measured ones. The results are considered agree reasonably well with the experimental data, indicating that the numerical model can be appropriately used to analyze the complex physics and mechanism involved in P-GMAW process. The main findings are summarized as follows.The thermal inertia and its effects on arc characteristics and stability during current fall time are investigated. The results show that arc stability is improved by thermal inertia through increasing the arc temperature and conductivity at the interface between arc and electrodes. Two high thermal inertia zones exist in arc region, one locates right under the wire tip, and another is above molten pool surface. Thermal inertia increases as the current decent rate improves for the same current difference. The effects of thermal inertia on arc stabilizing becomes more obvious as the current decent rate increases, and it will be limited if it is lower than the critical value. For the given condition, thermal inertia remarkably influences the arc temperature and stability while current decent rate is higher than 260 A?ms-1.The dynamic behaviors of arc plasma and its action mechanism on molten pool oscillation during P-GMAW process are analyzed. The calculated results show that arc pressure is the key factor causing the molten pool to oscillate. The periodically variation in arc morphology results in a great difference in the heat input and arc pressure acting on the molten pool. The surface deformation of the molten pool due to the varying degrees of arc pressure induces the molten pool to oscillate. The magnitude depends on the current difference between peak current and base current, and increases as the current decent rates improves. For the given condition, the arc pressure at peak time is more than six times higher than that at base time, bringing an oscillation magnitude of 0.8 mm.The distribution of metal vapor and its effects on arc characteristics, heat input and molten pool dynamic behaviors for various welding currents have been studied. It is found that the metal vapor mainly comes from electrode, whose evaporation rate is more than 12 times larger than that of the molten pool. As the current increases, the mass fraction of metal vapor increases and becomes more concentrated in the center of arc plasma and in the region surrounding the wire tip. The presence of metal vapor reduces the arc temperature in the central column region through reducing ohmic heat and increasing radiation loss, making current density go through the arc column more homogeneously. Consequently, the metal vapor decreases the heat conduction from the arc to workpiece. The reduction in the energy input to the workpiece associated with the presence of metal vapor reaches approximately 20~30%.