Research On Defects Mechanism In Laser + GMAW Hybrid Welding

Laser + GMAW hybrid welding combines the merits of both laser welding and GMAW, which can achieve high quality and high efficiency welding. However, compared with single heat source welding, hybrid welding involves more process parameters, leading to difficult optimization. In welding, when the process parameters does not match well, the weld defects are still generated. In this project, using numerical simulation technology and experimental detection method, keyhole dynamic behavior and fluid dynamics feature were investigated in laser +GMAW hybrid welding, on the basis of which, generation and suppression mechanisms of weld defect in high speed hybrid welding was analyzed, which provides technical support for enhancing the process reliability of hybrid welding.Based on heat transfer theory and fluid dynamics, with considering the effects of droplet and keyhole on weld pool, a three dimensional transient model is developed to numerically analyze fluid flow in weld pool of laser +GMAW hybrid welding based on FLUENT software. Arc heat input is modeled using an double-ellipsoid heat source; laser heat input is regarded as a conical heat source with changing peak power density, its distribution parameters being determined based on the simplified model for keyhole geometry and size. Droplet transfer is describes as the process of high temperature liquid metal flowing into weld pool from the certain domain above the weld pool. And the droplet transfer frequency was considered through applying the time pulse function to flow velocity of liquid metal. In the simulation, the calculation of keyhole was simplified and the influence of keyhole on fluid flow mode was mainly taken into account. This model considers the coupling of keyhole, droplet, weld pool and temperature field as well as the interaction of laser and arc, which is able to realize the simulation of bent keyhole, thermal and flow fields in hybrid welding, thus reflecting keyhole behavior and fluid flow more reasonably. Meanwhile, the calculation efficiency was also raised largely.The built model was used to simulate the keyhole, velocity field and weld formation in laser +GAMW hybrid welding for varied welding parameters and was validated and optimized according to the experimental results of weld geometry and size. The keyhole behavior and fluid flow regime were studied quantitatively and the mechanism of its influence to weld formation was analyzed. The fluid flow pattern in high speed welding was obtained when the bead hump, undercut and pore emerges. The rule for effect of laser power, welding speed and arc power on keyhole and fluid flow was discussed and generation and suppression mechanisms for weld defect was proposed. The research results show that, compared with GMAW, the temperature field and fluid flow pattern were changed in hybrid welding because of existence of laser heat input, thereby suppressing bead hump and undercut during the high speed welding process effectively. But, with an increase in welding speed, the influence of laser weakened and weld defect took place again. In the case of 1 m/min, weld bead hump is generated in single GMAW(laser power 0 W) when laser power is 500 W, bead hump disappears in welding, but there is no keyhole emerging in hybrid weld pool and fluid flow pattern is close to that in GMAW. When increasing laser power to 2000 W, keyhole is formed, which makes the fluid flow in weld pool more complicated. The predicted weld geometries and dimensions for varied laser powers are compared with the measured data, which are in good agreement, thereby indicating accuracy and applicability of the established model.