A Study On The Transient Keyhole And Moving Weld Pool Behaviors And Mechanisms Of Deep Penetration Laser Welding

With the advantages of highly concentrated energy, low residual stresses, high precision and high quality weld joints over traditional welding processes, laser based deep penetration welding technologies, such as autogenous laser welding and laser welding with filler wire etc., have great potential usages in aerospace, weapon, high speed railway and large-sized ship manufacturing processes. However, the physics of deep penetration laser welding process are very complex and the current understanding of many fundamental problems of this process, such as keyhole instability, weld pool dynamics and dynamic dilution behaviors of filler wire in weld pool is still not quite clear. The lack of sufficient understanding of these problems brings serious restrictions to the applications of this advanced welding technology. Supported by a national 973 project on welding science and technology, in order to elucidate aforementioned fundamental problems, this thesis theoretically and experimentally investigates the coupled behaviors of transient keyhole and moving weld pool of deep penetration laser welding, the transient coupling of wire, keyhole and weld pool in laser welding with filler wire, the mechanisms of dynamic dilution process of filler wire in the moving weld pool of laser welding. The major findings are listed below:(1) A three-dimensional sharp interface keyhole and weld pool model is firstly developed to simulate deep penetration laser welding process. The corresponding sharp interface boundary conditions of the present model are systematically derived and an in-house code is also successfully developed. For the first time, the simulated transient keyhole behaviors and weld pool dynamics of the present model are in good accordance with the widely accepted X-Ray transmission imaging experimental results.(2) For the first time, effects of different physical factors including recoil pressure, surface tension, themocapillary force and multiple reflection Fresnel absorptions, effects of different thermal physical properties including heat transfer rate and kinematic viscosity, and effect of different welding parameters including laser power, welding speed and beam radius on the depth oscillations of transient keyhole and the fluid dynamics of transient weld pool are systematically studied. It is found that the keyhole instability is closely associated with the hump behaviors on the keyhole wall and the mechanism of the formation of humps is the force imbalance of the keyhole wall.(3) For the first time, a three-dimensional mathematical model is also developed to investigate the transient coupling of wire, keyhole and weld pool during laser welding with filler wire. The transient behaviors of welding process, with filler wire in the leading direction, are theoretically and experimentally studied, under the assumption that the wire can regularly enter the weld pool in a mode of free droplet transition or liquid bridge transition, respectively. The results indicate that the oscillation of the maximum velocity of weld pool can be used to characterize the instability of weld pool, which deepens the physics meanings of the stability of weld pool during laser welding with filler wire.(4) Finally, a three-dimensional mathematical model is developed to investigate the dynamic dilution process in the moving weld pool of non-autogenous laser welding. The three-dimensional concentration field of the alloy elements in the moving weld pool at the instant time of solidification is numerically simulated and experimentally verified. The successful development of this model offers a possibility to precisely modify and control the alloy element distribution of the wire in weld pool at the instant time of solidification and also a possibility to further develop a new theory of multi-phase continuous metallurgy reaction dynamics of weld pool for non-autogenous welding process.In summary, this thesis has tentatively developed a systematic theoretical framework for numerical simulation of deep penetration laser welding. This framework can provide some science guidelines and references for the development of proper laser welding process and the development of theoretical models and numerical simulation methods for electron beam welding,plasma arc welding and laser-arc hybrid welding process.