Numerical Simulation Of Transport Phenomena During Laser Keyhole Welding

Laser deep penetration welding has been widely used in the automobile, shipbuilding, machine manufacture and other industrial fields due to its high energy density, high speed and high surface quality. These locally concentrated heat sources rapidly melt and evaporate the material surface. As a result, there exists a high vapour pressure on the molten surface referred to as recoil pressure and it acts as a repulsive force on the surface to make a narrow and deep geometrical shape called a keyhole. Liquid/vapor, solid/liquid phases changes occurred, and transport phenomenon is quite complex during laser keyhole welding. To enhance performance of weld bead, understand dynamic process of keyhole formation during laser deep penetration welding, three phase unified mathematical models of vapor phase, liquid phase and solid phase were established. The VOF (volume of fluid) method was employed to track free surfaces. Roles of surface tension, recoil pressure, thermal buoyancy and Lorentz force (during hybrid laser-MIG welding) were considered in the model. Dynamic process of keyhole and its corresponding temperature distributions were obtained by numerical modeling. The models were validated by comparing the computed results with the experimental data.Mathematical models developed including pulse laser deep penetration welding mathematical model, continuous laser deep penetration welding mathematical model and hybrid Laser-MIG keyhole welding mathematical model, are capable of simulating the fluid flow, heat and mass transfer and other transport phenomena during laser welding. The research contents include: (1) laser keyhole shape and dynamic formation process; (2) melt fluid flow and velocity field of laser keyhole welding; (3) temperature field and dynamic process of laser welding. (4) temperature field distribution and dynamic formation process of weld pool during hybrid laser-MIG welding.Mathematical models for the simulation of heat transfer and fluid flow in weld pool phenomena during deep penetration laser beam welding, based on a numerical solution of the conservation equations of energy, momentum and mass were presented. Surface tension, buoyancy, recoil pressure and Lorentz force were considered for the calculation of weld pool convection. The liquid/solid interface, i.e., the weld pool boundary, is represented by using an enthalpy-porosity technique in a fixed Cartesian coordinate system. Liquid/vapor interface was tracing using VOF (volume of fluid) method. Control equations were discretized by control volumetric method, and solved by the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm after the user defined functions of the source items, boundary conditions and thermal physical parameters in transport equations are linked to the solover of commercial software FLUENT of CFD.Firstly, heat source model was developed based on ray tracing method. The behaviour of the laser beam in the keyhole is analysed using geometrical optics theory. The laser heat flux comes from the Fresnel absorption of the incident intensity directly from the laser beam plus the incident intensity from the multiple refections. When an intense laser beam interacts with the vapor in the keyhole, a significant amount of laser radiation is absorbed by the ionized particles through IB absorption. According to path of laser beam propagation in keyhole, a heat source model was developed. The heat source conforms to the physical process of laser keyhole welding.Secondly, dynamic formation process of keyhole, formaton procees of weld pool and temperature field were studied during pluse laser keyhole welding based on developed mathematical model of pluse laser deep penetration welding.Thirdly, dynamic formation process of keyhole, formaton procees of weld pool temperature field and velocity vector were studied during continuous laser keyhole welding based on developed mathematical model, when the welding speed is 0.05m/s. And formaton procees of weld pool and temperature field were calculated during continuous laser conduction welding when the welding speed is 0.05m/s.Finally, dynamic formation process of keyhole and transient temperature field were studied during hybrid laser-MIG welding based on developed the mathematical model for hybrid laser-MIG welding.Good agreement has been found between the numerical simulation and experimental data. The results clearly demonstrate the proposed mathematical models can be used as a solid base for future studies.