Finite element models of laser welding.

Abstract

A numerical study was undertaken in an effort to understand the complex heat and fluid flow during the laser beam welding process. The finite element method was applied, and a horizontal cross sectional model and a vertical cross sectional model have been suggested. In the horizontal cross sectional model, heat transfer in the deep penetration mode laser welding was discussed with the assumption that a keyhole has been formed and heat and fluid flow are two dimensional. Even though the driving forces of the weld convection are missing in the horizontal cross sectional model and the heat and fluid flow in the weld pool are more complicated than can be explained by it, the model included the penetration mode concept of the keyhole and is a helpful conceptual stepping stone toward the understanding of laser welding. It also served as a point of introduction for the enthalpy method in finite element analysis. The weld pool convection induced by the surface tension gradient in the conduction mode laser welding has been discussed in the vertical cross sectional model with and without a flat free surface assumption. It was found that a typical surface tension gradient during welding created violent convection in the weld pool. This model explains the elements of weld pool convection in conduction mode laser welding. Free surface problems with kinematic boundary conditions were also introduced as a part of the vertical cross sectional model. It was found that the presence of normal stress on the free surface places a severe restriction on the time step in the computations of time dependent problems. An enthalpy method has been shown to be useful in solving the Stefan problems with moving multi-phase interfaces. The presence of latent heat in the simulations of welding influences heat and fluid flow such that the weld pool is stretched in the direction of welding when the latent heat is included in the model.