Effect of enhanced heat and mass transport and flow reversal during cool down on weld pool shapes in laser spot welding of steel

In the literature on numerical simulations of Marangoni driven hydrodynamics in laser spot weld pools, it has been proven impossible to obtain good agreement between simulated and experimentally observed weld shapes without artificially enhancing the thermal conductivity and the dynamic viscosity of the liquid steel by one to two orders of magnitude. This has mostly been ascribed to flow instabilities that are not properly accounted for in the simulations. However, whereas experimental weld shapes are obtained post solidification, the cooling and solidification stage is generally neglected in reported simulations. In the present work, we report a detailed study on the role of the artificial diffusivity enhancement factors in weld pool simulations, and we extend the simulations into the cooling and solidification stage. We show that during the cooling stage, flow reversal may occur in the weld pool, which enhances the downward heat and momentum transfer. This leads to a deeper weld pool that agrees better with experimental results. By including the cooling and solidification stage into the simulations of weld pool hydrodynamics, an improved agreement with experimentally observed weld shapes can be obtained with a reduced necessity to artificially enhance the thermal conductivity and the dynamic viscosity. (C) 2013 Elsevier Ltd. All rights reserved.