  Heat and fluid flow in low Prandtl number melting pools during laser
processing of materials are sensitive to the prescribed boundary conditions,
and the responses are highly nonlinear. Previous studies have shown that fluid
flow in melt pools with surfactants can be unstable at high Marangoni numbers.
In numerical simulations of molten metal flow in melt pools, surface
deformations and its influence on the energy absorbed by the material are often
neglected. However, this simplifying assumption may reduce the level of
accuracy of numerical predictions with surface deformations. In the present
study, we carry out three-dimensional numerical simulations to realise the
effects of surface deformations on thermocapillary flow instabilities in laser
melting of a metallic alloy with surfactants. Our computational model is based
on the finite-volume method and utilises the volume-of-fluid (VOF) method for
gas-metal interface tracking. Additionally, we employ a dynamically adjusted
heat source model and discuss its influence on numerical predictions of the
melt pool behaviour. Our results demonstrate that including free surface
deformations in numerical simulations enhances the predicted flow instabilities
and, thus, the predicted solid-liquid interface morphologies.
