Marangoni driven turbulence in high energy surface melting processes

TL;DR

This study uses direct numerical simulations to investigate turbulence in high-energy laser melting processes, revealing that flow instabilities and turbulence significantly influence melt pool dynamics, which are often underestimated in traditional models.

Contribution

The paper demonstrates the presence of turbulence in laser melting flows through DNS, challenging the common assumption of laminar flow and highlighting the importance of turbulence in modeling such processes.

Findings

Flow instabilities lead to turbulence-like behavior in melt pools.

Two competing vortices are driven by thermocapillary forces.

Turbulence does not fully explain differences in observed and simulated pool shapes.

Abstract

Experimental observations of high-energy surface melting processes, such as laser welding, have revealed unsteady, often violent, motion of the free surface of the melt pool. Surprisingly, no similar observations have been reported in numerical simulation studies of such flows. Moreover, the published simulation results fail to predict the post-solidification pool shape without adapting non-physical values for input parameters, suggesting the neglect of significant physics in the models employed. The experimentally observed violent flow surface instabilities, scaling analyses for the occurrence of turbulence in Marangoni driven flows, and the fact that in simulations transport coefficients generally have to be increased by an order of magnitude to match experimentally observed pool shapes, suggest the common assumption of laminar flow in the pool may not hold, and that the flow is actually turbulent. Here, we use direct numerical simulations (DNS) to investigate the role of turbulence in laser melting of a steel alloy with surface active elements. Our results reveal the presence of two competing vortices driven by thermocapillary forces towards a local surface tension maximum. The jet away from this location at the free surface, separating the two vortices, is found to be unstable and highly oscillatory, indeed leading to turbulence-like flow in the pool. The resulting additional heat transport, however, is insufficient to account for the observed differences in pool shapes between experiment and simulations.