A simplified thermal approximation method to include the effects of Marangoni convection in the melt pools of processes that involve moving point heat sources

Abstract

Processes that use moving point heat sources to temporarily create localized melt pools (metal additive manufacture and fusion welding) have a flow phenomenon due to the surface tension gradient. Surface tension of the liquid metal reduces with temperature and this, coupled with the high temperature gradients associated with point heat sources, creates Marangoni convection in the melt. The Marangoni convection tends to reduce the temperature and change the melt pool geometry (increases width but reduces depth). Computational Fluid Dynamics (CFD) models can simulate the phenomenon of Marangoni convection but are computationally intensive. A simpler thermal model involving heat conduction and latent heat, but with the liquid’s thermal conductivity artificially increased by a constant factor, exhibits similar thermal effects to the Marangoni convection. The heat conduction models are computationally less intensive than CFD, but the trial-and-error exercise needed to obtain an appropriate multiplying factor is time consuming. With an aim to improve the process of factor selection, the present study investigates the correlation between the surface tension gradient and correction factors. For a Ti-6Al-4V under typical additive manufacturing parameters, the corresponding correction factor to be applied to liquid thermal conductivity was 1.76.