Modelling of Marangoni effects in electron beam melting

Electron beam melting processes exhibit large thermal gradients in the region where the electron beam intercepts the melt; this leads to variations in the surface energy of the melt close to the beam inducing thermocapillary (Marangoni) flow. During melt processing of many materials the Marangoni contribution can dominate the fluid flow, influencing the trajectories of inclusions within the melt and providing a potential mechanism for controlling the removal and/or distribution of inclusions. A model of the macroscopic fluid flow and heat transfer, incorporating Marangoni effects, during electron beam melting has been developed and validated against surface flow observations during the electron beam button melting (EBBM) of IN718. The model indicates, and experimental observation confirms, that fluid flow in the molten pool is dominated by thermocapillary (Marangoni) forces, for the scale and operating conditions of the EBBM process. It is, therefore, possible to reverse the fluid flow through modification of the surface energy. The effect of altering the concentration of sulphur, which is a highly surface active element, upon the Marangoni flow was determined both experimentally and computationally. The implications of altering this concentration on the effectiveness of inclusion removal and final material quality are discussed.