Active solute effects on surface ripples in electron-beam welding solidification

Abstract

Surface rippling on workpieces containing surface-active solutes after solidification, during electron-beam welding or melting, is experimentally and analytically investigated. Most alloys contain one or more surface-active solutes such as O, S, Se, and Te. This leads to surface-tension coefficients being positive at low temperatures. In this work, the accelerating voltage and welding currents of the electron-beam welder used are 50 kV and 10 and 15 mA, respectively, with welding speeds of 20, 30, and 40 mm/s, while the workpieces are different steels containing sulfur of 0.0035, 0.008, and 0.014 wt pct and oxygen of 0.002 wt pct. The average amplitudes of ripples are measured for different beam powers, welding speeds, and sulfur contents. Extending a previous work studying rippling on alloys having negative surface-tension coefficients, the present work predicts average amplitudes of ripples by accounting for heat transfer and fluid flow induced by a positive temperature-dependent surface-tension gradient in the molten pool near the solidification front. It is found for the first time that dimensionless average amplitudes are increased by increasing the product of the Prandtl and Marangoni numbers, the product of the adsorption coefficient and the active solute content, and the elasticity number. The predicted results show good agreement with experimental data.