Surface Tension Effects on Fluid Flow in Laser Beam Welding of Cerium Metal

The presence of surface-active elements, at trace concentrations, can dramatically change the morphology of welds in both arc and high energy density welding processes. In various low carbon and stainless steel alloys, the presence of oxygen, sulfur, and selenium has been shown to reverse the direction of surface flow, leading to deeper and narrower weld profiles. For alloys outside of the ferrous systems, however, there is little experimental data available concerning the effect of surface-active elements. In this study, the effect of oxygen on the surface tension and weld morphology of cerium metal was investigated.

Surface tension of pure cerium was measured using the Electrostatic Levitation technique. Cerium samples were prepared with a range of bulk oxygen concentrations from 130 to 1500 ppm. Measured surface tension was shown to be dependent upon the initial bulk oxygen concentration. Additionally, the isothermal surface tension was found to increase significantly over time under high vacuum. This effect, attributed to the evaporation of bulk oxygen from the liquid-vapor interface, led to the measurement of a pure state surface tension value for cerium (1191 ± 35 mN/m at 798 C) which was much higher than reported in previous literature (700 – 800 mN/m).

Laser welding experiments on cerium showed a dramatic change in weld morphology as a function of metal purity. Cerium with low impurity concentrations demonstrated weld morphologies consistent with inward Marangoni flow. It is proposed that the evaporation of oxygen is responsible for the drastic change in morphology. For cerium metal with a high concentration of bulk oxygen, the evaporation was not sufficient to clean the highly oxidized surface; for the “cleaner” cerium metal, however, the evaporation of oxygen was sufficient to create a strong compositionally-dependent gradient in surface tension at the surface, and a corresponding reversal of surface-tension driven fluid flow.