Surface rippling induced by surface-tension gradients during laser surface melting and alloying

During laser surface melting and alloying, temperature gradients on the melt surface between the laser-beam impact point and the intersection line of the solid-liquid interface with the surface generate surface-tension gradients that sweep liquid away from beam impact point. The resulting flow of liquid creates a depression of the liquid surface beneath the beam and ridging of the liquid surface elsewhere. As the beam passes to other areas of the surface, this distortion of the liquid surface is frozen in, creating a roughened rippled surface. If the laser-beam sweep velocity exceeds a critical velocity, the liquid does not have sufficient time to form ripples, and rippling from surface-tension gradients can be avoided.