In Situ Observations of the Formation of Surface Fine Crevice Structures Created by Laser Irradiation

Abstract

We used laser irradiation to create surface fine crevice structures, which induced region-selective super-spread wetting. These crevice structures enabled joining of Cu–Cu. We recently reported that laser irradiation can form surface fine crevice structures, not only on Cu but also on Fe surfaces. However, the formation mechanism of such structures is not yet understood. To clarify this mechanism, we performed in situ observations of Cu and Fe surfaces during laser irradiation, with the use of a high-speed camera and laser illumination. In this way we directly observed swelling and spattering caused by ejection of molten metal from laser-irradiated regions. We conclude that the formation of the surface fine crevice structure was caused by the accumulation of ejected molten metal, that is, entanglement of protuberances and spatter.

Fig. 12 Illustration of formation mechanism for the surface fine crevice structure; (a) metal surface during a single line scanning of laser irradiation process and (b) expanded view of region surrounding keyhole which shows the phenomena of swelling and spattering, (c) metal surface during parallel lines scanning of laser irradiation process (b) expanded view of region surrounding keyhole which shows the phenomena of accumulation of re-melted protuberance/newly ejected melts, previously formed protuberances-spatters. (e) 3-dimentional view of surface fine crevice structure.