Effect of scanning speed on molten metal behaviour under angled irradiation with a continuous-wave laser

Laser processing is one of the most suitable technologies for fabricating controlled structures consisting of grooves or holes having a depth of several tens of micrometres on a metal surface. These structures are expected to improve the characteristics of the surface, such as its adhesiveness to metal and plastics. To control the surface structures, it is important to understand the dynamic behaviours during laser processing. Surface processing using an angled continuous-wave (CW) laser irradiation has been proposed for high-speed surface structuring. Distinct structures consisting of high upheavals on the incident side of the irradiated laser beam and wide grooves on the reflection side are formed on metal surfaces using this proposed method. The dynamic phenomena of molten metal during laser irradiation were investigated by examining the effect of scanning speed via experiments and thermal fluid analyses under the conditions of an average power of 200 W, an irradiation angle of 60°, and a scanning speed of 0.5–2.0 m/s. High-speed camera observations and fluid analysis revealed that the molten metal flow velocity from the reflection side to incident side significantly increased with an increase in scanning speed. Thus, the molten metal was dispersed on the reflection side and accumulated on the incident side. These asymmetric molten metal behaviours resulted in the formation of distinct surface structures. The detailed analyses of molten metal behaviours conducted in this study will help clarify the phenomena associated with high-speed laser surface structuring.