The laser surface processing has been widely used for controlling the surface function of materials such as roughness, brightness, adhesivity, and so on. A pulsed laser has been usually applied for the surface treatment due to its high peak power. On the other hand, a single mode CW fiber laser, even in the range of 100W, enables high power density more than 10 MW/cm² because of its high beam quality, and the key-hole can be generated at high-speed processing of low average power. In high-speed processing of CW laser beam with tiny spot and high power density, the fast flow and re-solidification of molten metal around the key-hole might contribute to the formation of micro-structure on its surface. Therefore, it is important to clarify the high-speed phenomena of molten metal dynamics in order to control surface structures.

The micro-structures formed on the surface of mild steel SS400, pure titanium and aluminum alloy A1050 by using a single mode CW fiber laser were investigated in this study. The experiment was performed in the conditions of 18 µm spot diameter and maximum average power of 200 W at the processing speed of 1 to 2 m/s. The incident angle of laser beam in the perpendicular plane to scanning axis was varied 30 to 60 degrees. The unique method of angled laser irradiation to the surface of specimen had the possibility to control the micro-structure. A relatively deep groove of 20 µm depth and high upheaval of 10 µm height were formed along the processing line at the incident angle of 30 degrees for SS400 and pure titanium, while there was no remarkable surface structure for A1050. The formation of surface microstructure was influenced by processing speed, average power, and thermophysical properties of metal. It is suggested that the inhomogeneous distribution and the flow rate of molten metal around the key-hole have important roles on the formation mechanism of micro-structures in high-speed processing by CW laser.