On the generation of transient capillary waves on the surface of laser irradiated targets

Abstract

Trails of IMI115 commercial purity titanium (CP-Ti) and IMI829 alloys, glazed in a pure nitrogen environment using a 1.35 kW CO2 laser beam with different specimen speeds, have been characterized in terms of surface conditions, hardness, melt depth and microstructure. Rippling marks and porous edges were found in all the trail surfaces. Pores and cracks along the width of the trails were evident. Cracks were observed to be connected with pores, and they were found to propagate down the melt depth where they were arrested close to the heat-affected zone. The cracks were found to increase in number with increasing speed. The melt region consists of a dendritic structure of TiN, and the dendrites were more prevalent in CP-Ti than in IMI829. The melt profile was irregular in shape, and the generation of a differential capillary flow pattern as well as the precipitation of TiN in the melt are considered to be responsible for this phenomenon. A very steep hardness profile was produced in all the trails. Nitrided CP-Ti and IMI829 both had a high Vickers surface hardness of around 2000 HV. The precipitation of TiN dendrites is thought to increase the hardness of the melt in general and, in particular, the larger dendrites developed a higher hardness in CP-Ti than in IMI829.