Application of high speed filming techniques to the study of rearwards melt ejection in laser drilling

Melt ejection is the dominant material removal mechanism in long, ms, pulse laser drilling of metals, a process with applications such as the drilling of cooling holes in turbine blades. Droplets of a molten material are ejected through the entrance hole and, after breakthrough, through the exit hole. High speed filming is used to study the ejected material in order to better understand how this debris may interact with the material in the immediate vicinity of the drilled hole. Existing studies have quantified various aspects of melt ejection; however, they usually focus on ejection through the entrance hole. This work concentrates on rear melt ejection and is relevant to issues such as rear wall impingement. A 2 kW IPG 200 S fiber laser is used to drill mild steel. High speed filming is combined with image analysis to characterize the rearward-ejected material. The particle size and velocity data are presented as a function of drilling parameters. It is concluded that high speed filming combined with image analysis and proper consideration of process limitations and optimization strategies can be a powerful tool in understanding resultant debris distributions.