[2][3][4][5] Manufacturers of turbine engines for aircraft propulsion and for power generation have benefited from the productivity of lasers for drilling small (0.3–1 mm diameter typical) cylindrical holes at 15–90° to the surface in cast, sheet metal and machined components.
Incremental improvements in laser process and control technologies have led to substantial increases in the number of cooling holes used in turbine engines.
[citation needed] Melt expulsion arises as a result of the rapid build-up of gas pressure (recoil force) within a cavity created by evaporation.
The value of Tcr[clarification needed] for which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion.
[citation needed] Generally speaking, droplet size decreases with increasing pulse intensity.
For the longer pulse duration, the greater total energy input helps form a thicker molten layer and results in the expulsion of correspondingly larger droplets.
A more rigorous treatment of melt expulsion has been presented by Ganesh, et al. (1997),[12] which is a 2-D transient generalized model to incorporate solid, fluid, temperature, and pressure during laser drilling, but it is computationally demanding.
At the melt-vapor front, the Stefan boundary condition is normally applied to describe the laser energy absorption (Kar and Mazumda, 1990; Yao, et al., 2001).
By considering the discontinuity across the Knudsen layer, Yao, et al. (2001) simulated the surface recess velocity Vv distribution, along the radial direction at different times, which indicates the material ablation rate is changing significantly across the Knudsen layer.
Ganesh's model for melt ejection is comprehensive and can be used for different stages of the hole drilling process.
When the 12 ns spike was added to the beginning of the long laser pulse, where no melt had been produced, no significant effect on removal was observed.
On the other hand, when the spike was added at the middle and the end of the long pulse, the improvement of the drilling efficiency was 80 and 90%, respectively.
Low and Li (2001)[18] showed that a pulse train of linearly increasing magnitude had a significant effect on expulsion processes.
Forsman, et al. (2007) demonstrated that a double pulse stream produced increased drilling and cutting rates with significantly cleaner holes.