Oxide formation in acute laser percussion drilled holes in single crystal nickel superalloy

One industrial application of laser drilling is the production of cooling holes at acute angles in single crystal nickel superalloy material. However, laser drilled holes are inherently associated with metallurgical defects such as recast layers and oxide formations. In previous studies it has been found that optimising laser parameters to minimise the recast layer increased the oxide layer thickness. In an attempt to better understand the formation of this oxide defect, a comparative study was performed to investigate the effects of drilling angle and peak power on the metallurgy of laser percussion drilled holes in turbine blade material - CMSX-4. The variation of the oxide layer thickness with peak power, drilling angle and hole depth is compared. The location and thickness of the oxide layer is found to be highly dependent on drilling angle – with the greatest thickness being observed on the leading edge corner at most acute angle (30⁰). To identify the point at which the oxide defect develops, holes were drilled at a range of pulse numbers and analysed. High speed photography was used to capture visual heat build up with time. Using computational fluid dynamics simulation, the thermal effect of melt and vapour ejection on the entrance hole section is presented.