Water-jet guided laser drilling of thermal barrier coated aerospace alloy

Drilling using quasi-continuous wave (QCW) millisecond pulse fibre laser is the state-of-the-art for producing film-cooling holes over uncoated aero-engine components. Laser drilling of coated components, including thermal barrier coated (TBC) components, is still a challenging task due to the risk of coating delamination. Water-jet guided (WJG) laser is nowadays increasingly used for machining of advanced materials such as ceramics and composites. This paper aims to study the basic fundamental characteristics of WJG lasers drilling of acute angular holes over TBC coated nickel superalloy and compare it with the state-of-the-art QCW millisecond fibre laser. Experiments were performed to investigate the characteristics of both QCW and WJG laser drilling in terms of TBC delamination, recast layer formation, hole surface topology and cycle time. Finite element analysis based numerical model was developed and used, to understand and confirm the mechanism of heat propagation and the subsequent material removal characteristics for both WJG and QCW laser. Experimental results show that the WJG laser drilling quality (in terms of thermal damages) is very similar to the process of cold-ablation (i.e. machining without inducing any thermal defects within the substrate material). The numerical model confirms the hypothesis that the high-temperature zone of WJG laser machining process is wholly confined within the small laser irradiation zone, which helps to avoid the TBC delamination typically observed with long pulse acute angle laser drilling process. The material removal mechanism of WJG laser machining seems to be the combination of melt ejection by vapour pressure, melt ejection by impulse shock pressure and by the action of water-jet pressure.