Continous wave fiber laser keyhole drilling for the production of zero tapered, low recast holes in Ni-alloys

Abstract

Until now, laser drilling has been achieved by pulsed beams through percussion, trepanning, and the combination of both. Problems often encountered for the production of cooling holes in aerospace nickel alloys include recast layer formation, tapering, hole repeatability (particularly when percussion drilling is used) and back wall strike for hollow components. This paper reports for the first time, a new laser drilling technique based on a continuous wave 1 kW single mode fibre laser. Vertical holes are drilled in nickel based super-alloy plates using a continuous wave (CW) and pulsed mode drilling methods. Zero tapered holes with negligible recast layer formation and free from micro-cracks are demonstrated when drilling in both single crystal CMSX-4 and Hayness 230 materials under the CW mode. The hole diameter has been reduced to less than 0.2 mm for a material thickness of 2 mm. The drilling and breakthrough time are measured using photodiode and high speed imaging techniques as 2.2 and 0.8 ms (100-200 times faster than standard Nd:YAG laser drilling). Process monitoring by a high speed camera images and the photodiode also revealed that material removal is characterized by a bulky downward ejection after the hole breakthrough. Elimination of the upward material ejection with a CW pulse provides less erosive flow on the walls. Unique process mechanisms leading to high speed production of recast free parallel holes with the high brightness fibre laser are presented.Until now, laser drilling has been achieved by pulsed beams through percussion, trepanning, and the combination of both. Problems often encountered for the production of cooling holes in aerospace nickel alloys include recast layer formation, tapering, hole repeatability (particularly when percussion drilling is used) and back wall strike for hollow components. This paper reports for the first time, a new laser drilling technique based on a continuous wave 1 kW single mode fibre laser. Vertical holes are drilled in nickel based super-alloy plates using a continuous wave (CW) and pulsed mode drilling methods. Zero tapered holes with negligible recast layer formation and free from micro-cracks are demonstrated when drilling in both single crystal CMSX-4 and Hayness 230 materials under the CW mode. The hole diameter has been reduced to less than 0.2 mm for a material thickness of 2 mm. The drilling and breakthrough time are measured using photodiode and high speed imaging techniques as 2.2 and 0.8 ms (100-20...