Drilling & Cutting Combustor Liners and Cans
Combustor liners and cans are typically manufactured from Inconel, Hastelloy, Mar-M and Haynes. These alloys require hole diameters from about 400µm to 900µm diameter, with most in the 500 – 600µm range.
The holes must be angled, enabling cool air to flow to the central core of blades and vanes. Lasers are able to drill angles as shallow as 15 degrees, even if the alloys are coated with ceramic thermal barrier coatings.
There are two methods of laser drilling holes. Most use a technique called percussion drilling. The laser’s focus spot is set so that it produces the correct hole diameter and a burst of Laser pulses produce the hole without any motion.
For holes larger than about 0.9mm then a trepan drilling method is needed to perform a static pierce followed by motion to cut the hole. This gives the Laser the ability to cut airfoil shapes and other features. Percussion drilling is faster but produces a larger diameter tolerance (generally about +/- 50µm) and has more recast than a trepanned hole.
Percentage taper and the roundness of the trepanned holes is generally better than percussion drilled holes. Processing times for a nominally 500µm diameter hole in 3mm thick aero alloy normal to the surface approximately 0.5 seconds for percussion drilling and 2.5 seconds for trepanning.
Because of the symmetrical arrangement of the holes in combustor liners, a special percussion drilling process called drilling-on-the-fly can be used. Using this method, high drilling speeds can be achieved by rotating the part at a constant rate and feeding back the encoder information to the system to trigger a Laser pulse at each position. If multiple pulses are needed to percussion drill the holes, the system creates many rotations, with each pulse striking in the same place as the previous hole.
Hole quality is determined by hole diameter tolerance, recast on the hole sides, cracks in the recast that penetrate the parent metal and flow rate of air through the holes. Recast thicknesses can be kept to less than 100µm for most alloys, with trepanning techniques reducing this to less than 30µm.
Diameter tolerance is often a guideline to hole size, though it is ultimately determined by the component’s air flow requirement. Turbine engines have a limited amount of air flow capability and designers set an air flow range for each.
The metallurgical aspects of hole recast and micro-cracking are met by proper laser parameters and set-up. Taper and hole wall parallelism also form part of the set-up process, choosing the correct beam diameter and focus lens focal length to match the laser beam to the hole depth.
Laser drilling requires very high peak power and good beam quality. At least 10kW peak power is needed for most high quality Laser drilling. Even higher peak power is necessary for larger diameter or deeper penetration drilling.
Holes in thinner components can be drilled with redENERGY Pulsed Fiber Lasers that employ. Laser drilling system integrators have the technology and techniques to incorporate drilling Lasers into productive manufacturing systems.
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