Drilling and Cutting Combustor Liners & Cans
Turbine engines are extremely reliable sources of power, both as aircraft engines and as power sources. Being fabricated from high nickel and high cobalt alloys to withstand high temperatures, they perform better and longer when cooled by cold air from the first stage compressors; an air process called skin film cooling. This is accomplished by ducting the cold air into the centre core of the blades and vanes, by drilling the holes at low angles to the surface. This can be done effectively using laser drilling.
Typically alloys such as Inconel, Hastelloy, MARM, and Haynes require hole diameters from about 400µm to 900µm diameter, with most holes in the 500-600µm range. Angles as shallow as 15 degrees to the surface can be laser drilled through these alloys even if they have been coated with ceramic thermal barrier coatings (TBC).
There are two methods of laser drilling holes. Most of these holes 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 the motion to cut out the hole. This gives the laser the ability to cut airfoil shapes and other feature shapes. Percussion drilling is fastest but produces a larger diameter tolerance, generally about +/- 50µm, and also will have more recast than a trepanned hole. Tolerances on trepanned holes are generally half that or less than percussion drilled holes. Drilling 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 the cylindrical can, a special percussion drilling process termed drilling-on-the-fly can be performed. By rotating the part at a constant rate and feeding back the encoder information to the system to trigger the laser to pulse at each of the hole positions in the row, the system can drill at a very high rate of speed. 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 into parent metal and flow rate of air through the holes. Recast thicknesses can be kept to less than 100um for most alloys and with trepanning techniques can be held to less than 30µm. Diameter tolerance is often a guideline to hole size. Simple go/no-go pin gauge tests can help set-up a process. Generally the real determination of hole size accuracy is 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 are also 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 from the laser. At least 10kW peak power is needed for most high quality laser drilling and even higher peak power is necessary for larger diameter or deeper penetration drilling. Beam quality is improved over standard welding lasers by laser resonator design and optimized systems for specific hole drilling tasks. Delivery of the laser beam via mirrors maintains good beam quality allowing the use of long focal length lenses for fast drilling processes with minimum taper. Holes in thinner components can be drilled with lasers that employ fiber optic beam delivery. Laser drilling system integrators have technology and techniques to incorporate drilling lasers with mirror or fiber delivery into productive manufacturing systems.
Typical lasers that have been used for this application
JK125P, JK300P & JK300HP
For Fiber-Delivered Cutting, Welding, Heat Treating
JK704
Lamp-Pumped Nd:YAG, 400W, 20kW Peak Power
JK704TR
Lamp-Pumped Nd:YAG, 370W, 30kW Peak Power
Please call us to discuss lasers for your specific application.