Laserdyne Systems, www.prima-na.com TL Precision Welding, www.tlprecision.com |
The airline industry might be in the dumps. But the turbine-engine repair and refurbishing business continues to grow, thanks to the sheer number of ground-based and aerospace turbines currently in operation and in need of regular maintenance. One leading aircraft turbine-engine manufacturer alone reports 25,000 units in service, all of which require periodic servicing.
High-value engine components such as combustors, nozzle guide vanes, blades, and other transition parts are regularly refurbished and put back into service. Leading engine manufacturers increasingly outsource this activity to repair shops like TL Precision Welding, Houston, Tex. The challenge, according to Mr. Quang Tran, the company’s president, is keeping costs down while performing high-quality work with fast turnaround. That requires specialized equipment, such as laser-machining systems, he says.
For one, lasers are ideal for machining difficult materials such as Inconel 617, titanium, and Hastelloy X that are frequently used in gasturbine engines. “And these lasers are often the same models used by OEM’s to make the parts originally, so hardware and software compatibility helps facilitate the refurbishing process,” says Tran.
A significant part of engine overhaul involves refurbishing airflow holes through engine components. Large combustor components contain thousands of small holes of various sizes and shapes. The holes help maximize engine thrust by selectively cooling critical components, and are precision drilled using lasers in carefully plotted patterns over a part’s contoured surfaces. The holes are also angled to the part surface to maximize airflow, reduce engine noise, and minimize fuel consumption. Hole angles vary from 90 to 20° to the surface.
The challenge with refurbishing the holes in these complex parts is that they must be redrilled in the same orientation and with identical precision and shape as the original, according to Tran. Recreating precision holes in a used part is often more difficult than generating new ones, reports Tran. “There’s no margin for error because most of these holes are close together, have diameters as small as 0.020 in., and require tolerances as tight as ±0.002 in.” To position and laser drill each hole accurately — without damaging the part — requires precise and agile multiaxis machining capabilities and highly skilled operators. His company uses a Laserdyne 790 BeamDirector from Laserdyne Systems, Champlin, Minn.
Using the same programming coordinates shared by new parts, operators reestablish hole locations, align the laser beam, and then confirm hole position and alignment so there is no chance of error. Laserdyne’s “through-the-lens” viewing magnifies holes 45×, letting operators individually process and verify each hole.
Lasers can cut small and precise holes in a variety of shapes and orientations. A high-power pulsed Nd:YAG laser is normally used, although CO2 lasers can handle nonmetallic parts. Different processes include:
Percussion drilling, most frequently used, generates a hole with only the laser’s focused beam. A single pulse or series of pulses remove the material in the beam path, with coaxial-gas flow helping clear debris. The process accurately cleans out slag, exhaust refuse, and other foreign material that has built up during engine operation.
Trepan drilling is similar but less frequently used for refurbishing holes. Trepanning does not rely on the size of the laser beam to generate the final dimensions of the finished hole. Instead, with the part held stationary, the laser beam moves and cuts the periphery of the hole.
The term “drilling” is generally used until the diameter of the hole or feature exceeds the material’s thickness. The advent of extremely accurate and repeatable laser-positioning systems allows for unique and tight-tolerance trepanned features. “While this hole refurbishing process often requires a manual point-and-shoot technique, it is consistently fast and accurate,” says Tran.
Shaped-hole drilling is an emerging variation of trepanning that gives designers of turbine components increased flexibility to create new cooling designs. For instance, holes can be tapered or shaped to enhance the amount and direction of air or liquid flow.
Drill-on-the-fly, a variation of percussion drilling, is another important capability. It delivers beam pulses with a stationary laser while the part rotates. Hole location is a function of rotational speed and laser pulse frequency. Laserdyne’s “drill-on-the-fly” software synchronizes movement of the part to the laser pulses, ensuring every pulse hits the exact position required. Adjusting the laser pulse energy, pulse count, and lens focal length helps control hole size, shape, taper, and other characteristics.