ndary of air between the flame, so-to-speak—the combustion—and the material, the components would melt,” Barry said. “And occasionally, when we go into MRO [maintenance repair and overhaul] facilities, you can see signs of combustors that have actually burnt through. No one gets on an airplane today that doesn’t have laser-drilled holes.”
A key trend in turbine design is using many different complex hole shapes, using a combination of cylindrical and shaped holes, that are more frequently laser-drilled, often with either high-power pulsed Nd:YAG lasers or increasingly with fiber lasers. Processing is typically either through percussion drilling, which pierces a hole in the material, or through trepanning, a method of moving either the laser, the workpiece, or a combination of these, to produce the holes. In either process, high-power density is accomplished by using a high-power laser with a focused spot size of 0.002–0.030" (0.05–0.75 mm). “To support the designer's goal for greater engine efficiency, Laserdyne had to develop new capabilities,” said Barry. “There are various shaped holes—fan tail and race track—that are designer buzzwords, and hole shapes and angle can vary across the surface.”
Use of fiber lasers for producing turbine components is increasing, Barry noted. Prima Power Laserdyne began an extensive R&D project in 2011, aiming to quantify the capability of fiber lasers with 12, 15, and 20-kW peak power, for drilling as a possible alternative or complement for Nd:YAG lasers, the predominant lasers used. In a paper presented in 2012, the company described its early experiments in fiber laser drilling. “Based on the experiments and experiences in the field, we’ve gone on to work with fiber laser makers. Not only is the fiber laser producing good holes at a good rate and meeting quality and throughput requirements, but it is doing so at a lower cost,” he said. The laser used in the early experiments was about $750,000. “We’ve driven the price point down,” Barry said. “We recognized the problem of laser cost and worked with the laser manufacturer [IPG Photonics] in the development of a QCW [Quasi Continuous Wave] laser with relatively high peak power and relatively low average power.”
Process Stability Is Key
Laser-drilling system designs mandate rigid structures and process controls. With the Laserdyne 795 laser system, users can drill, cut and weld medium to large 3D parts with a unique moving beam motion system. “Accuracy, ease of use, and process stability are critical,” Barry said of drilling effusion holes in turbine components that can cost thousands of dollars by the time the part reaches the laser system.
Much of Laserdyne’s software development centers on the requirement for consistent holemaking techniques, Barry added. “Back in the early days, everybody drilled round holes. When I say holes, you imagine holes that are circular. The idea of a shaped hole is foreign to most people,” he said. “With shaped holes, a question is ‘How exactly do you measure the hole?’ In the past 10 years, we have seen more requirements based on airflow through the hole. The airflow was almost a second requirement to the hole diameter. Blueprints today often specify an airflow with the hole diameter as a reference only. It challenges the people that are making the part.”
With the company’s current laser systems, users can measure the flow, and in the background, Laserdyne software adjusts hole size up or down, assuring the part meets design criteria. “Some parts are relatively simple, and you can drill the entire part and adjust,” Barry said. “All of this is done in the background with statistical process control. Another trend is to have different flow in different areas of the part; you can drill a portion of the part, adjust hole size if needed to achieve the correct overall flow, drill the next area, and repeat the process until the part is complete. The in-process flow check is the basis for these adjustments. The critical part of all laser drilling is to have the process under control. If the user has a good feeling