Laser technology for drilling precision holes has taken a leap forward with faster, cheaper, high-accuracy fiber lasers, which are used in the aerospace industry for turbine engine hole-drilling and other industries. Short-pulse picosecond fiber lasers are likewise making inroads, drilling small, precise holes for the medical and microelectronics industries.
For aerospace turbine engines, drilling ultra-precise holes that enable the effusion cooling used in the hot sections of the modern aircraft turbine engine is a mission-critical task. Builders of aircraft and power-generation turbine engines worldwide have adopted laser-drilling techniques that replace mechanical and EDM holemaking operations with laser technologies.
Many aircraft engine builders have turned to laser technologies as aerospace engineers adopted new designs more suited to laser drilling, noted Terry VanderWert, president, Prima Power Laserdyne LLC (Champlin, MN), a developer of laser-drilling systems. “Pratt & Whitney’s aircraft engine manufacturing was one of the early customers when we got involved with the turbine engine business. Suppliers all over the world now produce turbine engine components using laser processes for turbine engine OEMs, particularly in the hot section, including turbine blades and vanes, combustors, various sheetmetal parts, and we’ve watched the evolution and participated in the adoption of lasers.
“What we found in the early days was that lasers were the new technology and lasers had to be demonstrated to be more productive—faster, better, and lower cost—than the incumbent technology around which the part was designed,” said VanderWert. “A lot of those sheetmetal parts were designed to be manufactured using mechanical processes such as punching or milling. But we were able to demonstrate that the laser could be more cost-effective and more precise.”
In the early 1990s, working with Prima Power Laserdyne customers, designers began to develop new engine component designs to take advantage of lasers, VanderWert said. “Having seen what lasers could do, they came up with a whole new design of the engine involving smaller, shallow [hole] angles that can be as shallow as 10º from the surface,” he noted. “A hole is typically 0.5 mm in diameter. Implementing the design, which they call effusion cooling, improves the energy efficiency of, and reduces the emissions from, turbine engines.”
With effusion cooling, the number of holes in aircraft turbine engine designs has expanded, noted Mark Barry, Prima Power Laserdyne vice president. An early design from Allison in Indianapolis of a turbine engine combustor had 2000 holes, Barry recalled. “We thought, ‘2000 holes, that’s going to take forever,’” he said. “A portion of the JSF engine has 2.5 million holes in it. That’s the difference in the designs of 20–25 years ago and those of today.”
Current designs involve very complex hole shapes and hole patterns that can change throughout the part, Barry noted. “Years ago, we might drill at one particular angle to the surface; today, it’s multiple angles, different hole sizes and different patterns,” he said. “It’s become very complex and you can understand that the designers have this incredible urge to run their engines hotter and hotter, because the hotter the engine runs, the more fuel-efficient it is. And that’s what it’s all about—reducing emissions, increasing fuel efficiency and prolonging the life of the engine components.”
With effusion cooling, thousands of holes are drilled in the combustors used in the hot sections of turbine engines, Barry noted. The designs enable cooling the hot section of the engine that frequently runs into temperatures above the metal’s melting point, he adds. “The temperatures that the engines run today are above the melt point of the materials. So if it wasn’t for the effusion cooling laying a boundary 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.”
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 about the hole size and geometry, then they can check it less.”
The latest capabilities enable users to drill production parts to air-flow tolerances of ±3% on parts with thermal barrier coating (TBC) or ±1% on simple metallic parts, Barry said.
Laser micromachining in medical or microelectronics applications often uses short-pulse fiber laser markers for drilling holes in applications including wafers, medical cannulae and microchannels for fluids, noted Mark L. Boyle, laser product engineer, Miyachi Unitek Corp. (Monrovia, CA).
A laser systems integrator, Miyachi Unitek integrates 1070-nm, pulsed fiber lasers into complete laser systems for drilling holes in metals and other materials for a range of different applications. “We also have the vanadate green and vanadate UV technologies, for plastics and for some metals, and also the picosecond lasers,” Boyle said. “We’re running the whole gamut of IR to UV and nanosecond to picosecond lasers, but metal processing with fiber lasers is really where we’ve been focused.
With short-pulse lasers, users can get very fine precision and very clean walls, Boyle noted. “It’s all dependent on what the user requires. What we find is for most applications, most people are fine with the results from an IR laser. Now and then they’ll have stiffer requirements on the edge quality of those holes. As you go smaller and smaller, into the hundreds of microns, to sub-hundred micron, everything becomes a lot more critical because you don’t want any melt around it, you don’t want it to re-form and grow back over the hole.”
Picosecond lasers offer the advantage of virtually no HAZ for holemaking operations, Boyle said. “Particularly from the picosecond lasers, those are no HAZ or very little HAZ. When you get to the nanosecond lasers, particularly the IR has a little bit more, but in some cases, you can still get a very good edge quality.”
These picosecond lasers have applications drilling metals, stainless, and aluminum, Boyle said, to about a 10-µm typical hole tolerance, and laser drilling to 20–150 µm on drilling of surgical needles. “We’ve done some ceramics and some silicon, all with IR and it’s just a question of what are the requirements for that hole? How clean does it have to be? What are the requirements of debris coming off? How to handle that debris that’s coming off?”
Drilling techniques can be either stationary, with percussion drilling, or employing the trepanning method, moving the laser in concentric circles, Boyle said. “In trepanning drilling, we are using the galvonometric scanning head, mapping out an X-Y plane, marking at about an inch to five inches a second.”
After starting out cost-prohibitive, picosecond lasers’ costs have fallen and these lasers have emerged as viable manufacturing solutions, noted Sascha Weiler, program manager, microprocessing applications, Trumpf Inc. (Farmington, CT). The company’s TruMicro series 5000 picosecond laser systems excel at hole drilling in injection nozzles for automotive and turbine blades for aerospace applications. The TruMicro 5000 short-pulse lasers offer pulse durations under 10 ps, as well as one model rated at 800 femtosecond (fs) pulse duration.
“Picosecond lasers have come quite a way down in costs,” Weiler said. “On the high-quality side, if you use picoseconds, there it’s really more ablation. You get the highest quality on 100-µm holes, and you need that on injection nozzles. It competes with EDM, not only in quality, accuracy and time, but also on price. The cost per part is incredibly low.”
An advantage in drilling with these lasers is the quality offered by ablation, with virtually no HAZ. “It’s more or less direct vaporization,” Weiler said. “In the real world, there’s always a little heat, but with picosecond lasers, there is very little HAZ.”
Drilling strategies vary, but Weiler said percussion drilling makes sense when the aspect ratio is low, say with a 300-µm hole and material that is 300-µm thick. “If it’s a 1:10 aspect ratio, it’s not possible with percussion drilling,” he said. “With trepanning, you move the laser, and it’s like you’re steering inside of the hole. In addition, you can do the so-called helical drilling, which allows you to make undercuts.” ME
This article was first published in the November 2013 edition of Manufacturing Engineering magazine.
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