Turbine components are not only among the most challenging parts to produce, but require high levels of traceability and visibility with machining processes. images: GF Machining SolutionsClick image to enlargeby Kip Hanson

Manufacturers use advanced EDM technology to improve aircraft safety and efficiency

 

Aerospace parts suppliers are accustomed to high expectations from their customers. But due in part to increasingly stringent fuel consumption and environmental standards, there’s a fundamental shift in the aerospace industry right now, one that places additional burden on the machine shops serving it. Part geometries and tolerances become a bit more difficult with each new aircraft model, and familiar, albeit challenging metals such as titanium Ti-6Al4V and Inconel 625 are being augmented by even tougher grades of intermetallic titanium and gamma prime Inconels. Isn’t it ironic then that EDM, a machining process that has no problems achieving close tolerances in difficult materials, has long been the only one not invited to the aerospace manufacturing party.

No more HAZ
Englishman John Barber spun up the first gas turbine in his Warwickshire shop in the late 1700’s. It was a crude, ineffective device, but it did pave the way for future developments. Since then, his invention has become one of the most important technologies in the industrial world, driving airplanes and helicopters, naval vessels, locomotives, power generation plants, and more.

Through it all, machine tool builders have continually provided better ways to form, machine, and now print the 40,000 metal components in a typical aircraft engine. One of the most important of these machine tools today is EDM. Unfortunately, EDM has long suffered a bad rap for creating HAZ, or heat affected zone, a layer of “white metal” prone to cracking and early fatigue.

Modern EDM hole drillers have additional axes to interpolate the complex-shaped holes found in turbine components. Image: MakinoClick image to enlargeThat all began to change about a decade ago. In a 2007 webinar, Brian Pfluger, EDM product line manager at Makino Inc. said improved circuitry has virtually eliminated HAZ. “Modern EDMs use high speed switching transistors and AC generators with sophisticated and highly adaptive control circuits to produce cleaner, more accurate sparks that are minimally damaging to the integrity of the material being machined. These technological advances have reduced the total thickness of recast and HAZ from 0.076 - 0.102 mm (.003 -.004 in.) to less than 0.010 mm (.0004 in.) total. The HAZ has been eliminated and the recast layer is spotty at worst.”

Because of these metallurgical improvements, EDM is now a welcome and indeed vital component of aerospace machining.

A hot topic
It’s a basic tenet of gas turbine operation: the hotter the exhaust gases, the more efficient the engine. In fact, temperatures in a modern turbofan engine may exceed 1,400° C (2,552° F). Adding insult to injury are extreme centrifugal forces, with some parts of the engine spinning at 10,000 rpm or more. Conditions such as these are hard on components, greatly increasing the rate of fatigue. Metal “creep” at high temperatures is especially problematic, and may lead to eventual failure in vanes, blades, and bearings alike.

To combat this, aerospace engineers are continually looking for ways to keep metals cool relative to gas temperatures. One example is drilling cooling film and diffuser holes in turbine engine blade and vane components. In the past, these holes were often “hole-popped” with an EDM drilling machine, but Pfluger says prior generation equipment are unable to machine today’s complex hole shapes. “Traditional hole poppers have only three axes, whereas Makino’s EDBV (Electrical Discharge Blade and Vane) machine have seven. Because of this, it can use a regular stick electrode and an interpolated toolpath to create the funnel-shaped holes necessary for improved cooling in blades and vanes.”

Sodick’s K3HS hole drilling machine provides superior surface finish and excellent drilling speed compared to competing holemaking processes, claims the company. Image: SodickClick image to enlargeMakino’s not alone. EDM builder Sodick Inc. has also developed its own aerospace hole popper, the K3HS. Marketing manager Evan Syverson says hole quality is a “hot topic” with aerospace manufacturers, as is hole quantity. “EDM provides superior surface finish compared to competing holemaking processes. It also provides excellent drilling speed without increasing the recast layer, something that’s important when drilling thousands of holes in a workpiece.”

Keeping (relatively) cool
Another thing that’s important is breakthrough detection. Syverson notes that a turbine blade is a narrow but hollow structure with holes on one side; hit the far wall with the electrode and the part is likely scrap. Makino’s Pfluger agrees, adding that the metal alloys used to make engine components like these are typically proprietary, designed by the engine manufacturer to provide the best maintenance and performance capabilities for their products. This has presented a steep and continually changing learning curve to EDM builders and users alike.

Wait a minute: the EDM industry cut its teeth on super hard tool steels and cemented carbide. What’s the big deal about machining Inconel? The problem isn’t so much the material, it’s the quota: aerospace demands high quality and high production. “The harder you push the machine to meet productivity requirements, the more the part’s metallurgical properties and dimensional quality may suffer,” Pfluger says. “It’s a double-edged sword, and the need to reconcile both of these requirements is something everyone in the industry is acutely aware of.”

Expensive ‘trodes’
Pat Crownhart concurs that productivity is a top priority, but not at the expense of electrode wear. He is the Mitsubishi sinker EDM product manager at MC Machinery Systems Inc., and says the electrodes used on aerospace parts are often very complex and therefore quite expensive; finding the sweet spot between machining speed and electrode wear in Rene 41 and other high nickel, high chromium alloys is a balancing act.

“It’s all about determining the right combination of amperage, on time, and voltage,” he says. “The internal circuitry of the machine is similarly important; for example, how does it recognize when a spark is starting to discharge in a way that will produce extra wear, and killing it before that can happen.”

Electrode construction is another factor. Crownhart describes aircraft turbine part features known as seal slots. These are generally narrow, around 152 mm (6 in.) or less in width, and up to 250 mm (10 in.) or more in length, where undercuts and sharp internal corners are the norm. Rather than assemble multiple pieces of ground graphite pieces into the desired electrode shape, some manufacturers are employing five axis machining centres to cut the entire electrode from a single piece of graphite. This reduces the chance of errors during assembly, but is also one of the main cost drivers of such electrodes.

“Five axis electrodes are more challenging to make up front, but they help reduce scrap and make the setup process much easier,” he says. “They’re also more repeatable than multi-piece electrodes. The companies that are using them are seeing some big advantages.”

Anything but a bandsaw
GF Machining, Makino, Mitsubishi, Sodick, and other EDM machine builders are seeing increased use of wire EDM for removing build platforms from 3D printed parts. This might seem like a straightforward “bandsawing” operation, but as aerospace market segment manager Ken Baeszler of GF Machining Solutions points out, it’s anything but. “3D printed parts have variable surfaces, with internal hollows and areas that are simply nasty to cut into. The people processing these parts will try to design the build plates to avoid the interruptions as much as possible, but if there are a dozen parts on a build plate, you’ll still wind up with lots of gaps. Unless you have excellent adaptive control capability on the machine, the wire’s going to break. You might even damage the workpiece.”

Between concerns over recast layers, machine speed, and part accuracy, those responsible for aerospace product safety insist on keeping a watchful eye over the manufacturing process, hopefully preventing catastrophic events in the first place but helping them perform forensic analyses if one does occur.

“No one wants to be in an airplane and wonder if something’s going to let go in the engine,” Baeszler says. “That’s why aerospace suppliers want to make sure everything went according to plan during machining. It all has to be certified, and easily traceable.”

GF Machining Solutions product manager Eric Ostini says this is made possible by software he helped develop.

“With track and trace software, the generator output and the time in cut is constantly monitored, so you have complete visibility of all machining events. We also have functions to verify the right wire was used, the correct setup parameters were entered, and the correct barcodes were scanned when loading the program. We’ve added all those capabilities into our e-tracking software, so we know that the job is being cut correctly. It’s complete process control. That’s what is needed in the aerospace industry, and that is exactly what we are providing.” SMT

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