Optimized Cutting Tools for Tomorrow’s Sustainable Aircraft

Tomorrow’s aircraft will be required to fly more sustainably and support net zero objectives.

“Good design is, for people and the planet, an increasingly critical focus,” according to the UK Government’s UK Innovation Strategy. That includes in aerospace, where tomorrow’s aircraft will be required to fly more sustainably and support net zero objectives around the world. Optimized cutting tools and process knowledge will be crucial to sustainability and innovation in aerospace, especially when working with increasingly difficult-to-machine materials.

Tomorrow’s sustainable aircraft will rely more on next generation powder-based heat resistant super alloys (HRSAs) and advanced ceramic matrix composites (CMCs), as they can withstand high temperatures for more efficient fuel burn and low emissions. However, these materials are required to be heat resistant and maintain their material properties under extreme temperatures. This presents challenges at the machining stage.

Blisks, comprising both a rotor disk and blades, present unique machining challenges. Blisks located on the cold compressor side of the engine are made of titanium, while the hot turbine side requires blisks made of heat-resistant super alloys.

The UK Innovation Strategy report says that new technologies and processes will be key to manufacturing and machining these advanced materials at scale. Collaboration within the industry will also be essential. For aerospace, if we’re talking about sustainability, then new technologies and processes should focus on the ability to combust new fuel types, like sustainable aircraft fuel and liquid hydrogen, to create lower emissions.

As always, the ability to run hotter means there is a more efficient fuel burn. If we couple this with higher compression ratios, which most new and future engines can support, then the result is greater efficiency. That means less fuel is combusted with increased power and reduced noise.

Material Properties and Engine Components

With aero engines, the core of the engine is relatively small and the fan on the front is relatively large. So, a limiting factor is how fast you can rotate the fan. To remedy this, over the last five to ten years, gearboxes have been introduced between the fan and the core of the engine. They enable the fan to run more slowly while the engine core runs faster for high compression and better fuel efficiency.

Sandvik Coromant’s CoroMill® Plura yielded a 198% productivity increase for one customer and has been applied to blisks and other components.

However, HRSA components are needed to make this work. Such materials are metallurgically composed to retain their properties when exposed to extreme temperatures. But this also means the stresses generated when machining these materials are high. The unique capability of these nickel, iron and cobalt-based super alloys to perform close to their melting point also gives them generally poor machinability.

HRSAs are so difficult to machine because they resist heat, and the process used to machine them generates heat. When shops machine a piece of steel, the chips that come off absorb heat from the machining process. That is why steel chips turn blue. In HRSAs, the chips resist rather than absorb the heat, sending it back into tools or the workpiece. The generated heat can turn the carbide of the cutting tool into a plasticized or sintered state, and inserts can literally melt or plasticize. If special considerations are not taken, this can be catastrophic by damaging a tool or even worse, the engine component.

To protect tools and workpieces, it’s important that the process produces as little heat as possible when machining HRSAs. One way to do this is to use tools that cut and shear HRSAs rather than push or plow material off. Another is to not take too much material off too quickly, like burying the insert too deep into the material and plowing through. Instead, a series of lighter and faster cuts is more effective and produces less heat. Most computer-aided manufacturing (CAM) packages offer this trochoidal, or dynamic, technique that makes it easier for shops to apply.

One component that is used in aerospace is the blisk, which comprises both a rotor disk and blades. Blisks are located on the cold and hot sides of aircraft engines and are generally made of titanium alloys on the compressor or cold side of the engine, then migrating to HRSA materials when they are closer to the combustor or turbine part of the engine, or hot side. Blisk components present unique machining challenges because they are often made from HRSAs. The components demand tight dimensional and geometrical tolerances, while maintaining high standards of surface integrity and surface finish. Machining these components effectively, and to the highest standards, requires optimized tools and process knowledge relevant to these advanced materials.

More Secure Machining

In response to these machining challenges, some cutting tool companies offer a number of tooling solutions to support cost-effective, high-quality machining of aero engine components. One such method is high-feed side milling. The technique involves a small radial engagement with the workpiece, which allows increased cutting speeds and feed rates and axial cutting depths with decreased heat, chip thickness and radial forces.

To utilize this method, aerospace machining operations need to consider specialized milling tools. Sandvik Coromant for example has developed the CoroMill® Plura HFS high-feed side milling range. The range features a series of end mills with unique geometries and grades and is made up of two end mill families. One family is optimized for titanium alloys, the other for nickel alloys. Chip evacuation and heat are specific challenges when machining titanium, so the first family presents a solid version of the tool for normal chip evacuation conditions. The second family features internal coolant and a cooling booster for optimum swarf and temperature control.

Increasing Metal Removal Rates

For turbine blisks that require high-feed side milling, small radial engagement allows for increased cutting speed, feed and cutting depth due to decreased heat, chip thickness and radial forces.

To illustrate as an example of where optimized tooling led to improved machining, a customer trial was performed to test a 12 mm diameter CoroMill® Plura HFS end mill against a same-sized competing tool. This trial involved machining a low-pressure turbine (LPT) made from aged Waspaloy nickel-based alloy, using a horizontal machining center with an increased axial depth-of-cut and reduced radial depth-of-cut. The outcome was that metal removal rates were increased substantially with CoroMill ® Plura, leading to a 198% productivity increase for the customer. The solution has also been applied to blisks as well as turbine disks and casings, machining blades, and weight reduction scallops.

Other solutions for optimized aerospace component machining include next-generation turning grades, in both carbide and polycrystalline cubic boron nitride (CBN), which are designed for the high-speed finish turning of components made from ISO S materials. The grades are complemented by next-generation ceramic rough turning grades designed for high performance. For example, the latest finishing grades from Sandvik Coromant are being tested and optimized to deliver consistent surface integrity that’s demanded by aerospace engine manufacturers, while also producing components consistently to tight tolerances.

The Future

As outlined in the UK Innovation Strategy report, global hubs for innovation will continue to see “companies of all sizes creating breakthrough new products, becoming more efficient and scaling to full growth, all with an eye to the global, as well as domestic, market.”

HRSA components, such as blisks, will also become more prevalent in tomorrow’s sustainable aircraft. One of the leading aerospace manufacturers that Sandvik Coromant works with is developing larger engines, to achieve super fuel-efficient designs that run on biofuels. Other key innovations include flexible resin-transfer molded blades that are designed to untwist as the fan’s rotational speed increases. These technologies are already prevalent in mid-sized, single-aisle planes such as the Airbus A321.

Other future predictions include one that mid-sized aircraft will be the first to run on hydrogen, while smaller domestic planes will drive electric ambitions. There are now many small start-up companies that produce smaller electric engines for aircraft, and CNBC reports that the market for flying cars — known as electric air taxis — could rise to $1.5 trillion globally by 2040. There might even be regionalized landing points in the future. For instance, passengers may board a hydrogen plane to travel more locally, say around Europe, or a biofuel plane to fly further afield to a location like the United States.

Chip evacuation and heat are specific challenges when machining titanium. A solid version end mill tool (left) is designed for normal chip evacuation conditions, while a second version (right) features internal coolant and a cooling booster for optimum swarf and temperature control.

Machining aerospace engine components isn’t a one-size-fits-all approach. There is an opportunity to carefully consider every detail, from base material, to component, to feature, to process, to the individual tool. At the component level, the applications mentioned above will rely on next-generation materials which require specialized tooling solutions and extensive machining process and application knowledge to ensure that tomorrow’s sustainable aircraft become reality for aerospace’s leading manufacturers, as well as for people and the planet.

This article was written by Steve Weston, Industry and Tech Center Manager - Aerospace; and Bill Durow, Manager - Global Project Office for Aerospace, Space and Defense; Sandvik Coromant (Mebane, NC). For more information, go here .