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ASK THE EXPERT: Sandvik Coromant’s Bill Durow on conquering the new challenges in aerospace manufacturing

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HRSAs are increasingly being used in aerospace because of they're heat resistant. However, many times that heat goes back into the cutting tool, which creates a new challenge for tooling. PHOTO courtesy Sandvik Coromant.

Tomorrow’s aircraft will be required to rise above the net zero emissions initiatives increasingly coming to the fore. Doing so will require changes in both component design and materials used. In a two-part interview, Bill Durow, manager Global Engineering Project Office for Sandvik Coromant, details how this will change the design and makeup of aerospace parts and the tools required to manufacture them. PART I starts below.

SHOP: Tomorrow’s aircraft will be required to fly in a more sustainable manner from an emissions standpoint. How do you see that affecting airplane component design?

DUROW: I think in the future you are going to see changes in not only the engine platforms but also in ways that alter what airplanes look like. Some of the OEMs are looking at a completely different design for wing structures that is more efficient when it comes to flying. They almost look like an albatross – very long and narrow. And then of course there is a move towards lighter materials.

SHOP: We are seeing increasing use of nickel-based heat resistant super alloys (HRSAs) and advanced ceramic matrix composites (CMCs) in aero engine components because they’re capable of withstanding higher temperatures. What challenges do these materials present at the machining stage?

DUROW: The changes we are seeing involve lighter materials so you are looking more towards composites and titaniums. The reason for HRSAs is because they can run a lot hotter. They are designed to resist heat. If you think about the machining process you are actually creating heat during the process. So if you have a material that is resisting heat, the heat has to go somewhere. Typically when you are cutting a steel or stainless steel, materials that are more friendly to carbide cutting tools, what’s happening is that the heat goes back into the chips. This doesn’t necessarily happen with HRSAs. They’re heat resistant. So many times that heat goes back into the cutting tool, which is where the challenge starts. How do we change the way we design tools? How do our applications change?  What are our engagements with the tools? Is high pressure coolant needed to get that heat away from the cutting zone? So not only are there changes in some of the carbide cutting tools but also changes in some of the techniques in how we apply the tools. Heat is the big issue when it comes to HRSAs.

SHOP: And the reason that heat is a such a big issue with regards to tooling is because it deforms the tool?

DUROW: You could potentially run the tools to a point where they return to almost a plasticized state. Typically your carbide goes into a big furnace, it gets sintered, the tools are shrunk to a size and become a carbide. So the binders that are in there actually dissipate. The heat that is generated with some HRSAs exceeds that degree of heat so what happens is you get what they call plastic deformation and the materials start failing, almost melting or notching. 

SHOP: Looking specifically at blisks, which are made from HRSAs and are already seeing increased use in gas turbine engines to improve fuel efficiency, what specific machining challenges do they pose?

DUROW: Blisks have been around a long time. The challenge that comes with blisks is that working on them has never got any easier. The materials are getting uglier. We deal with a lot of the nickel-based materials on the smaller blisks for the hot section on the compression side but you also have blisks on the front side that are titanium. Those are two different types of challenges. The titanium ones are usually taller so you need a longer reach and you need tools that are either lollipop style as we call them or conical to reach down in there with the 5-axis machines. The nickel-based HRSA style blisks are more difficult because of the nature of the material. We are seeing developments in some newer materials, some of these more proprietary materials that are very specific to certain OEMs, that are even uglier to cut.

Another challenge is that a lot of our customers are utilizing smaller 5-axis machines and they’re trying to go lighter and faster. Years ago you used to be able to plow through such materials but they’re not doing that so much now. They’re trying dynamic tool paths, trochoidal tool paths, and the reason for that is to try and get away from that heat issue with smaller engagements on the tools. The area where the heat builds up (the heat propagation zone) isn’t as large and you’re moving the tool faster so it’s in and out of the cut a lot quicker and there is time to get the coolant around it to cool down that heat propagation area.  

SHOP: What performance limitations do conventional solid-carbide end mills face when used to work on HRSAs?

DUROW: We are trying to look at it in a holistic way. We want to put high-pressure coolant down into the cutting zone. You don’t want to flood the area with coolant anymore because it just evaporates at the temperatures of the cutting zone. We are also designing specific optimized tools for these areas. What I mean by that is we are creating tools that are designed for light engagement – long cuts, lighter engagements. And we are optimizing the geometries and the grades specific to the materials. Years ago you would take a good enough solution – this tool works well enough in stainless steel and we are just going to apply it into the nickel materials as well. That’s not the case anymore. Now we are looking very specifically to those types of materials. Your nickel-based HRSA materials are completely different than titanium so now we are designing grades and geometries very specific to those types of materials and the types of applications they will be used in.

SHOP: What about using ceramics?

DUROW: You can use ceramics on nickel-based materials. There is a niche area for them.  You’re not going to use them for finishing, but some folks are looking at the turning side and investigating that. The problem with the ceramics when it gets down to smaller sizes is going to be the rpms of the machines. You need to elevate your rpms on a milling application so high to run those tools where it’s almost putting that nickel material to a liquid state. For instance, a 10 mm 3/8ths tool you are talking about exceeding 20,000 rpms. The issue is typically when you are dealing with these nickel materials you are using machines that don’t have that capability. You might run a solid carbide tool at 6,000-8,000 rpm. So which way do you go? Do you want to optimize things based on the carbide tools that you have or do you want to look at utilizing ceramics along with the carbides? So how do you go about that? Now if you use larger tools, outside of the area of blisks where you are using almost a shell mill or face mill, then you’re fine, you’re in an area of that machine where the machine can handle either the carbide or the ceramic. But ceramics are typically used for roughing applications on nickel materials. But I emphasize do not use ceramics on titanium applications.

SHOP: Ceramics can deliver more speed, is that correct?

DUROW: Yes. If you take a carbide milling tool, same exact size as a ceramic tool, and say a 46-48 Rockwell and run it between 90 and 120 surface feet. That same tool in ceramics you can run that up to 3000-4000 surface feet. The speed is exponentially more. That’s a huge advantage. It’s a little bit more money than carbide tooling but the gains you get on productivity far exceed the extra bit of cost.

Bill Durow, is the manager of the Global Engineering Project Office, aerospace, space and defense, for Sandvik Coromant.

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