Can you stand the heat?

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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. For Canadian metal manufacturers looking to continue soaring with this $27 billion national industry, this will mean keeping up with the resulting innovation that will be necessary in tooling and machining processes. 

While the automotive industry is already down the path of electric and hybrid systems, the major aerospace manufacturers are still working on their counterparts. That does give aerospace suppliers more breathing room, but many of these developments will start to find widespread use by 2035 and with smaller aircraft that change could happen sooner. One approach will be to explore different aircraft shapes, such as the delta shape, blended wing body and strut-braced wing, or shapes in which the engines are better integrated into the fuselage. Another big design change will be a greater focus on single-aisled rather than twin-aisled fuselage. In all cases the emphasis will be on lightweighting to compensate for the weight added by becoming battery powered and still be able to achieve a longer flying range.

In terms of materials, plane manufacturers are heading in two different directions. One approach is increased use of aluminum, particularly for the fuselage, while experimenting with new types of aluminum that offer aircraft components greater strength and fatigue resistance than traditional aluminum. The other approach is turning to next generation heat resistant super alloys (HRSAs) and advanced ceramic matrix composites (CMCs), as they can withstand higher temperatures for a more efficient fuel burn and lower emissions. These materials will also be expected to meet the aerospace industry’s high safety standards for heat resistance, creep resistance and maintaining good material properties under extreme temperatures. Such materials, however, present significant challenges for the traditional carbide tooling and machining processes aerospace parts manufacturers are used to employing.

Another thing to take into account when machining HRSAs is the vibration generated during the machining process. Because the material is harder, it results in more vibration, which can be a challenge for machine stability and maintenance, something Soraluce is combatting with its latest innovation, the Dynamic Active Stabilizer feature. IMAGE: Soraluce

Heat is the big issue when it comes to HRSAs. The machining process creates heat so if you
have a material that is resisting heat, the heat has to go somewhere. 

“Typically, when you are cutting 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,” explains Bill Durow, manager Global Engineering Project Office, aerospace, space, and defense for Sandvik Coromant. “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. 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.”

Such dire consequences lead to a whole host of questions for both the tool suppliers and the aerospace part manufacturers who use those tools: 

  • How should tool design be altered to compensate for these high temperatures?
  • Do the tools require different materials? 
  • How should engagements with the tools change?  
  • Is high pressure coolant needed to get that heat away from the cutting zone? 
HRSAs are heat resistant so many times that heat goes back into the cutting tool, which is where the challenge starts.
IMAGE: Sandvik Coromant

“We are designing specific optimized tools. 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,” Durow says. “Your nickel-based HRSA materials are completely different than titanium so we are designing grades and geometries very specific to those types of materials and the types of applications they will be used in.”

Harder carbide substrates are one solution to working on these extremely difficult to machine materials but as Ashton Cherry, industry driver, aerospace solution specialist for Walter USA explains, they’re not without their own challenges.

“The harder carbide substrates are more brittle, so there is really an emphasis on proper programming technique to make sure that you are making in-tolerance parts consistently. As a result of being difficult to machine, there is a long cycle time, so we see shops that must order new machines to meet their rate requirements,” Cherry notes. “Even though there are cost savings efforts and efforts to reduce the machining time, the cycles are still quite long and require additional equipment. There is shorter tool life with a lot of the nickel-based alloys, so we are looking at higher tool consumption, larger machine magazines, and larger tool rooms to accommodate all that.”

To improve cycle times there is the option of leveraging the properties of cubic boron nitride (CBN), the second-hardest known material and capable of tolerating cutting temperatures of 982C.

“CBN is expensive to produce so you are always weighing the cost versus the improvement in time,” Cherry says. “Tool pressure is extremely high with nickel materials, so the tools need to be rigid and that plays into the smaller components that we are seeing and having to provide unique and special solutions. Once the tool starts to wear there is an increase in tool pressure and concerns over surface finish, microstructure and overall part quality.” 

Ceramics present another niche solution for nickel-based materials because they can deliver more speed. Ceramic end mills offer up to 20-30 times more machining speed in comparison with solid-carbide tools for operations such as shoulder and face milling. Such impressive gains can be achieved largely because ceramic cutters retain their hardness at the high temperatures which arise when machining nickel-based alloys.

“If you take a carbide milling tool, same exact size as a ceramic tool, and say a 46-48 Rockwell, you can 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,” explains Durow. 

But ceramics also require changes in practices such as coolant management. Sandvik Coromant advises machine shops to use pressurized air instead as coolant would simply burn at the high temperatures involved. In addition, the company says the use of coolant in such cases promotes thermal shocks and has a negative effect on tool life. Importantly, high spindle speeds of at least 13000 rpm are required, and a programmed tool path that keeps the tool in constant contact with the material. 

“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,” says Durow. “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?”

What all this complexity means for job shops machining these tough to work with aerospace component materials is that the days of opting for a “good enough” solution – that tool that worked well enough in stainless steel and was applied in nickel materials as well – won’t cut it anymore, pun intended.

“As a general rule, we recommend getting away from general-purpose tooling. These are high production components, so they really need focused tooling, which means material-specific tools,” says Cherry, explaining that Walter Tool offers what it calls Supreme, Advance and Perform tools. “If you are working on a high production component, we would be looking at a Supreme line tool which is going to be a bit more expensive, but the performance is going to be better and it’s going to be more reliable and have advanced geometries and coatings. As a result, the cost per edge is going to make for a cheaper option because we are talking about long-running jobs in high production.”

Cherry adds that it’s important to appreciate how unique each job could be. Every component needs to be considered individually when a shop is looking to optimize a process. 

“The process we like to implement starts with the customer contacting their field sales engineer, who will contact our engineering team and provide a total package – we can look at the tools, the speeds and feeds, the processing, fixtures, we can even provide programming support. There is a whole host of solutions that we offer other than just the carbide that goes on the end. That’s the best way to optimize a process,” Cherry advises.

The challenges don’t stop with the tooling; they extend to the machining aspect as well. Another thing to take into account when machining such alloys, explains Oier Elguezabal, vice president sales for Soraluce, is the vibration generated during the machining process. Because the material is harder, it results in more vibration, which can be a challenge for machine stability and maintenance. Elguezabal says Soraluce engineers have been working for the last 20 years on technology that can eliminate the vibration in the process. Soraluce’s latest innovation towards this goal is a feature called Dynamic Active Stabilizer (DAS).

“This technology is able to greet the vibration that comes from the machining process. If you have vibration coming from a particular direction, we have magnetic actuators that make a contact force against that vibration so the machining process stays stable, similar to sound cancelling headphones,” explains Elguezabal, cautioning that applying such technology in the right way requires a lot of experience. The movement can be faster or slower, but vibration is always on one frequency and that resonance frequency can be from the part or the tool or the machine and you need to apply different technologies accordingly to deal with it.

‘If you don’t have that type of technology, the only way you can continue is with reducing the cutting specifications, which reduces the productivity. If you reduce but don’t eliminate the vibration, tool life, surface finishing and machine life suffers,” he says.

Continuing to supply Canada’s aerospace sector is without doubt becoming more complicated. As a job shop you don’t have to take on this task alone, however. Your tool and machine suppliers are a source of expertise you should be leveraging, right from the quoting phase on new work.

“If they are looking at getting into the industry or if they’re looking at bidding on new components, I would say give your tooling rep a call as soon as possible,” advises Cherry. “Even if you are in the quoting phase, we can give you an idea of what we would recommend, what the run time would be, and if there are any features that would require special tools and inserts.”

This kind of information provided in advance has proved helpful for Walter customers in the past, Cherry explains as no company wants to take on a job they might lose money on. 

“Customers may change their pricing strategy to cover the difficult features of the job or choose another project. With our help upfront, the customer will have a really good idea of what it would cost to manufacture the parts they are bidding on. It makes for a more intelligent bid. The earlier we can be involved, the more support we can provide.” SMT

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