Taking on Heat-Resistant Superalloys

Ceramic inserts need to run at very high cutting speed with reinforced geometries, and in order for it to work, it needs to generate an extreme amount of heat to plasticize the material. Photo courtesy of Sandvik Coromant.

Heat-resistant superalloys (HRSA) include a number of high-alloyed iron, nickel, and cobalt-based materials. They are very similar to the ISO M materials, but are much more difficult to machine. With such a wide spread of materials under the generic heading of HRSA, the machining behaviour can vary greatly even within the same alloy group.

“New HRSA materials are constantly being developed,” said Steve Howard, engineering and marketing manager for NTK Cutting Tools, Wixom, Mich. “These new materials will withstand the demanding environments in which certain components must function, and they can have higher hardness characteristics, even exceeding 45 Rockwell C.”

However, these materials generally tend to be sticky, create built-up edges, work-harden, and generate heat, which is often given back to the tool. While some HRSA are very abrasive because of their material composition, almost all place particular demands on the cutting tools. This is why many shops are turning to ceramics for milling of HRSA.

Ceramic Cutting Tools

Because machinability of HRSA tends to be poor, the experts agree that opting for a ceramic cutting tool is the best choice.

“The transition to ceramic cutting tools is typically a learning experience for the machinist, as what is happening in the shear zone is completely different than what we expect with carbide tooling,” said Jeremy Corneil, milling product manager for Iscar Tools Canada, Oakville, Ont.

Choosing ceramics for HRSA helps reduce the time in cut, which can mean significant cost and time savings, which are important because machine times can be expensive in the component segment. Ceramics provide higher metal removal rates and can achieve up to 20 to 30 times the speed of conventional carbide in HRSA.

“This speed is a requirement for the tool,” explained Brian MacNeil, milling products and application specialist for Sandvik Coromant, Mississauga, Ont. “The minimum speed to generate enough heat for ceramic usually starts at 550 m/minute or 1,700 SFM. Depending on the material and its condition, you could achieve 975m/min. or 3,200 SFM, whereas carbide in this area typical averages 36 m/min. or 120 SFM.”

Ceramic inserts need to run at very high cutting speed with reinforced geometries, and in order for it to work, it needs to generate an extreme amount of heat to plasticize the material and then the tool will displace the heat. Ceramics can have a negative effect on the surface integrity and topography so are not used for machining close to the finished component shape; rather, they are recommended for roughing operations.

“If your chips are not glowing orange or white, ceramic milling tools simply won’t work,” said Corneil. He added that making the choice to experiment and implement ceramic milling tools on HRSA applications is an exciting and efficient transition. Due to the increased wear resistance of the ceramic substrate, machine rigidity, fixturing stability, gauge length, and proper parameter selection are of the utmost importance.

Tooling Recommendations

Compared to other tooling such as carbide, fewer options are available when it comes to ceramics, particularly geometry.

Operators should constantly try to promote a thick-to-thin chip milling strategy to help reduce the amount of shock forces on the tool. Photo courtesy of Iscar Cutting Tools.

“Positive geometry inserts typically provide the best results,” said Howard. “These styles generate less tool pressure compared to negative-style inserts.”

Positive geometry, with an optimized edge-rounding, will also prevent chip adherence at the point where the edge exits the cut. However, MacNeil noted that tool geometry with a negative land can provide a stronger edge line.

Corneil echoed this by adding that negative round button inserts are one of the most popular insert geometries with the trend moving toward solid ceramic end mills.

Beyond geometry, a shop also must decide what ceramic grade is best suited for its applications. According to MacNeil, generally speaking, two types of ceramic cutting tools are available: SiAlON and whiskered.

“SiAlON (silicon, aluminum, oxygen, nitrogen) is a mixture of silicon nitride and aluminum oxide,” he said. “It has the best chemical stability and resists notch wear. There are a number of variations under this category. Whiskered ceramic provides improved toughness and bulk strength compared to the traditional ceramic, and fibres are included.”

He added that ceramic tools can be produced as indexable inserts for larger features and as a solid round tool for smaller features.

Machining Parameters

Ceramics, unless whiskered, tend to be extremely brittle. This makes it important to use the best milling practices.

“Toolpaths are critical,” said Howard. “If you are nice to the cutter, the cutter and inserts will be nice to you. It’s important to treat them gently.”

When it comes to milling HRSA, the experts agree that developing a suitable cutting strategy is important. Avoiding interruptions and keeping the tool in the cut is ideal. Climb milling is preferred to conventional milling. Soft entry or arching into the cut is also highly recommended.

“It’s also important to make sure the SFM is high enough to plasticize the material,” said Corneil. “If the material ahead of the shear zone is not softened from the heat of cut, the ceramic inserts will simply chip and break. Due to the brittle nature of the ceramic insert a moderate feed rate should be tested first. Typically, a good starting feed rate is between 0.0025 in./z to 0.004 in./z depending on the geometry.”

To produce a proper chip thickness, the feed should be selected that is high enough to not work-harden the material but not so high that it causes edge frittering. Photo courtesy of Sandvik Coromant.

He added that at all times operators should try to promote a thick-to-thin chip milling strategy to help reduce the amount of shock forces on the tool. Also, coolant should never be used as the inserts will suffer from catastrophic thermal shock.

Operators and programmers need to work together to ensure consistent depth of cut, engagement, and chip thickness. Howard also noted that the depth of cut can be larger with certain ceramic geometries when compared to CBN inserts.

MacNeil pointed out that higher feeds and depths of cut require a reduced cutting speed. However, these boundaries will change depending on the component material hardness and grain size.

“Once initial tool life and productivity baselines are achieved, experimentation with increased depth of cut and SFM should be completed to maximize productivity,” added Corneil.

Tool setup and maintenance are also critical for ceramic milling.

“With ceramic indexable cutting tools, the insert pockets should be inspected and replaced if there is any damage,” said Corneil. “Due to the high RPM required to run these tools properly, it is also recommended that the insert screws be tightened with a torque wrench to proper specs.”

Beyond the cutting tools, quality toolholding with a low axial or radial runout is recommended to ensure long, consistent wear, according to MacNeil. When it comes to solid round ceramic tools, a quality hydraulic or heat-shrink toolholder is recommended to reduce the tool’s tendency to pull out in difficult materials.

Maximizing Tool Life

As previously mentioned, programming toolpaths specifically for ceramic inserts is necessary to ensure long tool life. The ceramic inserts should stay in contact with the part as much as possible. Every time the inserts come off the part, the insert’s tool life is decreased.

“Speed is not an enemy of ceramic inserts,” said Howard. “If ceramic is run too slow, a result will be chipping of the insert cutting edge. Push the feed rate as hard as possible; this will give you better tool life.”

However, the speed should be balanced between creating enough heat in the cutting zone to plasticize the chip but not so high to as unbalance the ceramic, added MacNeil. To produce a proper chip thickness, the feed should be selected that is high enough to not work-harden the material but not so high that it causes edge frittering. To get the most out of the tool, air is recommended to clear chips away and prevent them from being recut.

Speed is not an enemy of ceramic inserts. If ceramic is run too slow, a result will be chipping of the insert cutting edge. Photo courtesy of NTK Technologies.

The most common cause of tool failure and poor surface finishing is tool wear. However, flank wear on a ceramic insert is a normal result. Wear is on the top of the insert is a result of a SFM or speed that is too high.

“Wear patterns for ceramic usually resemble edge frittering, top slice, and notch wear,” said MacNeil. “When you get it right, you can achieve nice, even flank wear just like carbide.”

Corneil added that ceramic inserts typically suffer from depth of cut notch wear, built-up edge, and chipping. In most cases he recommends changing inserts based on an excessive burr, a large increase in machine power consumption, or wear that is 0.04 in. wide.

Associate Editor Lindsay Luminoso can be reached at lluminoso@canadianmetalworking.com.

Iscar Cutting Tools, www.iscar.ca

NTK Technologies, www.ntktech.com

Sandvik Coromant, www.sandvik.coromant.com