Essential tips for machining nickel alloys

Nickel-based alloys have widely become the material of choice for manufacturing aerospace engine components. Offering superior corrosion resistance and high strength, heat-resistant super alloys (HRSAs) provide a rare combination of beneficial properties ideal for the extreme conditions found inside a jet engine.

Nickel alloys are typically used for turbine discs, blisks, and engine casings. But as aircraft designs become more advanced, more manufacturers are also beginning to leverage these alloys for engine-adjacent frame components.

The exceptional performance of these alloys makes them difficult to machine. Nickel-based super alloys are notoriously hard to work because they can notch and wear down cutting tools much faster than a typical material. Combine these material challenges with the complex, thin-walled features in aerospace engine components, plus the most exacting tolerances typically required in aerospace applications, and you have a recipe for tough work ahead.

Know your nickel alloys

Before taking on the challenges of machining a nickel alloy component, it’s essential shops understand exactly what kind of nickel alloy they’re dealing with. There are more than a dozen types of nickel alloys commonly used in aerospace – such as Inconel 718, Waspaloy, or Rene 65 to name a few – and each alloy can also come in a variety of grades, hardness levels, and heat-treated states. They can also have a variety of form factors, as a forging, casting, or in powdered billet plates.

These material variables affect how the component should be machined. Too often, these factors are overlooked, and the exact material specifications are mistakenly assumed. This can lead to shops choosing the wrong cutting tool or running inefficient machining processes.

For example, many aerospace engine components use Inconel 718, but the hardness of this alloy can vary greatly depending on the heat-treated state of the material. This variance in hardness will completely change the cutting data and the inserts needed to cut the material. Many times, original equipment manufacturers (OEMs) will deliver the nickel alloy component in a preheated, softer state for roughing operations to allow more aggressive cuts. Once roughing is finished, the component is sent away for heat treatment and then returned in a hardened state for subsequent stages.

Once the exact material spec is clearly defined, it’ll be much easier to determine the most effective approach to machining. Because of the complexities of nickel alloys and aerospace components, it’s recommended to machine these parts in three stages, usually referred to as first-stage, intermediate-stage, and last-stage machining.

Turning heavy cuts in first-stage machining

First-stage machining of nickel alloy aerospace components requires significant material removal. In most cases, this can be done most efficiently with turning operations on a vertical turning lathe, due to the conical nature of engine parts.

While this stage of machining is more forgiving than later stages, there are several challenges to consider for efficient metal cutting. The biggest challenge in this stage is handling interrupted cuts caused by variance in the stock or surface conditions such as forge scale or casting skins. Differences in how castings solidify can cause slight variations in hardness, or uneven saw cuts in a billet plate can create interrupted cuts.

To overcome these challenges, shops should use cutting tools with grade geometries having high notch resistance to handle heavy, interrupted cutting. Using a 45° square insert with a 45° angle of cut can be particularly effective in early turning operations. Similarly, round inserts can also provide strong notch resistance. Choosing inserts with heavy-duty chip breakers is also important for handling large, thick chips generated by the aggressive cuts needed for roughing stages. With these types of inserts and techniques, shops can handle the demanding nature of first-stage machining without compromising tool life.

Managing stress in intermediate-stage machining

At the intermediate stage, shops will need to transition to milling. If there’s enough material on the component, a ceramic mill is excellent for efficient metal removal. However, ceramic mills will leave a work-hardened zone on the material, so they shouldn’t be used for thin-walled features. A general rule of thumb when milling with ceramic tools is to always leave 0.04" to 0.25" of material, including the final wall thickness. This allowance ensures enough material remains to eliminate the hardened layer in subsequent operations.

For semi-finishing, shops should switch to a carbide tool with sharper inserts and a sharper radius for making lighter cuts. This will lead to less tool pressure and provide more accurate cuts needed for transitioning to last-stage machining. To avoid notching that’s prone to happen in nickel alloy aerospace components, use round milling tools and inserts, such as a milling button cutter, to minimize wear and improve tool life.

The primary challenge during this stage of machining nickel-based alloys is managing stresses to avoid deformation while also maintaining efficient metal removal. Using the right combination of ceramic and carbide tools allows manufacturers to strike a balance between efficient machining and stress relief to prepare components for the final machining stages.

Refining finishing operations in last-stage machining

Final-stage machining is where precision matters. By this point, nickel alloy components are closer to their finished dimensions, often with thin walls and intricate features. The challenges here are dealing with stress relief, avoiding warping, and ensuring flawless profiles. These parts may need to be machined to tolerances within 0.001" or tighter. One misapplication could push the part out of tolerance or leave seam lines prone to fractures.

Accounting for these stress releases is crucial to maintaining precise operations, especially with thin-walled features. To address this, it’s important to sneak up on your finished cuts. Make check passes, then probe the part feature dimensions to see if you’re holding the right size. That way, if there is any deformation, you still have enough material to bring it back to size.

Cutting tool selection plays a critical role, too. Sharper inserts with a sharper radius will minimize tool pressure and enable the light cutting action needed for finishing processes. Cubic boron nitride (CBN) inserts are also an excellent option for light cuts on finishing passes, providing high metal removal rates and consistency.

Shops may also need to deal with complex features including hooked flange pockets, blisk blades, or seal fins requiring specialized tooling for effective finishing. In this case, a special taper tool in a 5-axis machine may provide the clearance needed for accessing hard-to-reach pockets without compromising part integrity. For components such as blisks for example, Sandvik Coromant has a line of CoroMill Plura solid barrel end mills, ball nose end mills, and tangent ball nose tools specifically designed for profiling nickel alloy blisk blades.

Leverage the expertise of your tool provider

Machining complex nickel alloy components for aircraft engines often takes a mixture of expertise, experience, and some experimentation. However, shops don’t have to face these challenges alone. Collaborating with a reliable tool provider can offer advantages that stretch far beyond tools. The right partner can provide comprehensive support, including advice on tooling strategies, CAM software optimization, in-house machining trials, and application engineering.

Working with global suppliers committed to aerospace manufacturing will also give you access to a network of experts who can help you enhance productivity at every stage of the process. Whether you’re working with Inconel 718 or Waspaloy, machining engine parts or frame components, they’ll have the experience and solutions to help you achieve optimal production outcomes throughout your operation.

Sandvik Coromant
https://www.sandvik.coromant.com

About the author: Tom Funke is aerospace technical leader for Sandvik Coromant.