Machining difficult aerospace materials

After giant aerospace companies duke it out for lucrative defense and commercial airline contracts, the subcontracting games begin. Opportunities can be plentiful, but shops in unfamiliar territory can get themselves into trouble when bidding on projects. Machine shops new to aerospace machining should be aware of the risks, not just the rewards.

Why it’s daunting

Machining aerospace parts is different than any other industry because security is paramount. Shops need to be prepared for original equipment manufacturers (OEMs) to drive hard negotiations and to not underbid on a contract. And while the opportunities are there, OEMs are anything but forgiving and make the highest demands for quality and precision. If suppliers are not aware of the complexities of aerospace machining, they can be stuck in a contract they can’t get out of and lose money every time they machine a part.

How to do it right

Anyone looking at processing complete components may want to do a little more homework to fully understand the OEM’s requirements before they get started. With roughing and semi-finishing, rules are more relaxed as there are fewer requirements for materials and components. However, suppliers who will do finishing of critical components will face strict requirements as process security and quality are of utmost importance.

The next step is to plan exactly how to adhere to the process control rules provided by the OEM. Shops should expect to know how to handle materials with lower machinability, reduced tool life, and finish requirements on critical features. Before signing a contract, suppliers need to fully understand the materials they are machining, the industry-specific tooling, and how to apply the right techniques for optimal machining.

Materials – For their first experiences machining ISO S materials, heat resistant super alloys (HRSAs), and titanium, suppliers need to understand tooling limitations. HRSA materials are split into three groups: Nickel-based, iron-based, and cobalt-based alloys.

Nickel-based materials such as Inconel 718 and Waspaloy are primarily used for aero components. They can be annealed, solution heat treated, aged, rolled, forged, or cast. These materials have a higher dynamic shear strength which means they require higher cutting forces and have poor thermal conductivity where the heat from the cut goes into the tooling rather than the chip. They are also prone to work hardening which leads to higher notch wear tendencies.

Shops must also be aware of the composition of the material they are machining. Practices that operators can get away with when machining steel are not possible for HRSAs or titanium. For example, a solid carbide end mill toolpath, not optimized for Inconel 718, could result in catastrophic tool failure and a scrap forging, resulting in expensive loss of material and production time.

With new materials being constantly developed, shops must know how to manage and adjust to them. The structure of the design of powder-based nickel materials will always be a challenge because they are more abrasive and harder to machine. To handle this abrasiveness and subsequent lower tool life, shops need to establish predetermined tool changes to avoid undesirable blend points so that the component design is not jeopardized.

With heat buildup, there is potential for work hardening, stress fractures or white layers that can develop when heat or stress is transferred to the material, creating a weak spot.

Engineered tooling – Select only tools engineered for the materials being machined. Shops should not be afraid to try ceramics for nickel-based materials for roughing applications. For those suppliers not experienced with these tools, it can be intimidating considering that the cutting speed in carbide is only from 60 surface feet per minute (sfm) to 120sfm and with ceramics, shops can run 900sfm to 1,200sfm for turning and upwards of 3,000sfm for milling.

The value in ceramics is the material removal rate (MRR), which is substantially higher when roughing than using carbide. Although the productivity gains are greater, ceramics have shorter tool lives than carbide. In some materials, ceramic inserts may last only around 4 to 6 minutes. It should also be noted that with ceramics, there is a need for more redundancy. This takes some adjustment for shops that are not used to having three or four back-to-back tool changes.

The best tooling optimized for aerospace machining is:

• SiAlON ceramics (silicon, aluminum, oxygen, and nitrogen) for nickel-based materials for roughing applications
• Solid carbide end mills (for high- speed machining techniques)
• Uncoated carbide inserts for titanium

Techniques – Proper machining techniques provide process security that is necessary to meet strict OEM requirements. Shops working with aerospace components should know about trochoidal and high-speed machining techniques which allow for low radial engagement and higher feed rates. Following process control rules supplied by the OEM, suppliers need to keep constant tool engagement in their process with no potential stress points, chatter, or vibration. Continuous runs are critical to the integrity and design of components, so security and performance are not compromised.

Today’s computer aided manufacturing (CAM) software providers offer standard functions in their packages to support some of these techniques. Shops should capitalize on CAM functionality with optimized speeds, feeds, and tooling to enhance their processes for critical components.

Suppliers should take advantage of the plentiful subcontracts available in the aerospace industry, but those new to aero machining need to fully understand OEM requirements so they don’t lose their shirts. Knowing what to expect with difficult-to-machine aerospace materials, industry-specific tooling, and how to apply the right techniques for optimal machining will lead to long-term success.

Sandvik Coromant
www.sandvik-coromant.com
IMTS 2018 Booth #431510

About the author: William Durow, manager of Sandvik Coromant's the Global Engineering Project Office can be reached at 201.475.5302 or william.durow@sandvik.com.