Superalloys retain their strength at high temperatures. This makes them the preferred choice for hot environments, like those downstream of the combustors in a gas turbine engine, but it also complicates their machining.

This blog reviews the challenge of precision machining superalloys. After explaining why they are so difficult to mill and turn, it reviews some of the favorite machine shop techniques. It also highlights an alternative approach. First though, an introduction to superalloys.

What Are Superalloys?

“Superalloys” are a class of metals with exceptional high-temperature strength. Most also have high melting points and excellent corrosion-resistance.

Other alloys, notably stainless steel, have high melting points too, but their tensile strength declines rapidly at elevated temperatures. In contrast, superalloys retain much of their strength until close to their melting point.

Superalloys are defined by their primary metal. Thus, the nickel-based superalloys contain at least 50% nickel and cobalt-based superalloys are more than 50% cobalt. Iron-based superalloys are somewhat different in that they may contain more nickel than iron.

Nickel-based superalloys are the most widely used. Common brands and grades include Inconel 718 and 625, plus Hastelloy X and Haynes 282. Cobalt-based superalloys include Haynes 188 and Stellite 6, while Incoloy 909 is among the best-known iron-based superalloys.

These alloys are used extensively in aerospace and power generation. They are also found in medical implants, chemical processing, oil and gas production, and the automotive industry.

The Superalloy Machining Challenge

Machining entails removing material by pushing a cutting edge into the surface. Pressure generates a shearing force, and when this exceeds the yield strength of the material, it shears away. The sheared metal slides over the surface of the cutting tool and, if sufficiently brittle, breaks into small curls.

As metals get harder and stronger their yield strength rises, and it takes more force to produce shearing. Temperature plays an important role in this by reducing yield strength.

Yield strength isn’t the only challenge with superalloys though. Other problems are:

• Low thermal diffusivity of superalloys: The heat generated by cutting remains mostly in the cutting zone rather than spreading out through the workpiece,
• Gummy, abrasive nature of the chips: Superalloys tend to stick to the cutting inserts, forming a “built-up edge” that raises cutting forces and increases friction. They also tend to wear inserts rapidly.
• Work-hardening: The shearing action produces strain hardening that requires even higher forces to cut through.

Consequences of these problems include:

• Difficulty maintaining tight tolerances, thanks to tool wear and local expansion
• Poor surface roughness
• High residual stress in the surface
• Frequent tool changes, which affect productivity
• Long stringy chips that are difficult to handle and potentially dangerous

Approaches to the Problem of Machining Superalloys

A well-equipped machine shop, staffed with experienced machinists and tooling engineers, can reduce the impact of these challenges. Important steps are:

• Use high quality CNC machine tools engineered with exceptional stiffness
• Select cutting insert geometry, base material and coating for compatibility with the specific superalloy composition
• Add chipbreakers as needed
• Optimize cutting feeds, speeds and depths of cut in accordance with the insert manufacturers’ recommendations

Another important, yet often overlooked tip, is to minimize the amount of machining required. This is accomplished by using a near-net-shape manufacturing process such as investment casting.

Investment casting uses wax patterns to produce detailed and highly accurate ceramic molds. Internal features such as cooling passages are formed by placing soluble cores in the wax pattern. The process can be performed under vacuum to minimize oxide formation.

Review Your Application With Precision Machining Experts

The hardness and high temperature of superalloys, coupled with their tendency to wear cutting tools, makes them difficult to machine. It’s possible to get good results with rigid CNC machine tools and lots of experience, but if possible, consider minimizing machining by using a near-net-shape manufacturing process like investment casting.

Impro performs high precision machining of superalloys and other challenging materials, but is also a leader in investment casting. If you’d like to explore manufacturing options for current or planned products, we’d like to help. Contact us to arrange a discussion.