Why is Inconel 718 difficult to machine?

Inconel 718 is a nickel-based alloy, normally supplied in a high-strength specification. Inconel is a trademark of special Metals Corporation. It has been used extensively in machined components but is considered more challenging to machine. Part of this difficulty can be attributed to its high strength, leading to high cutting forces at the tool tip. This means that more robust equipment is needed to cope. However, there are several more specific ‘metallurgical’ characteristics of Alloy 718 that make it more challenging to machine.

Firstly, it will work-harden during the machining process which will increase the strength of the metal at the surface appreciably.

As nickel-based alloys such as Inconel 718 have relatively low thermal conductivity, there is the potential for a build-up of heat at the tool tip during machining. This can lead to excessive tool wear, reducing tool life or requiring slower machining speeds.

Finally, grades such as Inconel 718 benefit from precipitation-strengthening, through the formation of very fine precipitates throughout the metal. However, these intermetallic particles are very hard, and can cause abrasive wear to the tooling.

Alloy 718 is produced in different specifications, including an even higher strength variant, as well as specifications for the aerospace industry. In that instance, it is possible to purchase Alloy 718 as AMS5662, which is lower strength as it has not undergone the final heat treatment that leads to higher strength (and the formation of those abrasive intermetallic particles). This allows complex parts to be machined more easily, but a final heat treatment process is still required which can cause its own challenges.

Langley Alloys stocks Alloy 718 as round bars from 1/2”-10” diameter (12.7-254mm).

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