AMS 5659: The Workhorse of Aerospace Engineering
The toughest challenge the engineers have when sitting down to design components for aircraft engines, is to find materials that can withstand the punishing conditions and maintain structural integrity over decades of service. AMS 5659 has become one answer to this problem, but its name tells you nothing about why it matters.
What Exactly Is AMS 5659?
AMS 5659 is a precipitation-hardening stainless steel commonly referred to as 15-5 PH. “AMS” stands for Aerospace Material Specification; which represents an alloy that meets the stringent requirements as put forth by SAE International (formerly the Society of Automotive Engineers). The composition contains about 17% chromium and 4% nickel, along with additions of copper and niobium that give it its unique properties.
Why Aerospace Relies on it
The alloy’s attractiveness is based on a combination of its extensive properties. There is good and appropriate corrosion resistance—very important when you are dealing with everything from coastal salt spray to de-icing chemicals. The material has reasonable strength at temperatures up to about 315°C though it is not meant for the hottest sections of gas turbine engines.
What makes this material so useful is how it responds to heat treatment: a process called precipitation hardening allows the manufacturer to tailor the material properties for specific applications. This flexibility means that a single specification can serve multiple purposes across an aircraft structure.
Engineers typically specify AMS 5659 for items such as actuator parts, valve bodies, fasteners, and various fittings. These are not particularly glamorous applications but are indeed critical ones. A failed actuator in a flight control system creates serious problems, regardless of how well the primary structure performs.
Working With the Material
Machinists find AMS 5659 to be cooperative compared to some alternatives. It machines reasonably well in the solution-treated condition before final hardening. This characteristic reduces tool wear and allows for efficient production of complex geometries.
The material can be welded, but this requires attention to detail. The heat associated with welding alters the precipitation-hardened structure, which could leave zones of different properties. Experienced fabricators account for this by developing careful procedures, and by undertaking post-weld heat treatment when necessary.
Forming operations are less of a problem than with some of the higher-strength alloys. Manufacturers can brake, roll, and otherwise shape the material before final heat treatment, then harden it to achieve the required strength.
Limitations Worth Noting
AMS 5659 exhibits reduced toughness at cryogenic temperature so it cannot be used in liquid hydrogen or oxygen systems. Its moderate temperature ceiling prohibits its use in hot section turbine components.
The alloy is also somewhat susceptible to stress-corrosion cracking in particular environments, especially in high-strength conditions. The risk is managed through good design and appropriate selection of heat treatment, but engineers should be aware of the possibility.
The Bigger Picture
AMS 5659 illustrates a common feature of aerospace manufacturing, which is a reliance on materials whose properties are well understood and fully characterised, rather than those which represent the latest developments. Decades of service history provide confidence that the material will perform as expected.