Precise heat treatment is vital for aerospace alloys
The stakes are high in aerospace research & development and manufacturing. In military, commercial and private aviation sectors, the industry has focused on developing better-engineered parts that withstand abusive service over long periods of time.
The pursuit of faster, lighter, more efficient aircraft has spurred the development of countless complex alloys. And for those alloys to perform as designed, equally complex heat treatments have emerged.
The development of these parts and the advanced heat treatments necessary for them to achieve superior mechanical properties will be forever linked.
Common aerospace alloy types
Aircraft feature a wide variety of complex materials. Manufacturers must balance important design and material selection concerns with pressure placed on them by aerospace OEMs to produce long-lasting parts at reasonable costs.
In a game of tradeoffs, the options are almost endless. Heat treaters can play an important part in sorting through the choices to achieve the right balance of a diverse array of alloys.
Aluminum alloys have been crucial to aerospace material and design advances for decades. As production techniques matured, aluminum alloys like 6061 helped make aircraft lighter and sturdier.
Today, modern aluminum alloys remain integral in some aerospace applications due to their high strength-to-weight ratio. Common types include:
- 5xxx alloys featuring silicon and manganese that are relatively workable and feature high hardness and high tensile strength despite low density.
- 2xxx alloys commonly used for structural service and as fasteners and rivets.
- 7xxx alloys that also perform well in structural applications and which boast high toughness and resistance to stress corrosion cracking.
Learn more about aluminum alloys here.
Titanium and titanium alloys are commonly used in critical aircraft applications where resistance to constant stress is necessary.
Ti6Al4V, a titanium formula containing aluminum and vanadium as alloying elements, is the most popular. It’s known for its performance under constant severe motion in the cold sections of jet engines. Ti6Al4V is often specified for discs, ducts, turbine blades and bearing housings. Slight tweaks to the formula result in excellent performance in cryogenic temperatures. It’s also sometimes used to make fasteners.
Titanium performs well in many aerospace settings, but it’s not as widely used as many in the industry would like. That’s because the element is hard to mine and process. It may become more widely used as mining and processing techniques improve.
Nickel alloys are also lauded for their performance in high-stress service environments. They’re also generally very resistant to corrosion, making them ideal for use in jet engine components in constant motion and subject to high heat and corrosive gases. Common aerospace nickel alloys include:
- WASPALOY and its variants, which are very strong at high temperatures.
- Inconel 718 and Inconel 625, each of which boast high strength in hot environments; 718 is stronger than 625 but 625 is more corrosion-resistant.
- Alloy 36, which retains dimensional properties in high temperatures and is often specified for complex aircraft instrumentation.
- Alloy 230, which resists corrosion under high stress and is suited for cold-section service.
- PWA 1484, a formula which is very resistant to high temperature creep. Alternatives include the René alloys N5 and N636 among others.
Cobalt alloys such as L-605 exhibit good oxidation resistance and high strength at elevated temperatures. They’re commonly used in turbine rotors, nozzle diaphragm valves, springs and other components.
Stainless steel alloys
Stainless steel alloys are among the most versatile in modern industry. From automotive to power generation to pharmaceutical applications and beyond, stainless steel alloys are widely specified for their excellent corrosion resistance.
The following stainless steel formulas are widely used in a broad range of aerospace settings:
- 304, used in fuel tanks and some structural components not subject to very hot temperatures.
- 309, a heat-resistant formula containing extra chromium and nickel.
- 316, 316L, 321, S30415 and S30815, all of which are more heat-resistant and suitable for use in exhaust systems, engines and some high-temperature structural service.
- 17-4Ph, a formula containing copper and niobium alloys that is comparable to 304 stainless but chosen when extra corrosion resistance is desired.
Vacuum heat treatment of aerospace alloys
Complex aerospace components require complex heat treatments. Vacuum furnaces are ideal for hardening and strengthening aerospace components because:
- Parts cannot oxidize or become otherwise contaminated when atmosphere is removed from the chamber.
- They allow metallurgists to design the complicated treatment cycles necessary for complex aerospace components.
- Our hot isostatic pressing (HIP) furnace applies very high pressure in an argon atmosphere to improve the characteristics of castings and complex additive manufactured aerospace parts.
In addition, the process controls that accompany vacuum furnaces make them capable of executing very precise, very specific treatment recipes to fulfill the industry’s incredibly strict specs. This is crucial: In aerospace, there’s no room for error.
Some projects come with specs that dictate the exact treatment parameters we must follow. Other orders specify treatments based on the properties parts must exhibit, and it’s up to us to develop the treatment that makes it happen. Either way, we rely on our experienced team of metallurgists and the industry’s most advanced equipment.
Vacuum furnaces operating in Paulo’s Cleveland Division routinely execute complex day-long treatments that top 2,400 degrees Fahrenheit. Consider these notable process control capabilities:
- We can create treatments that feature very slow ramp rates (down to around 1 degree per minute). This ensures that parts reach the required treatment temperature slowly and evenly, which promotes proper solutionizing.
- Fine-tuned thermocouples help our furnaces sustain temperatures to within +/- 2 degrees for an extended period.
- Using programmable logic controllers (PLCs), we can apply correction factors to treatments in progress.
- Dozens of sensors monitor around 1,000 different variables in real time and trigger alarms if out-of-spec conditions are observed.
We also conduct key tests as specified by aerospace customers to optimize furnace performance. Tests include temperature uniformity surveys (TUS), system accuracy tests (SAT) and leak rate monitoring.
We even monitor the RPM and Hertz frequency of motors powering the fans that push air or other gases in and out of vacuum chambers. Even the slightest issues in secondary systems can affect the success of a heat treatment.
Aerospace stakeholders trust Paulo
Paulo has provided the highest-quality heat treatments to customers across the industry spectrum since its founding in 1943. For much of our 75 years in business, we’ve helped develop the critical thermal processes that ensure commercial, military and private planes stay in the air. Learn more about how we keep pace with a changing industry in our Comprehensive guide to advanced aerospace thermal processing.
Our résumé includes treating countless commercial aerospace parts, rocket booster engine components used during NASA’s Space Shuttle program and sensitive assemblies currently in orbit on the International Space Station.