Why do aerospace parts have particularly strict requirements for heat treatment?

1. Extreme working circumstances test the limits of how well materials work.
Heat treatment techniques have a hard time meeting the many competing performance needs of aerospace parts at the same time.
High temperature strength and creep resistance: Turbine blades need to stay strong at a high temperature of 1300 °C. Heat treatment needs to form γ 'phase precipitation strengthening through solid solution and aging treatment. This can make nickel-based high-temperature alloys last more than three times longer before they break due to creep. For instance, the high-temperature endurance strength of a certain type of aircraft engine blade went from 400MPa to 650MPa after directed solidification and heat treatment.
To raise the yield strength from 150MPa to 350MPa while keeping the density at only one-third that of steel, aluminum alloy fuselage structural parts must go through T6 thermal treatment (solid solution plus artificial aging). 7075 aluminum alloy has a specific strength of 200MPa/(g/cm ³) after heat treatment. This is why it is the most common aluminum alloy used in the aviation industry.
The landing gear needs to be able to handle 10 ⁷ cycles of load, and the heat treatment process needs to create a lower bainite+martensite dual phase structure through bainite isothermal quenching. This raises the fatigue limit of 40CrNi2MoA steel from 450MPa to 650MPa. After being heated, the fracture propagation rate of a certain type of airplane landing gear went down by 60% when put under simulated service conditions.
2. Process control is particularly harder with complicated structures.
The intricate geometric characteristics of aerospace components present a significant obstacle to the consistency of heat treatment:
Controlling the deformation of thin-walled structures: Thin-walled parts (with wall thicknesses of 0.5 to 2 mm) in engine combustion chambers tend to warp during quenching because they cool at different rates. The vacuum high-pressure gas quenching technology carefully manages the nitrogen pressure (2–6 bar) to keep thin-walled parts from bending too much, from 0.3% to 0.05%, which is what is needed for precision assembly.
The turbine disk of a certain type of aviation engine has a diameter of 800mm and a thickness of 200mm. This means that the heating is even throughout all areas. When heating with a typical air furnace, the difference in temperature between the core and the surface might be as much as 150 degrees Celsius. The temperature uniformity is kept within ± 5 ℃ after switching to a multi-zone intelligent temperature control vacuum furnace. This is to stop early failure caused by uneven organization.
Hard to process the flow channels in the interior cavity: The internal cavity cooling flow channel of the whole blade disc is just 2–3 mm wide, therefore it is hard to get a uniform organization with normal heat treatment. Using induction heating and spray quenching techniques, the difference in hardness between the flow channel's surface and core was lowered from 15HRC to 5HRC. This made the flow channel far more resistant to thermal fatigue.
3. Quality traceability requirements must be followed throughout the whole life cycle.
The aerospace industry has set up a full closed-loop system for checking the quality of heat treatment:
Process database support: One aviation manufacturing company has made a heat treatment process database that comprises more than 2000 varieties of materials. Each process has to call the right parameters. The beta phase transition temperature of TC4 titanium alloy is 980 ± 5 °C. The database accurately maintains the solid solution temperature between 975 and 985 °C to prevent overburning or microstructure coarsening.
Full process record traceability: More than 30 things need to be recorded and kept for at least 15 years during the heat treatment process. These include the heating curve, cooling rate, and vacuum degree. After five years of use, a certain type of rocket engine nozzle started to break. By looking at the heat treatment records, it was found that the quenching medium's concentration deviation was 0.5%. This was finally found to be the main cause of the crack.
Non-destructive testing is a must: All important parts must be tested with ultrasonic waves 100% of the time, with a sensitivity of up to 0.2mm for flat-bottomed holes. After being heated, a phased array ultrasonic test detected a micro crack of 0.1mm at the grain boundary of a specific aviation bearing. Rework was done on time to prevent serious accidents.
4. Industry-specific needs motivate the constant improvement of technology.
The aerospace industry is pushing for the advancement of heat treatment technologies in the direction of "three highs and one low":
High vacuum environment: Titanium alloy reacts with oxygen easily at temperatures above 600 °C. Vacuum heat treatment can keep the oxygen level below 10 ppm, which makes TC11 titanium alloy 25% stronger against fatigue. Vacuum heat treatment has increased the operational life of a certain type of satellite bracket in orbit from 5 years to 8 years.
Very precise temperature control: To heat treat a special type of aviation engine single crystal blade, the temperature must stay within ± 1.5 °C. An infrared temperature monitoring and closed-loop management system are utilized to lower the standard deviation of the blade's initial alpha phase content from 3% to 0.5%. This makes the blade's high-temperature performance much more stable.
High energy beam processing: Laser surface strengthening technology may create a hardened layer up to 0.5mm deep on the part. This increases the contact fatigue life of a certain type of helicopter gear from 10 ⁷ times to 10 ⁸ times and makes it 15% lighter.
Aviation heat treatment has entirely gotten rid of quenching media that contains cyanide and switched to an aqueous solution of polyvinyl alcohol (PVA). This has lowered the COD value of wastewater from 5000mg/L to 200mg/L, which is in line with environmental rules.