Heat Treatment for Aluminum Castings
After the primary manufacturing and cooling processes, comprehensive heat treatment of aluminum castings can ensure they are capable of meeting the end user’s exact specifications. This controlled process alters the castings’ physical properties to increase hardness or tensile strength, and obtain the specified combination of ductility and strength.
Heat treatment is a process where the castings are heated to a predetermined temperature at a controlled rate, to achieve the desired structural changes. The casting is then held at this temperature for a specific period of time, and cooled down at a controlled rate. This process transforms the aluminum’s internal structure, thereby enhancing mechanical properties and wear resistance, and improving machinability and dimensional stability. This article looks at the various types of heat treatment for aluminum castings.
What Types of Heat Treatment are Available for Aluminum Castings?
Different aluminum castings may require different heat treatments, depending on the type of casting, desired outcome and the alloy structure. Specialized furnaces are employed to ensure that the aluminum casting will retain its original shape without cracking. Types of heat treatment include:
- TF (fully heat treated): The aluminum casting is heated to between 515-535°C for a period of 4-12 hours. It is then quenched in warm water to prevent cracking, and finally heated again in another oven at 150-160°C for a period of 4-16 hours. This process can result in a hardening of almost double that of the original aluminum casting.
- TB Condition (T4): Depending on the aluminum alloy’s composition, the castings are heated to a temperature just below their alloy melting point. They are held at this temperature until the alloy elements enter a solid solution, and then quenched in water, boiling water or polymer. The choice of the quenching medium depends on maintaining a balance between achieving certain mechanical properties, managing casting distortion, and minimizing internal stresses.
- TB7 (solution heat treated and stabilized): Similar to TF, but the casting is heated to a higher temperature in the second stage. The temperature is held between 240-270°C for about 2-4 hours. The end result is a slightly more malleable casting than that achieved with the TF process.
- TE (age hardening): For more intricate castings, or those with fine features, the casting is only heated to a temperature of 150-170°C for 4-12 hours to speed up the natural age hardening process. No quenching is involved in age hardening.
- Precipitation Aging – TE Condition (T5 or T51): This is an artificial heat treatment aging process which is usually carried out at temperatures in the range of 150-200˚C. This relatively low temperature stabilizes the castings and improves machinability. Soak, or hold, times range from 2-24 hours, depending on the alloy and thickness of the casting.
- T6 Temper: The aluminum casting is first heated at a very high temperature of 538⁰C for about 12 hours. The casting is rapidly quenched in a water or glycol solution at a temperature of 66-100°C. The casting is finally aged in a furnace, where it is heated to 154°C for about 3-5 hours, and then allowed to cool naturally. A straightening process may be performed after the quench to ensure the castings remain straight. T6 temper is most often utilized for castings used in structural applications such as aerospace, automotive, defense and marine industries. T6 results in a harder aluminum with less deformation under the stress of heavy loads.
- Solution Treated and Stabilized – TF7 Condition (T7 or T71): Those castings which may be used in higher-temperature conditions will sometimes receive a solution treatment and stabilization of between 200-250˚C. This will improve the castings’ mechanical properties and increase stability. The resulting mechanical properties are slightly different from T6, as there is usually a lower tensile and yield strength.
- Stress Relief and Annealing – TS Condition: These can be used to remove stresses or to soften the casting for further mechanical operations. Stress relief is usually performed between 200-250˚C, and annealing is done between 300-400˚C.
- Polymer quenching: A polymer is used instead of water, resulting in a slower quenching rate and less stress on the casting. This works for intricate castings that do not require a higher degree of hardness.
After heat treatment, castings may be put through a crack detection analysis process, to look for any defects.
Learn More About Heat Treatment for Aluminum Castings
Impro is a global, integrated manufacturer of high-precision, high-complexity and mission-critical components for diversified markets. We use state-of-the-art process technologies and equipment throughout our manufacturing operations. If you are interested in finding more information about our precision casting, machining and heat treatment capabilities, please contact our team of experts today.