Aluminum heat treatment is a critical process used to enhance the properties of aluminum alloys, making them suitable for a wide range of industrial applications. By applying controlled heating and cooling, the strength, hardness, and durability of aluminum can be significantly improved.
In this article, we’ll explore the various types of aluminum heat treatment, the materials used, and the advantages and disadvantages of each method, along with the practical applications of these treated alloys in industries such as automotive, aerospace, and construction.
What is Aluminum Heat Treatment?
Heat Treatment of Aluminum is a controlled process involving the heating and cooling of aluminum products to alter their physical and mechanical properties without changing their shape. This deliberate restructuring of the microstructure of aluminum helps to improve its strength, making it suitable for various applications.
Types of Aluminum Heat Treatment
It is important to note that each of the aluminum heat treatment types has its processes and specifications. Your choice of aluminum heat treatment depends on the product, desired result, type of aluminum alloy, and its application.
Here are the different types of aluminum heat treatment:
Annealing Heat Treatment
Annealing is an aluminum heat treatment used for softening aluminum, improving ductility, and relieving internal stress usually caused by cold working aluminum.
Forging aluminum by cold working allows the material to be hardened and stronger. This obvious advantage is, however, not beneficial if the cold worked material needs to be further shaped or bent, as it can cause breakage.
Hence, the annealing of aluminum alloys is carried out to redistribute the dislocations or slip planes of the aluminum grain structure that occurred during cold working.
This aluminum heat treatment process involves heating the material to a temperature between 570°F and 770°F (300°C to 410°C) and holding (soaking) it at this temperature for some time, usually determined by the thickness and type of aluminum. After this, the aluminum undergoes slow cooling in the furnace with still air.
Homogenizing
Homogenizing is an aluminum heat treatment procedure used when there is a need for a uniform distribution of alloying elements throughout the casting. This is because aluminum used for casting products hardens from the outer part of the mold to the center, which is away from the mold.
Additionally, when this solidification occurs, pure aluminum, which by nature has a lower melting point, remains at the edges, while the alloying elements, which have a higher melting point, migrate to the center and solidify last.
The combined effect of these is that the casting will be too soft at the edge and too hard at the center, leading to a malformed, brittle, and low-quality casting. This is why homogenizing is used to correct the error.
This aluminum heat treatment process is carried out by first heating the casting to a high temperature, just below its melting point, usually between 900°F and 1000°F (480°C to 538°C). The aluminum casting is then held at this temperature for some time and then cooled slowly to ensure the alloying elements remain evenly distributed throughout the aluminum in the casting.
Solution Heat Treatment
Solution heat treatment is used when there is a need to dissolve alloying elements in aluminum to achieve a solid solution, which in turn improves strength.
Similar to annealing aluminum, the aluminum is heated to a high temperature, usually between 825°F to 980°F (441°C to 527°C). However, unlike annealing, which is slowly cooled, aluminum subject to solution heat treatment is rapidly quenched in boiling water, brine, glycols, polymers, forced air blasts, still air, or oil.
Quenching allows the solid solution to freeze in place and prevents the alloying elements from precipitating. Hence, a delay in introducing the quenchant to the heated solid solution can cause natural aging (precipitation) to begin and defeat the aim of the process.
Natural Aging
Natural aging is a process that happens after alloying elements in aluminum are dissolved into a solid solution, just as explained under the aluminum solution treatment method. This aluminum heat treatment process is enabled if there is a need to increase the strength and hardness of aluminum through precipitation.
So, after a solution heat treatment where the aluminum is heated, dissolved into a solution, and then quenched to lock the alloying elements in place, the component is left to age at room temperature for a couple of days.
This aging heat treatment usually lasts between four to five days. During this time, the alloying elements reform into tiny particles, i.e., precipitate, to increase the strength of the component.
Artificial Aging
Unlike natural aging, artificial aging, also known as precipitation hardening, accelerates the strengthening process by heating the aluminum after quenching. Hence, after solution heat treatment is completed, as seen in the diagram above, the aluminum is reheated to a lower temperature, usually between 240°F and 460°F (116°C to 238°C).
It is then held at that temperature for a couple of hours to allow the alloying elements to precipitate quickly and uniformly. Finally, it is cooled to room temperature with the precipitation intact, thereby increasing strength and hardness.
Materials Suitable for Aluminum Heat Treatment
Not all aluminum alloys are suitable for heat treatment. This subheading highlights the ones suitable for heat treatment and those that are not.
Aluminum Alloys Suitable for Heat Treatment
Heat treatable alloys include 2xxx, 6xxx, and 7xxx series. This is mainly because they gain strength through the process. Prime examples of these alloys are 2024, 6061, and 7075.
Aluminium Alloys Not Suitable for Heat Treatment
Some aluminum alloys cannot pass through heat treatment. This is mainly because they rely on cold working, also known as mechanical deformation, to become strong. Non-heat treatable aluminum alloys are the 1xxx, 3xxx, 4xxx, 5xxx, 8xxx, and 9xxx series, like 1100, 3003, and 5052.
Benefits of Aluminum Heat Treatment
Aluminum heat treatment benefits are as follows:
- It increases the strength of aluminum products, especially when it is needed for structural applications that require load-bearing capacity.
- Heat treatment of aluminum improves ductility, thereby making it more formable and less likely to crack during manufacturing.
- It enhances corrosion resistance for aluminum applications, which require resistance to environmental conditions.
- Aluminum heat treatment exponentially improves the machinability of aluminum products, making them easy to cut or shape.
Disadvantages of Aluminum Heat Treatment
- Heat treatment of aluminum alloys is not cost-effective because of its large energy consumption and the need for specialized and high-scale equipment.
- The processes involved in each heat treatment of aluminum are complex and require precise application, control of temperature, and cooling. Hence, a miscalculation can distort the product.
- For instance, an unnecessary delay in quenching during a solution heat treatment can cause natural aging to begin, thus, defeating the entire process.
- The process of rapidly heating and cooling an already-formed aluminum component may cause changes in shape, which then require further processing for correction.
Aluminum Heat Treatment Applications
Heat-treated aluminum is used for the following applications:
- Aerospace: It is used for high-strength, lightweight components like wings, fuselages, and landing gear.
- Automotive: It is used for engine blocks, wheels, and structural components for reduced weight and improved fuel efficiency.
- Construction: Useful for structural frameworks and cladding materials requiring good strength and corrosion resistance.
- Electronics: Applicable for housings and components where a balance of strength, weight, and thermal conductivity is needed.
If you’re looking for the perfect aluminum heat treatment solution, contact Sanon Casting now!
Final Thoughts!
Aluminum heat treatment, when carried out with precision, can help strengthen it and improve flexibility as required. This is, of course, after determining if the material is suitable for this heat treatment procedure and if the method chosen is perfect for the type of component in production. After these are determined it will be possible to manufacture strong, and high corrosion applications in industries like aerospace, construction, and automotive.
