Heat treatment of aluminum and aluminum alloy materials is a crucial processing method involving heating to specific temperatures and maintaining them for a certain duration to adjust the structure and properties, achieving the desired product characteristics. This article will explore the main classifications, fundamental principles, and the differences in quenching and cooling speeds, as well as the distinctions between air and water quenching for aluminum profiles.
Fundamental Principles of Aluminum and Aluminum Alloy Heat Treatment
Annealing involves heating the product to a specific temperature, maintaining it for a defined time, and then cooling it to room temperature. This process improves the uniformity, stability, and stress relief of the structure. It includes:
- Homogenization Annealing of Cast Ingots: Long-term insulation at high temperatures followed by controlled cooling to homogenize chemical composition, improving material ductility by around 20% and reducing extrusion pressure.
- Intermediate Annealing: Also known as local or inter-process annealing, it enhances material ductility, eliminates internal processing stresses, and involves shorter insulation at lower temperatures.
- Complete Annealing: Also known as final annealing, it results in a fully recrystallized state at higher temperatures, offering the best plasticity and lower strength.
Solution Quenching Treatment
Solution quenching involves heating aluminum alloy materials to high temperatures and holding them for a certain duration. This dissolves second-phase or other soluble components into the aluminum matrix, forming supersaturated solid solution. Fast cooling maintains this unstable state, allowing for subsequent cold working or straightening processes.
- In-line Quenching: For alloys with low sensitivity to quenching, solubilization can occur during extrusion at high temperatures, followed by air cooling (T5) or water mist quenching (T6).
- Offline Quenching: For alloys highly sensitive to quenching, a specialized heat treatment furnace reheats the material to high temperatures, holds it for a specific time, and then quenches it in water or oil within a transfer time not exceeding 15 seconds. This can include salt bath quenching, air quenching, vertical quenching, or horizontal quenching, depending on the equipment.
After solution quenching, aging involves keeping the material at room temperature or a higher temperature for a period. The unstable supersaturated solid solution decomposes, and second-phase particles precipitate, leading to precipitation (or precipitation) strengthening.
- Natural Aging: Some alloys (such as 2024) can undergo precipitation strengthening at room temperature, known as natural aging.
- Artificial Aging: For alloys like 7075, where room temperature precipitation is less significant, artificial aging at higher temperatures results in pronounced precipitation strengthening. This can be categorized as under aging and over aging.
To enhance plasticity and facilitate cold bending or correction of form tolerances, reversion treatment involves briefly heating products that have undergone quenching and aging to high temperatures.
Aluminum Alloy Quenching Cooling Speed
The cooling speed during quenching of aluminum alloy furnaces must ensure that the supersaturated solid solution is fixed without decomposition. The choice of cooling speed depends on the alloy and the shape and size of the product. Water is a commonly used quenching medium, but adjusting water temperature or adding different solvents can regulate cooling capacity.
- Different Cooling Speed Requirements: Alloys 2A11 and 2A12 require a cooling speed of over 50°C/s, while 7A04 alloy is highly sensitive, necessitating a speed of over 170°C/s.
- Selection of Quenching Medium: Water is a common quenching medium due to its low viscosity, high heat capacity, rapid evaporation, strong cooling ability, and economic feasibility. However, its drawback is reduced cooling capacity after heating. Quenching in water involves three stages: film boiling, bubble boiling, and heat convection, and to overcome the first stage quickly, compressed air pipes are often installed in quenching tanks for stirring. Proper agitation is necessary upon immersion to prevent uneven cooling.
In addition to adjusting water temperature to control the quenching speed of aluminum alloy furnaces, different solvents can be added to the cooling water.
- Commonly Used Solvent: A polyethylene glycol-water solution is commonly used as a cooling medium. Adjusting the concentration of this solution can control the cooling speed during quenching. This solution is frequently used for quenching easily deformable products.
Differences Between Air Cooling and Water Quenching for Aluminum Alloy Profiles
Aluminum alloy quenching involves either air cooling or water cooling. T5 and T6 states differ concerning cooling speed, with water cooling achieving T6 effects and vice versa. The T6 state can be achieved through online or offline quenching, depending on customer requirements and product specifications.
- Choice Between T5 and T6 States: T5 is suitable for most door and window profiles, while T6 is suitable for profiles requiring higher hardness, although water cooling may cause deformation.
- Relation Between Cooling Effect and Hardness: A better cooling effect results in higher hardness after aging. The choice between air and water cooling depends on product shape and requirements.
By employing scientifically sound heat treatment processes, aluminum and aluminum alloy profiles can attain the desired structure and properties, meeting the diverse needs of different products. When selecting heat treatment methods, it is crucial to consider material characteristics, shape, and customer requirements to ensure optimal results.