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Heat treatment of aluminum alloys

The role of heat treatment on aluminum and aluminum alloys

Heating aluminum and aluminum alloy materials to a certain temperature and holding for a certain period of time to obtain the expected product structure and properties

01 Annealing

The product is heated to a certain temperature and held for a certain period of time, then cooled to room temperature at a certain cooling rate. By atomic diffusion and migration, the organization can be made more uniform, stable, and stress relieved, which can greatly improve the plasticity of the material, but the strength will decrease.

① Uniformization annealing of ingots: Long term insulation at high temperature, followed by cooling at a certain speed (high, medium, low, slow) to homogenize the chemical composition, structure, and properties of ingots, can increase material plasticity by about 20%, reduce extrusion pressure by about 20%, increase extrusion speed by about 15%, and improve the quality of material surface treatment.

② Intermediate annealing, also known as local annealing or inter process annealing, is a process to improve the plasticity of materials, eliminate internal processing stresses, and maintain insulation at lower temperatures for a shorter period of time, in order to facilitate continuous processing or obtain a combination of certain properties.

③ Complete annealing: also known as finished product annealing, is a process of holding at a higher temperature for a certain period of time to obtain a softened structure in a fully recrystallized state, with the best plasticity and lower strength.

02  solution quenching treatment

Heating the heat treatable reinforced aluminum alloy material to a higher temperature and maintaining it for a certain period of time, so that the second phase or other soluble components in the material are fully dissolved into the aluminum matrix, forming a supersaturated solid solution. Then, the supersaturated solid solution is maintained at room temperature by rapid cooling. It is an unstable state because it is in a high-energy state, and solute atoms may precipitate at any time. But at this time, the material has high plasticity and can be subjected to cold working or straightening processes.

① Online quenching: For some alloy materials with low quenching sensitivity, high temperature during extrusion can be used for solid solution treatment, and then air cooling (T5) or water mist cooling (T6) can be used for quenching to obtain certain microstructure and properties.

② Offline quenching: For some alloy materials with high quenching sensitivity, they must be reheated to a higher temperature and held for a certain period of time in a specialized heat treatment furnace, and then quenched into water or oil with a transfer time of no more than 15 seconds to obtain certain structure and properties. Depending on the equipment, they can be divided into salt bath quenching, air quenching, vertical quenching, and horizontal quenching.

03 aging 

After solid solution quenching, the material is maintained at room temperature or higher temperature for a period of time. Unstable supersaturated solid solution will decompose, and second phase particles will precipitate (or precipitate) from the supersaturated solid solution, distributed around the α (AL) aluminum grains, thus producing a strengthening effect called precipitation strengthening. nature

Aging: Some alloys (such as 2024) can undergo precipitation strengthening at room temperature, which is called natural aging. Artificial aging: Some alloys (such as 7075) exhibit insignificant strengthening at room temperature, while the strengthening effect is significant at higher temperatures, which is called artificial aging. Artificial time efficiency can be divided into under time and over time.

① Underaging: In order to achieve certain performance, control a lower aging temperature and maintain a shorter aging time.

② Over time: The aging process carried out at a higher temperature or for a longer period of time to obtain certain special properties and better overall performance.

③ Multi level aging: In order to achieve certain special performance and good overall performance, the aging process is divided into several stages. It can be divided into two stages and three stages of timeliness

04 Regression Processing

In order to improve plasticity, facilitate cold bending forming or correct positional tolerances, the product that has been quenched and aged can be restored to its new quenched state in a short period of time by heating it at high temperature, which is called regression treatment.