The properties of metal materials often fluctuate during use and processing. Factors such as time, environment, and stress can trigger a phenomenon known as aging. Aging alters the metal's structure, affecting its strength, plasticity, toughness, and corrosion resistance.
Metal aging refers to the process by which the structure and properties of metal materials change over time at room temperature or under heated conditions. This typically manifests as hardening, increased strength, decreased plasticity, and even embrittlement in some cases.
The aging process is closely related to the diffusion of solute atoms, the formation of precipitated phases, and dislocation motion within the metal. This microstructural evolution is the root cause of changes in metal properties.
Natural aging is an aging process that occurs spontaneously at room temperature or lower, without the need for an external heat source. A typical example is the gradual increase in strength of aluminum alloys (such as 2024 and 6061 alloys) after quenching and aging. This is natural aging.
Characteristics:
Relatively low temperature (usually 20-30°C).
Requires a relatively long time (several hours to several days).
Gradual increase in strength.
Poor controllability.
Typical Materials:
Age-hardenable aluminum alloys (Al-Cu and Al-Mg-Si systems).
Some copper alloys and low-carbon steel.
Artificial aging accelerates the aging process by heating to an appropriate temperature (generally 100-200°C) and holding the temperature for a specified period. This process, also known as "aging," is a crucial step in the heat treatment process.
Characteristics:
Higher temperature and controllable time.
More pronounced precipitation strengthening.
Higher strength, but with some potential loss of ductility.
Optimal properties can be achieved by adjusting temperature and time (called "peak aging").
Typical Materials:
2000, 6000, and 7000 series aluminum alloys.
Maraging steel.
Copper-nickel-silicon alloy.
Temperature: Higher temperatures accelerate diffusion and aging; however, excessively high temperatures may lead to overaging.
Time: Aging time determines the number and size of precipitates; a balance must be struck between strength and toughness.
Alloy composition: The type and concentration of solute atoms determine the type of precipitates and the degree of strengthening.
Pretreatment: Quenching rate, cooling medium, and initial microstructure influence aging effects.
Stress and environment: Stress or corrosive environments experienced during service can accelerate or alter aging behavior.
Positive Effects:
Artificial aging is a key method for achieving high strength in modern aluminum alloys, titanium alloys, steel, and other materials.
Negative Effects:
Certain low-carbon steels, copper alloys, or welded components can experience decreased plasticity, embrittlement, or uneven performance due to natural aging over long periods of service.
Therefore, in actual production, it is necessary to select a suitable heat treatment system according to the purpose of the material in order to utilize aging strengthening or avoid aging damage.
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