Ageing
Ageing, or age hardening, is a heat treatment process used primarily with certain alloys, notably some aluminium, copper, and certain types of steel alloys. The primary goal of ageing is to enhance the mechanical properties of the material by precipitating fine particles within its microstructure, which strengthen the material. There are two main types of ageing: precipitation hardening and natural ageing.
1. Precipitation Hardening (Artificial Ageing) :
Solution Treatment : The first step is to heat the alloy to an elevated temperature known as the solutionizing or solution treatment temperature. This temperature varies depending on the alloy and typically exceeds the solidus temperature. This process dissolves the alloying elements and any precipitates that may have formed during previous processing.
Quenching: After solution treatment, the alloy is rapidly quenched in a specialized medium, such as water or oil, to "lock in" the dissolved alloying elements and prevent them from reprecipitating too soon.
Ageing: The quenched material is then aged at a lower temperature for a specific period. During this ageing stage, fine precipitates form within the material, which impede dislocation movement and increase the material's strength and hardness. The ageing temperature and time are carefully controlled to achieve the desired properties.
2. Natural Ageing :
Solution Treatment: Similar to the artificial ageing process, the material is heated to the solution treatment temperature to dissolve alloying elements.
Air Cooling: Instead of rapid quenching, the material is allowed to cool in ambient air. This slower cooling rate promotes a gradual precipitation of fine particles within the material over an extended period.
Natural Ageing: The material is then stored at room temperature for an extended period, allowing for the slow formation of precipitates and an increase in strength and hardness. This process is often used with materials like aluminium alloys.
The main objectives of ageing are to:
- Enhance the strength and hardness of the material while maintaining or improving its ductility.
- Achieve a balance between strength and toughness for specific applications.
- Tailor the material's properties to meet specific engineering requirements.
