Basic construction metals tend to physically transform over time due to natural environmental conditions. The texture and color of the metal surface changes with an oxide layer forming on it in the initial stages. Soon this layer converts to a hydroxide layer. Later this hydroxide layer combines with other elements in the atmosphere and finally the metal surface gains a stable mineral composition that is very resistant to any further alteration.
This weathering or aging process can be clearly seen in natural aluminum, copper and copper alloys, lead, steel and zinc.
Aging has been extensively studied and many companies are working towards perfecting the ability to accelerate and increase the weathering or aging process to bring the metal surface to a desired texture and color. The aging process used by several companies is simply an extended heat-treatment process. This process is important for strengthening heat treatment of alloys containing Al, Cu, Mg, and Ni.
Different Aging Techniques
Artificial aging is the treatment of a metal alloy at elevated temperatures so as to accelerate the changes in the properties of an alloy as a result of the casting and forging process. Generally, the chemical properties of newly cast and forged metals naturally change and settle very slowly at room temperature. Artificial aging will speed up this change more rapidly at higher temperatures. This process ensures quality and accuracy in close tolerance specifications. It also helps manufacturers make machine-ready parts available much more quickly to machinists and distributors.
Aging that occurs at room temperature is referred as natural aging.
The aging process is one of the three important steps involved in precipitation hardening, the other two being - solution treatment and quenching. During precipitation heat treatment, the supersaturated solution is heated to an intermediate temperature so as to induce precipitation and held at that temperature for a specific amount of time.
To perform precipitation hardening, there has to be three criteria - appreciable maximum solubility, a solubility curve that reduces quickly with temperature, and composition of the alloy that is less than the maximum solubility. Care should be taken to ensure that the aging is accomplished below the equilibrium solvus temperature, and also below a metastable miscibility gap called the Guinier-Preston (GP) zone solvus line.
Aging is an essential step that ensures that the materials in the alloy do not revert to their original configuration after a time period. Aging is performed under controlled conditions so that the resultant grain structure will create a greater tensile strength in the metal than in its former state.
If the process is continued beyond the specified time, eventually the hardness decreases. This is termed as overaging.
This technique refers to aging or hardening process that occurs due to cold working the alloy.
This technique refers to another type of aging process that freezes or stops the internal changes that would generally take place in the alloy at room temperature.
Benefits of Aging
Aging not only helps to enhance high strength of alloys but helps them to acquire other valuable properties such as high coercivity. In addition, aging tends to restore the equilibrium in the metal and to eliminate any unstable conditions brought upon by a prior operation.
It has been shown that deformation and aging greatly reduce the resilience and ductility of alloys. However, the adverse effects of aging can be reduced to a large extent by using special alloying and heat treatment.
Aging is used in the following industries:
- Tools and dies
- Die casting and high-performance shafting
- Aircraft manufacturing and repair industries
Sources and Further Reading