Heat treatment is applied to increase the strength of aluminum alloys produced by mechanical processes. These processes consist of 3 steps:
- Immersion in Solution
In solution immersion heat treatment, after the material is heated to a temperature above the solvus curve, a supersaturated solid solution structure is formed by rapidly cooling from this temperature. The aim here is to dissolve the elements that will cause precipitation hardening and to imprison them in solid solution. Three phases are seen in the solution taking step. The first is the dissolution of the alloying elements to form a rich aluminum matrix phase, which will then create precipitation hardening, the second is the spherification of the insoluble components, and the third is the homogenization of the microstructure. Rapid cooling, that is, quenching, ensures that the dissolved elements remain in solution. It is known that the stress that occurs during quenching has negative effects on properties such as dimensional stability and fatigue strength in later use of the part. The quenching method to be used is determined by the weight, geometry and expected properties of the part. The dissolved elements remaining in solution after quenching begin to precipitate slowly at room temperature. After a while, some alloys can become quite hard at room temperature. This is called natural aging. The process called artificial aging is the acceleration of aging by keeping the cast parts at temperatures such as 100-200 °C. As the time at the aging temperature increases, a precipitate gradually forms and the hardness increases towards its maximum value. If further aging is continued, a state of extreme aging occurs, in which a decrease in hardness occurs.
HARDENABLE ALUMINUM ALLOYS
- 2xxx Series Alloys: 4.5% copper, 1.5% magnesium and 0.6% manganese
- 6xxx Series Alloys: 1.0% magnesium, 0.6% silicon, 0.3% copper
- 7xxx Series Alloys: 5.6% zinc, 2.5% magnesium, 1.6% copper and 0.25% chromium