Heat treatment is done by raising the alloy temperature to about 980 degrees F and holding it there for about an hour. The purpose of this is to dissolve all the alloying elements in a solid solution in the aluminum. Then the alloy is quenched in water. The purpose of quenching isn’t really to strengthen the alloy, although it does somewhat; it is to cool it rapidly enough to prevent the alloying elements from precipitating on cooling.
So we have a solid solution of magnesium, silicon, and other elements in aluminum at room temperature. This is called the T4 temper. If we take this material and heat treat it at a temperature between 325 and 400 degrees F, the alloying elements begin to form ordered arrays of atoms in the aluminum matrix. These arrays are called GP zones, and they strengthen the aluminum considerably. This heat treatment is called aging, which results in material with a T6 temper.
Three commonly used time/temperature cycles are used for aging—one hour at 400 degrees F; five hours at 350 degrees F; and eight hours at 325 degrees F. All are equally effective.
As an aside, you often see designations of T3 and T5 for 6061. What are they? To be considered T4, the aluminum plate (or extrusion, etc.) must be produced, allowed to cool, and then solution treated and quenched. However, aluminum producers quench extrusions right out of the extrusion press while they are still hot. Technically, this produces T3 material, not T4. If you age T3 material, you get T5 material, not T6. Just remember that for our purposes, T3 and T4 materials are the same, as are T5 and T6 materials.
Below you can see a comparison of base strengths between alloys and tempers. Please note that many factors affect the allowable design strengths used in design which goes well beyond this article.
|(KSI)||Ultimate Tensile Strength||Yield Tensile Strength|
The Aluminum Design Manual (2015, IV-26 alloy values)