March 14, 2011
Q: Can you tell me the heat treatment steps necessary to produce 6061-T6? I have heard that heat treating it after welding will increase its mechanical properties.
A:Let's first describe what is happening when you heat treat 2XXX, 6XXX, or 7XXX alloys. All of these alloys are heat treated by precipitation hardening. This involves two steps—solution heat treating and aging.
Solution 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 we quench the alloy 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.
The question, however, is whether you can perform this yourself. There is no doubt that the result will be 6061-T6 properties if you do it properly. The main difficulty is that the component usually distorts quite a bit during quenching and requires significant mechanical straightening before aging. This is often very difficult or even impossible, especially on large weldments. That's why most people use 6061-T6 in the as-welded condition.
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.
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