March 24, 2009
Frank Armao discusses why weld strength increases when incorporating an aging cycle prior to welding in the T6 temper.
In the last Aluminum Workshop, you talked about improving the welded strength of 6061 by welding in the T4 temper and aging after welding. But what happens if we weld in the T6 temper and age after welding? Does the weld get stronger, or does the extra aging cycle overage the T6 base material and make it weaker?
First, let's define what an aging cycle is. The two common aging cycles for 6061 are 400 degrees F for one hour or 350 degrees F for four hours—both will result in the same mechanical properties of the aged 6061-T6. If you weld 6061-T6 and then age it using either of these aging cycles, the transverse tensile properties of the weld actually increase slightly, but not much—only 1 or 2 KSI.
Why does the strength increase? During welding the HAZ is subjected to various thermal cycles, depending on how far it is from the fusion line. Part of the HAZ is actually re-solution-heat-treated by the welding. So when we age the weld in the T6 material, this part of the HAZ actually increases in tensile strength.
So why doesn't the extra aging cycle overage the 6061-T6 parent material and make it weaker? In some aluminum alloys it would. For such alloys, exposure to an extra aging cycle would overage them and reduce the strength of the parent material. However, the 6XXX alloys like 6061 and 6063 have a relatively flat aging curve. That is to say, while the T4 properties rise quickly on exposure to the elevated aging temperature, they don't fall off quickly. So exposing welds in 6061-T6 or 6063-T6 to an extra aging cycle doesn't reduce the strength of the base material at all and actually increases the strength of the HAZ a bit. Is the small improvement in tensile strength worth the cost of the extra aging cycle? That's up to you.