Q: We fabricate and weld 6061-T6 aluminum in various product forms. We know that the ultimate tensile strength (UTS) of welded 6061-T6 is significantly lower than the original T6 strength. Recently, some people mentioned that a low-temperature aging treatment after welding can restore welded components to T6 strength. Is this accurate?
A: I wish that were the case, as it would simplify things considerably. Unfortunately, it's not.
The minimum required UTS for most 6061-T6 product forms is 40 KSI. However, fabrication codes such as AWS D1.2 or the ASME Boiler and Pressure Vessel Code only require a minimum of 24 KSI UTS after welding. This is the value needed when tensile testing is part of qualifying a welding procedure, and it's usually not difficult to achieve. Tensile tests on welded samples typically yield 25–26 KSI.
If you attempt low-temperature aging on the welded sample before testing, the heat-affected zones (HAZs) will gain a little strength, but testing will usually show only 1–2 KSI improvement-about 25–27 KSI-far from full T6 properties.
Restoring 6061-T6 Tensile Strength
The only method to fully recover T6 strength is a complete reheat treatment: solution heat treatment at approximately 1,000°F, rapid cold-water quenching, followed by low-temperature aging at around 400°F. This process is feasible but complex and requires significant experience. A common challenge is distortion during quenching, which must then be mechanically straightened before aging-a difficult step.
An alternative approach is to source the starting material in the T4 temper rather than T6. Weld the T4 material, then perform a low-temperature aging treatment at 400°F for one hour (or other schedules, e.g., 325°F for eight hours). Tensile testing after this treatment usually shows around 35 KSI UTS-still below T6 levels but significantly higher than the typical 24 KSI achieved immediately after welding.
Two potential issues to consider:
T4 temper aluminum can be harder to source for small end users, though larger users generally have no problem.
Fabrication codes do not provide any credit for this method; designs still must assume welded strength of 24 KSI. However, for applications not bound by a code, this approach can improve performance.
GNEE recommends carefully evaluating your welding and heat-treatment capabilities before attempting full T6 restoration, and considering T4 starting material with post-weld aging as a practical alternative.
