The material has excellent weldability. The problem is you are going to adversely affect the mechanical properties of the heat treated 6061 base material along the weld region. I would recommend you use two weld coupons simulating weld joint geometry to qualify the welding procedure with hardness testing in the as-welded and re-heat treated condition. I would suggest using the GTAW process with 4043 filler metal. You can use a higher strength filler metal -5654.
Either way, you will probably need to reheat treat the welded component to restore original mechanical properties.
Precipitation heat treatment - 345 deg F for 8 hours
For the T652 - see the note below
Stress relieved by 1 to 5% cold reduction subsequent to solution heat treatment and prior to precipitation heat treatment.
You can restore forgings to a T6 condition by simply performing a solution and preciptation hardening treatment by using the same parameters above. You will need to evaluate if this condition is permitted by design.
Wrought 6061-T6 has a published tensile strength of 42 ksi. Most welding standards dealing with aluminum consider the weld to be "good" if it can develop a UTS of 24 ksi after welding. The degradation in the HAZ isn't a function of the filler metal, but on the welding technique used.
I've had good results when using GMAW in the spray mode when welding multipass grooved joints. The results using GTAW isn't as favorable especially if the weld and HAZ isn't cooled down between passes. In any event, the interpass temperature and the time at temperature must be limited to minimize the width of the HAZ. Cooling the weld and HAZ between weld beads with blowers can be beneficial.
Allowing the 6061 to remain at elevated temperature too long can result in over aging in the HAZ. Over aging does nothing beneficial for the mechanical properties.
One problem I see is that with the existing product developing fatigue cracks, your design is not robust. You should investigate a stronger material, like 2014-T6 or 7075-T6, as well as surface enhancement by shot peening.
I was researching the concept for repairing the 6061-T652 structure vs replacement. (thank you mrtrngr for the help)
The structures have been in service 20 years. Our FEA analysis shows 2 modes of vibration 10 hz forward and aft and 14 hz side to side. Many have been operated in unbalanced conditions. We are installing strain guages to measure the actual stress. Hope our analysis will determine cause.
swall thanks for providing material and surface finish ideas
