From the info You posted and what Stephan posted My guess is that the length of the "soak" is related to the temperature, so it might be practical to use a temperature that suits the overnight timeframe, if You can find such a chart.

Hi Rebekah!

Here's what I got from the Alcotech site with respect to me making a somewhat "educated guess" as to why 4643 was chosen as opposed to a 4043:

"Using 6061-T6 to fabricate a large safety-critical lifting device that is required to undergo extensive welding during fabrication, followed by postweld solution heat treatment and artificial aging in order to restore some strength and return the structure to the -T6 temper. In this application our concerns may be the strength of our weld after it has been exposed to postweld heat treatment. Most filler alloys commonly used for welding this base material will not respond favorably to this type of heat treatment. The 5356, 5183, and 5556 filler alloys are non-heat treatable alloys which can undergo undesirable changes if subjected to this form of heat treatment. The 4043 filler alloy, on its own, is non-heat treatable and would be totally dependent on dilution with the base material in order to achieve any significant response to the heat treatment. In this application we should seriously consider the use of filler alloy 4643, which is a heat treatable filler alloy and will, therefore, respond to the heat treatment after welding and provide a weld of comparable strength to that of the base material."

This was referenced from "AlcoTechnics": http://www.alcotec.com/qanda5.htm

Here is a "Weld Data" sheet on alloy 4643 which explians in better detail as to why this filler was chosen as opposed to a 4043 or some other alloy instead:

http://www.alcotec.com/a4643tds.pdf

So the differences between 4043 & 40643 are:
1.) One alloy (4643) is a heat treatable alloy and one is NOT(4043).

2.) The 4643 is slightly better in corrosion resistance than 4043 and since this component will be headed below into a salty, somewhat corrosive environment such as the bottom of the ocean... Any advantage in corrosion resistance will help - no matter how little.

3.) 4643 has anywhere from 0.10 to 0.30 % of Magnesium as opposed to 0.05 % Mg in 4043.
However, I do not know if the Mg compatability would be a primary issue here if, and I say this without knowing what the actual joint configurations that were used in welding the component together and the specific weld process or the intended application of this component. It would certainly make a difference in groove welds when one compares the dilution of Mg in square groove as opposed to a vee groove or even a single bevel goove joint configurations when one uses the GMAW process.

All one has to do is to do the math here... For instance, the 6061-T6 has an approx content of 1% Mg... 4043 filler has an approx 0.05% Mg... In a square groove joint, it has been observed that approx 20% Mg (0.01%) is diluted into the weld from the filler and approx 80% (0.8%) of the rest of the total Mg content comes from the parent or base 6061-T6 alloy which would bring the total Mg content to approx: 0.81% Mg.

   0.01% = 20% of total Mg coalesced in weld from 4043 filler.
+ 0.80% = 80% of total Mg coalesced in weld from 6061-T6 base.
---------------------------------------------------------------
  0.81% = 100% of total Mg in weld found in a square groove joint.
Note: These are nominal values referenced from "AlcoTechnics" written by Tony Anderson.
You can find these values here:
http://www.alcotec.com/qanda3.htm
http://www.alcotec.com/a4043tds.pdf
http://www.alcotec.com/a4643tds.pdf

Now compare the total Mg content that has been found when one uses an alternative joint configuration... Instead of a square groove, a single bevel is used... Up to 60% (0.03%Mg) of the 0.05% Mg found in 4043 will dilute into the weld, with the balance of 40% (0.40%) coming from the 1% found in the 6061-T6 base...

   0.03% = total % of Mg coalesced in weld from 4043 filler.
+ 0.40% = total % of Mg coalesced in weld from 6061-T6 base metal.
------------------------------------------------------------------
   0.43% = total % of Mg found in weld when filler and base metal are coalesced together in alternative joint configuration.

This would therefore give one an approx total Mg content in the weld of: 0.43% Mg. Notice the difference in total nominal Mg percentage found in the weld when one compares between a square groove and a single bevel or vee groove??? Clearly there's less Mg in the weld when a single bevel as opposed to a square groove joint configuration is considered.

Now compare the total Mg content when 4643 is used as the filler instead of 4043...
Once again, we start with a square groove joint configuration whereby approx 1% Mg is found in the base  alloy 6061-T6, and 0.1 to 0.3% Mg found in the 4643 filler metal... We then use the same percentages of Mg found when one compares square vs single bevel or vee grooves whereby in a square groove, 20% of the Mg found in the weld comes from the filler and the 80% balance comes from the base metal... Everybody follow me so far? Okay, let's do the math: For a square groove, 20% of the 0.1 to 0.3% Mg found in the 4643 filler would equal to: 0.02 to 0.06% Mg... Now add that amount with the 80% balance of Mg found in the weld which came from the 6061-T6 base metal... this amount of Mg would be: .8% Mg... Now add the two percentage amounts which would be:

   0.02% to 0.06% = total % of Mg coalesced in weld from 4643 filler metal.
+ 0.80%     0.80% = total % of Mg coalesced in weld from 6061-T6 base metal.
-----------------------------------------------------------------------------
   0.82% to 0.86% = total Mg found in weld when filler and base metal are coalesced together in square groove joint configuration.

Last step is to compare the square groove total Mg content in the weld with what would be approximately found when one considers the use of either a single bevel or vee groove joint configuration...
So, the percentages change to 60% of the Mg is diluted into the weld, and the balance of 40% coming from the base metal... Hence - 0.06 to 0.18% of the total Mg found in the weld comes from the filler which would equal to 60% of the Mg found in the 4643 filler... Then the balance of 40% would come from the base metal and would amount to approx: 0.40% of Mg... Now let's add the two together...

   0.06 to 0.18% = total % of Mg coalesced in weld from 4643 filler metal.
+ 0.40     0.40% = total % of Mg coalesced in weld from 6061-T6 base metal.
---------------------------------------------------------------------------
   0.46 to 0.58% = total % Mg coalesced in weld when filler and base metal  are coalesced together in alternative joint configuration.

Now let's consider this from AlcoTechnics:
"Solidification crack sensitivity increases sharply with an increased Mg content up to about 1.5% and then starts to decrease with further Mg additions. (Figure 1 at Al-Mg curve)".

We have:
4043 filler+6061-T6 = Approx 0.81% total Mg content found in weld of a square groove joint.
4643 filler+6061T-6 = Approx 0.82 to 0.86 % total Mg content found in weld of a square groove joint.
Note: There's not too much of a difference in solidification crack sensitivity here! Well - maybe slightly more when using 4643 but only a difference of a few hundredths of a %.

4043 filler+6061-T6 = Approx 0.43% total Mg content found in weld of single bevel or vee groove joint.
4643 filler+6061-T6 = Approx 0.46 to 0.58% total Mg content found in weld of single bevel or vee groove.
Note: There is a slight, marginal difference in solidification crack sensitivity where one would consider between the use of 4043 and 4643 filler.

So this data would suggest that 4643 is slightly more sensitive to solidification cracking when compared to 4043... However, since 4643 is a heat treatable alloy and 4043 is NOT! Then I believe that is the primary reason why 4643 was chosen as opposed to 4043 when considering the use of solution heat treatment that would be applied post weld in order to regain a majority of the tensile strength lost in the adjacent base metal due to welding.

Now we are left with the questions as to what would be the recommended temperature and time for post weld solution heat treatment and artificial aging to bring back up 90% of the tensile strength of the unwelded base metal because as we should know, after welding of 6061-T6 - the surrounding base metal has lost quite a bit of it's original tensile strength that it had prior to welding.

So when we read this:  http://www.alcotec.com/qanda1.htm
We get a better understanding of what we're actually working with as far as the base metal is concerned.
We also understand why one would want to apply post weld solution heat treatment and artificial aging to the component in order to regain as much of the lost tensile strength due to welding as possible without the potential of overaging the component resulting in a useless component. By the way, this is AlvoTech's explanation for PWHT when welding 6061 T-6 Al: "Post Weld Heat Treatment: Typically, the common heat treatable base alloys, such as 6061-T6, lose a substantial proportion of their mechanical strength after welding. Alloy 6061-T6 has typically 45,000 PSI tensile strength prior to welding and typically 27,000 PSI in the as-welded condition. Consequently, on occasion it is desirable to perform post weld heat treatment to return the mechanical strength to the manufactured component. If post weld heat treatment is the option, it is necessary to evaluate the filler alloy used with regards to its ability to respond to the heat treatment. Most of the commonly used filler alloys will not respond to post weld heat treatment without substantial dilution with the heat treatable base alloy. This is not always easy to achieve and can be difficult to control consistently. For this reason, there are some special filler alloys which have been developed to provide a heat treatable filler alloy which guarantees that the weld will respond to the heat treatment. Filler alloy 4643 was developed for welding the 6xxx series base alloys and developing high mechanical properties in the post weld heat-treated condition. This filler alloy was developed by taking the well-known alloy 4043 and reducing the silicon and adding .10 to .30 % magnesium. This chemistry introduces >Mg2Si< into the weld metal and provides a weld that will respond to heat treatment."
So you were on the right track Rebekah!

I would agree with Stephan's suggestions for post weld heat treatment and artificial aging. I would also suggest to read Eutectic's response in the "Shop Talk" section of the forum in which he gets into more metallurgical detail than myself in explaining what happens when one applies post weld heat treatment to 6061 T-6. Concerns of overaging the component is explained in better detail in the other thread as well.

I would also observe that Rebekah was slightly conservative in her temperature and time which should be okay considering the presumption that the least amount of heat input possible was used during the welding of the component, and that the proper steps were taken to maintain the minimal amount of heat input allowable while still resulting in an acceptable weldment.

I do'nt know if it's true with the 5082 Al filler Stephan but here in the US, it can be difficult or should I say, less cost effective to procure an alloy that is more commonly found in EN designations hence, European Union nations such as Germany. Also ASME and some other codes do'nt always recognize certain alloy designations... I found that out when I first started to work with 316Ti alloy which is not commonly sold here in the US. However in Germany, the use of 316Ti is widespread and especially in high heat applications, the appropriate filler metals are also more readily available.. At least this has been my experience.

Good luck with your project there Rebekah, and just think of the Andes Mountains and all their astonishing beauty when things get rough... Hey - it works for me!

Respectfully,
Henry