Rebekah,
As soon as someone starts figurin they're an expert something will rear up and give em a reality check. We pick ourselves up, dust ourselves off, and dive into it again.
We do the best we can.
And as emplyees we generally do what we are asked to do. We don't learn if we are never asked to do things unfamiliar, so to speak. But we research, do our due diligence, talk to those who have been there, read, read, read, and take a shot. You obviously have done that, and done well.
You are right, 4643 is the recommended alloy for HT response with 6XXX's, due to Mg.
But with Scott, Lawrence, and TJ's advice you should see improvement the next time around.
Sometimes companies unreasonably expect us to nail it first time around. It doesn't always work out that way. But I think thats maybe the beauty of it. Long as we have enough butt left over after the chewins.
Have worked with the 4643 filler metal using GMAW with good results. At the end of the day the part we welded was too big to do the PWHT ourselves and it was sent out for PWHT so unfortunately I can't give you a this worked for us story about it.
In researching this subject (we were considered doing the PWHT) we were going to use a similar HT process except we were going to heat and soak our assembly at a temperature of approximately 530°C (985°F) for a given amount of time (can't remember). Keep in mind that the complexity and chemistry of the part have an effect on the heating/soaking temperature. The heating rate and soaking time will also depend on the type of furnace used and the time to bring the through thickness of the assembly to the soaking temperature. Can you pump an inert gas into your furnace to help minimize contamination of the Al during PWHT? The assembly was to be quenched in cold water. It was to be quenched as quickly as possible after the solution heat treatment to minimize precipitation... you need to make sure that the amount of water you have will draw out the heat very quickly so that it cools your assembly to room temperature without interruption. The colder you can get the water the better. Also, we found that in our research that typically 6061 was aged at 160°C (320°F) for 18 hours; however, there are a number of curves that will work depending on the yield strength you need.
Just out of curiousity rather than going to all this trouble would it be possible to make things a little thicker to compensate for the lower strength of the assembly?
Keep trucking and at the end of this "adventure" you'll be able to weld and PWHT 6061 back to T4 or T6.
Sean
4643 Also gives good mechanical properties in the as welded condition on 6061-t6 so it could be kept around the shop for other weldments if your worried about never haveing to use it again. But it sounds like you have your situation under control.;-)
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