Aluminum Welding Question?

I have an aluminum welding question for the experts out there. I have been welding Steel for a number of years now and I decided to build something out
of aluminum. I am using 1/8th 6061 aluminum 2 inch square tubing for this project. I am also using an Miller 250 MIG welder with 100% argon and .35 4043 wire. Anyhow, it seems that no matter what heat or wire speed, my weld ends up "falling-in" a little at the end. It does not fall all the way through, but it looks no where near the rest of the weld. It looks like it could crack and it is not very nice to look at. I have tried double backing and even re-striking the arc, but it just moves the "falling-in" spot to wherever I stop welding. Just for a reference I have tried using between 19-22 volts and wire speed between 300-500. Just about every setting seems to give me the same results. If someone knows what I am doing wrong, please tell me before I pull all my hair out! Thank you!
-Bob
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wrote:

You already say that you're doubling back a little at the end, which is good.
What happens with the gas ? Could this "falling in" be burning ? Ideally your shield gas feed continues for a little delay after stopping the weld (this is adjustable on some machines) and you should hold the torch in place until things are cool.
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Yes, but even when I double back, the "crater" moves to where ever I stop the gun. I have tried leaving the gas on it until it is cool, but it still happens. I can literally watch the weld stay solid up to one-second after I stop welding, it is at that point that it falls in.
-Thanks, -Bob

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This "falling in" is a new one on me. I have been welding Al with mig for some time and have not seen this particular problem. I have had a problem with Al tubing "blowing out" on me when making the last connecting pass. This is remedied by drilling a "weep hole". Good luck with finding a solution. Tom
wrotE:

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What you are looking at is a severe crater crack. You should make sure your gun angle is set to about a 15 degree push. Set the voltage to 23 and the wfs to 450. Weld just like normal and when you get to the end you can either double back by dragging back or what I do is hit the trigger like 5 or 6 times to fill the crack. When the arc is broken there is a crater that is caused by rapid cooling, it will ALWAYS crack so proper arc stopping is probably more important that arc striking. You will get it with practice. You can read more at alcotecs site in the equipment document in "Crater-Fill".
(if nothing else works try preheat, this will slow down the cooling after welding)
- Mike
Bob Bristow wrote:

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