Welding Tips and Tricks

Yeah, probably an old subject.
Miller 251 with spoolmatic 30A gun
tank with pin holes to repair - 5052? aluminum plate?
3/16" x 3" aluminum flat bar for repair job - 6061?
Filler wire - ER4043

I cut off a piece of 3" flat bar (aluminum 3/16") to patch a hole in a aluminum tank where a pin hole corroded through. I had 2 holes so I had 2 weld patches to do.
The first patch weld had no problems. it was a 3" by 3" square I cut and welded. It looked fabulous.
The second one however was a bit bigger - 3" by 6" patch. I finished welding and it also looked good and then BAM! - one of the side welds cracked the whole 6" length on one side. So I cleaned it up and welded it again. You guessed it, BAM! It cracked again.

What gives here? I imagine the tank is 5052? series plate aluminum. The flat bar was probable 6061 series. I used ER4043 filler wire.

Anyone know what I'm doing wrong? Should I shorten up the welds to let them cool properly? Preheat the weld area? change filler alloy to a higher alloy say 5356?

It was a bitch taking this tank out of my boat. So, I need to be sure I do this right so I can confidently put it back without worries. What do you think?

Jim

Spool guns tend to have that issue because you can't cool the weld with argon after you lay it down. It is my opinion that 5356 tends to hold better than 4043 without the heat cracks afterwards. Preheat can help, shorter welds can help too. The last time I welded an aluminum boat with my spool gun I had this same issue. I "bumped" the weld and had much better success. Basically independent hot tacks. This also allows the base metal to rest without becoming heat soaked.The weld looked more like a TIG bead than a MIG bead because each advance was so deliberate.

UPDATE

After several years of occasional use of this spool gun, I discovered (smh) it is running much hotter than what the dial read-out says. When the chart says to turn my dial to 21 volts, I found that 19 volts is closer to correct. I should not have put that much faith in what the machine dial said and used my own judgment on what I was seeing. Looking back I can recall when I should have thought, "this is not right" but just shrugged my shoulders.
Anyway, I ended up reducing the voltage and preheating the aluminum and the welds turned out fabulous - even looking professional. I am relieved.
I don't know why Miller welders doesn't make this right (maybe they did and I missed it). At any rate, I've ruined several projects thinking I was regressing as a welder and was a bit confused as to why I could not weld aluminum anymore. Anyway, turns out it was miller or at least there spool gun (30A) as the main welder (I use on steel) on the machine works perfectly.
I don't know, is there an adjustment on the spool gun or welder to dial in the voltage so 21 volts means 21 volts instead of 23 or higher?

jim

You might see if there is a software update for your machine. Especially, where the spool gun is concerned.