Welding Tips and Tricks

Hi all,
I have problems in keeping consistent quality in but welding of thin, (.040) alu sheet. I make frequently small reservoirs, out of .040 AlMg3 alu sheet. Lots of butwelding herefore is used. For most of the welds, there is no "back support" possible.
(When putting a SS sheet against the backside of the weld bead, no problems at all, but when that is not possible, the problem is there.
Usually, the weld puddle progresses in the form of a "tongue", and in that case I can produce exellent welds.Exellent contolabillity of weld puddle, control of penetration and all the goodies, but... sometimes, the tongue form changes (all of a sudden) to an "O" shape with' you guessed it already, a small hole in the middle It is not a matter of sagging of the puddle, but more like the puddle seems to loose cohesion in the middle, and wants to crawl up to the edges....No way to get that hole "repaired". Adding in more filler rod will only increase the sise of the walls..For one or another reason, the weld puddle seems obstinate refusing to flow together....whereas the molten alu looks nice and shiny. What am I missing here?
Weldor setting is Dia electrode 1 mm., 45% cleaning, 80Hz, 35/45 Amps, filler rod dia 1.5mm
Looking forward for your reply,
Kind regards,
Des.

Hmmm, wish I could see it happening. At first thought though I suppose it could be some form of steel contamination or maybe even a tungsten inclusion.

If you move off of the hole and continue does it happen again?

Can you post a picture?

Are you finding this near the end of welding the container up? If so this could be a hot air problem, expanding gas (and it's filling up from your shielding gas) are trying to escape causing an updraught. Maybe try letting it cool then do a quick run before it heats.
Cheers

Here's a little trick I learned. Get yourself some 6 or 8 ga copper wire and flatten it with a 3 lb hammer. You can cut it to length lay it under the piece or beside the weld and it will help to keep the haz area in check. Sometimes it works and sometimes it doesn't but mostly it does. A buddy of mine uses copper pennies and that's where i got the idea.

Hi all,
no, it is not an air pressure problem, since I leave the filler cap off. If I have to weld a closed cavity, I always drill a small hole , wich I close up as a last action. (Common practice I tought)
As I mentioned before, if possible, I put a SS strip under the bead, wich works fine. ( Does a copper strip not contaminate the alu?) To be complete, I weld with a 2% lanthaan electrode. I know some welders who also select a thicker sheet dimension to do away with the problem. Changing to a .065 sheet is already a night/day difference, but it is heavy, and more expensive, and an "overkill" in strenght.
Best regards,
Des.