Welding Tips and Tricks

How best to manipulate the torch to control the keyhole in MIG in an open root butt weld? For example, you see the keyhole opening up too much, what can you do? You can't exactly whip as with stick, at least not along the root gap. I have seen how, if you reduce the stickout ie push the torch closer, the voltage can fall quite a bit and so, if the current were to stay the same, the heat input would be less, but I wonder if it works in practice? I have tried momentarily stopping and restarting, letting the pool cool for a fraction of a second and that works but I'm not convinced it's good practice.

Advice please.

Martin

Wow,

I have never done or seen an open root mig keyholing joint.

But I think that if you reduce you stickout as you mentioned, that that keyhole woule close up a bit.

Stopping the weld while it cools may work as well, but runs down your production time.

My best guess is to turn down the voltage and wire speed a bit, until you can run a steady bead without stopping. Maybe you could even close the root gap a bit?

I hope this helps a bit,

-Rick

Thanks Rick.

The reason for the open root MIG joint is for the City and Guilds Level 2 qualification (a UK qual); so that explains why you won't have come across it. I imagine MIG would not be the first choice for such a joint, all other things being equal.

You are absolutely right and if this were a production joint, I'm sure you could set it up so that you could find the ideal voltage and wire speed. However, when you grind the bevel and root face by hand there is scope for inconsistency; nevertheless, I use 2 pieces of 1.5mm stainless to set the root gap the same every time. (Much rather have the root gap a bit too wide than too close, because if you don't get the penetration it's a fail.) I've found with both MIG and stick that the root gap is ctitcal: a very slight change and the settings that worked previously are now either too hot or too cold. The root face size is also important but not quite as much, I've found.

So I've tried to be as consistent as I can within the confines of the facilities in preparing my test pieces prior to welding. I can see why they say the butt joint is the hardest, but as ever the more practice I get the better I get.

I really appreciate your kind advice, which I think is spot on.

Martin

Martin, a few techniques come to mind. 1. Perform a manual pulse by cycling the arc on and off, say in 1 second intervals. This will cool the puddle just as if you were pulse arc welding. 2. Drive the wire back into the puddle by dropping he gun down flatter so you are not pushing down through the weld. 3. You can also do a step back weld by running the wire back onto the the bead and back into the groove. This is not a whip action, but a back stitch and a with forward progression.
Tighten up the gap and reduce the land thickness almost down to a sharp edge. Your gap is about 1/16" what is the thickness of your land and what size wire are you using?

Tom, hi, and many thanks. Very interesting ideas -they makes excellent sense and I look forwards to trying them out.

The wire dia is 0.8mm and we use Coogar 5 gas: 93% AR, 5% CO2 and 2% O2. The root face I set is about 2mm. I try not to use anything less and there are times when it might go to 2.5mm.

I understand what you say about feathering the edges and butting them together. In fact, the tutor, when I did my MMA Level 2 course told me that some people prefer to set up that way rather than leave a gap. That said, I persevered with my root gap and 2~2.5mm root face and slowly got to grips with the setup. I didn't have the time on the course to go back and try the feathered-edge method. And some of the experience I gained from MMA has carried over to MIG and, with this course being slightly shorter, I didn't want to lose time to a new technique, so I've stuck with the gap and root face. Nevertheless, I have, thanks to MMA, found the MIG butt welds in the flat and the vertical, a lot easier, so I think I might have a bit of time free at the end to go back and try the feathered method. But first, I really do want to try your ideas: they appeal very much.

All being well, I hope to post back and let you know how it went but I really thank you for you excellent suggestions and I intend to pass them on to the other students taking Level 2.

Many thanks for your kind advice.

By the way, last term I did a few trials to satisfy myself about the effects of voltage and wire speed on penetration and I put the result together in a Word file (20Mb in size) at:

http://dl.dropbox.com/u/534872/MIG%20Cl ... risons.doc

If the link doesn't open or you get a 404 message, just copy the address into the address box in your web browser and hit Enter.

I hope they might be af interest to anyone interested in MIG. I found the results most helpful to some of the questions I had.

Martin