MIG Penetration???

[BillC]

I've just finished my first two nights of MIG at welding classes, and it is going really well so far. My question stems from RockyD's statement that you only get penetration where the arc touches. I can see the arc digging as I move it, but what is happening under the rest of the puddle as it fills? Is the molten puddle melting the surface of the plate to provide fusion or is the puddle only mechanically bonding to the plate, like brazing, but without any penetration or fusion?

Thanks!

[Seldom]

Well BillC, you’re a lucky fella to be able to see mig-SC (short-circuiting) digging into the base metal. Mig-SC really is no different in principle then any other weld joining process actually. It relies on and requires sufficient electric current to melt both the wire and a proper amount of base metal. Proper amount of base metal directly relates to the amount of dilution (mixing) of filler material and base metal. Since Mig-SC’s arc transfer mode is of low heat input, it inherently produces a small, fast-freezing weld puddle characterized by its shallow depth of fusion into the base metal. You would see this in an etched cross-section of weld.

As was similarly stated in another reply, welding is a process that produces coalescence of metals (base metal & filler metal) by heating them to a welding temperature. The process of brazing is a joining process where the base metal doesn’t melt so there’s no dilution of filler and base metal.

Your concern for asking the question is correct though, if you have no degree of dilution, which is what’s called “cold-lap” with mig-SC, there’s no fusion!

I have to say I’m a little disappointed in your instructor for teaching a welding process without first explaining the principles of it. This seems to be the trend today in many training centers I’ve noticed, teach the student to pass a test but don’t teach the student much about “welding”.

[BillC]

Thanks for the interesting reply, Seldom! I guess there must have been a similar phenomenon when I was learning stick welding. When I was dragging 7018 rod, although the arc was much larger than the MIG arc, I think the resulting stringer bead was still wider than the arc. So, either stick welding introduced a lot more heat into the base metal and melted beyond the arc, or there was still a cold lap zone; but it was just a lower percentage of the weld area than with MIG-SC.

So what's the trick with MIG-SC? Do I have to run an intense weave to melt more base metal and maximize fusion?

[Sberry]

I am going to add something sort of related here about fusion and wire welding. I have a small machine also (240v but small, SP130T) I have a larger one also but the same principle applys. I dont like .023 wire and use .030 just because it can make more heat and after welding for almost 30 yrs thru technique I can carry a lot of heat. Someone else picks up my machine and burns holes or cant control the puddle very well so they turn down to make it managable. I had heard about penetration problems with wire thru the grapevine and I woulds have said nonsense till I began reading here. I would have said it was an operator problem and not inherient. Personally I havnt had a problem and I made a post earlier about heavy welding with wire just to hear more about it. After welding for 1000's ? of hrs I know what fusion looks like and you are doin right to look for it. Crank the machine up to the limit for best results.

[Seldom]

Hi BillC,
Yes you’re exactly right, the stick electrode’s arc is a lot hotter thereby increasing the dilution rate which then increases the depth of fusion into the base material well beyond that of mig-SC (short-circuit).

You ask- “So what's the trick with MIG-SC? Do I have to run an intense weave to melt more base metal and maximize fusion?”
Nope, you don’t want to do that. The slower your travel speed, the wider the puddle, the more you set back on the puddle (like you could with stick) the less fusion you’ll get with mig-SC! Mig-SC generates an arc of low heat input, that’s why the puddle freezes so fast. This is also why the process was originally invented was to weld thinner materials which it’s great at. The heat that’s generated at the arc is so low that if the welder allows the wire to settle back onto the puddle as little as 1/16” while he’s welding, the puddle becomes an insulator between the arc and the base metal! (Even with stick there’s a limit of weave or puddle size where the weld metal just rolls over onto the base material and doesn’t fuse) This is called “cold-lap’ and is a true form of lack of fusion because there is no slag entrapped in the areas of LOF (lack of fusion), just weld metal against unmelted base metal!

With mig-SC, the faster your speed of travel while still maintaining an arc and puddle will afford the best fusion because your arc (wire) will be out front on the leading edge of the puddle where optimum fusion is happening. Very, important! Most important!

[chip hayden]

try not to weave or whip when using gma, unless you have an excessive gap or worried about burnthru.[or if you are looking for that washover/beachmark appearance].
if you are welding .250 or thicker, and edge prep and fit-up is correct, it's pretty much a "straight forward" weld with little oscillation. that's one reason why it works so well as an automated process.
also, remember that gma is not as good as gta or sma for sidewall and root fusion. that's why edge prep and fit-up can be a little more critical with thicker sections when using gma.

[Seldom]

Hi Sberry27,
Yes it is inherent to mig-sc with industry-wide documentation supporting the fact. So inherent in fact that industry codes of construction impose extra limits on it’s use specifically for pressure vessel, pressure piping, and structural steel fabrication!

So here you have a process with a transfer mode (short-circuiting) that has this inherent LOF problem and couple it with the eye-hand coordination variability of a welder. Now is when the difference between a quality weld (meeting specific requirements) and a substandard weld gets real fine! I’ve seen and tested many mig-sc welds (multi-pass) that looked like pieces of artwork but were absolutely worthless and the welder may only have missed by 1/16”!

I’ll jump up on my stump here a little.
The weld training and some percentage of welding equipment supply industry as a whole have done a poor job in my opinion by focusing on mig-sc’s “easability” of use rather then it’s inherent short-circuit transfer mode limitation

[chip hayden]

you are soooo right!, Seldom. "Let's sell a product that makes everyone a weldor." let me think, how many pre-qualified joints are there in the short-circuit mode?

[Seldom]

You're right on the money Chip my man! I can see you've "been there-done that."

[morpheus]

hey Mike, what's the best way for a guy at home to test a weld ?

- jack

[Big Nate]

I usualy take test pieces to my buddies shop and we use his 100 ton press and see what they can handle before the weld breaks. the highest # so far was 28 tons on 1/2" mild steel fillet weld with 7018. I think his pressure gauge is off

[Seldom]

Hi Jack,
One of the easiest/quickest tests for a “home-type” welder is also a very common and elementary industry standard test. In addition it’s probably the first “real” test a trainee takes. That’s the fillet weld break test.

Take two pieces of scrap plate or bar and setup a “T” configuration weldment. Make it so you can weld about a 4” fillet weld or so and weld one side of the “T” only using whatever position you want to test yourself in. (vertical, flat, overhead, etc.) Use as many passes as you normally would with the thickness of plate that you’re using to make the test weld on. Don’t forget to add a start & stop when doing this test. As an example, if you’re using 3/8” plate, the test weld should not be larger then a 3/8” fillet weld measured by weld leg size (less then a 3/8” weld on either plate) a two pass ¼” weld would be a nice size.

After you’ve made the weld let it cool to the touch. When cooled, lay the “T” weldment onto its side with the welded side up and facing you. The intention is to either bend or break the weld by applying force to the welded side. You want to apply a loading to the weld test assembly so that the root of the weld is in tension and it bends over onto itself or breaks onto itself. However you accomplish this will force the open the side of the “T” that isn’t welded and either bend the plates back onto the weld or breaking the weld onto itself. You can do this with a sledgehammer or a press as Big Nate used.
Years ago I tested welders in some of the darnedest places with only a sledgehammer and a concrete floor or driveway many times in order to get a job going.
Be very careful sometimes they bounce and go flying off!

If the plate bends flat over onto itself without breaking- you pass!
If the weld breaks and has a fracture surface showing complete fusion for the full width of the weld from the outside weld edge down to the bottom of the root (“V”)-you pass!
If you see any LOF (lack of fusion)- it fails!
If there’s LOF or “cold-lap” present it’ll look like a flat spot and no depth into the plate surface will be seen. (sort of like the backside of a spot of solder dropped on the floor)

[Seldom]

To supplement my explanation above, I just posted an image of what the fillet weld test coupon is suppose to look like when you got to break it on METALWORKER.com You'll find it in (Fillet Test2.img"

[morpheus]

Thanks for the info and image Mike. I have a test coupon at home where I was practicing overhead welds and was curious about it's strength and did pretty much what you describe above but had the weldment in my vise. I wasn't sure what I was looking for (or even doing for that matter) but the plate is almost folded back on itself and the weld didn't break.

I've done this other times with different welds and the weld has kind of torn little pieces of the base metal out after bending the piece over quite a bit. From your description above I can't tell if that passed or failed ?

here's the pic he put on the metalworking.com site:


for future reference you can attach pictures of the appropriate size and type to your posts here by using the Browse button on the Post Reply screen.

- jack

[Seldom]

Hi ya Jack,
Listen; if you bent the weld over pretty much onto itself and nothing gave, she’s a good one!

If the weld broke or tore down the center, she’s a good one!

If the weld pulled away from one of the plates in a ragged or jagged fashion, she’s a good one!

If the weld pulled away and either part (could be only the size of a match head) or all of common surface (weld interface) has a relatively smooth side edge to it where it was up against the plate instead of into it, she’s a bad one!

It sounds as though both or all of yours were good ones!

Now how in the heck did you get that image inserted? I’ve tried and tried starting with those setting tables a few days ago and can’t do it! I thought I’d be the same as inserting an image in an email message but there’s a screen that doesn’t pop up. I had thought my files size was too larger but I knew that was wrong this morning when I scanned the test file down to a tiny 1K and it wouldn’t go.
Here’s what I’ve been doing without success-
Hit “Browse”-takes me to MY Documents (where the jpg’s are)
Highlight file
Hit “open”-takes me back to the “Browse” box where the file path now shows up.
That’s it! Nothing! Nada! No insertion or attachment! With an email image insertion, there’s another little screen that pops up and you have to click “OK” to put it in but not here. What am I doing wrong or missing? It doesn’t say much for a trained observer now does it! LOL