Hey, thanks for the response Dee. My main interest (right now) is understanding polarity, but in welding, everything works together so much that you don't just get to discuss one thing. Most of what I know comes from watching the puddle, I've never been what you'd call a technical welder. Wire feed is changing that. There is a lot more to the technical side of wire feed, and to use it effectively, you have to understand it. Read all my statements as questions. I don't mind being wrong, Im just intersted in why I see this differently.
I don't think you can test the penetration characteristics of FC by changing the polarity. It was engineered to run on DCEN and it probably won't work at all any other way.
I seem to be having a problem with current density and arc energy. I have only ever heard these terms used to discribe the arc between the electrode and the work. Both are determined by the characteristics of the shielding gas, whether it's from burning flux, or an external gas source. Argon, for example, transfers current very well, producing a dense narrow arc stream. This results in a secondary arc stream with very little arc energy. That's why Ar rich gasses are not used with short circuit GMAW. There is not enough energy at the lower voltages to break up surface oxides. Even in spray mixes, O2 is sometimes added to kind of stir things back up. CO2 on the other hand, does not transfer current well. The current is less dense, causing a broader arc stream with more arc energy. It does require more voltage and works great, it just isn't any fun to weld with. I have never read or heard of the wire size effecting arc density. By the way, you guys talking about EN and tig aluminum. The spiked penetration characteristics you described are exactly like the characteristics of the arc stream with an Ar shielding gas.
As far a CV power sources, somewhere I was lead to beleive that the voltage is set and remains constant (not perfectly, but we'll let that slide for the sake of the discussion). The current (amperage) varies as is necessary to maintain the arc. This is hard for an old stick rod hand like me to comprehend, it's like everything is backwards. In wirefeed, reducing the wire size automatically reduces the amperage. You don't get more by going smaller, you get less. (Maybe more weld time with the low duty cyle machines, but not more heat). Also, increasing the wire extension reduces the amperage. The long wire stickout is preheated, reducing the amperage necessary to burn off the wire. This is pretty obvious if you do it during a weld. Increasing the wire speed increases the amperage, it takes more to burn it off and maintain the arc. For what it's worth, on 10ga, FC self shieding uses 18% less volts, with a 45% lower wire speed than solid wire with 75/25 (according to the chart on my MM250). The deposition rates of FC self shielding are no where near that of solid wire.
That brings us to polarity. I'm lost on that one. FC self shieling penetrating? I bought one of those little 110 machines to do some tack up on a job site (there wasn't any 220 available and I didn't want to string out 400' of leads). I played with it a little back at my shop. Cranked it up to maxeen and tuned the wire, then welded on some 16ga. Made a cute little weld, but penetrate? I didn't even come close to burning through. I just don't get why everybody says DCEN provides penetration. Kind of changing the subject, I sure don't see how the EP cycle in AC tig welding provides surface cleaning, either. Anybody kind enough to explain that to me?
Over,
For the moment I want to clear up a few misimpressions left over from my last post. Not to get me wrong, read in a note of humility. My process experience is limited, and I am no man of letters... so read that into all of the following:
It was my belief that changing a SOLID wire which was being run DCEP to DCEN would serve to establish a feel for the difference in affect between the two polarities... presuming a familiarity with at least one of them. I may have said something else or been unclear, which I didnt intend. As you say, there are interrelated variables, and I was trying rehtorrically to change just one to illustrate its effect which was brought to question.
CURRENT DENSITY can be verified through any convenient technical reference. I know of only one definition of the term and it is calculated by dividing the amps being used by the cross sectional area (square cm or square inches) of the electrode. Technically, for the purpose of understanding deeper physics, any other applications of the term would be misapplied... it's helpfull to be on the same sheet of musing.
I'm not sure "arc energy" is technically respected terminology, but we all understand it to be a sum total of variables that relate to what we might think of as BTUs and heat; unofficially I would suggest it holds a similarity to Watts or horsepower or any other measure of power. I would suggest the two are proportionally related but I cant go much further out than that. The shielding gas and other variables you mention may prompt a change in the amperage setting (wire feed speed) of your machine but turning that knob or changing the wire size is what changes the current density, not the atmosphere... I am thinking of the dynamics in that context.
I dont know that I mentioned "arc density"; I never intended to use the term and if I did I misspoke... apparently I did.
(I was surprised to read that Ar rich gasses cannot break up surface oxides; they dont dip well on clean steel any much better, and I have never been shown that Ar/O2 mix on steel provides anything I cannot get from Ar/CO2 except potential trouble)
I am a little confused about your paragraph opening with CV sources; I was under the impression typical GMAW power supplies were of the constant voltage type, and that a small arc voltage change (like from altering stick-out) yields a large change in welding current. Welding current, furthermore, is increased according to the wire feed setting; I am unaware of any significant effect the wire diameter would have on weld current. I am confident this is correct and should also turn up in some kind of reference; I think it might clear up a small point of misunderstanding and could be a help.
Your clarification on wire feed rates apparently stands me corrected and my inaccuracy puzzles me. I plead no contest to your reference. Somewhere, though, in an AWS forum discussion, mention was made that deposition rates (actually feed rates) were higher for FCAW than for solid, and that a complex array of transfer dynamics were involved within the arc of flux core wire... we may have been discussing GSFC?... did I misinterpret?... have I inhaled too many fumes?.. at this point I am convinced I should have kept quiet at the get-go.
Finally, at an intuitive level, we know that larger electrode diameter requires higher power parameters to work. I am not focusing on the depth or width of penetration profiles but the idea that penetration is penetration and fusion is fusion; beyond that depends on technique.
With a small, low power, SMAW power supply I was given a small diameter electrode to learn with. When I tried an electrode that was too large and had difficulty I could see it didnt work. I dont know what would have happened or how I would have gotten confused if I started with the larger one. Somewhere herein lies the phenomenon to support my statement. A mind greater than mine may frame it into words and distill it from the complication of comparing a CV source to a CC source. I cannot... in fact the "trueisms" I understood intuitively are becoming lost as I over think the subject. This sometimes indicates a fundamental error.
It would be great if a bona fide authority would help.
Regards
d
Overthehill-
I had a heck of a time posting this; I keep locking up with memory conflicts... and my computer's giving me trouble too :-)
More info is available on another site; I referred the question to Ed Craig he posted his answer today at www.weldreality.com
from home page go to "weld product questions"; it's the thumbs up/thumbs down icon to the left, and look for the topic posted today's date.
You may want to peruse the entire site and particularly the area on consumables/ gasses for other enlightenments.
It is a current density issue that relates to penetration
regards
d