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- - By Lawrence (*****) Date 10-16-2012 17:05
Ok guys.. I had a student question that I could not answer in real simple terms today...  It went something like this.

AC GTAW operations on aluminum are thought to require superimposed High Frequency (HF) in order to avoid rectification, assuming a sine wave or traditional transformer square wave power supply...  However those same power supplies can peform AC SMAW operations seemingly without rectification or HF.  This is true because we can observe it; but why?

I gave some temporary blather about SMAW having typically higher currents and voltage (electrical pressure), which produces a more conductive plasma column that allows an easier restart of the arc at the hot and conductive electrode end when the current passes through the zero point.... In addition the mill scale, oxides and nitrides found on steel surfaces for SMAW produce a less formidable barrier for the current than does aluminum oxide for the AC GTAW.  Along with the possibility that a little rectification during an SMAW weld on steel would not be as detrimental as it would be with Aluminum and GTAW

I'm not satisfied with my own answer; and in fact am not sure it isn't simply baffeling bs.

Al, Henry, Brent, Joe, Allan, Obewan, Max?   46, Electrode?  Anybody?

Somebody must have a better explaination that I can share with a student worthy of a better answer.

Thanks in advance folks

Lar
Parent - By rlitman (***) Date 10-16-2012 18:02 Edited 10-16-2012 18:18
Well, my understanding of arc rectification is that it is because of the breakdown voltage of the arc gap (this is related to ionization potential).  That it is different, depending on the polarity of the arc is what causes rectification.

In a GTAW argon (or helium) plasma, the arc is made up of ionized argon plus a small amount of ionized oxides. 
At the zero crossing, the argon will return to a neutral charge very rapidly (it is not an element that likes being charged), so at each zero crossing there is the potential for the arc to be extinguished and re-established only after the voltage rises above the breakdown voltage.  With the voltage require to re-light the arc being different depending on the polarity, you get rectification, as the clipped waveform (which otherwise is basically the waveform created by an SCR dimmer) is asymmetric.

Now to your point.  In an SMAW arc, the arc is full of heavier ions that stick around for much longer.  Basically it is "smoke" which maintains the plasma state of the arc during the zero crossing.  It is the fact that the GTAW arc is so "clean", that it extinguishes to rapidly.  Since it isn't extinguished at the zero crossing, there is no gap in the waveform to allow for rectification.

As a thought experiment, I suspect that if you were to have a candle flame passing through a sinewave AC GTAW arc, you would get a result that might actually work without HF.
Parent - By OBEWAN (***) Date 10-17-2012 03:04 Edited 10-17-2012 03:32
I agree with the ionization needs for GTAW stated above. What I seem to recall about SMAW from my welding classes back in 1979 is that the electrode coatings and chemical constituents contain arc stabilizers.  I do not recall specific chemicals but in many electrodes they are proprietary and are developed by chemists and metallurgists and welding engineers that do a lot of hands on testing.  Maybe someone else can give a better answer though.  I have not used the SMAW process since 1979 and never since I got out of school.  My work has all been SAW, GMAW, FCAW, GTAW, PAW, RSW, EB, and Laser.  A young engineering intern once asked me if I actually knew how to weld.  I promptly answered:  Yes, with a robot!  I still have a great respect and admiration for people that are talented enough to do manual pipe jobs though.  It is not something I have ever attempted.
Parent - By jwright650 (*****) Date 10-22-2012 11:13
Lawrence,
Thankyou for asking this question on our forum....it has been a great question/topic that I feel has benefited the forum in many aspects. Lots of info has been laid out on the table for digestion, and even when there were items here that everyone didn't fully agree on, they were discussed with a great deal of professionalism!..Woot! I love seeing this.

Now, I need to sit down and read back through this, as much of what was explained in here went straight over my head.

Gentlemen....to all who participated..."bravo".
Parent - By welderbrent (*****) Date 10-22-2012 15:32
Gentlemen,

All I can say is... WWOOWW!!

Lawrence,  I am honored to have had my name included in your original post question.  I'm afraid when it comes to that portion of the 'Science of Welding' that I consider myself to be even less than yourself a very simple man who welds comfortably with GTAW but is no expert in it's science in order to explain it's operation. 

I actually feel, that for a simple person desiring only to understand the basics and have a simple question answered, this thread has done that and much more.

I understand too the questions of some desiring deeper answers with more complete and, to some extent, more correct descriptions/explanations (only more correct because at times a lack of full explanations in the science of the process can lead to misunderstood applications).

But we must at times concentrate our answers on the LEVEL of the person asking the question.  We can at times totally confuse the issue with too much information.  Though we should still desire to be as correctly accurate as possible with our answers. 

Now, I know, I have not contributed to the OP or the ongoing conversation with my rather lengthy post.  I jumped in only to say I am very impressed with the level of knowledge and the willingness to share with others.  As inferred by others, it can become a very exacting discussion way past any of us that would require someone with very vast resources of time and money to get to the bottom of it and maybe even start a specific research project if it has not already been done. 

This has been very enlightening to me.  Thank you all for your participation and provoking all of us to higher levels of knowledge and understanding. 

Have a Great Day,  Brent
Parent - - By 357max (***) Date 10-27-2012 14:12
A few SMAW experiments may be useful.
1. Have the students weld with a 1/4" AAC carbon electrode and RG45 filler wire. Start with DCEP than switch to DCEN.
2. Cut 12" lengths of RG45 and center grip ~ not end grip as an electrode weld both with DCEN & DCEP.
3. Let a few rust overnight in the water tank repeat with DCEP & DCEN arc.
4. Now make covered electrodes with RG45 and wet newspaper rolled tight and cut for center grip.
5. If you have some calcium carbide ~ gas it off and use the sludge to make a wash coat on the electrodes.
6. "Butter" wet newspaper with the calcium carbide sludge and weld.
7. Repeat all above with AC.
8. Repeat all above with AC & high frequency arc starter
9. Repeat #6 with addition of potassium from the chemistry department.
10. Write up observations.

GTAW
Set up identical AC & DCEN aluminum welding parameters using argon shielding.
Repeat using helium shielding.

Report findings to forum.
Parent - - By DaveBoyer (*****) Date 10-28-2012 04:45 Edited 10-30-2012 04:53
The book My Dad learned to weld from "Lessons in Arc Welding" from the Hobart Trade School has students learn with bare electrodes if using a DC machine because they are harder to run. It also shows how to determine machine polarity by burning a carbon electrode.

The experiments You suggest would go a long way toward giving students insight into why things are done the way they are.
Parent - By 357max (***) Date 10-29-2012 01:17
whoops ~ makeup one with sodium don't use table salt it has chloride. fyi the sodium will be the 6010 & the potassium will be the 6011
- - By 803056 (*****) Date 10-16-2012 23:09 Edited 10-17-2012 04:17
High frequency is used with AC to make it easier to initiate and maintain the arc.

The welding arc is extinguished every time the voltage drops below the ionization voltage required to strip an electron from the outer shell of the inert shielding gas. The arc extinguishes 60 times a second and may be reestablished when the ionization voltage rises above the ionization voltage. When HF is used the arc does not extinguish because the superimposed HF provides sufficient voltage to maintain the arc.

If the welder has a square wave generator, HF is not needed to maintain the arc. You can turn off the HF on a Miller Syncrowave and the arc will be maintained, i.e., it will not extinguish. Granted, if HF is used, the arc is much more stable.

As shown in the attached sketch, the traditional power supply, without the high frequency oscillator, has rather long periods when the ionization potential drops below that needed to keep the arc "ignited". The arc is extinguished. The high frequency, superimposed over the basic wave form prevents the arc from extinguishing. That is, the arc stays on because the HF ensures the voltage necessary to ionize the shielding gas is available. Self-rectification can be very problematic if the welder does not remove the oxide immediately before welding. Wave balance control is used to provide the lower half of the wave with a voltage boost to keep the transformer from overheating as suggested by 357 and to ensure there is sufficient voltage to reestablish the arc.

The square wave generator, as used in the Miller Syncrowave, provides a very fast switching mechanism to ensure the time when the voltage drops below that needed to ionize the shielding gas is nearly zero. Essentially the arc does not extinguish even when HF is not utilized. Welding oxidized aluminum will cause self-rectification and a corresponding voltage drop during one half of the cycle which may not be sufficient to keep a stable arc (the arc wants to go out). Again, wave balance control can boost the voltage to compensate for the voltage drop due to self-rectification.

As a youngster I remember seeing a welder attach a car battery in series with the GTAW torch to boost the voltage to compensate for the self-rectification. Modern machines have a knob (controlling an electronic circuit) that provides the voltage necessary to overcome the voltage loss due to self-rectification. At the time I asked the welder why he had the car battery in the welding circuit. His response was, "It welds better kid." End of story.

As shown by the sketch of the traditional power supply wave form, the arc tends to extinguish when the voltage drops below the threshold voltage required to ionize the shielding gas. HF (not shown by the sketch) maintains the arc by providing sufficient ionization voltage to prevent the arc from extinguishing.

Best regards - Al
Parent - - By Blaster (***) Date 10-16-2012 23:15
Does it extinguish only during one particular polarity change - 60 times per second rather than 120?
Parent - By 357max (***) Date 10-17-2012 03:03
The oxide layer is acting as a rectifier like the old selenium or silicon rectifier wafers. Hi Frequency is a high voltage with a very high frequency. The square wave ac machines switch from positive to negative at a rate of a micro second versus the sine wave the direction changes 1/120 of second. Alot of time in the "0" value so the high voltage high frequency "kicks" the sine wave through zero instead of rectifying.
If you have an old tube style oscilloscope (don't use a new oscilloscope!!!) the high freq will destroy it. You can show the dc component of the sine wave when the oxide rectifies the ac. note that is energy and it is not destroyed it goes back into the welder as heat. Generate enough heat and the insulation will melt and components will short out.
A square wave ac machine can maintain an ac arc with high frequency arc start only.
Parent - By rlitman (***) Date 10-17-2012 19:24
No, it is 120 times a second on a 60hz sine wave, however the amount of time it spends extinguished will be different depending on the direction at the crossing, which is why there is rectification (if it were symmetric, there would be no rectification).
Parent - - By DaveBoyer (*****) Date 10-17-2012 06:35
Old literature for the Lincoln HF box add on for the Idealarc 250 "Toomb Stone" machines showed the car battery in the circuit.

Am intrum technology I have seen on a machine in a scrap yard was the Airco "CB" Capacitor Balanced machine. These looked like a Miller A-B/P machine, but had a large box of capacitors to keep the half cycles in balance.

I guess Syncrowave was the next technology.
Parent - By 46.00 (****) Date 10-17-2012 07:36
I don't mean to be belligerent here, but unless I am badly mistaken, no one is answering the original question!?
Parent - - By 357max (***) Date 10-19-2012 02:16
The Airco/Miller ABP capacitors would have been there for power factor correction to lower the primary/input amperage. The car battery would have been used to offset the ac rectification dc component.
FYI the high frequency high voltage arc stabilizer was developed for SMAW and then found itself used for GTAW.
Parent - By DaveBoyer (*****) Date 10-20-2012 01:21
These were not the PFC caps. This was a good size enclosure on the left side of the machine with a whole lot of smaller caps in it.

This thing was an odd duck, the only one of them I have seen. The nameplate was designated CB, and said capacitor balanced.

I don't know what year it was built, it was orange with a black top and had the older oval Airco logo. I believe it had HF, and may have been an AC only machine. I was stripping parts out of it that were common to the A/B-P I needed to fix.

I have (2) A/B-P type machines, and know what the PFC caps look like.
Parent - - By Lawrence (*****) Date 10-17-2012 09:51
Thanks Al for the lengthy reply and pictures too!   

You said  "If the welder has a square wave generator, HF is not needed to maintain the arc. You can turn off the HF on a Miller Syncrowave and the arc will be maintained, i.e., it will not extinguish. Granted, if HF is used, the arc is much more stable. "

This is getting near the core issue I think.....   But what you said does not support itself...   "If the HF is used the arc is much more stable"  

If there is any "instability" it MUST be because of rectification (arc not being maintained/extinguished)

Explain as if to a child   :)      Then maybe I will get it.
Parent - - By 803056 (*****) Date 10-17-2012 10:48 Edited 10-17-2012 10:54
With a traditional power supply the "time" that the arc is off is lengthy enough to cause severe arc instability to the point where the HF is needed to keep the arc on as the ionization voltage drops below the ionization voltage required. This happens every time the "polarity" is switched, i.e., each half cycle of the 60 hertz sinusoidal waveform.

Even with the square wave generator the time element involved when the polarity is reversed is not actually "zero". The switching is not instantaneous, thus the arc still tends to extinguish momentarily as the voltage decays and passes through zero volts. As the voltage decays, polarity is switched; there is a brief moment when the arc wants to extinguish. This results in arc instability is noticeable, but not to the same extent as the arc instability of a traditional power supply that has a true sinusoidal waveform.

High frequency is essential with the traditional power supply, but not as necessary with the square wave generator. Still, the HF is helpful with the square wave because it ensures the arc has sufficient voltage to ionize the shielding gas and maintain the arc. Think of the HF as a "pilot light" in that it adds no significant current, but it does provide the necessary voltage to ensure arc stability. Then there is the added benefit that the HF jumps the air gap more easily than the 60 Hz main welding voltage and current provided by the step down transformer of the welding machine. This chqracteristic makes it easier to initiate the welding arc when the welder first initiates the arc. Without the HF feature, the arc is initiated by scratching the tungsten against the work piece. HF permits the welder to simply bring the end of the tungsten close to the base metal and the arc jumps the air gap to initiate the arc.

The self rectification of the aluminum oxide makes the situation worse. If the oxide is thick enough it can literally prevent reignition of the welding arc on the half cycle when HF is not superimposed on the welding circuit. The problem is mitigated with the voltage boost provided by the wave balance control which is simply an electronic version of the car battery that used to be used to overcome the self-rectification caused by the aluminum oxide. The self-rectification effect can be minimized by removing the oxide just prior to welding. Thick aluminum oxide causes a couple of problems. First: the voltage drop on the half cycle due to self-rectification and second: the high melting temperature of the oxide prevents "wetting" of the molten aluminum base metal. I'm sure the experienced aluminum welder has seen the effects of the aluminum oxide when it appears the aluminum is molten and yet retains the shape, i.e., the molten metal does not flow. Thorough cleaning, i.e., wire brushing removes the heavy oxide that forms on the surface thereby mitigating the self-rectification and the wetting problem thus the metal flows and wets more easily.

I have to run. I have a class to teach.

Best regards - Al
Parent - - By Lawrence (*****) Date 10-17-2012 12:19
Thanks Al,  Thanks everybody.

I shared this posting with my student and we had a good conversation (making for more questions)

I was worried that a 19 year old farm girl would not be able to digest this conversation... This proves how poorly I can judge my students.  She got it just fine and she thanks you all.
Parent - - By 46.00 (****) Date 10-17-2012 15:38
In the UK, most apprentice welders and students of welding used to have a couple of 'must have' books, these were 'The Science and Practice of Welding Vol 1 & 2' by A.C Davis.
Below is a link to what Mr Davis has to say on the subject:

http://books.google.com/books?id=qF4fJJcW5vEC&pg=PA150&lpg=PA150&dq=inherent+rectification+of+the+AC+arc&source=bl&ots=qQszfbkSFz&sig=zUz6z1A3JzbABv2gfbY78gvaOhU&hl=en&sa=X&ei=t81-UJX6IpHviQK34oDoDw&ved=0CCgQ6AEwAQ#v=onepage&q=inherent%20rectification%20of%20the%20AC%20arc&f=false
Parent - - By rlitman (***) Date 10-17-2012 19:52
OMG that sinewave graph is SO wrong it isn't funny.
Parent - - By 46.00 (****) Date 10-20-2012 07:28
.....and how many book's have you published?
Parent - - By Lawrence (*****) Date 10-20-2012 14:24
I've published six books and contributed to another half dozen, and was the one who asked the original question.

Maybe a better way to go would be to ask rlitman exactly how the presentation is inaccurate in his view.

You don't really think everything is perfect in a technical textbook do you?

If technical texts were perfectly accurate, there would be no need for anything but first editions :)
Parent - - By 46.00 (****) Date 10-20-2012 14:35
Lawrence, I was directing the question to rlitman, hence my post UNDER his!
I have stated before and will continue to do so, the method of posting on this forum leads to misunderstanding and conflict!
Parent - - By Lawrence (*****) Date 10-20-2012 15:28
No conflict here 46  :)

I too want to know what led to rlitman's dissatisfaction with your linked data.
Parent - By 46.00 (****) Date 10-20-2012 16:31
:lol:
Parent - - By rlitman (***) Date 10-17-2012 19:30 Edited 10-17-2012 19:50
That graph has a major flaw.  The ionization voltage is what is required to establish the plasma.  It is NOT the threshold below which the plasma will extinguish.  That is much lower.  The result is that the time that the arc is extinguished is not symmetric around the zero crossing.
Instead, the arc will continue to exist up until the voltage is nearly zero, however there is a delay between the time that the voltage starts to rise, and the time that current is reestablished.  Something more like the graph I attached.

As I said before, rectification is caused by the fact that the voltage required to restrike the arc is different depending on the polarity.  This causes a different delay in the time it takes to reestablish the arc depending on whether it is in the EN or EP half of the phase.  This in turn will make the arc spend more time either EN half, and less time in the EP half (as surface oxides will make it more difficult for electrons to escape from the workpiece).

But back to my first point.  The arc doesn't necessarily have to be extinguished at the zero crossing.
Note that when there is matter in a plasma state in the location of the arc (regardless of whether or not current is flowing, I'm just talking about a large number of charged particles in the space), the voltage required to establish a current flow is pretty low.
This is why the arc does not extinguish as it approaches 0V.

In a 60hz sine waveform, there is plenty of time after the zero crossing for the plasma in a GTAW arc to break up after the current stops.  Not so in a sharp enough square wave.  If the plasma is always present, there is no time delay to reestablishing current flow after the zero crossing, as you do not have to wait for the voltage to rise above the ionization voltage (since the space is already ionized).

What makes the SMAW process different, is the nature of the ions within the arc.  As I said in my first response, argon is not a molecule that likes to be ionized.  Noble gases are really happy with their neutral electron states, and shed off the extra electrons they have while ionized quite quickly.  That's why the plasma breaks up so fast.
Nitrogen and oxygen are happier carrying a negative charge, and since they're the primary component of air, they are also the primary component of the plasma that forms an SMAW arc.  They will still try to return to their neutral diatomic molecular states, but they're more stable than A-, so the arc won't be extinguished as rapidly.
I guess (this goes beyond my specific knowledge of the subject) that they stick around long enough to stay charged across the zero crossing of a 60hz arc.
Attachment: dimmer4.gif (6k)
Parent - - By Lawrence (*****) Date 10-17-2012 20:30
Ok...... Sounded great until this:

"Nitrogen and oxygen are happier carrying a negative charge, and since they're the primary component of air, they are also the primary component of the plasma that forms an SMAW arc."

I thought the primary SMAW plasma componant was thought to be CO2, a byproduct of the flux burn.   I understand that the primary arc start goes through Air (oxygen and nitrogen) but once the arc was established that the plasma/shield functions were a carbon dioxide thing...

Not trying to be picky.. This is a great conversation and I appreciate you taking the time to explain so thoroughly
Parent - By rlitman (***) Date 10-17-2012 21:26
Good point.
I would think it would primarily be CO-, N-, and O-.  My mind went straight to N- and O-, as they are the more reactive ions of the bunch.
Parent - By electrode (***) Date 10-18-2012 08:57
Sir,

permit me to say: "You are not picky."

You see me confused, as well.

What a thread.
Parent - By 803056 (*****) Date 10-18-2012 02:19
Good know. Al
Parent - - By 46.00 (****) Date 10-18-2012 12:27
We don't all work with 60Hz!

Nitrogen and oxygen are not really the main gases involved in SMAW Arc, are you sure it is classed as a 'Plasma'?
Parent - - By Lawrence (*****) Date 10-18-2012 13:20
I can help with part of that 46

In the American Welding Society lexicon  (AWS 3.0 Standard Welding Terms and Definitions)

Arc Plasma:
A gas that has been heated by an arc to at
least a partially inonized condition, enabling
it to conduct an electrical current
Parent - - By electrode (***) Date 10-19-2012 04:35
Lawrence,

"Arc Plasma:
A gas that has been heated by an arc to at
least a partially inonized condition, enabling
it to conduct an electrical current."

An arc, heated by an arc is a plasma?

Now it is I not trying to be picky but - is all this really satisfactory?

Does all this, being said and done along this thread, really answer your student's question?

In brief - are you happy?

I'm yet hoping somebody may pop up to cleanse this.

Maybe 'ssbn727'? I could read he seems to be seriously ill.

Thus I hope he may be o.k. and capable of clearing this puzzle up finally.
Parent - - By rlitman (***) Date 10-19-2012 21:04 Edited 10-19-2012 21:08

> An arc, heated by an arc is a plasma?


No.  The arc is the manifestation of current flow within the plasma.  The plasma is the conductive channel of super hot charged particles that allows for the current flow.
In this case, it is called an arc plasma, because the arc is used to supply the heat to maintain the plasma.  A flame however will also create plasma, and you can even create plasma in your microwave oven.

To be truly technical, many consider plasma to be a distinct state of matter from a gas (which is why I would consider that quoted definition to be a bit dated).
But, without the gas in the middle to start with, you have no matter to form a plasma, which is why you cannot arc weld in a vacuum (although you can weld with an electron beam).
Parent - - By electrode (***) Date 10-20-2012 06:54 Edited 10-20-2012 08:56
"Come on let's eat, Grandpa!
Come on let's eat Grandpa!"

Sometimes it's just a tiny detail that can make a significant difference.

I'm wondering whether somebody has really understood what I did actually mean.

Unsure I am also, whether I should apologise for being that dramatcially humourless, or "picky", when it goes about welding.

But that is so. I do become that strangely serious when it comes to welding. I thus decide to not apologise.

So, the 'plasma' definition according to: "AWS 3.0 Standard Welding Terms and Definitions".

Hard for me to recall a situation where I could read something heavier misleading, than this statement.

Reasonably to gain a common understanding, we should, of course, set our boundary condtions first.

If we presume an electric welding 'arc' to entirely equal the physical state of a 'plasma'.

Then the AWS 3.0 statement:

"Arc Plasma:
A gas that has been heated by an arc to at
least a partially inonized condition, enabling
it to conduct an electrical current."

could, after shortening, be simply converted into:

"Plasma Plasma:
A plasma heated by a plasma."

There is, per defintion, no real physical difference between an "arc" and a "plasma".

Because both is physically equal. Hence, if I shall want, I could use it equivalently to describe any status connected to it. You see my point?

Don't get me wrong, Sir. No disrespect intended against you. In my humble opinion you gave some very good input.

But, nonetheless, reading: "The plasma is the conductive channel of super hot charged particles that allows for the current flow." makes me wonder "what" actually is "super hot"?

No offence. I seem to understand where you're coming from. And, whether you like it or not, I do appreciate your efforts in this connection.

However, Lawrence unfortunately did not answer my question. Maybe it's because that it was I this time who had overlooked some tiny detail(s) leading him again to misunderstand my - respectful - intentions.

Yet, I'm waiting.

EDIT: I had to correct a typo.
Parent - - By Lawrence (*****) Date 10-20-2012 14:39
Friend electrode,

I diddn't answer your question because I can't  :)

I offered the AWS term because I assumed you are not an American and my not be familliar with that particular part of the AWS lexicon.

I'm mostly an old welder who has had many good teachers... Not a physics man.

Plasma is a tricky thing...  Over the years I have heard very bright science women and men describe "plasma" as anything between the low temp charged light producing particle exchange within the tube of a neon beer sign to a lightning bolt.

The bright addition to our conversation that plasma is a 4th state of matter is also something that has been taken en passant for several decades.  I'm not sure if the introduction broadens this conversation or if it even adds anything at all to the seemingly distant hope that the original question by the farm girl will be ever answered.

I do know it's been fun watching the inputs on this thread .   I'm not here to moderate the conversation, only to enjoy it.
Parent - By electrode (***) Date 10-21-2012 10:24
Lawrence,

Thank you.

You see me honestly impressed. Very interesting read this is to say. And reading all this I gladly can see my statement approved suggesting that a row of really fantastic fellows is around here.

Amongst others I have very much liked the comments from 'Tommyjoking', 'CWI555' and – again and fore sure – the amount of information from 'ssbn727'.

Not to forget, I need to express my gratitude to you, for recalling the root cause for all this:

"I'm not sure if the introduction broadens this conversation or if it even adds anything at all to the seemingly distant hope that the original question by the farm girl will be ever answered."

Btw, please pass my best personal regards to your student; tell her she should keep her excellent attitude in scratching at the surface of obviously simple facts, and should never stop for digging deeper.

Thereby reaching more fascinating but, fortunately, also more intricate dimensions.

So. After we were introduced to some quite interesting, thorough and deep-going (Large Hadron Collider) information, I'd finally like to add my, as you US-American fellows say: "Humble $ .02."

Before I'll be trying that, let me mention to not further deal here in particular with what’s already been said and written such thoroughly by 'ssbn727'. That is, the coherences in SMAW.

Even though, from my personal perspective, also here could be added some further tricky details (as usual), especially as when it comes to:

"…an electrically conductive path must be maintained in the arc stream. Potassium compounds in the electrode covering provide ionized gaseous particles that remain ionized during the fraction of a second that the arc is extinguished with AC cycle reversal."

this shall not be done.

I very suppose what’s been written in this respect is quite comprehensible to your student due to at least sufficiently covering the basics.

What I would like to deal with rather is the question: "What causes rectification in AC-GTAW?"

And before I begin, I need to express my gratitude to 'ssbn727' for saying something very important:

"So before I try to narrow this part of the question down or rather clarify/simplify it somewhat, It would have been most helpful if the base/parent metal were also included as part of the query in order to avoid any confusion as to what material was being used when the student observed the AC SMAW(MMA) operations... I believe most of us assumed that the student was referring to carbon steel as opposed to Aluminum (I really do not find any joy using that word!)..."

This is very well said. Because, if we all do not have a common understanding of the boundary conditions, we could stretch this thread to infinity.

Thus, I would like to try to generally treat the physics behind the rectification effect in GTAW in a way to nonetheless attain some level of understanding.

This, for making the whole subject comprehensible to your student - at the end of the day, the most important person here among us. We'll see whether or not I'll be successful in this respect.

O.k.

As I was mentioning you to have been the first one actually who had held the key to answering your students' question; I was meaning the part of your statement saying:

"…that allows an easier restart of the arc at the hot and conductive electrode."

I also could read something similar when having a look at A.C. Davis' statement, saying:

"… but actual re-ignition depends upon many factors including the surface condition of the weld pool and electrode, the temperature of the pool (…)."

And, last but not least, I was happy to read it once more in 'ssbn727’s' post saying:

"… intended to make the electrode thermionic (electrons emitted from materials at high temperatures)."

You can already see it?

Yes. It is the temperature in a direct connection to the materials used.

As mentioned for improving understanding, let us simplify the complicated facts as far as even possible. And(!) before somebody cries out now that either even "this" or even "that" could be said or explained or treated differently - I'm aware of this and sure moreover this to be true.

However, again, we common should prove willingness to come to some sort of common understanding in order to achieve and answer comprehensible to your student(s) at least basically.

Therefore we may suggest (our boundary conditions):

•  The electric welding arc is a glow-discharge.
•  Ionisation is caused through particle collision.
•  The major part of the electric current transported in the arc is due to electrons.
•  Electrons (negative charge carriers) are emitted from the "hot" cathode.

We should agree thus that the main driving force for establishing and maintaining the arc is the electrodes' temperature, physically considered referring it to as "Thermionic Emission".

As I say 'Physics'. Somebody very famous (O.W. Richardson), even a Nobel Laureate, was granted with this prize for improving our understanding of this.

He formulated a quite impressive 'RICHARDSON' formula, and I beg your forgiveness for necessarily putting this down here.

It says:

j = A_0 T^2 e^(-W/kT  )                                                                     (1)

Here is j the current density at the "hot" cathode, A_0 is a constant, T is temperature (absolute, i.e. defined as °K), W is the material work function (i.e. the amount of energy needed to "kick an electron in the a$$ to be stripped from the outer shell"), and finally k is another, the so-called BOLTZMANN constant.

Now, forgive me but it needs it, for A_0 the theory proves a value feasible to calculate by applying:

A_0 =  (4 π e m k^2)/h^3 = 120  A/(cm^2 °K^2)                                                (2)

Here m is for the electrons’ rest mass and e is for the electrons elementary charge. h is the so-called PLANCK constant. A is for Ampere.

Good. What we have seen as yet is that the major effect for any arc is the emission of electrons from the cathode and that temperature plays the most important part in this relation, for achieving "thermionic" conditions. The latter again, we could see, may be expressed mathematically what makes it possible to us to quantify-, and not only "assuming" things.

This is important at all.

Back to emission. We can see, that the specific material work function plays also a major part in all this. In brief. We may suppose now that, in case of using different materials, e.g. Aluminium (workpiece) and Tungsten (non-consumable electrode), the work functions of both materials are either different. And true this is. Of course we do have different work functions, and thus need different energies for allowing an electron to escape from the material surface in to the 'plasma'.

And now we do approach the core.

As the temperatures of both cathode and anode may appear different (different melting points) their effective work functions may be suggested to be different. Hence, the amount of thermionically emitted electrons may be suggested different either, quite depending on which material serves actually as either the cathode or the anode (just as a reminder we are speaking of AC welding).

The emissivity-difference again leads to say an imbalance, apparent as 'rectification' of the electric current, due to different current heights in either direction between anode and cathode.

Different electric currents transported (mainly) along the arc axis again lead to different current densities at both anode and cathode. And finally, it is exactly this ratio between the current densities at the point of time where the arc voltage required for re-ignition is at maximum, what mostly affects the level of rectification.

So. Knowing all this, one could modify equation (1) implementing appropriate indices thereby achieving some sort of “rectification factor” (remember, we should try to quantify not assuming things). Terming this factor e.g. "R" we could write:

R = j_1/j_2 = (T_1/T_2 )^2  e^((W_2 T_1) - (W_1 T_2)/kT_1 T_2))                    (3)

Now, however, as we come to the rather end it becomes a little complicated. Because for applying equation (3) we actually need the values T_1, T_2, W_1, and W_2.

For tungsten the work function W_1 is rather well-known (4.53 eV at its melting temperature, if memory serves me correctly) but as also repeatedly noted along this thread the work function for aluminium oxide (not the pure aluminium) is, even nowadays, intensively discussed among the experts, since depending on some quite intricate conditions again. Short. It may vary significantly.

However, to come to a brief, and again I need to repeat, simplified summary, we may be able to write:

Limited to our specific set boundary conditions thermionic emission may be assessed the major driving force or mechanism for establishing and maintaining a welding arc. In our particular case this is a gas shielded tungsten arc without dealing with further peripheral parameters (shielding gas etc.). The temperature in connection to the materials paired has significant influence on rectification, being nothing else than the external noticeable result from an imbalance between the current densities at the either electric poles surfaces.

I hope this may prove somewhat understandable. But this simply is what I was allowed to learn… some while ago.
Parent - - By 803056 (*****) Date 10-19-2012 21:27 Edited 10-19-2012 21:44
I was trying to simplify an explanation with my sketch. Someone wanted to bring in the finer points (justifiably), but does it clarify/simplify the principle I was trying to show so a student or welder can understand the fundamentals of the problem?

A plasma is gas to which energy has been added (heated and usually a voltage is applied to kick it in the ass) to the point where an electron is stripped from the outer shell and it is able to conduct electricity. That is exactly what a welding arc is. The fluorescent light is another example of plasma.

Al
Parent - By electrode (***) Date 10-20-2012 07:15 Edited 10-20-2012 07:20
Thank you, Sir.

“I was trying to simplify an explanation with my sketch. Someone wanted to bring in the finer points (justifiably), but does it clarify/simplify the principle I was trying to show so a student or welder can understand the fundamentals of the problem?”

I can see the truth of it.

A pleasure it is to me to fully agree.

Particularly I do like: “…but does it clarify/simplify the principle I was trying to show so a student or welder can understand the fundamentals of the problem?”

That was my question, wasn’t it?

Nonetheless, it should be worth it to slightly analyse what’s been said and done here.

Because, in particular it is the bunch of different theories, hypotheses, assumptions, suggestions and so forth, what makes it so tremendously confusing (to me).

And I was suspecting a welding student to must have felt similarly finally. The reason for me to ask 'Lawrence' whether he, as a welding educator, could live with all this confusion – at least at this present status.

Don’t get me wrong. This is not meant disrespectful, but as a matter of fact I would like to list some quotes from all the different posts:

“… the arc is full of heavier ions that stick around for much longer.”

“… it is "smoke" which maintains the plasma state of the arc.”

"It is the fact that the GTAW arc is so "clean", that it extinguishes to rapidly.”

“The oxide layer is acting as a rectifier like the old selenium or silicon rectifier wafers.”

“The self rectification of the aluminum oxide makes the situation worse.”

“Thorough cleaning, i.e., wire brushing removes the heavy oxide that forms on the surface thereby mitigating the self-rectification.”

“What makes the SMAW process different, is the nature of the ions within the arc.”

“…argon is not a molecule that likes to be ionized.”

“Noble gases are really happy with their neutral electron states.”

“Nitrogen and oxygen are happier carrying a negative charge, and since they're the primary component of air, they are also the primary component of the plasma that forms an SMAW arc.”

“They will still try to return to their neutral diatomic molecular states, but they're more stable than A-, so the arc won't be extinguished as rapidly.”

“…they stick around long enough to stay charged across the zero crossing of a 60hz arc.”

“I would think it would primarily be CO-, N-, and O-. My mind went straight to N- and O-, as they are the more reactive ions of the bunch.”

“Arc Plasma: A gas that has been heated by an arc to at least a partially ionized condition, enabling it to conduct an electrical current.”

Just a selection from what’s been suggested.

And now, not to forget, and finally, one of the best:

“A plasma is gas to which energy has been added (heated and usually a voltage is applied to kick it in the ass) to the point where an electron is stripped from the outer shell…”

Marvellous.

I must admit. I am a little surprised that apparently nobody, and here's a row of extremely good people around, seems to recognise what actually appears here so obviously to everybody.

Actually it was Lawrence himself who had used the key(word) in his very first explanation to his student(s). I could read it another time in the post from 46.00, or better, within what A.C. Davis had explained in his book.

But an avalanche of, in my eyes, unneeded and, actually, extremely complex terms was used to bury the - actually - simple* truth.

And hence, although apparently buried underneath all this misleading and confusing stuff, the “secrets'” solution is still on hand.

I do not want to repeat myself here again saying, I am yet waiting that somebody clears this up. And although I am heavily struggling, I should not like to be the one to give it a try.

Finally. I do very much appreciate what yourself and Lawrence and all educators are supplying to the human society. Invaluable actually, in these crazy times we’re living in. You are battling ignorance. This deserves respect from all of us.

*) Of course physically it is not that "simple", but I'm sure one could try to find an understandable explanation.
Parent - - By ssbn727 (*****) Date 10-20-2012 08:44 Edited 10-21-2012 01:02
Thank you for your kind words electrode...

Btw, I do have volume 1, "Welding Science and Technology" by A.C. Davies...
The volume you have in the link 46.00 posted is "The Science and Practice of Welding" which is different in format and content although interesting as is the volume I have.

I'm going to quote from "The Science and Practice of Welding" starting from page 160.

Quote; "Square wave power output units (AC/DC). Power units are now available in which the voltage and current waves are not sinusoidal, but have been modified by solid state technology and using printed circuit boards (P.C.B's) to give a very rapid rise in the AC wave from zero value to maximum value to give a square wave output.

When GTA welding of aluminum using a sinusoidal wave form current, the arc tends to become unstable and the electrode is easily overloaded.
This gives tungsten inclusions in the weld (spitting) and leads to faults in the weld bead and more rapid consumption of the tungsten electrode.

Square wave current overcomes these drawbacks and the arc is greatly stabilized and the risk of inclusions greatly reduced.
In addition, MMA (SMAW) welding characteristics are greatly improved and the arc is smooth with reduced spatter.

The units are designed for AC precision welding of Aluminum etc. and for DC TIG (GTAW) and Manual Metal Arc welding or MMA (SMAW).
they have a transformer and silicon bridge rectifier, SCR (Silicon Controlled Rectifier) or thyristor with a square wave output. A memory core stores energy proportional to the previous half cycle and then injects it into the circuit just as the wave passes through zero at the beginning of the next half cycle.

The rapid rise of the wave from zero to maximum, of about 80 microseconds from peak to peak means that the high frequency and high voltage at the beginning of the cycle needed in order to initiate the arc and often to keep it ionized without touchdown, need only be applied when the arc is first initiated. After this first initiation the HF is switched off automatically."

Well this doesn't give me a complete answer to your student's query Larry so I decided to look in a publication more suitable for folks on this side of the pond and I came across with this: AWS Welding Handbook ninth edition, Volume 2, Welding Processes part 1, Chapter 2 Shielded Metal Arc Welding, age 94, Arc Stability.

A stable arc is required if high quality welds are to be produced. Discontinuities , such as incomplete fusion, entrapped slag and porosity (blowholes), can be the result of an unstable arc. The following are important factors influencing arc stability:

1. The open circuit of the power source.
2. Transient voltage response characteristics of the power source.
3.Size of the molten drops of filler metal and slag in the arc.
4. Ionization of the arc path from the electrode to the workpiece.
5. Manipulation of the electrode.

The first two factors are related to the design and operating characteristics of the power source. The next two are dependent on the type of welding electrode. The last one represents the skill of the welder.

The arc of a covered electrode is a transient element in the circuit even when the welder maintains a fairly constant arc length. The welding machine must be able to respond rapidly when the arc tends to extinguish because of the resistance of the arc momentarily increases or when a large droplet partially or totally short circuits in the gap. In these instances, a surge of current is needed to restore the arc. With AC, it is imperative that the voltage cycle leads the current cycle to sustain the arc. If the voltage and current were in phase, the arc would be very unstable. This phase shift must be designed into the welding machine/power source.

Some ingredients of the electrode covering tend to stabilize the arc. These are necessary ingredients for an electrode to operate well on AC. A few of these ingredients are titanium dioxide, feldspar,  and various potassium compounds (including the binder, potassium silicate). The inclusion of one or more of these arc stabilizing compounds in the covering provides arc plasma that readily ionizes and achieves a quiet arc. Thus, the electrode, the power source, and the welder all contribute to arc stability."

This was simple enough to go over yet I wasn't satisfied either Lawrence, so I kept digging for more... I'll post it tomorrow because I need some rest... I'm worn out from all the drugs and the crap of being ill...

Respectfully,
Henry
Parent - - By Lawrence (*****) Date 10-20-2012 14:51
Exactly so Henry.

1)     AC GTAW requries "help" of some kind to avoid rectification.  (Assuming anything but a true square wave inverter power supply is involved.)

2)     AC SMAW has been blasting away practically since alternating current and wire first met. with no need for any kind of help.

electrode says I'm knocking on the door of what he thinks is correct...  I just have a feeling that there should be a rather straight-forward way to reconsile 1 & 2   :)

It makes me happy every time you pitch in Henry.
Parent - - By 803056 (*****) Date 10-20-2012 16:00 Edited 10-20-2012 21:28
The physics of the welding arc is exactly what our friend Stephen has been working on. If he finds it difficult, we mere mortals can be simply dumfounded.

Being a simpleton, I need to keep things simple in order to digest the concept. This is no simple concept and has been studied, but I am led to think that we would have to give due consideration of the specifics of the particular circuit design in order to proceed too much further. There have been different approaches taken by different manufactures in their quest to solve the problems we are discussing. The square wave generator is one of those solutions, as is the inverter that employs variable frequency as well as the idea of variable polarity. Details aside, I think the "physics" of the problem have been touched upon in a rudimentary fashion.

All this discussion brings back an incident that happened several years ago. I was qualifying two welders on aluminum pipe in the 6G position. The welders had the same WPS that was qualified previously and both welders were using the same voltage and amperage based on the meter readings. One welder was producing a beautiful root bead (no backing was used), yet the second welder was getting absolutely no root penetrations. As stated, both welders were using the same voltage and same amperage. Then I noticed that the difference in machine setting was with the balance control. A simple twist of the knob and perfect results were obtained without changing either the voltage or the amperage. Another instance of a welding variable that is listed neither as an essential or nonessential variable biting you in the ass. ASME Section IX be damned!

Gotta catch my connection to Houston. I'm interested to see some more insightful response. There is much to be learned from our fellow experts. Keep it coming gents.

Al
Parent - - By Tommyjoking (****) Date 10-20-2012 20:30
Al I thought your explanations were extremely straightforward...and logically/technically accurate.  It takes skill to converse on such an issue in a way it reaches all listeners.   LOL ....remember the actual physical performance of electricity is still a theory..(oh boy)...we cannot actually observe electrons hopping from atom to atom...we can only measure the results of EMF.  Great thread..above my head..but good read. Thanks
Parent - - By ssbn727 (*****) Date 10-21-2012 01:07 Edited 10-21-2012 02:13
Oh yes we can Tommy!!! We can go down to observe sub-atomic particles buddy!!!:yell::twisted::lol::wink::eek::eek::eek::cool:

Respectfully,
Hen:wink:ry
Parent - - By CWI555 (*****) Date 10-21-2012 04:02
Henry,

We can observe secondary affects, but not observe the 'electron', or electricity directly.
Below comes from the LHC. As you can see, it's secondary information.


If there is a direct measurement of radiation, decay, or sub atomic particles, I’d be real interested in knowing who, how, and when it was done.
To my knowledge, the LHC is state of the art, and it can only measure secondary affects. Therefore it remains a theory.

Respectfully,
Gerald
Parent - By ssbn727 (*****) Date 10-21-2012 05:13
The Large Hadron Collider in Europe is state of the art and I believe that there have been for quite some time validations of measuring some sub atomic particles... What you're talking about is really deep man!!! I mean the the Higgins Boson??? There working on finding stuff that's very much smaller than what I'm referring to...

Hey man, is the national ignition lab online yet??? You know, that friggin huge laser that was built for Lawrence Livermore Labs... The French have been building one almost identical in design... Anywho, good to see you around again Gerald!:wink:

Respectfully,
Henry
Parent - - By Tommyjoking (****) Date 10-23-2012 03:06 Edited 10-23-2012 03:08
but Henry by simply observing it you alter it's behavior...they do not like an audience.  :grin::grin::grin:  I dearly do not like Quantum Mechanics...everything I read on it usually results in a headache.  Lets get Einsteins dna and boot up another copy of him so he can make QM actually work for those guys with an addendum to relativity.  Sorry off subject.
Parent - - By ssbn727 (*****) Date 10-23-2012 04:54
There are always consequences for everything Tommy... How about trying to wrestle with the string theory for a few minutes and see how confused you'll get afterwards.:eek::confused::lol::wink::twisted:

Respectfully,
Henry
Parent - - By Dualie (***) Date 10-23-2012 05:43
I wrestled with sting theory once,  afterward i think the bottle won.     That will drive you to drink, and i fancy myself fairly physics minded.
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