TIG arc temp ?

Date 02-24-2005 07:10

The temperature of the arc is not the same for all currents. Generally, higher currents will produce a plasma of a higher energy. The temperature of the arc is predominantly governed by the shape of the arc. If a constricting orifice is placed before the arc to constrict the plasma, this will increase the temperature of the arc.
This phenomenon is utilised in the plasma welding process for obtainig higher welding speeds for key-hole welding techniques.

By rugg

Date 02-24-2005 17:54

dont get me wrong, i am no expert. i am trying to understand what i read. i very much appreciate your input. i have read that the arc temp using a tunsten electrode is in the range of 35k deg F. the power source manipulates the arc characteristics for given applications (current, polarity, frequency, ect...). the "force" of the arc and the surface area of the arc is not what my question is. if a "typical" arc is 35K degrees, this is roughly 17X the melting temp of the metals that are welded. heavy plate does not require higher arc temperatures vs thin plate; higher arc temperatures is not the reason to use high current and other variables. is this flawed reasoning? thanks

By OSUtigger (**)

Date 02-24-2005 19:18

Ill take a shot at this one, but someone please correct me if I am wrong...

Temperature is a relative value, and is normally thought of as being much, much more than it actually is. Yes, in order to melt metal, it does take a certain temperature, and an arc *can* output much greater than this temperature (could a spark plug arc melt metal?). The defining factor is electron density, or amperage vs. area. if you were to take a large plate or a small plate and push electrons through it with an even distribution (one big ground clamp and one big electrode), nothing would melt because the electrons are dispersed and are flowing easily. But when you require the electrons to jump across an arc with a small cross sectional area, there are a lot of electrons in a small space, reqiring everything in that space to raise in temperature (higher electron state) and melt. Perhaps raising the amperage will create more electron flow, but at the same time will increase cross sectional size, and give the exact same temperature. If you restrict the cross sectional area and then raise the flow, temperature increases.

Simply put, you cannot definitively say that amperage does or does not raise or lower temperatures. You can say that amperage plays a part as ONE of the variables that effects temperature. Others are the arc's effective cross sectional area, specific gas characteristics, heat transfer coefficients of the steel and atmosphere, characteristics of any solids being transferred across the arc, among many, many others. There just is no simple explanation. Think of a water jet cutting machine. The flow rate (amperage) of water is extremely low (how long would it take to fill a 5 gallon bucket with the water from the jet?), but the pressure and velocity is very high, and it can cut through steel. However, multiply that flow times one hundred (increase amperage) and divide speed and pressure by the same, and you get a waterhose, barely capable of ripping through a paper bag.

Hope that helps,
gls

By rugg

Date 02-25-2005 15:29

this is why forums are great; exchange ideas and maybe you look at things a little different. i am not a chemist, nor physicist (sp?). i think what you have said makes perfect sense. the underlying question remains, purely; does the arc temp itself vary with changes in current. i have not read that a tungsten arc @ 50amps is 35K deg F and @ 200amps it is 45K deg F. if it is higher with higher amperage, it appears insignificant and not proportional. the water jet example does not apply. in DCEP, e- travel from the work to the electrode. plasma is created at the expense of the electricity and the result is energy in the form of light and temp. oxy/acetylene is a good example; the same ratio of oxy/acet at different flows rates results in the same temp of the flame. would you aggree? heavy plate will dissipate the heat quick enough so that low flow and a small torch will not cut it.

By OSUtigger (**)

Date 02-25-2005 16:13

Rugg,

First, I think you are still confusing temperature with heat. Temperature is an indirect measurement of heat, a relative value. A plasma stream with a certain mass has a whole lot different amount of "heat" than the same mass of steel at the same temperature due to the electron valence shells, if I remember physics right. Temperature does not equal heat, but is a good indicator of change in heat for a particular substance. Heat capacity's are a good example of this quality.

What I was trying to say with the water jet example was that you are asking what happens to a whole complex proccess when you change one variable. Simply adding gas delivery rates to a carburetor does not necessarily increase horsepower. Having a certain flow of water does not mean that you can cut steel. Simply adding fertilizer to the ground does not mean anything will grow better. Generally, it does, however. But "generally" is not a good way explain something. There are a million different ways to increase penetration on a joint, but generally, people just increase amperage, which may or may not be the best thing to do.

All that said, focusing electrons in a single solitary spot is what increases temperature in an arc. If a small volume of plasma has a large number of electrons compared to a large volume of plasma with the same number of electrons, the small volume will have greater temperature. The number of electrons that flow into the plate are also flowing out of the plate or being somewhat retained in the plate. That is why electron flow is so important to the heat imput. You need this to figure out how many electrons are in one place at one instant in time, which, combined with heat capacity, will provide a temperature. Increasing amperage may or may not increase the amount of electrons in one place, dependant on the conditions that the electrons must travel through and how spread out they are allowed to be. Therefore, temperature may or may not increase with increased amperage. How much TIG allows the amperage to "spread" is what I do not know, but a little bit of research will provide arc cross sections and the information needed to provide your own answer. As you have heard, the numbers cannot lie.

gls

By agong

Date 02-25-2005 20:19

Rugg,
Thanks for "sticking" on this question. Here's my thought about your question, I welcome other folks' comments.
The arc temperature has a range 5000-30,000K (K is absolute temperature).
For the same diameter electrode, the temp of arc itself changes if current changes. It's also different if the electrode material, shielding gas, environmental temp are different.
Arc temperature represents heat of the arc, the heat is generated by the electrons and ions striking anode and cathode. Electrons flowing represents current, so if current increases, the quantity of electrons and ions are increased. Then more heat is generated and the arc temperature is increased.
In the same arc, temperatures are different in different point. The center of the plasma has the highest temperature, and the temperature in anode area is higher than in cathode area.

But in welding, the term “heat input” makes more sense than arc temperature. When increases current, for the same diameter electrode, it means increasing energy density or heat input. E=IU (I: Current, U: Voltage ). ED=E/A (ED: Energy Density, A: Electrode Area).
The water jet cutting is a good example may explain as this: For the same diameter jet holes, increasing current (water flow rate) will increase temperature (velocity), then increase the energy density. Assume the pressure has samilar function as arc voltage.

Thanks
Gong

By OSUtigger (**)

Date 02-25-2005 20:33

Exactly. I, however, would note that just because you use a certain size diameter electrode does not mean that the arc is that diameter, not to mention the effects of tapering and rounded points, or balled tungstens. All of these have an effect on the shape of an arc, and will subsequently produce different temperatures (Taper the tungsten and what happens? More electrons flow all the way to the point and are transferred across the shortest distance of plasma, creating the hottest portion of the arc in one focused area. Ball the tungsten and spread the arc a little bit, with a more even distribution, and so on...)

gls

By agong

Date 02-25-2005 20:48

I agree.
As I mentioned, there are many other factors affecting arc temperatures, of course including electrode size, type.The question from Rugg is how the current affects arc temperature all other things being the same.
Thanks
Gong

By rugg

Date 02-26-2005 03:56

agong and OSU (ohio state??). yes, my curiosity is the temp of the arc itself, not what other characteristics that the arc may have as a result of several ways to manipulate arc performance. performance meaning accomplishing a task. agong, you stated that a tungsten arc temp can range from 5K to 30K. someone must have documented that. i (we) could look at the document (journal article, lecture, ect) or a reference article and see what has been probably already studied, my guess a long time ago. i am not familiar with engineering and physics research publications/academia. that is, i dont dont how or where to do a search to find information, from a research perspective. i am not even a welder by trade, although i do have a synchrowave.

ponder this and you will understand my conflict: i hope i am not comparing apples and oranges or creating more tangents: it is known that welding aluminum with "balance control", biased towards electrode negative, will create deeper penetration, all things otherwise remaining the same. you can see the difference when sectioning through the parent material and the weld. was the arc temp higher? if it was, why didnt the surface of the metal "burn up"; it behaved the same. does your travel speed need to be increased to avoid this? my goal is not to understand plasma physics and the multiple ways that the arc can be modified. i did look at some published research articles on this site. they were more recent. not helpful. a review article in a welding engineering journal might be helpful?

this is my hypothesis: arc temp is independent of current.

i dont necessarily think this is true, i just have not read anything to refute it.

thanks for your comments.

By OSUtigger (**)

Date 02-26-2005 17:35

Rugg,

OSU, not THE osu. Oklahoma State University. We really don't have to emphasize anything down this direction (poke, poke, prod, prod...)

That was what you needed to clarify. Balance control has absolutely nothing to do with current. When you weld in a certain direction, dc-, dc+, or AC, the part that is recieving the electrons, the negative side, is receiving about 70% of the heat from the current that is there, while the other side, the positive side, is receiving 30%. The aluminum does not "burn up" for the same reason that when you put a torch on ice, the ice does not boil. You are not putting enough energy in in a fast enough rate to boil the water that is forming on the ice. Travel speed does not need to be increased because the aluminum that is melting has enough energy to stay melted AND transfer energy to the al below it to melt it as well. None of the aluminum save the thin surface on top is actually being exposed to the plasma, the rest of the energy is simply transported through conduction.

hope that helps,
gls

By agong

Date 02-28-2005 14:50

Rugg, I am confusing what you are looking for.
First, the arc temperature is high enough to melt base metal to be welded, so you can say the arc temp is independent of current in welding. Second, you can search from the internet you may get some information about arc temperature. My state of arc temperature 5000-30,000K is from my book in university, that's 15 years ago.It is not specific for TIG welding. I believe there are articles in the net about that is in similar range.
As OSU stated before, you are still confused with Arc temp and heat. If you really talk about welding, forget arc temp because it's high enough to do it. Focus on heat input so you can understand arc welding application easily.
Now AC Tig aluminum, Yes DC- has good penetration but no cleaning action. DC+ has good cleaning action but very low penetration. So AC Tig is best selection for both cleaning action and penetration. If you have a change to test, you use DC-, you increase current, you will see the base aluminum Collapse but the surface doesn't burn up. You have to use special movement to break the surface.
So, aluminum welding is nothing to do with arc temperature, nor heat input? Polarity is more important?
That's why welding is so complicated. You can't only stick on one point or you'll get more confusion.

Gong