Dseman,
I'll gladly accept your explanation since I nearly flunked out of my metallurgy course while studying for my chemical engineering degree at Stanford, eons ago. The word around our chem. eng. circles was that metallurgy was one dull course that should be cut often. Poor Dr. Shepard (I still remember his name!) was the butt of pranks sometimes. One story was told how a student, during the course's final exam, rose to his feet shouting "Dr. Shepard, you've got me this time! He then threw his exam papers down and stormed out of the class. It later turned out that he wasn't even enrolled in the class. Be that as it may, I cut a lot of metallurgy classes. Now that I'm wiser and have an interest in welding and metalworking, I wish that I hadn't cut those classes!
I guess that the molten pool of metal can react very rapidly as you say. I myself am impressed that pulsing works in welding thin to thin or thick to thin. Yesterday, for example, I was working on a new welder cart (isn't that always the first project after buying a new welder?). At 120 amps EN and pulsing set at 200, I was amazed at how nicely and quickly I could lay down a bead between 0.125" thick square tubing and 0.035" thick rectangular tubing (scrap from Chinese pallet). Previously when I tried welding these two together with 75 amps EN without pulsing, I'd burn through the thin stuff at least once just about every time.
Lawrence, you need to try pulse GTAW on thin material for yourself and give us your opinion on it.
Hi dseman
I too am sceptical of the explanation that the weld pool solidifies in such a short space of time. I say this because when welding with a low frequency pulse (0.5 - 1 Hz) one can clearly see the weld pool solidifying between pulses. This solidification taking place rather slowly. If we argue that at a low average amperage, the temperature will be very close to the solidification temperature, then the weld pool size will also be correspondingly smaller. If it is able to solidify and again melt within the space of a few micro seconds, then the weld pool will also be so small that it will not be practical to weld with. (How do you dip your filler into a weld pool that is 0.01mm in diameter?)
I do however not have a definitive answer, but will present another theory that somebody else will probably shoot down!
I believe that at these higher pulse frequencies, the welder merely has better control of the heat input and penetration characteristics than with an arc that does not pulse. In essence however, it is the same as welding with a lower, non-pulsing current. Let me explain where I am coming from:
There is a general misconception that when welding with the same heat input, the penetration into the base metal will be the same. This is incorrect. For example, welding with the following parameters will give the same heat input:
CASE 1: 100A, 10V, 20mm/min
CASE 2: 150A, 10V, 30mm/min
However, case 2 will give a substantially higher penetration into the base metal than for case 1. To understand this, we need to think about what happens to the heat that is introduced into the metal by the welding arc.
A great deal of the heat (energy) is conducted away from the area where the arc inpinges onto the plate. Some of the energy is used to melt the base metal, and some is used to melt the filler metal.
According to conventional wisdom, by reducing the amperage you can just make up for it by reducing the travel speed. However, when the amperage becomes low enough, no melting of the base metal will occur, even if you stand completely still. (Infinite heat input!) (Ever try to weld a thick piece of Copper with a low amperage TIG torch?) By increasing the amperage, the same heat input will be achieved by increasing the welding speed, but we must remember that the rate at which the energy is conducted away from the weld pool is very much a function of the weld pool temperature and thermal conductivity of the base material. As such, as the amperage is increased, and welding speed also increased the penetration into the base metal increases, becausue the thermal gradients in the base metal increases. This is a good situation when welding thick materials, but is a liability when welding thin materials. As such, when welding thin materials we need to reduce the amperage so that the percentage of energy that is "lost" into the base metal through thermal conductivity is increased, and the corresponding weld pool is much smaller. This reduction in amperage is however limited to the point where the arc stability is affected. Here is where the high frequency pulsing helps, because a stable arc can be maintained at a much lower average amperage setting.
I hope you could follow my reasoning here.
Regards
Niekie Jooste
Fabristruct Solutions
Nieke,
In your example of low freq. pulsing you noted that you saw the weld pool freezing. I guess what I should have stated in my earlier post is a better definition of a 'frozen pool'. Based on your example, I believe you were indicating 'frozen' to imply a state where a solid object could no longer penetrate, as in a filler rod or chipping hammer. My definition would be where 'sufficient grain growth occurs at a temperature, where the heat flow out due to thermal conductivity is greater than the heat flow in to support a molten state'. The temperatures I provided were given as a concrete example of how narrow that temperature band is for steel. The issue becomes how can the cooling rate occur so quickly.
I agree with your caseI, caseII examples describing the heat into a weldment where Hinput = ( I * E )/ V .
I = amperes, E=volts, V = travel speed
where Hcase1 = Hcase2, yet case 1 had lower penetration and a wider HAZ than case 2.
Linnert, from "Welding Metallurgy" has a similar example:
Case 3: 800amps,26volts,27.4 in/min produces Hinput = 45.5KJ/in
Case 4: 135amps,26volts,4.7 in/min produces Hinput = 44.6KJ/in
the kicker is that the weld nugget area was 0.185 sq.in. for Case 3
and only 0.044 sq.in. for Case 4. He indicates that a greater amount of heat was absorbed from the arc for Case 3,indicating a greater transfer efficiency over the thermal conductivity and cooling of the nearby metal. Hot and fast produces greater thermal gradients in temp over shorter distances away from the bead, and this implies less heat loss due to conduction.
I do disagree, (however quite friendly!!) , on your theory that an averaged pulsed waveform (consisting of high amperage, short duration pulses and a longer duration pulse of low amperage) has a Hnet equivalent to E * I from a non-pulsed waveform, and that the problem is mainly an issue of arc stabilty at lower amperages. My experience with inverters has indicated to me that they can be very stable at amperages as low as 5 amps. The good doctor's example had a high pulse of 125amps,a low pulse of 24amps, and I just can't believe this is really that much more stable than his averaged pulse-waveform value of 58 or so amps.
The high heat and very short duration of the pulsed waveform would seem to me to also have very high thermal gradients. Since the arc is only on for a brief period of time it should be very concentrated and have a narrower distribution of peak temps then say a constant DC arc. Steep temp distributions would tend to indicate a rapid cooling of the pool. If travel continues at too slow of a pace however, I can see the HAZ increasing as you continue to pour heat into relatively the same physical spot. To be sure, if you want to reduce penetration, you will want to reduce the heat transferred immediately after pool establishment and prior to it sinking downward and creating a nugget. The fastest way to do that is to cut the amperage low enough so thermal conductivity is greater than the heat input. I don't quite have a good mathematical expression to show for how pulsing differs from the continuous amperage cases that you and I both have found. I need to get a better understanding of how the short term , almost instantaneous, heat transfer differs from the long term found with a constant amperage arc. If and when I find out more, I will post.
I think I'm off to the library.....
-dseman
Hi dseman
Let us know what you find in the library. It should be quite interesting.
Regards
Niekie Jooste