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Effect of AC, DCEP, DCEN on Stick, TIG and MIG welding and underlying mechanism

StevSmar

(Steven)
Premium Member
In This thread we started talking about the difference between AC, DC electrode positive and DC electrode negative and the underlying mechanisms.

Rightly or wrongly, I thought it might be good to have a thread where this could be specifically discussed.

I found the following partial description in the booklet produced by Millar and Hobart:
Welding And The World Of Metals, page 25
A Major Technical Development
In 1941, Mr. Merideth devised a method for hand-feeding magnesium wire through a capped
nozzle in which an inert gas was introduced through copper tubing. This was not satisfactory because of the high burnoff rate of the magnesium. Additional research indicated that a refractory, non-consumable electrode would be better suited to the process.

The first tungsten inert gas torch was simply a standard electrode holder of the type used for metallic arc welding. A 1/8" tungsten was inserted in the jaws of the holder. This passed through an improvised copper tubing nozzle that had helium induced through an attached copper tube.

A patent was applied for in October, 1941, by Mr. Merideth and was issued to him in February, 1942.

In the meantime, further development work had been done on the torch itself and a workable unit was designed. It was air cooled and had a 75 ampere capacity.

It is interesting to note that the development work was done with DC reverse polarity. This follows the pattern of thought then in use, namely, that DC reverse polarity was THE type of current to use. Remember, this was in 1942, just a few years ago in comparison with welding's long history.

Since that time, as we shall see, the use of DC reverse polarity has disappeared almost entirely from tungsten inert gas (TIG) welding applications. The reason is the type of electrode used with this process. Let us explore this thought a bit further.

Stick electrode metallic arc welding depends on massive heat at the electrode tip to superheat the filler metal. This superheated metal is transferred across the arc to the relatively cold base metal where the heat dissipates into the base metal bringing it up to a temperature where fusion can occur. DC reverse polarity is good for this type of welding because approximately 65% of the welding heat energy is in the electrode. Thus, our hypothesis works very well.

Now, however, let us look at the tungsten electrode for TIG welding. Contrary to metallic arc welding, we do not want to melt the electrode. Indeed, the inclusion of tungsten particles in the weld is usually very harmful. Yet, with DC reverse polarity, we find that a very large tungsten is required for relatively low amperages. For example, it requires a 1⁄4" diameter tungsten to operate at 125 amperes DC reverse polarity and that is the maximum amperage one can use with this setup. This is because the electron flow is from the negative-charged base metal to the positive-charged electrode with DC reverse polarity.


So we have the phenomena of one type of current being excellent for one process and rather undesirable for another process.

This leaflet is available Here
 
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