POINTED TUNGSTEN ELECTRODE

By jok1974 Date 12-04-2005 12:10

First of all, sorry for my bad english :(

I´m a new member from Spain. I´ve found this fantastic forum about welding and wanted to congratulate all of you for your contribution to this site.

In order to begin, here is my first question:

I´ve always thought that for a DCEN, a pointed tungsten electrode is preferred, with these fact:

Minor electrode tip angle degrees -> arc concentrated with more penetration

I´ve readed this for example in “The proceeding handbook of arc welding” (by Lincoln) and in many documentation and books printed here in Spain.

But, two days ago I found this image (you can see in the link below. Image courtesy of American Welding Society - Welding Handbook,8th ed., Volume 2, “Welding Processes.” ):

http://www.cesol.es/forosold/viewtopic.php?t=169&highlight=

What´s wrong with this?.. If you see, mayor electrode tip angle degrees produces more penetration.

Is book of Lincoln wrong?.. cause an image is better than a hundred of words ;)

Thanks for your help.

By MDG Custom Weld (***)

Date 12-05-2005 15:32

Your photobucket link does not work.

By SWP

Date 12-05-2005 16:01

Jok,
the photo series shows that as the electrode angle is increased (from 15 to 120 deg.), the arc is more concentrated (increased power density) and thus the resulting spot weld is smaller diameter and deeper.
A couple of comments on this:

1. The metal shown in the photos is not identified, but it displays the weld shape characteristics of a "high sulfur" type Marangoni flow, wherein surface active elements such as sulfur or oxygen cause surface tension to be highest at the high temperature weld center and lowest at the colder weld toe, thus generating an inward and downward convection flow within the weld pool, and a deep/narrow weld profile.
2. As the electode angle increases, power density increases, the temperature at the weld pool center increases, the driving force of Marangoni flow increases, the weld for this "high sulfur" alloy becomes deeper and narrower.
3. The effect shown in the photos will not occur on all alloys. On a different heat of material, such as a more pure alloy that is lacking the surface active elements, the Marangoni flow is opposite, it is outward form the weld center, thus producing ever wider and shallower welds as power density (central temperature) is increased.
4. Arc initiation and low current arc stability is better with a sharper electrode, thus this shape may be favored for manual welding.
5. The distance between the electrode and the weld surface may be more important than electrode angle in affecting the arc power density and the resultant weld penetration.
6. This photo series was surely done in a highly controlled mechanized welding setup, and these are stationary spot welds, so these effects may not be so pronounced in manual spot welds, or in traveling welds.

By jok1974 Date 01-15-2006 16:53

Now.. the link works.

Thanks for your answer, SWP. I´ll try to translate and understand what you wrote above ;))