Tungsten Size

[Will_Davis]

May be a stupid question but:

Why is the tungsten size important? After all a 40 amp arc from a 1.6 tungsten would still be a 40 amp arc from a 2.4?

I appreciate the size of the filler rod makes a huge difference and it is vitally important to match the filler to the pool size and the amps.

I do use the recommended sizes but am curious about the theorey (Nerd Alert)

Will

 

[shenion]

A smaller tungsten can focus the arc better; it has less area for a small arc to wander over.

 

[Shox Dr]

The larger the dia, the harder it is the start the arc and to keep it stable, at low amps. You can also change the arc width by altering the angle of grind on the tungsten: smaller the dia the more pointed grind

 

[gordon stephenson]

I may be wrong, I think i read a long time ago that larger dia tungstens do not pass the lower amp currents very well required for thinner materials.

 

[Fazerruss]

Also if you are welding in AC a bigger diameter is needed to cope with the amount heat put into it or it just melts

 

[hotrodder]

http://www.pro-fusiononline.com/tungsten/diameter.htm and http://www.pro-fusiononline.com/tungsten/geometry.htm

For the 'full nerd' everything you ever wanted to know about tungsten electrodes (and a lot you probably didn't)

 

[Will_Davis]

Many thanks Hotrodder - the nerd is in his element!

 

[blueytoo]

As another curious beginner....

I figure the real reason is probably cost/efficiency. From a purely electrical perspective, electrode diameter is not going to make much difference if the tip profile is the same angle.

A larger electrode for an equivalent current will:
* transmit more heat away
* require a bigger ceramic cup
* require higher gas flow
* cost more
* require more grinding time

So the smallest electrode that will work without melting is what seems to be recommended.

For more nerdy stuff, try these research papers by Matsuda, English ones at least full text open access (I found some in the references of a Huntingdon Fusion paper) http://ci.nii.ac.jp/search?q=gas+tungsten+electrode+matsuda&range=0&count=&sortorder=&type=0

Start with Study on Gas Tungsten Arc Electrode (report 1) 1986.

 

[blueytoo]

This one is interesting. Ground tip angle and electrode diameter effect on arc pressure using Thoriated electrode. Optimum tip angle is 45deg sharp (vs 30deg and 60 deg) apparently due to lowest arc heated electrode surface area. Arc pressure of 45deg pointed tip increases with electrode diameter, more so with higher current. More tip flattening reduced arc pressure. Article in Japanese but the abstract and graphs are english.

Effect of Electrode Geometry on Maximum Arc Pressure in Gas Tungsten Arc Welding [in Japanese]
HIRAOKA Kazuo , OKADA Akira , INAGAKI Michio
1985 Japan Welding Society
http://ci.nii.ac.jp/lognavi?name=nels&lang=en&type=pdf&id=ART0003954419


So this research seems the opposite to "accepted wisdom", in that for a given current, the larger diameter electrode generated more arc force, not less. But there is a question of how "arc force" at 90 deg translates into "penetration" at 70deg (20 deg off vertical).

Here is a more recent one modelling a tilted torch 70deg and with experimental metal results. Curiously and contrary to accepted wisdom, the widest tip angle of 120deg shows the deepest penetration in both model and metal.
Abid "Effect of different electrode tip angles with tilted torch in stationary gas tungsten arc welding: A 3D simulation"
http://strathprints.strath.ac.uk/44444/

The model and metal results on a flat surface could only apply to a butt weld with flat edges. That the widest angle (bluntest tip) generated the deepest penetration is also contrary to accepted wisdom. Presumably an alternative tip geometry would be better for a fillet weld. I'm trying to find if there is a proper comparison of a pointed vs rounded cone tip. This rounded cone tip shape is suggested by the EWM Welding Dictionary (which also claims pointy electrodes have deeper penetration).

That blunter electrodes have deeper penetration is consistent with the basic physics of electrons requiring more energy to depart a flat vs pointy metal surface.

 

[blueytoo]

So just found a Miller article on pointy vs blunter
http://www.millerwelds.com/resources/articles/Tips-to-Improve-TIG-Arc-Starts/

"Fig. 11 The Impact of Point Angles. Contrary to popular belief, a pointed tungsten does NOT provide deeper penetration (but it DOES provide better directional control)."

And the Miller TIG handbook, page 36 has more photos credited to the American Welding Society showing penetration on flat plate - deeper with blunt electrodes.

 

[blueytoo]

So, been trying to make sense of the Hiraoka paper and the Abid paper.

According to Abid (who presently don't study electrode diameter effects) - results from model reproduced in metal that maximises penetration:
a. wide tip angle miminises arc temperature and concentrates temperature distribution
b. wide tip angle and short arc length minimise arc velocity
c. wide tip angle produces lower current density at the tip and makes no difference to current density on the workpiece
d. short arc length increases heat flux to the workpiece, angle makes no difference
e. wide tip angle reduces gas shear
f. short arc length increases electromagnetic force, tip angle makes no difference
g. wide tip angle increases buoyancy force
h. wide tip angle reduces pool convection

The implication for the Hiraoka results is that one probably sees deeper penetration with lowest arc pressure and highest heated electrode surface area. (The opposite of what one would infer from reading the paper which implies that high arc pressure should increase penetration.) That makes the 45 deg angle the least good. Lower arc pressure with both very pointy electrodes and wider angles seems in keeping with practice, where very pointy electrodes appear favoured for thin work and very blunt electrodes for thick work. For a truncated cone, the arc pressure minimises at about 1.5mm for a 3.2mm electrode. Testing 90 deg tip angle, arc pressure minimises with electrodes between 1 and 2mm from 100-300A.

At this stage, I'm getting the impression the optimal tip geometry could be a thin balled electrode (which makes a quiet arc).