Thread: Tungsten Preheating - any white papers?

i don,t see the point of it, can,t tell what industry its used in? are you making this stuff up, been watching too many movies.

i,ll ask the welding gods at work, we have orbital pipe welding machine , dc welding, maybe its on it. ill see if they have heard of it.

Some information from years back.


"The electron stream is from work to electrode while the heavier positive ions travel from electrode to work - piece.

With aluminium and magnesium there is always a thin layer of refractory oxide of a melting point

of around 2000 C which has to be dispersed in other processes by fluxes by means of a corrosive

flux to enable weldability. The positive ions in the TIG arc bombard this oxide and together with

the electron emission from the plate, break up and disperse the oxide film.


This a characteristic which has made the process so successful for the welding of light alloys.

The electrons streaming to the tungsten electrons generate a great heat, so its diameter must be

relatively large and forms a bulbous end.


It is this overheating with consequent vaporisation of the tungsten and and possibility on tungsten

being transferred to the molten pool ( pick up) and contaminating it is the drawback to the use of

the process with electrode positive. Very much heat less is generated at the molten pool and

therefore this is wide and shallow."

Reference : page 375. The Science and Practice of Arc Welding by A.C.Davies

Cambridge University Press Seventh edition 1977

Originally Posted by Grahame Collins

Some information from years back.

...

Reference : page 375. The Science and Practice of Arc Welding by A.C.Davies

Cambridge University Press Seventh edition 1977

Hi Grahame,

I have the later version of that book "The Science and Practice of Welding" by A. C. Davies, volumes 1 and 2. The section to which you refer is in volume 2 on page 148.

This describes the "cleaning cycle" of aluminium etc AC welding. The function I am concerned with uses DCEP but not as part of the normal cleaning cycle. I believe it has two functions.

1. Tungsten ball maintenance as described previously and
2. Tungsten preheating

The electron emissivity of tungsten increases with temperature and preheating it creates a more stable arc when the trigger is pulled.

Regards
Jack

semantics, but I think DCEP and cleaning action go hand in hand.
As for electron emissivity - which we spoke about separately, I have a hunch this is why we alloy tungsten with Thorium, zirconium, lanthanum, ytterium, cerium etc etc. Helping to create a clean arc start before tungsten reaches an ideal working temp.

Originally Posted by Commander_Keen

semantics, but I think DCEP and cleaning action go hand in hand.
As for electron emissivity - which we spoke about separately, I have a hunch this is why we alloy tungsten with Thorium, zirconium, lanthanum, ytterium, cerium etc etc. Helping to create a clean arc start before tungsten reaches an ideal working temp.

I haven't made a proper study of the effects of alloying elements in tungsten but they do improve arc starting and stability. Improving the electron emissivity may well be the mechanism of that.

In addition though, adding alloying elements to the tungsten improves the current carrying capacity of the electrode and reduces the incidence of breakaway parts leading to inclusions in the weld.

Jack