"Weldcome" jef1 to the World's Greatest Welding Forum of the AWS!!!
"In most applications, welders use tungsten that contains an emission-enhancing oxide such as Thorium, Cerium, or Lanthanum. These oxides naturally migrate from inside the tungsten to the heat at the point of the electrode, where they give off their oxide element in the arc, and leave a film of the metal alloy on the tip. This causes the electrode to have a different temperature at the tip based on the work function of that element. The oxides that are emitted at the tip serve to improve arc starting and stability. They also cause the electrode to provide the same level emission as pure tungsten at much lower temperatures. Lower temperatures improve the longevity of the tungsten and keep the grains within the tungsten smaller for improved arc stability. Thus, oxides are a very important part of tungsten. Each oxide has unique physical characteristics that affect tungsten performance. In addition, the technique used in manufacturing the tungsten will also affect its performance.
Manufacturing Variables: Grain Size and Structure:
Oxides migrate to the tip of the tungsten primarily along the boundaries or borders or these grains. It is much easier for the oxides to migrate from inside the tungsten to the tip on the grain boundaries than it is for them to migrate within the crystallized grains. In manufacturing the tungsten, smaller sized grains are better, because they produce more paths and therefore the oxides can more easily migrate to the tip. However, it is a difficult manufacturing process to minimize the size of the grains while maximizing the consistency of the oxide distribution and maintaining the proper quantity of oxides. This difficulty in the manufacturing process is the primary reason for the differences in tungsten performance quality that is produced by the different manufacturers.
During the extreme temperatures of welding, the grains have a tendency to combine with larger neighboring grains to form one large grain in a process called “grain growth.” However, if a continuous flow of oxides at the grain boundaries is maintained, this serves to surround the grains and keep them from combining. Thus, oxides are grain growth inhibitors. When tungsten runs out of oxides in any area, the grains combine readily, and the tungsten performs poorly because the oxides lack avenues to move to the surface.
Oxide Distribution and Size:
Oxide distribution is a key indicator of quality. Oxides should be distributed homogenously throughout the tungsten. Uneven distribution results in poor performance: areas with little or no oxides will tend to suffer from grain growth, whereas areas with too much oxide will tend to “bottleneck” and prevent the oxides from getting to the point. Higher quality oxides are smaller in size, which allows them to migrate to the tip easier.
Physical Characteristics of Different Oxides:
Electron Work Function (eV) - Work function is the energy needed to remove an electron from an atom, and it is measured in electron volts (eV). The lower the work function of an electrode, the lower the voltage necessary to strike an arc, thus the easier the arc starts. The oxides added to a tungsten serve to promote the electron emission by lowering the work function of the tungsten. The lower the eV for the oxide, the easier it will start. The metal work function is important, because upon emitting an oxide, the metal is left as a film on the tip. The lower the eV of the metal at the tip, the lower the temperature will be at the tip which will decrease grain growth and provide a constant flow of oxides and longer service life.
The more of an oxide added to tungsten, the lower the work function that tungsten will have, and thus the better it will arc start. A 2% Lanthanated tungsten will arc start more easily than a 1% Lanthanated tungsten (assuming they were made using the same manufacturing process). Density: Each oxide has a different density. Thus a 2% by weight thorium, cerium, or
lanthanum electrode will all have different amounts of oxides by volume. Although each material has 2% by weight of the oxides, a 2% Lanthanated tungsten has a significantly higher volume of oxides than 2% Thoriated tungsten to feed to the tip.
Note: Even if electrodes are the same type, electrodes of different manufacturers cannot be compared using only the work function and volume of oxides, because this comparison would
not take into account the important manufacturing variables such as grain size and structure of the oxide size and distribution. Therefore, the work function and oxide density numbers should only be used as a general guide. Testing is always the best way to determine which tungsten will be best for you.
The migration rate, or diffusion rate as it is often called, is the rate at which each of the different oxides naturally travels from inside the tungsten to the heat at the tip of the electrode. The evaporation rate is the rate at which the oxides separate from their metal component and are emitted at the tip of the electrode. The optimum-performing electrode is one that has a balance of good migration and evaporation rates. If the migration rate is slower than the evaporation rate, then there will be an inadequate amount of oxides arriving at the tip to maintain a consistent arc and the tungsten may be reduced to the performance level of pure tungsten. If the evaporation rate is slower than the migration rate, the oxides will be crowded at the point. If both of the rates are very high, welding properties at the beginning of welding will be great, but all of the oxides may be used up quickly."
There are figures and tables in the .pdf below with the url: diamondground.com/TugstenGuidebook2013.pdf and has all of what I posted and more... I suggest that anyone reading it should download the .pdf for reference...
http://www.diamondground.com/TungstenGuidebook2013.pdfThis link is for some information regarding the radioactive considerations in the use of thoriated tungsten/wulfram electrodes.
http://www.iem-inc.com/information/radiation-sources/useful-radioactivity/welding-rodsEdit: Here's some Tungsten data from Pro-fusion and there's a wealth of information in this website to take advantage of:
http://www.pro-fusiononline.com/tungsten/http://www.pro-fusiononline.com/feedback/articles.htmhttp://www.pro-fusiononline.com/Enjoy the reading!
Respectfully,
Henry
