For each type of oxide, the more of an oxide added to tungsten by the manufacturer, the lower the work function that tungsten will have and thus the better it will arc start. For example, ABC Tungsten Manufacturer makes both 2% Lanthanated and 1% Lanthanated tungsten using the same manufacturing process. 2% Lanthanated will have a lower work function because of a higher volume of oxides and thus the arc will start better.

It is difficult to compare different types of electrodes without testing because of the different properties of the oxides. However, another issue that is important is the density of the oxide. All of the oxides have different densities and thus a 2% by weight thorium, cerium, or lanthanum electrode will all have different amounts of oxides by volume. The following is a list clarifying this:

Therefore, even though they all have 2% by weight of the oxides, 2% Lanthanated tungsten has a significantly higher volume of oxides than 2% Thoriated tungsten to feed to the tip.

Warning: even if electrodes are the same type, you can not compare electrodes of different manufacturers 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.

Migration and Evaporation Rates- 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 once they finish migrating there. The optimum-performing electrode is one, which 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. This is due to an inadequate replenishment of oxides from inside the tungsten to the surface. 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. As mentioned earlier, the migration of oxides to the surface does not only depend on the physical properties of the oxide but also the manufacturing techniques used (i.e. oxide distribution, grain size, and structure).

Below is a list of the common oxide types and some of their general characteristics. Once again, testing is the only way to determine which tungsten will optimize your welding performance.

1. Thoriated - This has been the most commonly used type of tungsten for a long time and thus is the standard that is used for comparison to other tungsten. However, since it is a low-level radioactive hazard many users have switched to other alternatives. Regarding the radioactivity, thorium is an alpha emitter but when it is enclosed in a tungsten matrix the risks are negligible. Thus holding a stick of Thoriated tungsten in your hand should not pose a great threat unless a welder has open cuts on their skin. Thoriated tungsten should not get in contact with open cuts or wounds. The more significant danger to welders can occur when thorium oxide gets into the lungs. This can happen from the exposure to vapors during welding or from ingestion of material/dust in the manufacture and grinding of the tungsten. Therefore, precautions should be taken when welding and use of an exhaust system should be implemented to remove the grinding dust from the work area when grinding tungsten. Proper disposal in an environmentally friendly way is also a responsibility. There are discussions under way in Europe about eliminating the use of thorium there altogether because of these problems. Please research this subject yourself to get a complete understanding. Notwithstanding these issues, 2% Thoriated tungsten is still the most commonly used tungsten in America and is a good general use tungsten. It has one of the lowest work functions, and it performs well when overloaded with extra amperage. However, it does not hold its point as well as some other non-radioactive tungsten that have been introduced. This tungsten is used primarily for DC welding and may split if used for AC welding.
2. Ceriated - Ceriated tungsten was introduced as the first non-radioactive alternative to Thoriated tungsten. 2% Ceriated tungsten is how it is most commonly offered and it is readily available. It is known to be especially good for DC welding with low amperage because it starts very easily at low amps and usually requires about 10% less amps than Thoriated material to operate. Thus it is most popular material used for orbital tube and pipe welding and is also commonly used for welding very small parts. Cerium also has the highest migration rate so it has the best delivery of oxides to the tip at the beginning of use. This gives it good welding properties at the beginning but later you have fewer oxides to surround the grains so you get grain growth (i.e. grains combining) and a significantly reduced migration rate. However, at lower amperages it should last longer than Thoriated tungsten. Because of its properties, it would generally be good for short welding cycles and also where a specific number of welds are called for and then the electrode is to be replaced. Higher amperage applications are best left to Thoriated or Lanthanated material. This tungsten is used primarily for DC welding and may split if used for AC welding.
3. Lanthanated - In Europe and Japan, this has been the most popular alternative to 2% Thoriated tungsten for most applications. It is available as 2%, 1.5%, and 1% Lanthanated tungsten. Lanthanum Trioxide has the lowest work function of any if the materials thus it usually starts easiest and has the lowest temperature at the tip, which resists grain growth and promotes longer service life. Testing of 2% Lanthanated material showed that it offers much longer life than Thoriated if not overloaded and better arc starting in most applications. It is also especially good at (a) resisting the "Thermal shock" of pulsing, (b) welding in situations where there are numerous re-ignitions with a short weld cycle, and (c) resistance to contamination. Welders with tube mill applications have been especially satisfied with this material because its longer life reduces down time. Also, as a general rule it will probably require about 15% less amps to start and sustain low current arcs. The Lanthanum in this tungsten is a "rare earth" material and is not radioactive. It has not been as heavily marketed and used in the United States as in Europe or Japan, however Diamond Ground Products, Inc. has been offering this material since 1993. This tungsten is primarily used for DC welding, but will also show good results for AC welding.
4. Zirconiated - This material is most commonly used for AC welding because it balls up well in AC welding and has a more stable arc than pure tungsten. It also resists contamination well in AC welding. Finally, while it has better current carrying and arc starting characteristics than pure tungsten, on the whole it is the worst non-radioactive tungsten from a performance standpoint.
5. Pure Tungsten - This material has a very high work function thus it is more difficult to start and produce a stable arc than other materials. Also, because of the high work function, the temperature at the tip is higher and grain growth occurs. This leads to an unstable arc, starting difficulty, and a shorter service life. This material is only used for AC welding; however; better alternatives are available.
6. Other Options - In addition to the materials listed above, there are other less common materials, such as 1% Thoriated, 4% Thoriated, 2% Yttriated, and also mixes of different oxides in the same tungsten. Diamond Ground Products, Inc. introduced tungsten called TRI MIXä , which combines three non-radioactive materials into one tungsten. The goal was to make the best possible tungsten by balancing the migration and evaporation rates, while keeping the work function down. It starts and re-ignites very well and offers a particularly excellent service life in welding situations where welding cycles of at least 15 minutes are used. Technical studies in Japan have shown that mixed tungsten is very successful in optimizing welding and thus it is expected that more of this type of product will become available in the U.S. market. The Japanese and European markets are already recognizing the benefits of combining three non-radioactive oxides into one electrode.

The best way to determine which tungsten material is best for your application and whether or not a particular manufacturer produces a high quality tungsten is to perform a series of tests for your particular application. When testing you should document the following properties of the different tungsten:

1. Ease of Ignition (i.e. arc starting)

1. Ease of first arc start
2. Ease of re-ignition of same tip after previous use(s)

2. Service Life

1. Maximum number of ignitions
2. Retention of tip geometry during use

3. Quality of Welds

1. Arc shape
2. Arc stability
3. Quality of welding joint
4. Depth of weld pool

After some trial and error you will likely discover that you can make improvements to your weld process and lower your costs by utilizing materials that last longer and improve arc starting. In addition, reduced down time in replacing and preparing new electrodes is a substantial savings to consider.

Selecting the proper electrode geometry for your particular application is an important consideration in the arc welding process. Tungsten electrodes may be used with a variety of tip geometries. In AC welding, pure or Zirconiated tungsten electrodes are usually used and are melted to form a balled end. This section of the guidebook is dedicated to grinding electrodes for DC welding. The complete geometry for DC welding is comprised of the electrode diameter, the included angle (a.k.a. taper) and the tip (flat) diameter. In addition, the surface finish of the grind is also important.

Your choice of geometry will always be a compromise that will effect the following results: shorter to longer electrode life, easier to more difficult arc starting, deeper to shallower weld penetration, and wider to narrower arc shape (and thus bead shape and size as well). No matter which geometry you select, it is very important that you consistently use the same geometry once a successful welding procedure is established. Electrode configuration is a welding variable that should be tested while welding procedures are being developed, noted as a critical process variable for the weld procedure, and held close tolerances for all subsequent welds.

Electrode Diameter- The welding equipment manufacturer’s recommendations are almost always the best way to choose which diameter electrode to use. There are also guidelines published by the American Welding Society, which are duplicated in the Current Ranges chart on page 3 of this guidebook. You will notice that larger diameters can accommodate higher amperages. In addition, if you have chosen the amperage you will weld with and are deciding between two different diameters of electrodes, keep in mind that larger diameter electrodes will last longer than smaller ones, but smaller ones will be easier to arc start. If higher current levels than those that are recommended for a given electrode size are used it will cause the tungsten to deteriorate or breakdown more rapidly. With the erosion of the tip, the probability of tungsten particles falling into the weld pool and defecting the weld much greater. If the current used is too low for a specific electrode diameter, arc instability can occur.

For a given level of current, direct current with the electrode positive requires a much larger diameter because the tip is not cooled by the evaporation of electrons but heated by their impact and thus will become hot and subject to erosion. In fact, an electrode used with DCEP can handle approximately on 10% of the current that it could with the electrode negative. With AC welding, the tip is cooled during the electrode negative cycle and heated when positive. Thus, an electrode on AC can handle the current somewhere between the capacity of an electrode on DCEN and DCEP and about 50% less than that of DCEN.

Electrode Tip/Flat- The shape of the tungsten electrode tip is an important process variable in precision arc welding. A good selection of tip/flat size will balance the need for several advantages. The bigger the flat the more likely arc wander will occur and the more difficult it will be to arc start. However, by increasing the flat to the maximum level that still allows you to start and eliminate arc wonder, you will improve the weld penetration and increase the electrode life by using the larger flat. Some welders still grind electrodes to a sharp point, which makes arc starting easier. However, they risk decreased welding performance from melting at the tip and the possibility of the point falling off in the weld pool. In situations where very low amperage is used or short weld cycles are used (i.e. one second or less) a pointed electrode is desirable, however, outside of these applications it would be beneficial to consider preparing a flat at the end of the electrode. Please refer to your welding equipment manufacturer’s recommendations as a starting point for your testing or use the chart below. During the welding operation, the accurately ground tip of a tungsten electrode is at a temperature in excess of 3000°C (5500° F). Without a correct and consistent diameter flat at the tip of the tungsten electrode the following problems can occur:

• Pointed electrode tip drops into weld pool creating weld defect
• Reduction in electrode life
• Arc instability
• Change in arc voltage from one electrode to another due to inconsistent tip shape