Proper preparation can lead to an increase in electrode life and better welds. Optimizing Tungsten Electrode Performance.
While hand grinding can be tolerated in some circumstances, producing welds for safety-critical applications and where weld properties must meet stringent inspection criteria needs a more controlled approach. Following are some tips for optimizing electrode preparation, which can lead to a significant increase in electrode life.
1. Choose the right diameter
Table 1 can be used to determine the electrode diameter best suited to the welding current you are using. This guide to the optimum diameter is based on decades of welding practice.
2. Choose a reputable supplier
Superficially, electrodes from different companies look much the same, but do consider that, in general, cost means quality. The better electrodes have been manufactured to ensure a small grain structure that allows for better migration of oxides to the tip, easier arc starting, improved arc time, and better weld quality with minimized contamination.
3. Choose the best electrode composition
For many years, manufacturers have been adding residual compounds to offer improved performance. Additions of stable oxides such as thoria, ceria, and lanthana provide the same level of emission as pure tungsten at lower temperatures, while improving arc time and stability.
Pure tungsten has a high work function, i.e., it takes a great deal of energy to operate. This makes it difficult to start and maintain a stable arc. It also has a high burn-off rate and thus a shorter service life.
Thoria stablized. It is now clear that thoria, although promoting better welding, is low-level radioactive and many manufacturers and welders have stopped using it because of the health concerns (Refs. 1, 2).
Zirconia stabilized. Zirconia is used for radiographic-quality welding where tungsten contamination must be minimized. It balls up easily in AC applica-tions, but has good arc starting and current- carrying capacity.
Ceria stabilized. Ceria electrodes are good for low-current, DC, orbital tube, pipe, and thin-sheet applications. This formula has low-current capacity but offers low arc ignition and good arc stability.
Lanthana stabilized. These electrodes are a nontoxic alternative to thoria-stabilized products. They offer excellent ignition and re-ignition properties and good service life.
Combination stabilized. Some companies manufacture electrodes with complex oxide stabilization. These advanced nonradioactive formulas combine three oxides with tungsten to produce excellent all-purpose electrodes. They offer long life, repeatable performance, and reliable arc starting even after numerous ignitions (Ref.e). An example of this type of electrode is MultiStrike® from Huntingdon Fusion Techniques.
Color coding is used on some electrodes but this practice is not standardized for all mixes and varies from Europe, Japan, and the United States. See AWS standard A5.12/A5.12M, Specification for Tungsten and Tungsten-Alloy Electrodes for Arc Welding and Cutting for U.S. designations.
Classification is undertaken on the basis of chemical compositions, as follows:
E: Electrode
W: Tungsten
P: Pure tungsten
Zr: Zirconia stabilized
Th: Thoria stabilized
Ce: Ceria stabilized
La: Lanthana stabilized
G: Unspecified oxide stabilization
The numbers on electrodes specify the nominal alloying composition (in wt-%). For instance, EWTh-2 is a thoria-stabilized tungsten electrode that contains 2% thoria.
4. Choose the best electrode grinder
A standard bench grinder just isn’t good enough. Cross contamination from other operations to the electrode surface can introduce impurities, which materially affect the welding process. Furthermore, an uneven wheel will produce a poorly shaped electrode tip.
Choose a grinding machine that has been designed specifically for electrode preparation and use it only for this purpose — Fig. 1 (photo to the right).
Select one with robust performance to cover the entire range of diameters to be used.
A diamond wheel is preferred and the operation should ensure that any grinding marks are parallel to the length: electrons flow along a surface and become erratic if they encounter cross-grinding marks — Fig. 2 (diagram below).
Ensure that an angle-setting jig is provided (0–90 deg) and extraction facilities are available to remove any radioactive dust safely.
Repeatability is a must if consistency of weld deposition is to be realized and this is where the advantages of employing a tungsten tip grinding machine start to be appreciated.
The alternative of manual preparation by the welder brings with it the probability not only of inconsistent geometry from electrode to electrode but the introduction of significant deviations from the optimum.
Compare machine grinding to manual grinding:
Fig.2
5. Choose the proper tip grinding procedure
The geometry and surface finish of the electrode point are crucial to good welding — Fig. 3 (diagram to the right).
Taper. Only experience — taking into consideration the arc current, welding power supply, welding torch, material to be welded and its thickness, and joint preparation — will define the truly optimum electrode configuration, but Table 3 for DC polarity can be used as a valuable guide.
In general, larger angles offer longer life, better penetration, a narrower arc, and the capability to sustain more current without erosion. Smaller angles result in less tendency for arc wander, give a wider and more stable arc, and can be used at lower currents.
Electrode tip finish. Current transfer takes place predominantly through the flow of electrons along the electrode surface and is influenced by the surface finish. Free flow of electrons is inhibited by scratches or grinding marks that do not run parallel to the axis, and for this reason, it is important that grinding should be longitudinal and concentric. For optimum operation, a typical surface finish of 0.5 Ra is essential. Electrodes that have been ground normal to the axis or that have a surface finish much coarser than 0.5 Ra will produce instability in current flow. This may result in the following: arc initiation away from the tip, arc wander, thermal shock at the tip, and reduced electrode life.
Truncation. For some specific applications, the use of a truncated tip is beneficial. A truncated cone of specified included angle, obtained by grinding, is often preferred for DCEN. Table 4 (below) shows recommended truncation dimensions for different electrode diameters.
By Drs Michael Fletcher and Ron A Sewell
References
1. Thoriated tungsten electrodes studied for effects on welders’ health. 1994. Welding Journal 73(5): 88, 89.
2. Statement of Commission VIII on health aspects in the use of thoriated tungsten electrodes. 1994. Soudage dans le Monde 33(4): 276.
3. Performance of TIG Electrodes. June 1993. Welding Institute Ref 220177/1/93.
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