Molybdenum has higher melting points and maximum working temperatures than niobium and platinum. It&#;s competitive with tantalum and tungsten for many applications. Perhaps most importantly, it&#;s less expensive than the alternatives, making it a good choice for a range of applications, so long as it satisfies temperature requirements.
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The primary challenge of molybdenum is its brittleness, especially when heated above its recrystallization temperature. Above recrystallization temperature, grain orientation randomizes and the material becomes brittle when returned to room temperature.
MoLa is a molybdenum alloy doped with between 0.3% and 0.7% lanthanum oxide (La2O3). The addition significantly elevates MoLa&#;s melting point and recrystallization temperature. As a result, it is ideal for very high-temperature applications. The alloy also has impressive creep resistance. MoLa is primarily supplied in sheet form; the increased ductility makes it easier to bend and deform without cracking the workpiece.
The trade-off is that MoLa costs 15% to 20% more than pure molybdenum metal.
Molybdenum TZM is an alloy of molybdenum with 0.5% titanium, 0.08% zirconium, and 0.02% carbon. This formulation boosts the recrystallization temperature of the material. The most important attribute of the alloy is its increased strength compared to the pure metal. Molybdenum TZM is available in all forms but most frequently supplied in round bar for loadbearing legs or racks.
The mix of characteristics enables OEMs to make cost/benefit trade-offs during product design. As an example, consider high-temperature vacuum furnaces. In a standard design, the walls might be pure molybdenum metal, but the shelves and supporting beams would be molybdenum TZM. In the extreme high-temperature version, some or all of the walls would be MoLa, with support structures of molybdenum TZM.
Other applications include:
Molybdenum and its alloys deliver excellent high-temperature performance at a fraction of the cost of competing metals. The key to success is to identify the specific needs of each part of the product. Consider the three common formulations discussed above, and don&#;t forget to consider a combination for the best balance of performance and price. Finally, molybdenum has a reputation for being difficult to machine, but that is not the case, see Sourcing Molybdenum Parts. All you need is a trusted supplier with the equipment and expertise to work with molybdenum and sourcing parts becomes straightforward.
The Vulcan team has decades of aggregate experience turning out precision molybdenum parts for customers in volume. Contact us to find out what we can do for you.
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.
Figure 2: Electrode Geometry
Choosing the best electrode geometry requires compromise among various attributes such as: 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). Whichever geometry is selected, it should be used consistently as part of a successful welding procedure.
For best results, electrode configuration should be tested while welding procedures are being developed; it should be noted as a critical process variable for the weld procedure; and close tolerances should be held 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 Table 2 of this guidebook. Note that larger diameters can accommodate higher amperages; and larger diameter electrodes will last longer than smaller ones, but smaller ones will be easier to arc start. Use of higher current levels than those that are recommended for a given electrode size will cause the tungsten to deteriorate or breakdown more rapidly. As the tip erodes, the probability of tungsten particles falling into the weld pool and contaminating the weld is 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 it will become hot and subject to erosion. In fact, an electrode used with DCEP can handle approximately only 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, increasing the flat to the maximum level that still allows arc start and eliminates arc wander will improve the weld penetration and increase the electrode life. 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, for other situations it would be beneficial to prepare a flat at the end of the electrode.
Guidelines for testing can be found in Table 6; also refer to the welding equipment manufacturer&#;s recommendations. During the welding operation, the accurately ground tip of a tungsten electrode is at a temperature in excess of ° C (° F). Incorrect or inconsistent diameter flat at the tip of the tungsten electrode can lead to the following problems:
In AC welding, pure or Zirconiated tungsten electrodes melt to form a hemispherical balled end. For DC welding, Thoriated, Ceriated, or Lanthanated tungsten electrodes are usually used. For the latter, the end is typically ground to a specific included angle, often with a truncated end. Various electrode tip geometries affect the weld bead shape and size. In general, as the included angle increases, the weld penetration increases and the width of the weld bead decreases. Although small diameter electrodes may be used with a square end preparation for DCEN (Direct Current Electrode Negative) welding, conical tips provide improved welding performance.
Table 6: Tip Recommendations by Electrode Diameter Size
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Electrode Included Angle/Taper: Electrodes for DC welding should be ground longitudinally and concentrically with diamond wheels to a specific included angle in conjunction with the tip/flat preparation. Different angles produce different arc shapes and offer different weld penetration capabilities. In general, blunter electrodes that have a larger included angle provide the following benefits:
Sharper electrodes with smaller included angle provide:
Larger tungsten diameters and higher currents are normally paired with larger tapers in the 25° to 45° included angle range in order to increase electrode service life and provide a more stable arc. More pointed tips in the 10° to 25° included angle range are used for lower current.
Electrode Angle Surface Finish: The smoothness of the finish on the prepared tip of the electrode will determine some of the characteristics of the welding process. In general, points should be ground as fine as possible to improve welding properties and increase the service life of the electrode. Electrodes that are ground too coarse result in unstable arcs.
Surface finish is typically expressed as a Root Mean Square (RMS) or as a Roughness Average (Ra). RMS is a comparative number as related to surface finishes measured with a profilometer. A fine finish is in the range of 20-40 RMS, a machined finish often is in the range of 80-120RMS, and grit blasted surfaces will be in the range of 400-500 RMS. The Ra value is defined as the average value of the departures from its centerline through a prescribed sampling length. Measured values expressed as RMS will read approximately eleven percent higher than values expressed in Ra. (Microinches x 1.11 = RMS).
A standard finish of around 20 RMS, which would still show the longitudinally ground lines to the naked eye, is an all-purpose, quality finish for any application. A high-polished or mirror-like finish of approximately 6-8 RMS, where few or no lines can be seen, is better for the longevity of the electrode because without any grit to the electrode surface, it is much less likely for contamination to &#;stick&#; to the electrode point and thus less erosion takes place. However, for welding power supplies that do not have strong arc starting characteristics, a finish of approximately 20 RMS is better because the longitudinally ground lines will help steadily lead the electrons to the extreme point of the electrode which assists in arc starting. Some manufacturers of pre-ground welding electrodes provide coarser finishes in the 30 to 40 RMS ranges; however, these do not last long, they provide unstable arcs, and they tend to be too gritty for extended, effective arc starting.
Typical Manufacturers&#; Recommended Geometries: Many manufacturers provide information on recommended electrode geometries, because they have already preformed testing to determine which electrode geometry is the most beneficial for their equipment in various applications. However, when this information is not available, Diamond Ground Products, Inc. or other industry experts are the best source for this information.
Tolerances Required for Different Applications: Many welding applications are deemed highly critical and require strict tolerances on the length, taper, and flat, in addition to a high-polished finish. These applications include orbital tube welding for high purity, pharmaceutical, aerospace applications, fitting manufacturing, and many others. Basic guidelines for tolerances in these applications are ± .002&#; for the length, ±½° for the taper, and ± .002&#; for the tip/flat. Where applications require electrodes to be manufactured to these extreme tolerances, it is necessary to use equipment such as an optical comparator, microscope, and micrometer in addition to the precision tungsten electrode grinder which is required for almost all applications. Other applications will often call for their own specific tolerances. Where not specified, keep reasonable tolerances for the type of work being performed and remain as consistent as possible.
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