Characteristics of Tungsten Wire
In the form of wire, tungsten maintains many of its valuable properties, including its high melting point, a low coefficient of thermal expansion, and a low vapor pressure at elevated temperatures. Because tungsten wire also demonstrates good electrical and thermal conductivity, it is used extensively for lightingelectronic devices, and thermocouples.
Wire diameters are generally expressed in millimeters or mils (thousandths of an inch). However, tungsten wire diameter is usually expressed in milligrams – 14.7 mg, 3.05 mg, 246.7 mg and so on. This practice dates back to the days when, lacking tools for accurately measuring very thin wires (.001″ up to .020″ in diameter), the convention was to measure the weight of 200 mm (about 8″) of tungsten wire and calculate the diameter (D) of tungsten wire based on the weight per unit length, using the following mathematical formula:
D = 0.71746 x square root (mg weight/200 mm length)”
The standard diameter tolerance 1s士3% of the weight measurement, although tighter tolerances are available, depending on the application for the wire product. This method of expressing diameter also assumes that the wire has a constant diameter, with no significant va「1ation, necking down, or other conical effects anywhere on the diameter.
For thicker wires (.020″ to .250″ diameter), the millmeter or mil measurement is used; the tolerances are expressed as a percentage of the diameter, with a standard tolerance of士1.5%
Most tungsten wire is doped with trace amounts of potassium creating an elongated, interlocking grain structure that exh心ts non-sag properties after recrystallization. This practice dates back to tungsten wire’s primary use in incandescent light bulbs, when white-hot temperatures would cause filament sag and lamp failure. The addition of the dopants alumina, silica, and potassium at the powder mixing stage would alter the mechanical properties of the tungsten wire. In the process of hot swaging and hot drawing the tungsten wire, the alumina and silica out-gas and the potassium remains, giving the wire its non-sag properties and enabling incandescent bulbs to operate without arcing and filament failure
While the use of tungsten wire today has expanded beyond filaments for incandescent lamps, the use of dopants in tungsten wire manufacturing continues. Processed to have a higher recrystallization temperature than when in its pure state, doped tungsten (as well as molybdenum wire) can remain ductile at room temperature and at very high operating temperatures. The resulting elongated, stacked structure also gives the doped wire properties such as good creep resistance dimensional stability, and slightly easier machining than the pure (undoped) product.
Doped tungsten wire is typically produced in sizes from less than 0.001″ up to 0.025″ in diameter and is still used for lamp filament and wire filament applications, as well as being beneficial in oven, deposition, and high-temperature applications. In addition, some companies (including Metal Cutting Corporation) offer pure, undoped tungsten wire for applications where purity is paramount. At this time, the purest tungsten wire available is 99.99% pure, made from 99.999% pure powder.
Unlike ferrous metal wire products — which can be ordered 1n different annealed states, from full hard to a wide range of softer final conditions — tungsten wire as a pure element (and aside from a limited choice of alloys) can never have such a range of properties. However, because processes and equipment vary, the mechanical properties of tungsten must vary between manufacturers, because no two manufacturers use the same pressed bar size, specific swaging equipment, and drawing and annealing schedules. Therefore, it would be a remarkably lucky coincidence if tungsten made by different companies had identical mechanical properties. In fact, they can vary by as much as 10%. But to ask a tungsten wire manufacturer to vary its own tensile values by 50% is impossible.