Tungsten Wire
Tungsten wire is a filament made by forging and drawing tungsten strips. Tungsten filament, except a small amount used as heating material for high temperature furnace, electron tube heater and reinforcement of composite materials, are mostly used in making filament of various incandescent lamp and halogen tungsten lamp and electrode of gas discharge.
Advantages of Tungsten Wire
Resistance to high temperature
Among all metals in their purest forms, tungsten has the highest melting point of 3,244°C. This means that it can be used in areas of high temperatures where other metals cannot be used.
High density
Tungsten has a very high density at 19 g/cm³ due to its microcrystalline shape. This property makes it useful in areas that require mass in small sizes.
Low thermal expansion
Of all pure metals, tungsten has the least coefficient of thermal expansion. Compared to steel, tungsten is very stable at high temperatures and does not alter its form or size.
Electronic structure
Because of its conductive properties and general inertness, tungsten is mostly used in electrical appliances where there are very high levels of radiation. It is also a perfect metal that can be used as electrodes in the electrolysis of substances.
Corrosion resistance
Tungsten is very resistant to corrosion and therefore can be used in a wide range of applications where corrosion is common. It can be used outdoors where water and acids are likely and still hold out its function. Common usage of the metal is in fishing lures and building ship bases and some jewelry.
Fabrication strengthening
Tungsten is a robust metal but it can be drawn into very thin wires without fracturing. A typical example are the bulb filaments which are heated to very high temperatures and still function.
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When an electric current passes through the tungsten filament in an electric bulb, the resistance of the filament to the flow of the electric current causes it to heat up. The filament becomes so hot that it begins to emit light, creating the bright glow we see in incandescent light bulbs. This process is known as incandescence.
By drawing the tungsten into a lengthy, thin wire, a procedure known as wire drawing creates the tungsten filament. To eliminate any impurities, the wire is then twisted into a coil and heated in a vacuum. Then, the highly-purified tungsten wire is heated in a furnace to a high temperature, which causes it to anneal and increase in ductility. The wire is then coiled tightly and put into a bulb. To prevent the filament from oxidising and burning out too soon, the bulb is next evacuated, meaning that all of the air is removed from it. It is then replaced with an inert gas, such as argon or nitrogen.
Since the filaments must be both thin and robust enough to endure the high heat and pressures of use in an electric bulb, making fine tungsten filaments for light bulbs needs an exceptionally high level of precision. The diameter of the wire is decreased and its length is increased throughout the wire drawing process by dragging the tungsten through a series of dies with progressively smaller holes. Until the wire achieves the desired thickness, which may only be a few micrometres, the procedure is repeated. The resulting wire is next heated, twisted into a coil, and treated as previously mentioned.
Process of Tungsten Wire
Pressing
Tungsten powder is sifted and mixed. A binder may be added. A fixed amount is weighed and loaded into a steel mold which is loaded into a press. The powder is compacted into a cohesive, yet fragile bar. The mold is taken apart and the bar removed.
Presintering
The fragile bar is placed into a refractory metal boat and loaded into a furnace with a hydrogen atmosphere. The high temperature begins to consolidate the material together. Material is about 60% – 70% of full density, with little or no grain growth.
Full sintering
Bar is loaded into a special water-cooled treating bottle. Electric current will be passed through the bar. The heat generated by this current will cause the bar to densify to about 85% to 95% of full density and to shrink by 15% or so. Additionally, tungsten crystals begin to form within the bar.
Swaging
The tungsten bar is now strong, but very brittle at room temperature. It can be made more malleable by raising its temperature to between 1200°C to 1500°C. At this temperature, the bar can be passed through a swager. A swager is a device which reduces the diameter of a rod by passing it through a die which is designed to hammer the rod at about 10,000 blows per minute. Typically a swager will reduce the diameter by about 12% per pass. Swaging elongates the crystals, creating a fibrous structure. Although this is desirable in the finished product for ductility and strength, at this point the rod must be stress-relieved by reheating. Swaging continues until the rod is between .25 and .10 inches.
Drawing
Swaged wire of about .10 inches can now be drawn through dies to reduce the diameter.
A wire is lubricated and drawn through dies of tungsten carbide or diamond.
The exact reductions in diameter depends on the exact chemistry and the final use of the wire. As the wire is drawn, fibers again elongate and tensile strength increases. At certain stages, it may be necessary to anneal the wire to allow further processing. A wire can be drawn as fine as .0005 inches in diameter.
To get from powder to wire, tungsten is subjected to pressing, sintering, swaging, drawing, and annealing at elevated temperatures in hydrogen atmospheres. So, you might think that the metal undergoes significant changes in properties during that process.
However, the wire ultimately does retain many of the valuable characteristics of tungsten, such as its:
High melting point
Low coefficient of thermal expansion
Low vapor pressure
In addition, tungsten wire demonstrates useful electrical and thermal conductivity, which explains why it is used extensively for lighting as well as in electronic devices and thermocouples.
Most tungsten wire today is doped, which means it has undergone an additional process that provides the wire with non-sag properties. Doped tungsten wire can remain ductile at room temperature as well as at very high operating temperatures.
While doping was initially developed to improve tungsten wire's use in incandescent light bulb filaments, it continues today in tungsten wire manufacturing and is an advantage for other high-temperature applications, such as industrial ovens and vacuum metalizing.
Unlike ferrous metal wires, which are available annealed to a wide range of tensile strengths, the tensile strength of pure tungsten wire varies only with diameter. The strength cannot be adjusted significantly by customized annealing schedules.
However, between different manufacturers, tungsten wire at the same diameter will have slightly different tensile strength values. This is due to differences in their respective tungsten wire manufacturing processes, such as the pressed bar size, swaging equipment, and drawing, reduction, and annealing schedules.
Incandescent Lamp
It is mainly used as filament in an incandescent lamp. An incandescent lamp heats a tungsten wire to incandescence by means of heat radiation that emits visible light, and the filament in an incandescent bulb is made of ultra-thin tungsten wire.
The normal working temperature of the lamp reaches more than 2000℃, which is not enough to reach the melting point of the tungsten wire 3370℃, but this high temperature can make a part of the tungsten wire surface atoms evaporate.
The chemical properties of tungsten filament are very stable and strong in oxidation resistance, and should not be changed with other substances in the air, which enhances the life of the tungsten bulb.
Photocopier
It is used as an electrode wire in photocopiers. The main type used is gold-plated tungsten wire, which refers to the tungsten wire coated with a layer of gold. It is a special tungsten wire, and the main purpose of gold plating is to prevent corrosion.
In the surface sense, the gold-plated tungsten wire becomes more beautiful, and the surface is brighter than the wolframite, scheelite, and sprayed tungsten wire. In a deeper sense, the corrosion resistance of gold-plated tungsten wire is much higher than that of ordinary tungsten wire.
In addition to the electrode used in photocopiers, gold-plated tungsten wire can also be used as an ideal corrosion-resistant electron emission material, which can be widely used in scientific research fields such as high-energy physics and meteorology.
Hairdryer
It is used as the heater wire of the hairdryer to generate heat and send out hot air. The hairdryer is made up of a group of tungsten wires as a heating wire and a small fan. When electrified, the heating wire will generate heat, and the windblown by the fan will pass through the heating wire and become the hot wind. If only a small fan turns, but the heating wire is not hot, then it will only blow out the wind and not hot.
The blower drives the rotor directly by an electric motor to drive the wind blades to rotate. When the wind blades rotate, the air is drawn in from the air inlet, and the resulting centrifugal airflow is then blown out from the front nozzle of the blower.
When air passes through, if the tungsten wire on the heating bracket installed in the wind mouth has been electrified and heated, it will blow out hot air; if the switch does not energize the heating wire, it will blow out cold air. A hairdryer is through this way to achieve drying and shaping purposes.
The heating element of the blower is made of tungsten wire, which is installed in the air outlet of the blower. The air discharged by the motor is heated by the heating wire in the air outlet, and then it is sent out as hot air.
Iodine-Tungsten Lamp
In addition to a small amount of tungsten wire used as heating materials for high-temperature furnaces, heating elements of electron tubes, and reinforcing bars of composite materials, most of the tungsten wire is used for making various filament of incandescent lamps, iodine tungsten lamps, and electrode of the gas discharge lamp.
The tungsten filament made by forging and drawing tungsten bars is mainly used in the incandescent lamp, halogen tungsten lamps, and other electric light sources. Iodine tungsten lamp has the characteristics of high brightness and long life. Ordinary iodine tungsten lamp is often used as a lighting source for cinematography, stage, factories, buildings, and squares.
Lodine tungsten lamp not only reduces the evaporation of tungsten, and extends the service life, but also improves the working temperature and luminous efficiency. In size, the iodine tungsten lamp is very small and delicate.
Ensuring Quality in Tungsten Wire Doping
The purity is important because impurities can have a negative impact on the workability of tungsten and their high vapor pressure can impair the function of the wire at high temperatures.
Tungsten oxide must be prepared from tungstic acid or apt so that it will pick up the dopant (aks). There are different “lower oxides” - for example, blue, violet, brown, and gray - and not all of them are suitable for doping.
This process of converting tungsten oxide to the proper lower oxide requires precisely controlling factors such as the temperature, rate, and atmosphere used in material reduction. The process is done with hydrogen and either rotary furnaces or tube furnaces; while the former optimizes the driest hydrogen, rotary furnaces are often preferred for their higher throughput.
This is followed by the proper blending of the oxide and dopant, and a highly controlled dispersion of the dopant during further reduction of the oxide by hydrogen. This homogenous blend helps to ensure the proper grain size and distribution in later steps of wire fabrication.
For quality control, the tungsten power is measured for characteristics that have an impact on the ability of the tungsten powder to be pressed and compacted, including:
Mean particle size
Apparent density
Tap density
Green density
Particle size distribution
Identification of impurities such as oxygen, iron, and potassium
These factors also affect how the tungsten particles interlock and grow, ultimately forming bubble rows of the correct size, density, and length.
Excess dopant is removed from the metal powder by acid washing prior to pressing and pre-sintering. Further evaporation of the dopant in sintering removes most of the aluminum and silicon, while potassium remains behind due to its high vapor pressure, causing the formation of voids and bubble rows in the tungsten material's structure.
From there the doped tungsten continues through the process of swaging, annealing, and drawing as described above. Taking care to ensure that there are no interruptions at any step in the process - from powder preparation through drawing - is crucial to avoiding issues of splitting, brittleness, cavitation, and sagging in the finished wire product.