Copper–tungsten

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Copper–tungsten (tungsten–copper, CuW, or WCu) alloy is a pseudo-alloy of copper and tungsten. As copper and tungsten are not mutually soluble, the material is composed of distinct particles of one metal dispersed in a matrix of the other one. The microstructure is therefore rather a metal matrix composite instead of a true alloy.

The material combines the properties of both metals, resulting in a material that is heat-resistant, ablation-resistant, highly thermally and electrically conductive, and easy to machine.

Parts are made from the CuW alloy by pressing the tungsten particles into a desired shape, sintering the compacted part, then infiltrating with molten copper. Sheets, rods and bars of the alloy are available as well.

Commonly used copper tungsten alloy contains 10–50 wt.% of copper, the remaining portion being mostly tungsten. The typical properties of the alloy depend on its composition. The alloy with less wt.% of copper has higher density, higher hardness and higher resistivity. The typical density of CuW90 alloy, with 10% of copper, is 16.75 g/cm3 and 11.85 g/cm3 for CuW50 alloy. CuW90 has higher hardness and resistivity of 260 HB kgf/mm2 and 6.5 µΩ.cm than CuW50.

Typical properties of commonly used copper tungsten composition

Composition Density Hardness Resistivity IACS Bending strength
wt. % g/cm3 HB Kgf/mm2 µΩ.cm≤  %≥ Mpa≥
W50/Cu50 11.85 115 3.2 54
W55/Cu45 12.30 125 3.5 49
W60/Cu40 12.75 140 3.7 47
W65/Cu35 13.30 155 3.9 44
W70/Cu30 13.80 175 4.1 42 790
W75/Cu25 14.50 195 4.5 38 885
W80/Cu20 15.15 220 5.0 34 980
W85/Cu15 15.90 240 5.7 30 1080
W90/Cu10 16.75 260 6.5 27 1160

[1]

Applications[edit]

CuW alloys are used where the combination of high heat resistance, high electrical and thermal conductivity, and low thermal expansion are needed. Some of the applications are in electric resistance welding, as electrical contacts, and as heat sinks. As contact material the alloy is resistant to erosion by electric arc. WCu alloys are also used in electrodes for electrical discharge machining and electrochemical machining.[2]

The CuW75 alloy, with 75% of tungsten, is widely used in chip carriers, substrates, flanges and frames for power semiconductor devices. The high thermal conductivity of copper together with the low thermal expansion of tungsten allows thermal expansion matching to silicon, gallium arsenide, and some ceramics. Other materials for this applications are CuMo alloy, AlSiC, and Dymalloy.

Alloy with 70–90% of tungsten is used in liners of some specialty shaped charges. The penetration is enhanced by factor 1.3 against copper for homogeneous steel target, as both the density and the break-up time are increased.[3] Tungsten powder based shaped charge liners are especially suitable for oil well completion. Other ductile metals can be used as binder in place of copper as well. Graphite can be added as lubricant to the powder.[4]

CuW can also be used as a contact material in a vacuum. When the contact is very fine grained (VFG) the electrical conductivity is much higher than a normal piece of Copper Tungsten.[5] Copper Tungsten is a good choice for a vacuum contact due to its low cost, resistance to arc erosion, conductivity, mechanical wear, and contact welding. CuW is usually a contact for vacuum, oil, and gas systems. It is not a good contact for air since the surface will oxidize when exposed. CuW is less likely to erode in air when the concentration of copper is higher in the material. The uses of CuW in the air are acting as an arc tip, arc plate, and an arc runner.[6]

Copper tungsten materials are often used for arcing contacts in SF6 circuit breakers that require medium and high voltages. The copper tungsten used in these circuit breakers is subject to temperatures above 20,000K, which is reached during arcing. Copper tungsten is a valuable material to use during arc contacts due to the high level of resistance to arc erosion. [7]

Copper tungsten has applications in Electrical discharge machining. The process uses copper tungsten as an electrode because it is able to withstand high voltage and current. [8]

The Spark Erosion (EDM) process calls for copper tungsten. Usually this process is used with graphite but tungsten has a high melting point (3420 0C). This allows the CuW electrodes to have a longer service life than the graphite electrodes. This is very crucial when the electrodes have been processed with complex machining. Since the electrodes are susceptible to wear the electrodes provide more geometrical accuracy than the other electrodes. These properties also let the rods and tubes manufactured for Spark Erosion be made smaller in diameter and have a longer length since the material is less likely to chip and warp. [9]

Properties[edit]

Tungsten % 55 68 70 75 78 80 85 90
UTS (MPa) 434.322 517.050 585.990 620.46 648.036 661.824 517.050 482.58
Thermal Conductivity (W/CM) 2.4 2.1 2.01 1.89 1.84 1.82 1.75 1.47
Electro Resistance at 20°C 3.16 3.33 3.41 3.51 3.71 3.9 4.71 6.1

As you can see from these varying levels of copper and tungsten used to create the copper tungsten alloy, increases in copper has its benefits, while an increase in tungsten has its benefits as well. Most notable of the increase in copper used to create the alloy is the thermal conductivity, which plays a huge part when being used in circuit breakers, as mentioned before, due to the extremely high temperatures that the material is subject to during peak arc contact (well above 20,000K) and is a factoring role in deciding how much percentage copper to use in the alloy. However, another interesting fact to note is the electro resistance at room temperature (20°C) and how this increases with an increase in the percentage of tungsten present in the alloy, ranging from 3.16 at 55% tungsten all the way up to nearly double the resistance at 6.1 when the alloy contains 90% tungsten. Finally, another important characteristic to keep track of is the ultimate tensile strength of the alloy at the varying levels of copper and tungsten in the alloy. An increase in tungsten leads to an increase in ultimate tensile strength up until the alloy reaches 80% tungsten and 20% copper with an ultimate tensile strength of 661.82GPa. After this mixture of copper and tungsten, the ultimate tensile strength then begins to decrease fairly rapidly. [10]

References[edit]

  1. ^ http://www.torreyhillstech.com/cuwdata.html
  2. ^ http://www.tungstenchina.com/product/Tungsten-Copper.html
  3. ^ http://www3.interscience.wiley.com/journal/109668096/abstract
  4. ^ http://www.freepatentsonline.com/5656791.html
  5. ^ http://www.plansee.com/en/Products-Electrical-contacts-Tungsten-copper-WCu-58.htm
  6. ^ http://www.cmwinc.com/copper-tungsten-contacts.php
  7. ^ http://www.plansee.com/en/Products-Electrical-contacts-Tungsten-copper-WCu-58.htm
  8. ^ http://www.credoreference.com/entry/mhmeh/the_principle_of_edm
  9. ^ http://www.plansee.com/en/Products-Forming-and-machining-tools-Erosion-electrodes-76.htm
  10. ^ http://www.eaglealloys.com/t-tungsalloyscopper.html