Tungsten Based Heavy MetalsDavid Fleckenstein and Jennifer Sniderman - CMW Inc.
CMW Inc. (the former Mallory Metallurgical Co.) produces Electrical Contacts, Tungsten Based Heavy Metals, and Resistance Welding Electrodes and auxiliary products. Tungsten Based Heavy Metals are produced from powdered metals by liquid phase sintering techniques. The 10 grades of material that CMW Inc. produces are used in diverse applications from high speed balancing and gyroscopes, to radiation shielding to kinetic energy uses, to tooling used in die casting and hot extrusion, to transpiration cooling applications.
There are several names which are used to refer to tungsten based heavy metals and products. WHA, Mallory Metal, Heavy Metal, and Machinable Tungsten are just a few examples. Whatever they are called, most are made by a process called "liquid phase sintering". What this means is that a compact of loose powder aggregate which is a very high percentage (>85%) tungsten is sintered with the balance of the chemistry being an alloy which will be in "liquid phase" at the sintering temperature. Usually the "liquid phase" is either a Cu-Ni, Fe-Ni, or Cu-Fe-Ni alloy which is pressed into the compact as elemental powders along with the elemental tungsten powder. At temperatures > 1000 C there is very limited solubility of tungsten (W) in Ni or in such alloys, however at temperatures > 1450 C the solubility (though still very limited in percentage [<0.2%]) will be accelerated in time. Tungsten itself does not become molten until over 3400 C and is very rarely processed in liquid state.
What occurs in liquid-phase sintering can be thought of like Munch-Man (the video game) where little particles of alloy metal gain power with added heat, and eventually alloy ever so slightly with the surface of the large tungsten particles. As this occurs the voids (or gas space) between the tungsten particles shrink dramatically until there is very little void space left. In actuality a pressed compact of tungsten (particles of typically 3 to 35 micrometer dia.) and elements forming the liquid phase will typically shrink in all three physical dimensions 11% or more. This dramatic shrinkage occurs fairly quickly (about 16 minutes) once the activation temperature for the liquid phase sinter is reached and is accompanied by consolidation and growth of the individual tungsten particles. Extended time will lead to additional particle growth to about 40 micro meters dia.
A typical liquid phase sinter process would be to:
Blend elemental powders
Press a near net shape but 12 - 20% oversize compact.
Sinter in the range of 1450 - 1500 C 20 minutes or more in reducing gas.
Following the sinter the tensile (or bending) strength of the compact will be increased from around 100 psi to over 110,000 psi, accompanied by dramatic increases in ductility, and density.
Most Tungsten Heavy Metals are classified by density. Common specifications assigning specific classes are AMS-T-21014 and AMS 7725. The density of the Tungsten Heavy Metals varies from 17gm/cm3 to 18.75gm/cm3. This means that in general these materials are more than 1.5 times more dense than lead, more than 2.25 times more dense than steel, and more than 6.5 times more dense than aluminum.
CMW Inc. makes a couple of tungsten heavy metals (Elkonite 40W3 and Elkonite 50W3) that are not made by liquid phase sintering. These are made by pressing and high temperature sintering a tungsten compact (the "sintered" tungsten without the liquid phase to consolidate it is still a fairly weak structure with great amounts of porosity) and then infiltrating the pores of this structure with liquid copper.
Tungsten heavy metals find their way into many varied products because of their unique properties. The foremost useful properties are density (hight weight for low space) and extreme high temperature tolerance, and well as the ability to transfer heat (thermal conductivity). Examples are:
Balancing weights
Counter weights
Collimators
Kinetic Energy devices
tooling for hot work such as die cast tool inserts, and hot extrusion dies
precision balancing
radiation shielding as in source holders and nuclear waste caskets, and nuclear medicine handling equipment
transpiration cooling as for a directional vane for a jet engine or a rocket nozzle
Electronics for thermal cycling with low stress due to well matched thermal expansion
Tungsten powder is won from the commodity (APT) Ammonium Para Tungstate, first by calcination to the oxide WO3, and then by reduction to powdered tungsten. Most of the world production is controlled by the Chinese where the richest mines are located. Until about 5 years ago the Chinese were only producing and exporting APT, but now they are producing the powder and many of the Tungsten Heavy Metal products as well. Currently Tungsten sells around $24-$25/lb. Currently other world mining sources of tungsten are unable to compete with the Chinese.
Following Mr. Fleckenstein's presentation Ms. Sniderman gave a quick preview of CMW Inc,'s new web site which covers the companies three products lines Electrical Contacts, High Density Metals, and Resistance Welding accessories. The site is organized by product line and application, and provides easy access to rich technical information about the various materials, their uses, and how to work with them.