Air Force

Process for making large tungsten alloy structures

Brick and mortar approach wherein smaller segments of this alloy (low length to diameter ratio) are joined together to produce a larger part with higher length to diameter ratio

Materials Military Technology

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The tungsten alloy shell of this kinetic energy penetrator becomes very difficult to form as the shell size increases. Photo by Sgt. Sarah Kirby.

The quest for a high-density material having a unique combination of high strength and ductility, high modulus, and good corrosion resistance, resulted in the development of a class of alloy known as tungsten heavy alloys. Though there are several other high-density metals, none of them can rival tungsten in terms of high density and affordability. These qualities have made it useful for kinetic energy penetrators, radiation shields, vibration dampening devices, containment of radioisotopes, gyroscopes, and even golf clubs.

Tungsten heavy alloys have some unique applications that require fabrication of very large (hundreds of kilogram) shapes such as hollow conical, hollow cylindrical, or even a one-face, open box-like structure. Processing of such shapes as one piece is usually very difficult and sometimes impossible. The problem stems from the basic manner by which the excellent properties of these alloys are achieved, which is by liquid phase sintering. During liquid phase sintering, a part of the structure becomes a liquid (volume being dependent on the composition and the processing conditions). If the structure has a large mass on top, it results in gravity induced slumping leading to distortion at the bottom of the structure, often termed as elephant foot syndrome.

In response, Air Force scientists have developed several processes to form large tungsten heavy alloy parts with a high length to diameter ratio. These approaches involve the joining of short pieces of liquid phase sintered tungsten alloys using a layer of joining material (W, Ni, Fe, Cu, B, Mo, Ta, Pt, Re, and Pd). Joining is done via sputtering, thermal evaporation, plasma, or electron beam vaporization. The joining materials can be made into a foil of the desired composition for ease of application. The uses of these structures can as radiation shields, smaller joint structures for the oil and gas industry, and of course large penetrators.

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