Flying High With Tungsten

Flying faster and higher with less fuel are goals shared by virtually all aircraft in – or soon to be in – service worldwide. It may then be surprising to some that weights made from tungsten heavy alloy (WHA), a special family of materials made from tungsten, nickel, and iron, are present in many of these aircraft. For many decades in both fixed and rotary wing aircraft, it has been the practice of aircraft designers to incorporate concentrated mass where it is needed to perform some engineering function. And with a density over 50% greater than lead, WHA puts a lot of mass in a relatively small space.

In contrast to most common metals which are melt, cast, and mill worked into standard shapes, the manufacture of WHAs begins with the blending of fine, high purity powders of tungsten, nickel, and iron. The uniform powder mix corresponding to the desired alloy chemistry is then compacted under high pressure and subsequently hydrogen sintered to essentially a fully dense, pore-free state. By utilizing this powder metallurgy (P/M) approach to making parts, blanks can be made very close to final part geometry, eliminating the material waste and machining time that would otherwise be required to fabricate the part from mill stock.

WHA weights can serve two distinctly different roles in aerostructures. A very common application is that of balancing – the use of strategically located counterweights to shift the center of gravity of a subassembly or an entire platform for optimal flight characteristics. In military aircraft, the use of ballast weights may also be employed to adjust for weight differences created as various avionics or weapons systems are changed out to meet the demands of new mission requirements. The second principal use of tungsten heavy alloy is that of inertial damping. Weights are often attached to control surface structures to suppress the onset of flutter mode vibration, thereby contributing to flight stability.

Whether for balancing or inertial damping, WHA weights offer distinct advantages over other dense materials – namely lead and depleted uranium (DU) – that have seen frequent use in past years. Both lead and uranium are very toxic metals, whereas tungsten heavy alloy provides high density in a much safer form. In addition to high chemical toxicity, DU is also radiotoxic, adding further to issues concerning its handling and use. DU also exhibits high chemical reactivity, making it susceptible to both pyrophoric behavior and high corrosion rate. A number of case studies exist that show the serious problems that can occur when areas are contaminated by uranium-containing smoke or dust. For these reasons, few aircraft counterweights today are fabricated from either lead or DU.

In addition to low toxicity, WHA also provides low surface reactivity compared to DU. It is not subject to the high corrosion rate or stress corrosion cracking susceptibility of DU. Nevertheless, given the weather extremes and exposure to strong cleaning agents that aircraft service imposes, a protective coating is generally recommended for WHA weights for additional corrosion protection. Older aircraft specifications commonly dictate the use of a cadmium plating, which is subsequently primed and painted. In today's world, cadmium is unfavorable considering its high toxicity. In many cases, a more environmentally friendly nickel electroplate will provide equal or better corrosion resistance. In the relatively few applications that additionally involve sliding wear, tungsten heavy alloy can be effectively protected from both corrosion and wear by the application of a hard chrome plate. The reality, however, is that in many cases, all that is needed to provide enhanced corrosion resistance is simply a durabl painted finish, such as an epoxy-based paint. Painting additionally makes for easy color coding and labeling when necessary.

Parts made from tungsten heavy alloy (WHA), a special family of materials made from tungsten, nickel, and iron, have many advantages over parts made from traditional aluminum or dense materials such as lead and depleted uranium.

Aerospace weights of WHA can take a wide variety of forms. One of the smaller forms is a thin weight adjustment shim, used for precision, local trimming of weight. These specific-use weights are typically fabricated as net shape pressed and sintered parts or trimmed from rolled sheet. Counterweights commonly take the form of 3D curved forms intended to match the surfaces to which they will be attached. In some cases, sintered tolerances are sufficient for these near-net-shape weights. More commonly, final machining is required to achieve closer geometric and weight tolerances. Machining of WHAs is straightforward. With a typical hardness of ~27 HRCc, they are easily shaped using common shop tools and techniques. Larger counterweight designs encompass many shapes from the simple to the complex, and in sizes up to 1,000 lb or more. Common weights, therefore, span a range of over four orders of magnitude – all of which can be addressed by P/M fabrication techniques.

Commercial specifications such as ASM-T-21014 define four density classes of WHA, with nominal tungsten content varying from 90 wt.% (Class 1) to 97 wt.% (Class 4). Nominal densities range from 17g/cc to 18.5 g/cc (0.614 lb/in3 to 0.668 lbs/in3). The various grades of tungsten heavy alloy offer yield strengths of at least 75 ksi, which is comparable to many steels. This strength level, coupled with ductility, makes for damage resistant parts that, unlike low strength lead alloys, can be securely fastened in place. "Non-magnetic" (low permeability) compositions are additionally available for use in locations near EM sensors, geomagnetic positioning detectors, or similar devices.

As described above, WHAs offer a unique set of engineering properties not available from any other high density material. All other high density metals are disfavored on the basis of toxicity, radioactivity, availability, and/or cost. WHAs provide the modern aerospace engineer with a robust materials solution for mass property applications.