Tungsten and its Alloys: Processing and Physical Fundamentals
Tungsten is a refractory metal with a high melting point (3410 °C) and high heat resistance. It has an atomic number (74) and symbol (W) on the periodic table and is one of the five naturally occurring refractory metals. It has a high hardness (8.5-9 on the Mohs scale) and a strong affinity for oxygen. It is also unique among its refractory metals peers by having the highest tensile strength and the highest melting point of the high-melting metals of their group.
The unique characteristics of tungsten render it an ideal choice for a variety of applications, particularly in the field of forming and machining of refractory materials. It is often used to form parts which require a high level of wear resistance, as well as parts which require a high temperature operating environment (such as rocket engine components). Tungsten-based alloys are also well suited for use as tooling material due to their high strength and resistance to wear and corrosion.
Tungsten-based alloys, more commonly known as “tungsten alloys”, are commonly composed of a unique combination of tungsten, nickel, and iron. The addition of these additional metals to the tungsten provides improved properties in areas such as strength, ductility, and wear/corrosion resistance.
Tungsten alloys can be produced for applications requiring low expansion coefficient at elevated temperatures and low density with good mechanical properties. The combination of superior properties makes tungsten alloys an ideal choice for a variety of applications in the aerospace, biomedical, and automotive industries.
The processing of tungsten-based alloys requires a different approach than that used for other metals. Tungsten alloys have much higher melting temperatures and higher shrinkage rates than many other metals as a result of their high atomic masses. This often creates process-specific considerations when designing tungsten alloy components such as designing for optimal shrinkage control and surface finish. Optimization of the processing parameters is required to ensure that components are produced to meet necessary performance and dimensional criteria.
Similar to many other metals, forming, casting, welding, and machining processes can be used to form tungsten alloys into their final shapes. Cold and hot rolling are common methods used to reduce tungsten alloy’s thickness. Tungsten alloys can also be formed into intricate shapes and sizes by cutting, drilling, sawing, and grinding. Working with tungsten alloys often includes the use of special cutting tools and fluids to reduce the cutting forces generated while machining.
The physical properties of tungsten alloys are governed by the alloys microstructure and composition. The basic microstructure typically consists of a matrix of primary tungsten grains which are held together with a network of intermetallic compounds and other precipitates. The size and shape of the primary tungsten grains, as well as the amount, distribution, and size of the intermetallic compounds and other precipitates, all influence the tensile strength and wear resistance of the alloy.
While tungsten alloys are usually forged or rolled and then machined, more recent advances in molding, casting, and extrusion techniques allow for improved processing and subsequent performance. Tungsten alloys are often used to produce components with relatively large surface area to improve the load carrying capacity and abuse resistance. Common components include valves and pump components, light fixtures, electrical terminals, and energy and electronics components.
Although tungsten alloys are stronger and harder than many other alloys, they can still be work-hardened. Work-hardening reduces the ductility and fracture strength of the alloy and therefore must be minimized during processing. This can be accomplished through careful control of the forming and machining processes, as well as the selection of appropriate surface finishes.
Tungsten alloys can also be prone to stress-corrosion cracking, which is caused by the reaction between residual stress in the component and corrosive environments. This type of failure can be largely prevented by minimizing contact with corroding environments, selecting appropriate lot variants, and using appropriate corrosion resistant protective coatings.
In summary, tungsten and its alloys require a different type of processing and physical considerations than other metals. The unique properties of tungsten alloys, such as their high temperature resistance, strength, and wear resistance, make them an ideal choice for a variety of applications. Adapting the appropriate processing techniques, understanding the microstructure, and taking measures to prevent stress-corrosion cracking are critical components of successful component design and manufacturing.
