The manufacturing value of tungsten alloys is found in their corrosion resistance against molten metal and high thermal conductivity for chill-mold casting processing of aluminum. Yet, because it is a heavy metal, with a density comparable to gold, tungsten is highly valued for tool manufacturing and for shielding from alpha and gamma radiation.
Note, however, that at around 3,400°C, tungsten has the highest melting point of all chemical elements and is therefore very difficult to work with, as well as due to its Mohs hardness of 7.5. As a result, components with more complex shapes, such as curves or conical bores, often have to be switched to hot-work tool steel, which is easier to form.
In order to make tungsten available for use for those more demanding geometries, and thus to increase the efficiency and longevity of the components to be formed, Bayerische Metallwerke GmbH has developed and patented (2021) a new manufacturing process for the tungsten alloys WNiFe and WNiCu. This process is noteworthy because the multi-phase mixed crystal alloy is obtained in a powder form that is suitable as a starting material for 3D printing and coating processes.
“Due to its resistance to corrosion and erosion from molten metals, as well as its excellent thermal conductivity, tungsten is the material of choice in aluminum casting processes,” explained Nabil Gdoura, research and development engineer at Bayerische Metallwerke. “The very high density of 19.25 g/cm3 in its pure form also makes it a good alternative to (toxic) lead, which is still used for radiation shielding in medicine, for example.”
In the particular case of molds used for aluminum casting, frequently the designer aims to establish long but at the same time very thin and sometimes conically shaped cooling channels, less than 1 mm in diameter, in order to ensure the most uniform and rapid heat dissipation. Otherwise, the material quality of the end product may be adversely affected by crack formation.
Such precise and sometimes curved shapes are impossible to model from tungsten (which has an extremely high melting point between 3,387 C and 3,422 C) using conventional machining or forming processing techniques. Therefore, for these complex components for the applications mentioned, up to now it has been necessary to switch to hot-work steel, which can be brought into almost any desired shape using 3D printing.
Tungsten alloy powders. Following a two-year development phase, in early 2020 Bayerische Metallwerke made a patent filing for its new manufacturing process for a tungsten alloy product and its further use. The patent was finally granted in January 2021.
“The special feature of our tungsten-nickel-iron alloy is that we obtain it in the form of a pre-alloyed powder,” explained Dr.-Ing. Hany Gobran, research and development manager at Bayerische Metallwerke and inventor of manufacturing technology. “This is suitable as a starting product for 3D printing and coating processes.”
In the absence of an alternative, to date only a mixed powder has been used to make tungsten usable for components with complex geometries. The main disadvantage of such mixtures, however, results from the different melting points of tungsten (around 3,400 C) and of nickel and iron, both of which change their physical state at around 1,500 C. As a result, a large part of the two added substances evaporates in an uncontrolled manner during the melting process in the further processing process. This is because the boiling points of nickel and iron are already around 2,700 C and 3,0000 C, respectively. On the other hand, thanks to the pre-alloying in the process developed by Gobran, all three elements are combined as a multiphase material in each individual powder particle, so that their composition and distribution in the end product can be precisely controlled and no loss of the binder metals has to be accepted.
According to the common standardized variants, the new alloy can be produced with 80 to 98.5% (weight) tungsten, 0.1 to 15% (weight) nickel and 0.1 to 10% (weight) iron and/or copper. This achieves an end-product density of 17 to 18.8 g/cm3, which is desirable for applications in the aluminum industry, tool manufacturing, and for alpha and gamma radiation shielding.
“The higher the proportion of tungsten in the end product, the more resistant it will be to molten aluminum, and the better its thermal conductivity,” Gobran explained. “If, on the other hand, good ductility and mechanical machinability play a greater role, the proportion of tungsten in the alloy also can be reduced accordingly. Therefore, the composition can always be adapted to the specific application and the respective complexity of the shape.”
During the comminution process as part of the manufacturing process, the flow behavior and the grain size of the powder between 10 and 200 µm also can be determined. In this way, the alloy is individually prepared for the desired type of further processing – such as plasma coating processes or additive manufacturing.
Materials for upcycling. If, for example, the hot-work steel once used for thin and conical cooling channels in cast aluminum chill-molds is replaced by the tungsten alloy developed by Gobran, the application benefits not only from the heavy metal’s resistance to corrosion and erosion. Compared to steel, tungsten also offers the advantage of much higher thermal conductivity, so that the wear on the chill-molds can be significantly reduced.
Due to its higher density, the alloy product also is an alternative to toxic lead, which is used not only for radiation shielding, but also as a stabiliser – for example in the tool industry. “Another special feature of our alloy is that we can make the powder from scraps or chips,” Gdoura added. “This is a big step forward from both the economic and environmental perspectives, as it allows us to recycle and ‘upcycle’ waste products from conventional processes.”