The last article published in the journal Additive manufacturing presents an in-depth review of additive manufacturing processes for refractory metal tungsten and its alloys.
Study: An update on the additive manufacturing of refractory tungsten and tungsten alloys. Image Credit: AkulininaOlga/Shutterstock.com
Benefits of Tungsten Alloys
Tungsten and tungsten alloys are refractory metals used for various industrial applications. This is due to their exceptionally high density and other outstanding properties such as superior heat capacities, increased toughness, low spray efficiency and interactions with hydrocarbons.
Photon projection modules for fusion power, plasma warheads, armour-piercing shells, and nuclear spacecraft propulsion are among the various industrial applications.
Advantages and limits of additive manufacturing
Additive Manufacturing (AM) offers exceptional morphological design freedom and rapid testing capability that traditional production processes cannot match.
Additionally, AM can create functionally graded transitions from tungsten to a variety of different materials.
Due to its exceptionally high melting temperature (Tm = 3422°C for pure tungsten) and brittleness, tungsten is notoriously difficult to produce using laser or electron beam AM processes. .
The most popular processes for making tungsten alloys are powder bed laser melting (L-PBF) – also known as selective laser melting (SLM) – laser directed energy deposition (L -DED) and electron beam powder bed fusion. (EB-PBF), also known as electron beam fusion (EBM).
Introduction to Laser-Powder Bed Fusion (L-PBF)
L-PBF is a popular AM-based process for metals and alloys as well as some ceramic substances. Powder is deposited layer by layer (20-150m thick) throughout this procedure, and a laser beam (both constant and pulsed frequency) is used to selectively vaporize the specific destination.
Topographic layer thickness, beam powder, sweep speed and hatch separation are critical elements that determine the build quality of materials.
Cracking phenomena in L-PBF
Cracking is identified as the most difficult challenge in L-PBF W. None of the relevant publications claim crack-free specimens except when a femtosecond laser diode was used. In general, two different types of cracks were discovered in L-PBF W: longitudinally propagating (cracks parallel to the axis of the laser scanners) and branching or transverse (direction of the fracture tilted with respect to the direction of the laser scanner ).
Solution of cracking phenomena in L-PBF
To reduce cracks in L-PBF W, the inclusion of rare earths or other compounds in highly purified tungsten has been investigated. The researchers combined pure tungsten powder with one percent, five percent, and ten percent tantalum (Ta) powder, and the inclusion of 5% Ta significantly reduced the grain size. However, at 10% Ta, no further grain refining was detected.
Other documented solutions for crack suppression in L-PBF W include modification of the tracking technique, recrystallization, and reheating of substrates. These procedures aim to reduce or eliminate residual stresses in the tungsten produced during printing.
What is Laser-Directed Energy Deposition (L-DED)?
L-DED is an additive manufacturing technology in which metal powder is injected into a fusion zone created by laser. After the first layer is coated, the powder injector rises and the second layer is accumulated. L-DED is usually performed in an argon environment with an argon pump. Due to the additional variables associated with the process, L-DED is suitable for the relatively rapid production of large parts while providing exceptional design freedom.
Cracks and punctures have been found in individual tungsten tracks, solitary bedding alloys, and multi-layered deposits. Liquation failure and latent stresses due to periods of rapid heating and cooling cause cracks.
Optimizing parameters and improving material integrity can help solve problems such as achieving desired build thicknesses and reducing permeability. Additionally, surface preheating has been proven to reduce latent stress induced cracking.
Introduction to Electron Beam Melting (EBM)?
AM technologies powder particle substrate series includes EBM or EB-PBF process.
During fusion, the electron beam has a relatively higher energy density, exceeding several kilowatts when focused on spot sizes of several hundred microns in diameter. EB-PBF takes place under regulated suction conditions to both preserve electron beam location diameter standards and counter pressure variations caused by the vaporization of liquid metals during melting of the beam.
There is currently a minimal study of the effectiveness of EBM W for thermomechanical activity. A three-point bend test was performed on an EBM W, and the bond strength was found to be 340MPa, which is much lower than standard forged tungsten.
This class of high temperature alloys should benefit from precise microstructure control and new alloy design methodologies.
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Albérico T. and. Al. (2022). A review on additive manufacturing of refractory tungsten and tungsten alloys, Additive Manufacturing. Available at: https://doi.org/10.1016/j.addma.2022.103009