Plasma centrifugal atomization is a method widely used in the production of spherical powders of metals and alloys with relatively low melting points. A novel plasma centrifugal atomization process suitable for high melting point materials (i.e. 3500 ᵒC and above) was developed by Metasphere Technology AB, currently Höganäs Sweden AB. In this process, feedstock material in the form of crushed powder with particle sizes in the range 400-1000 µm is fed into a rotating crucible and subsequently melted by the glow discharge of a plasmatron. Due to high rotational speeds, a melt film forms at the edge of the crucible and breaks into fine droplets that are ejected into the reactor chamber and solidified in a whirl of cold inert gases. Capability of the plasmatron to reach very high temperatures, combined with extremely rapid cooling of the ejected droplets, allow for the fabrication of fine powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise. 

Oil drilling, ore processing and metal shaping applications, among other, require tool materials capable of withstanding harsh working conditions under heavy loads. Owing to their physical, chemical and mechanical properties, tungsten-carbon alloys are among the most suited materials for such applications. Melting followed by rapid solidification of tungsten-carbon mixtures with 3.9 wt.% C results in a biphasic structure composed of WC lamellae inserted in a W₂C matrix, known as cast tungsten carbide (CTC). Due to the metastable nature of both phases present, CTC exhibits exceptional mechanical properties. CTC is mainly used as reinforcing dispersed phase in metal matrix composite hardfacing overlays, which are deposited by plasma transferred arc (PTA) welding or laser cladding onto steel tools.

High-entropy alloys (HEAs) are defined as multi-component solid solutions with equimolar or near-equimolar concentration of all principal elements. Owing to their outstanding mechanical, corrosion, erosion, oxidation and radiation resistance properties compared to conventional alloys, HEAs are among the most suited materials for aerospace and nuclear applications. Several processing routes have allowed for laboratory-scale production of HEAs. Nevertheless, size and shape of bulk components that can be thus produced are largely limited. In a quest for up-scaling the processing of high-end bulk HEA components, plasma centrifugal atomization of pre-alloyed refractory HEA spherical powders suitable for additive manufacturing was envisaged.

In this work, capabilities of the novel plasma centrifugal atomization for processing of refractory alloys into fine spherical powders have been evaluated based on two different material systems, namely CTC and a refractory HEA containing Ti, V, Zr, Nb, Mo, Hf, Ta, W. Challenges of local mechanical characterization of micron-sized powders have been addressed and a robust method for testing of individual particles has been developed. Mechanical properties such as hardness and fracture toughness of plasma atomized CTC powders have been extensively investigated and related to the corresponding thermal stories. Experimental results suggest significant straining of the crystal lattice in the case of as-atomized CTC, possibly due to extremely high cooling rates experienced by the solidifying particles. This has been ruled out the main reason for the outstanding mechanical properties of plasma atomized CTC compared to both spheroidized CTC and conventional cast & crushed CTC. Effective stress relieve was possible upon heat treatment. Plasma atomization of the refractory HEA yielded similar results, where an extremely fine microstructure with no noticeable chemical segregation was obtained. Indentation hardness of this novel microstructure was found to be approximately 25% higher than that of similar alloys reported in literature. HEA powder thus produced was then consolidated into bulk HEAs with very simple geometries, proving that this powder can be further processed into components of more or less complexity for pre-defined applications.

Near-net-shaping through powder metallurgy results not only in reduced material waste, but also reduced energy consumption, and strict control over the structure and properties of the final materials. The development of fabrication technologies suitable for the production of high-quality, fine metallic powders of conventional and novel alloys, with optimized mechanical, physical, and functional properties, is crucial to this manufacturing approach. A plasma-assisted centrifugal atomization unit optimized for the production of spherical cast tungsten carbide (CTC) powder was developed by Metasphere Technology AB, and subsequently acquired by Höganäs AB. In the standard implementation of this process, feedstock material in the form of crushed powder with particle sizes in the range of 400-1000 µm is fed into a rotating crucible, melted by the glow discharge of a plasma torch, and atomized into a dispersion of fine droplets that are ejected into the reactor chamber, and solidified in a whirl of cold gases. The capability of the plasma torch to melt materials with melting temperatures above 3 000 ᵒC, combined with the extremely rapid solidification of the ejected droplets, allows for the fabrication of spherical powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise.

The main objectives of this work were to better understand the role of the centrifugal atomization mechanism on the microstructure and the mechanical properties of spherical CTC powders thus produced, particularly compared to other conventional powder fabrication routes, and to explore the capabilities of the pilot-scale plasma centrifugal atomization unit at Höganäs Sweden AB (Luleå, Sweden) for the design and development of novel refractory alloys.

The challenges of local mechanical characterization of micron-sized hard carbide powders have been addressed. A robust method for testing individual particles has been developed, based on Vickers microindentation of polished powder specimens and atomic force microscopy (AFM) topography imaging of the indented surfaces. This method enabled a reliable comparison among CTC powders fabricated by different methods, evidencing the mechanical superiority of the centrifugally-atomized spherical powder. Subsequently, the microindentation hardness, the micro-pillar compressive strength, and the resistance to cyclic compressive loading of entire particles were extensively investigated in centrifugally-atomized CTC powders subjected to different heat treatments. The extremely high cooling rates experienced by the solidifying particles were concluded to result in the refinement of the CTC lamellar structure and significant straining of the crystal lattice. Moreover, rather complex stress relaxation phenomena through the entire particles were observed as a result of the different heat treatments, and attributed to local WC-to-W₂C phase transformations at the surface of the particles.

In order to showcase the capabilities of the plasma-assisted centrifugal atomization unit for alloy development, a suitable processing route for the fabrication of spherical powders of refractory multi-principal element alloys has been developed. In particular, a near-equiatomic refractory high-entropy alloy containing Ti, V, Zr, Nb, Mo, Hf, Ta, and W has been used as model alloy. Starting from a blend of the corresponding elemental powders, the preparation of suitable granulated feedstock material by partial sintering followed by cryogenic milling was considered. Subsequently, in-situ alloying of the powder blend in the melt and simultaneous atomization was envisaged in order to avoid the very time-consuming step of cryogenic milling. Size and microstructure refinement, chemical homogenization, and degradation of the fabricated spherical powders were investigated upon successive re-atomization runs. The indentation hardness, phase stability, prospects of consolidation into bulk alloys by spark plasma sintering, and the hydrogen storage and permeability properties of the alloys thus produced have been extensively investigated.