Höganäs (Sweden)

Abstract In this work, the capabilities of plasma‐assisted centrifugal atomization for the fabrication of spherical powders of Ti‐V‐Zr‐Nb‐Mo‐Hf‐Ta‐W refractory high‐entropy alloys (R‐HEAs) are thoroughly studied. Atomization in the transient regime by direct droplet formation is investigated through batch processing by simultaneous melting, alloying, and atomization from a blend of intimately mixed elemental powders pre‐packed in the atomization crucible. In this processing approach, selective depletion of the heaviest elements is attributed to solid‐solid centrifugal separation during the early stages of melting by plasma arc. Conversely, atomization in the steady‐state regime by ligament formation is investigated through continuous processing by plasma‐assisted centrifugal re‐atomization of pre‐alloyed feedstock fed into the rotating crucible from the top. In this case, diffusion‐driven elemental redistribution in the liquid state occurs only within a thin molten metal film, leading to reduced selective depletion of the heaviest elements. The effect of the rotational speed of the atomizing unit on the particle sizes, the chemical composition, the crystal structure, and the microstructure of the atomized powders is investigated. The use of both inert (Ar) and reactive (Ar+N2) plasma carrier gases is considered for the in situ, one‐step fabrication of purely metallic and metal‐ceramic composite powders, respectively.

Powder bed fusion-laser beam (PBF-LB), an additive manufacturing process, takes advantage of rapid cooling rates (103-106 K/s) to enable novel aluminium alloys. This study reports the mechanical properties of one such alloy system (Al-Mn-Cr-Zr based). The alloys based on this system are designed to be precipitation hardenable with high service temperatures. To elucidate the precipitation hardening, three alloy variants were studied involving different heat-treated conditions. Uniaxial tensile testing at room temperature revealed yield strengths between 250–500 MPa with elongation to fracture of 5–25 % with high repeatability. To demonstrate high-temperature resistance, two alloy variants in peak hardened condition were tested at temperatures of up to 573 K. Yield strength up to 170 MPa at 573 K was observed. These properties in combination demonstrate highly competitive Al-alloys for high-temperature applications.

This work represents a preliminary study of atmospheric plasma-sprayed (APS) Yttria-Stabilized Zirconia (YSZ)-based thermal barrier coatings (TBCs) deposited on forged and additive manufactured (AM) HAYNES®282® (H282) superalloy substrates. The effect of different feedstock morphologies and spray gun designs with radial and axial injection on APS-deposited YSZ layer characteristics such as microstructure, porosity content, roughness, etc., has been investigated. The performance of TBCs in terms of thermal cycling fatigue (TCF) lifetime and erosion behaviour were also comprehensively investigated. In view of the high surface roughness of as-built AM surfaces compared to forged substrates, two different types of NiCoCrAlY bond coats were examined: one involved high-velocity air fuel (HVAF) spraying of a finer powder, and the other involved APS deposition of a coarser feedstock. Despite the process and feedstock differences, the above two routes yielded comparable bond coat surface roughness on both types of substrates. Variation in porosity level in the APS topcoat was observed when deposited using different YSZ feedstock powders employing axial or radial injection. However, the resultant TBCs on AM-derived substrates were observed to possess similar microstructures and functional properties as TBCs deposited on reference (forged) substrates for any given YSZ deposition process and feedstock.

Defects in materials manufactured via laser powder bed fusion challenge this manufacturing process' dependability and may prevent it from being comprehensively used for structural components, particularly those intended to operate under dynamic loading conditions. This study aims to investigate the effect of spatter-driven lack of fusion on the mechanical properties of Hastelloy X, with a particular focus on fatigue performance. Mechanical test specimens were manufactured in two builds with different build rates and monitored through in-situ optical tomography. The images acquired in situ were analyzed to detect spatter redeposits to predict the defect content in the gauge section. Selected specimens were measured ex-situ using X-ray computed tomography to map the defect populations. Afterward, the specimens were tensile and fatigue tested, and their performance was analyzed based on the measured and expected defect populations. It was confirmed that a higher build rate is associated with more extensive detection of spatter redeposits in optical tomography images and lack of fusion defects. The fatigue lives of specimens manufactured at higher build rates presented higher scatter but significantly higher average, despite the more critical defect population. Surprisingly, the confirmed presence of lack of fusion defects in the gauge section of test specimens did not necessarily result in a poorer fatigue performance. It was concluded that the grain refinement obtained through the increase in nominal layer thickness has a life-prolonging effect that overrides the effects of spatter-induced lack of fusion defects.

Abstract For the last few years, the HVAF process has been established as a commercially used process and has gained an increasing share in the market of thermal spraying. The main thermal spray materials being used for HVAF spraying have been those based on the tungsten carbide family. Economical aspects and European regulations on chemicals management REACH (Registration, Evaluation and Authorisation of Chemicals) have motivated the demand for thinner WC based coatings, which are still dense and wear resistant. This demand has progressively increased, and the trend shows a further growth in the need for thermal spray feedstock for HVAF sprayed net shape coatings. The challenge for powder producers lies in providing suitable spray powders, with high and consistent quality as well as in considerable volume, to be able to make reliable recommendations to the users of HVAF technology. A deeper understanding of powder requirements for net shape coatings, matching the needs with new powder solutions, and appreciation of the differences in behavior or performance depending on powder type are essential to address the above challenges and constitutes the theme of this paper.

In isotropic pressure-less sintering continuum mechanics models, densification kinetics is driven by the balance between the effective sintering stress and bulk viscosity. In components manufactured by binder jetting (BJ), the green structure created by the arrangement of spherical powder particles during printing is characterized by its high porosity (40-50%). This leads to a wide porosity range for the initial and intermediate sintering stages, where a complex combination of diffusion mechanisms drives matter redistribution through the porous compact. In this paper, a comprehensive analysis of the porosity effect on the resistance to densification of 316L BJ during sintering was performed by avoiding other highly influencing factors like δ-ferrite phase transformation. Different normalized bulk moduli expressions, inspired by Skorohod, Hsueh, and Abouaf sintering models, are used in the framework of the continuum theory of sintering. A new material constants determination algorithm based on the sintering experiments design and non-linear analysis of the model was proposed. This evidenced the critical importance of the experimental data collection for the determination of the required sintering model constants. Accordingly, material shear viscosity and normalized bulk viscosity constants were successfully determined based on dilatometry and grain size experimental data. The bulk moduli proposed comprise physical parameters which depend on the interparticle stress distribution or the initial high reactivity of the BJ compacts. The variation of powder size distribution and/or arrangement would potentially impact the determination of these constants in the normalized bulk moduli.

This study investigates the influences of intraparticle and interparticle diffusions on the reaction rates of char gasification in a packed bed without forced convective flows. The main objective is to elucidate how the dominant scales of mass diffusion resistance change based on particle size distributions (PSD). CO2 gasification rates were measured by thermogravimetric analyses (TGA) of spruce char produced from pilot-scale reactors. Experimental setups using two TGA devices highlighted the effects on different rate-limiting steps. Effects of intraparticle diffusion were investigated with a single layer of monodispersed particles between 75 µm and 6.3 mm using a commercial TGA. Effects of interparticle diffusion were investigated with a packed bed of monodispersed and polydispersed particles using a macro-TG. At the particle scale, gasification rate decreased with the increase of particle size when the reaction was controlled by intraparticle diffusion. This effect can be described by the effectiveness factor with Thiele modulus. At the bed scale, void fraction and tortuosity of the packed bed are influential parameters on diffusivity of CO2 through the bed channels. Due to its non-sphericity of the char particles, the bed of relatively large particles had high void fraction and the presence of smaller particles were essential to lower the bed void size. Consequently, smaller size fraction in the PSD had a major impact on the diffusion resistance at bed scale. It means that the diffusion resistances at particle and bed scales are sensitive to different size fractions in the PSD. It allows one to tweak the overall reaction rates in packed beds by manipulating the PSD if the dominant mass transport mechanism is diffusion.

Electrical machines for automotive applications require high efficiency and power density while maintaining low production costs. Soft magnetic composites are commonly used in auxiliary drives. Their usage in axial flux traction machines however is so far limited to high-end electric-or hybrid sports cars reaching state-of-the art power densities. In this publication, an individual test bench for a single stator tooth is used to investigate the potential of soft magnetic composites in high-speed radial flux machines. Several grades of soft magnetic composites and reference electrical steel grades are characterized. The introduced separation approach for stator tooth losses allows the estimation of manufacturing influences for different material types and grades. This gives the opportunity to adapt core loss models to additional losses due to manufacturing and packaging influences. A better understanding of manufacturing influences helps with decreasing uncertainties in the design stage of electrical machines.

In this study, the concept of enhancing the in-situ oxide precipitation in laser powder-bed fusion processed parts is investigated using powder produced by water and gas atomization. By using water-atomized 316L powder, compared to gas-atomized powder, more oxide precipitates were introduced into the microstructure with the intent to enhance the strength of the material, as an alternative path to oxide dispersion strengthened materials. The results showed that oxide precipitation was homogenous, with higher-number densities of oxides in the sample built using the water-atomized powder. The oxides were observed to be amorphous and enriched in Si and Cr. The average size of the oxides was ~56 nm. After an annealing heat-treatment at 900 °C, the oxides were observed to remain within the microstructure with only minor changes in size and composition. Mechanical testing at room temperature and at elevated temperature did not show any increase in strength relative to the sample built using gas-atomized powder. However, it was shown that the use of water atomized powder in the L-PBF process provides a viable method of introducing and tailoring the number of oxide particles within a built component relative to a conventional gas atomized powder.