Instytut Podstawowych Problemów Techniki

Streszczenie:

Within the framework of the EuroFusion WPMAG project, an automatic laser-welding line was constructed to produce a 1 km long, empty stainless steel jacket demonstrator for the React & Wind (RW) conductor for EU-DEMO. Four 500 m long C-profiles made of 316 L austenitic stainless steel were fabricated from ∼8 m long sections using the manual TIG (Tungsten Inert Gas) welding method. A series of experimental investigations was carried out on the welded samples, including ferrite content measurements, microhardness tests, residual stress measurements, and shape deviation assessments. The results revealed that part of the austenitic structure transformed into ferromagnetic phase—ferrite—around the heat affected zone (HAZ), with up to 7% ferrite observed in the laser welds and up to 10% in the TIG welds. Due to the relative magnetic permeability of ferrite (μᵣ > 1), electromagnetic (EM) forces will be present in that region of the jacket during magnet operation.
The microhardness measurements revealed an increased hardness in the welded region—up to 40.6%—due to material hardening and the presence of harder ferrite in the microstructure. Residual stresses were measured using the hole-drilling technique for both TIG and laser welds, revealing mostly compressive stresses in the TIG welds and tensile stresses in the laser welds. Considerable compressive stresses were introduced into the TIG welds during grinding. To assess the equivalent residual stress, a method of approximating the lower bound of the von Mises residual stress was proposed, revealing increasing values with the depth up to 1 mm, exceeding the initial yield stress at depths greater than 0.5 mm and reaching up to 425 MPa.
The shape deviations around the TIG weld reached 0.41 mm, with deformations toward the centre of the wide side of the jacket, resulting in a concave shape. Such deviations are considerable and could impact subsequent assembly steps of the superconducting Cable-In-Conduit Conductor (CICC).
This study presents a procedure for evaluating weld quality in conductor jackets, focusing on residual stresses, phase transformations, and welding-induced property changes.

Słowa kluczowe:

Welded jacket, Residual stresses, RW conductor, EU-DEMO

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Streszczenie:

This study focuses on improving the wear resistance of cutting tools and extending their service life under intense mechanical, thermal, and radiation loads in nuclear power plant environments. This research investigates the potential of electrospark alloying (ESA) using W–Zr–B system electrodes obtained from disks synthesised by spark plasma sintering (SPS). The novelty of this work lies in the use of SPS-synthesised W–Zr–B ceramics, which are promising for nuclear applications due to their high thermal stability, radiation resistance and neutron absorption, as ESA electrodes. This work also establishes the relationship between discharge energy, coating microstructure and performance. The alloying electrode material exhibited a heterogeneous microstructure containing WB2, ZrB2, and minor zirconium oxides, with high hardness (26.6 ± 1.8 GPa) and density (8.88 g/cm3, porosity < 10%). ESA coatings formed on HS6-5-2 steel showed a hardened layer up to 30 µm thick and microhardness up to 1492 HV, nearly twice that of the substrate (~850 HV). Elemental analysis revealed enrichment of the surface with W, Zr, and B, which gradually decreased toward the substrate, confirming diffusion bonding. XRD analysis revealed a multiphase structure comprising WB2, ZrB2, WB4, and BCC/FCC solid solutions, indicating the formation of complex boride phases during the ESA process. Tribological tests demonstrated significantly enhanced wear resistance of ESA coatings. The results confirm the efficiency of ESA as a simple, low-cost, and energy-efficient method for local strengthening and restoration of cutting tools.

Słowa kluczowe:

electrospark alloying, W–Zr–B electrodes, SPS, coatings, phase composition, microstructure, hardness, steel

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Streszczenie:

This work compares experimentally measue properties of W-Al-B thin films with mechanical properties, density, and thermal conductivity values calculated using DFT methods. Theoretical modelling was conducted to simulate two WB2 stable structures alloyed with varying amounts of aluminium: α-WB2 (P6/mmm) and ω-WB2 (P63/mmc), as well as α-AlB2 (P6/mmm). Using the HiPIMS-DC magnetron sputtering technique, films with α-WB2 structure and varying aluminium contents were deposited at 400 °C. When layers are composed with x = 1.4% aluminium (where x = at%Al / (at%Al + at%W)), their microstructure changes from amorphous to crystalline columnar. A back transformation to an amorphous microstructure occurs when the amount of aluminium exceeds x = 7.3%. An original method was used for the film density studies, which combined mass measurements and microscopic observation. These measurements were then used to determine the layers' thermal conductivity using the thermoreflectance method. The measured conductivity of the deposited ceramic films range from 3 to 6 W/(mK). Moreover, the obtained films are very hard, e.g. H = 36.1 ± 1.7 GPa for x = 1.4% Al, but exhibit a much lower Young's modulus than the theoretical values. The relatively high H/E⁎ ratio > 0.1 for films with low aluminium content indicates anmore elastic character. Ab-initio calculations showed that, based on the criteria of Cauchy pressure (C12-C44) and Pugh's ratio (B/G), the α-WB2 structure may have a ductile nature in contrast to the other structures. However, the deposited films are rather brittle in nature, resulting from an excess of boron. The fracture toughness measurements show higher KIC values for low aluminium content. They are 3.8 MPa√m for WB2, 2.8 MPa√m for x = 1.4%, and 3 MPa√m for x = 7.3% aluminium

Słowa kluczowe:

thin films, high-power impulse magnetron sputtering, density, thermal conductivity, fracture toughness, stiffness tensor

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Streszczenie:

Four-hour oxidation processes of niobium in an air atmosphere at temperatures of 400 °C, 425 °C, 450 °C and 500 °C were carried out. In order to characterise the layers produced, the cross-sectional microstructure, chemical and phase composition as well as surface roughness were examined. The mechanical properties of the surface were determined by performing Vickers microhardness tests. In order to verify the properties from a biological point of view, contact angle analysis and corrosion tests in Ringer's solution were carried out. The results revealed the formation of layers composed of a solid solution of oxygen in niobium Nb(O) at oxidation temperatures of 400 °C, a solution of Nb(O) and niobium pentoxide Nb2O5 at 425 °C, and Nb2O5 at 450 °C and 500 °C. Increased oxidation temperature resulted in an increase in hardness and surface roughness, and each process contributed to improved corrosion resistance. Oxidation at too high temperature (≥450 °C) caused degradation of the material's surface due to niobium's low heat resistance. At 450 °C the first cracks in the material were visible, and at 500 °C the layer was inhomogeneous, brittle and underwent significant chipping. The highest hardness, roughness and hydrophobic properties were shown by niobium oxidised at 500 °C, which underwent surface degradation at this temperature. In turn, niobium oxidised at 400 °C and 425 °C showed outstanding properties in the biological aspect, achieving both high hydrophilicity and the highest corrosion resistance.

Słowa kluczowe:

Niobium, Oxidation, Microstructure, Corrosion, Contact angle, Surface engineering

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Słowa kluczowe:

HiPIMS magnetron sputtering, pulsed laser deposition, nanocomposite, transition metal borides, silver

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