Search Results (7)

Aluminum-based ceramics exhibit excellent wear resistance and hot hardness that are suitable for various responsible applications allowing products to work under extreme mechanical and thermal loads (up to 1000 °C). The problem of high-precision forming complex-shaped parts is a known engineering challenge due to the insulating properties of aluminum-containing ceramics and the formation of chemically active carbides in a hydrocarbon medium. The alternative approach for electrical discharge machining non-conductive sintered Al₂O₃ in the water-based medium using nickel-chrome plasma-vapor-deposed coating of 12 mm, SnO powder suspension (particle diameter of ⌀10 µm, concentration of 150 g/L), and brass wire-tool is proposed. The productivity was evaluated by calculating the material removal rate and discharge gap for various combinations of pulse frequency and duration. The maximal material removal rate of 0.0014 mm³/s was achieved for a pulse frequency of 30 kHz and pulse duration of 1.7–2.5 μs. The recommended value of the interelectrode gap is 48.0 ± 4.9 µm. The possibility of electrical discharge machining aluminum-containing insulating ceramics without using hydrocarbons, carbon and copper-group assisting measures was proposed and shown for the first time. The chemical content of the debris in the interelectrode gap between components of the materials was thermochemically analyzed. Full article

In electrical machines, laminated steels are commonly adopted as soft magnetic materials, while for permanent magnets, sintered ferrites and NdFeB are the most common solutions. On the other hand, the growing demand for volume reduction with the increment of efficiency leads to the necessity of exploring other magnetic materials able to face the challenge better than the traditional ones. Bonded magnets have been used to replace sintered magnets, obtaining a better use of space and particular magnetic properties. Instead, for the magnetic circuit, Soft Magnetic Composites (SMC) allow realizing very complex magnetic design (3D path for flux) with iron loss reduction at medium-high frequencies, especially for the eddy currents loss contribution. On the other hand, SMC materials have such drawbacks as low mechanical properties and high hysteresis losses. For this reason, in this work, different studies considering several variables have been carried out. SMCs were produced through a moulding process; inorganic and organic layers to cover ferromagnetic particles were used, adopting different coating processes. Particular tests have been performed for a quicker and more indicative overview of the materials obtained. The single sheet tester (SST) is easier than traditional toroidal methods; on the other hand, the multiplicity of variables affects the SMC materials and their process. For this reason, coercivity and conductibility tests permit rapid measurement and provide a direct classification of the produced SMCs, providing the main information needed to select suitable materials. Results highlighted that choosing the more appropriate SMC material is possible after using these simple preliminary tests. After these tests, it was possible to argue that with 0.2 wt% of phenolic resin as the organic layer (and compaction pressure of 800 MPa), it is possible to produce a good SMC. On the other hand, the SMC with 0.2 wt% of epoxy resin (and compaction pressure of 800 MPa) gives a minor coercivity value. Additionally, despite the SMC with the inorganic layer, 0.2 wt% of nano-ferrites showing the best coercivity values (specifically for vacuum treatment at 600 °C), their resistivity was unsatisfactory. Full article

The fabrication of soft magnetic Fe parts by the medium-frequency electrical resistance sintering (MF-ERS) technique is studied in this paper. This consolidation technique involves the simultaneous application to metallic powders of pressure and heat, the latter coming from the Joule effect of a low-voltage and high-intensity electric current. Commercially pure iron powder was used in the consolidation experiences. The porosity distribution, microhardness, electrical resistivity and hysteresis curves of the final compacts were determined and analysed. The results obtained were compared both with those of compacts consolidated by the conventional powder metallurgy (PM) route of cold pressing and vacuum furnace sintering, and with fully dense compacts obtained by double cycle of cold pressing and furnace sintering in hydrogen atmosphere. Full article

A commercially pure (c.p.) nickel powder was consolidated by Medium-Frequency Electrical Resistance Sintering (MF-ERS). In this consolidation technique, a pressure and the heat released by a high-intensity and low-voltage electrical current are concurrently applied to a metal powder mass. A nickel powder with a high tap porosity (86%) and a low applied pressure (only 100 MPa) is chosen in order to be able to obtain compacts with different levels of porosity, to facilitate the study of the porosity influence on the compact properties. The influence of current intensity and heating time on the global porosity values, the porosity and microhardness distribution, and the electrical conductivity of the sintered compacts is studied. The properties of the compacts consolidated by MF-ERS are compared with the results obtained by the conventional powder metallurgy route, consisting of cold pressing and furnace sintering. A universal equation to describe the porosity influence on all the analyzed properties of powder aggregates and sintered compacts is proposed and validated. Full article

The influence of the applied pressure and electrical parameters on the macrostructure of specimens consolidated by the medium-frequency electrical resistance sintering technique (MF-ERS) is analysed in this work. This technique is based on the application of pressure to a mass of conductive powder that, simultaneously, is being crossed by a high intensity and low voltage electric current. The simultaneous action of the pressure and the heat released by the Joule effect causes the densification and consolidation of the powder mass in a very short time. The effect of the current intensity and heating time on the global porosity, the porosity distribution, and the microhardness of sintered compacts is studied for two applied pressures (100 and 150 MPa). For the different experiments of electrical consolidation, a commercially available pure iron powder was chosen. For comparison purposes, the properties of the compacts consolidated by MF-ERS are compared with the results obtained by the conventional powder metallurgy route (cold pressing and furnace sintering). Results show that, as expected, higher current intensities and dwelling times, as well as higher pressures and the consolidation of compacts with lower aspects ratios, produce denser materials. Full article

In this work, powders of cemented ultrafine WC-6 wt.% Co were consolidated. The feasibility of the medium frequency electrical resistance sintering (MF-ERS) technique were studied to prevent WC grain growth during consolidation. Porosity and hardness were measured at different zones of the MF-ERS compacts. The compacts showed a slight inhomogeneity in their properties across their section, but it was controlled by choosing suitable values of the processing parameters. The optimal values for the material studied were current intensities between 7 and 8 kA and sintering times between 600 and 800 ms. The main achievement using this consolidation method was that sintered compacts essentially maintained the initial WC grain size. This was attained to processing times of less than 2 s, and without the need for using protective atmospheres. Full article

Commercially pure (C.P.) iron powders with a deliberate high degree of oxidation were consolidated by medium-frequency electrical resistance sintering (MF-ERS). This is a consolidation technique where pressure, and heat coming from a low-voltage and high-intensity electrical current, are simultaneously applied to a powder mass. In this work, the achieved densification rate is interpreted according to a qualitative microscopic model, based on the compacts global porosity and electrical resistance evolution. The effect of current intensity and sintering time on compacts was studied on the basis of micrographs revealing the porosity distribution inside the sintered compact. The microstructural characteristics of compacts consolidated by the traditional cold-press and furnace-sinter powder metallurgy route are compared with results of MF-ERS consolidation. The goodness of MF-ERS versus the problems of conventional sintering when working with oxidized powders is analyzed. The electrical consolidation can obtain higher densifications than the traditional route under non-reducing atmospheres. Full article