Papers by Keyword: Mechanical Milling

Abstract: The harmonic structure composites with Ti-Ni alloy and Cu were fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and were investigated mechanical and thermal properties in detail. Fine Ti-Ni alloy powder and coarse Cu powder were mechanically milled using planetary ball mill equipment at cryogenic temperature. The MM powder was sintered by using the SPS apparatus at 1073 to 1273 K. Tensile tests carried out at 383 K as mechanical properties evaluation. Thermal expansion to 1073 K was evaluated using thermomechanical analyzer equipment. Microstructural observation of the MM powders and SPS compacts was achieved using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SPS compacts is the harmonic structure composite having the network structure with Ti-Ni alloy and the dispersive area with Cu. Such a Ti-Ni/Cu harmonic structure composite exhibits unique mechanical properties. T ensile strength and elongation increase with increasing the sintering temperature in the Ti-Ni/Cu harmonic structure composite. The coefficient of linear thermal expansion of the Ti-Ni/Cu harmonic structure composite lies between that of Ti-Ni alloy and Cu, and a sufficient reduction in the coefficient of linear thermal expansion is confirmed.

Abstract: AlCoCrFeNi high entropy alloys (HEA) have superior strength and corrosion resistance at both room and high temperatures and are expected to application in elevated temperature environments. However, it is not clear the relationship between the harmonic structure and the mechanical properties of these HEAs at elevated temperatures. The harmonic structure is composed of dispersed coarse grains and fine grains that are networked around them. In this study, the harmonic structure AlCoCrFeNi HEA was fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and the microstructure and elevated temperature mechanical properties of AlCoCrFeNi HEA are investigated in detail. AlCoCrFeNi mixed powders with average particle sizes of 14.6 and 82.4 μm were treated with MM. The MM powders were consolidated by SPS at 1173 to 1373 K. Mechanical properties were evaluated by compression tests at room temperature to 1073 K. Microstructural observation was performed using a scanning electron microscope, electron back scattered diffraction and energy dispersive X-ray spectrometer. The conventional SPS compacts have modulated structure with BCC and B2 phase and grain boundary precipitates with FCC phase. While the MM-SPS compacts have a similar structure of the conventional compacts at dispersed region and an equiaxed nanograins including a σ phase at network region. MM compacts with harmonic microstructure demonstrate high compression strength compared to conventional compacts at room temperature to 673 K. However, conventional microstructure compacts have higher strength than harmonic structure above 873 K. These results suggest that the harmonic structure has unique deformation behavior at elevated temperatures.

Abstract: In this paper, we present a microwave characterization of some copper-based nanocomposite materials. They are composed of epoxy resin reinforced with copper powders nanostructured at different mechanical milling durations. Main purpose of this work is to probe the material properties such as complex permittivity and conductivity. For that, elementary copper powder were nanostructured via high-energy mechanical milling process. Milled powders were sampled at 3, 12, 33 and 58 hours milling and characterized via X-Ray Diffraction (XRD). The materials were then incorporated into epoxy resin to mold bulk composite structures of 1 mm thickness and a section that matches the R120 waveguide one. Microwave characterization of the bulk composite samples was carried out by a two- port measure of the scattering parameters using a vector network analyzer. The computation of complex dielectric permittivity followed the noniterative approach proposed by A.H. Boughriet. Obtained results are presented in the form of spectra. XRD spectra attest to the structural refinement during mechanical milling. Complex dielectric permittivity and microwave conductivity spectra exhibit the effect of the structural refinement on the microwave absorbing properties.

Abstract: The formation of LiFeO₂ lithium ferrite from unmilled and milled Fe₂O₃-Li₂CO₃ mixture was studied by X-ray powder diffraction (XRD). The ball milling was perform via AGO-2S high-energy planetary ball mill at a rotational speed of 2220 rpm for 60 min. Solid-phase synthesis was carried out by conventional laboratory furnace at 600 °C. Using PowderCell 2.4 software, the structural parameters of the reagents and ferrite obtained from these were determined. According to the XRD data, the crystallite sizes of the milled reagents decreased, while the strains increased. It was found that the synthesized ferrite is characterized by multiphase composition consisting of unreacted initial reagents, α-LiFeO₂, γ-LiFeO₂ and α-Li_0.5Fe_2.5O₄ phases, the concentration of which depends on the prehistory of the mixture.

Abstract: An electromagnetic interferences (EMI) shielding is a material that attenuates radiated electromagnetic energy. Polymer nanocomposites is a class of materials that combine electrical, thermal, dielectric, magnetic and/ or mechanical properties, which are useful for the suppression of electromagnetic interferences. In this work, we looked over the effectiveness of the electromagnetic interferences shielding of polymer-based nanocomposites. These are thin samples of epoxy resin strengthened with nanostructured Cu powders. Nanostructured Cu powders were obtained by mechanical milling using the high-energy RETSCH PM400 ball mill (200 rpm). A powder sampling was conducted after 3h, 6h, 12h, 24h, 33h, 46h and 58h milling for characterization requirements. XRD analysis via the Williamson-Hall method shows that the mean crystallites size decreases from 151.6 nm (pure Cu phase) to 13.8 nm (58 h milling). Simultaneously, the lattice strain increases from 0.1% (pure Cu phase) to 0.59% (58 h milling). The elaboration of thin samples was performed by mixing a vol./3 fractions of nanostructured Cu powder, epoxy resin and hardener. Thin slabs of 1 mm thickness were moulded for use in a rectangular wave-guide. The EMI shielding experimental involved a two ports S parameters cell measurement made of R120 metallic wave-guides of rectangular section (19.05x9.525 mm²) and operational over the frequency band of 9.84 to 15 GHz associated to a network analyser. Obtained results show moderate EMI shielding effectiveness for the milled Cu-based slabs.

Abstract: Fine powders of strontium hexaferrite are widely used in powder metallurgy for the production of permanent magnets resistant to atmospheric oxygen and high working temperatures. Obtaining powders with predefined technological characteristics in minimal time and with minimal energy consumption is an actual problem of powder metallurgy. The paper provides the results of experimental studies of technological characteristics of strontium hexaferrite powder (SrFe₁₂O₁₉) during milling in a beater mill. Mechanical milling of coarse strontium hexaferrite was carried out in the mill with the system of rotating beaters for 120 minutes without and with the creation of a pseudo fluidized bed. The fluidization was formed by a perpendicular constant and alternating magnetic field with induction gradients of 150 and 210 mT/m. Average particle size and powder bulk density dependencies from milling time were studied. Experimental data show that milling with the formation of a magneto fluidized bed allows intensifying the process. Beginning from 70 minutes, the dependencies of average particle size and bulk density come to almost asymptotic behavior making further milling rather ineffective. Carried out research allows choosing optimal milling duration for obtaining the required average particle size.

Abstract: Problem of obtaining fine powders of strontium hexa-ferrite is actual because of its wide applications. The paper provides the results of studies of particle size distribution and structural characteristic changes of strontium hexa-ferrite powder (SrFe₁₂O₁₉) during milling in impact mill and after its consequent annealing. Mechanical processing of coarse particulate system was carried out in the mill for 120 minutes without electromagnetic effect and with creation of magneto fluidized bed, formed by perpendicular constant and alternating magnetic field with induction gradient of 210 mT/m, providing reciprocating motion of particles and aggregates with sizes of 3 – 4 mm. It was shown that milling of coarse strontium hexa-ferrite with average particle size 1558.5 μm and the most possible size 1500 μm in magneto fluidized bed allowed to intensify milling process and to provide a significant increase of powder particle sizes uniformity. It was found out, that milling in magneto fluidized bed leads to a great decrease of coherent scattering regions sizes and an increase of lattice micro-deformations and relative dislocation density. Consequent annealing of the powder for 2 hours at 850°C refined structural characteristics significantly. The carried out research allows to choose the optimal milling duration for solution of practical problems of powder metallurgy.

Abstract: The objective of this work was to provide information about the behaviour of Fe-based nanocomposites when exposed to microwaves. It is about rectangular bulk samples of epoxy resin reinforced by nanocrystalline Fe powders and shaped in accordance to the internal section of the R100 metallic waveguide (8.2 to 12.4 GHz) at a fixed thickness of 7 mm. The nanocrystalline Fe powders were obtained by high-energy mechanical milling process using a planetary Retsch PM 400-ball mill. The milling speed was fixed at 200 rpm for three durations and the milling process were performed under Argon atmosphere. The bulk nanocomposites were obtained by dispersion of 30% vol. of the nanocrystalline Fe powders in the resin matrix. Electromagnetic parameters as complex relative dielectric permittivity and magnetic permeability, electric and magnetic loss tangent and reflection loss were calculated using reordered S parameters. The scattering parameters were characterized using a measure cell made off two metallic R100 wave-guides associated to an Agilent 8719 network analyser according to the reflection-transmission technique. The obtained spectra inform on the new electromagnetic properties as well as the absorption characteristic acquired by the bulk nanocomposites due to the presence of the nanocrystalline Fe powders.

Abstract: In this study, amorphous Fe₇₈Si₉B₁₃ alloy was successfully synthesized by mechanical alloying (MA) of pure elemental powders which were milled under an argon gas atmosphere. Effects of milling time on the phase transformation, microstructure and morphological evolution were studied by X-ray diffraction (XRD), laser diffraction (Granulometry), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results showed that by increasing the milling time, the nanocrystalline and amorphous phase content increases and alloys with good properties are obtained at 100 h of milling.

Abstract: This work studies the production of melt spun Fe₇₈Si₉B₁₃ ribbons with amorphous or nanocrystalline structure. The main objective is the preservation of the amorphous structure after obtaining powders by mechanical milling of the ribbons, as well as the study of the influence of the milling conditions on the size distribution and structure of the obtained powders. In order to obtain high quality amorphous ribbons, the wheel rotation speed, crucible-wheel distance, melt homogenization time, ejection pressure and the ejection temperature were optimized in the melt spinning process. Different mills were used for powder production, studying the size distribution, efficiency, and preservation of the amorphous character as a function of the milling time. Ribbons and powders were characterized by X-ray diffraction (XRD) and electron microscopy (SEM and TEM); laser diffraction was used for powder granulometry.