Papers by Keyword: Mechanical Milling

Abstract: It is a difficult problem how to obtain the density of powder metallurgy products. Sintering billets had been prepared by mechanical milled graphite/copper compound powders firstly. Their microstructures had been analyzed by such means as scanning electron microscope (SEM) and optical microscope. Influence of technological parameters on relative density had also been investigated. The results show that sintering of non-milled powders are intensely affected by sintering temperature, contrary to mechanical milled compound powders. Hot pressed sintering under vacuum can promote densification effectively. By prolonging time, elevating temperature or pressure of hot pressed sintering, relative density of sintering billets can be increased accordingly. Under the same conditions, relative density decreases with mechanical milling time of compound powders.

Abstract: We report on the preparation and hydrogen desorption/absorption kinetics of nanocrystalline magnesium hydride (MgH₂) added commercial Ti by high-energy ball milling. The phase and composition of the as-milled powders are characterized by X-ray diffraction (XRD). The results show that the milled sample contained MgH₂ phase, Ti phase and small amount of MgO phase. When the milling time is 30 h, the hydrogen desorption property of MgH₂ has been investigated and found that the sample releases 0.43, 0.86 and 0.90 wt% H₂ in 200 minutes at 280, 290 and 300 ^oC , respectively. Moreover, the sample absorbs 0.48, 0.0.58 and 0.61 wt% H₂ in 15 minutes at 280, 290 and 300 ^oC , respectively. It can be seen that the kinetics of hydrogen desorption/absorption of MgH₂-Ti composite has been greatly enhanced compared to the pure MgH₂.

Abstract: Al₂O₃/Ni nanocomposite powder was obtained by high-energy mechanical milling starting from a mixture of Al₂O₃ and Ni commercially powders. The Al₂O₃+15%vol. Ni mixture was homogenized for 15 minutes in the Turbula-type blender and then was milled in a planetary ball mill (Fritsch, Pulverisette 4) under argon atmosphere up to 120 min. Several milling times were used: 10, 30, 60, 90 and 120 minutes respectively. The evolution of the powders during milling and the stability of the composite phases were investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX). The SEM and OM images show a high level of homogenization of the Ni and Al₂O₃ phases for milling times larger than 90 minutes. The X-ray studies indicate no mixing between the two phases. The crystallite grain size is decreasing with the milling time.

Abstract: A sum of mixed nickel-manganese ferrites, Ni_xMn_1-xFe₂O₄ (x=0, 0.3, 0.5, 0.7) were synthesized by classical ceramic route starting from stoichiometric mixtures of commercially MnO₂, NiO and Fe₂O₃. The polycrystalline ferrites obtained by ceramic route were subjected to the mechanical milling procedure in order to reduce the particles size and to refine de crystallites size. A planetary high energy ball mill Fritch Pulverisette 4 was used and the milling time was up to 120 minutes. The ceramic and as-milled ferrites samples were investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and laser particles size analysis (LPSA). After 15 minutes of milling the mean crystallites size for each one of the nickel-manganese ferrites is in nanometric range. After 120 minutes of mechanical milling for all ferrites types the mean crystallites size is at 6-8 nm, depending on Ni/Mn ratio. According to the SEM and LPSA investigations the milled ferrites powders consists in nanometric particles alongside of the micrometric ones. The micrometric particles are formed by multiple nanocrystallites.

Abstract: We report on the preparation and hydrogen desorption/absorption kinetics of nanocrystalline magnesium hydride (MgH₂) added commercial TiO₂ by high-energy ball milling. The phase and composition of the as-milled powders are characterized by X-ray diffraction (XRD). The results show that the milled sample contained MgH₂ phase, small amount of Mg and various phases of TiO₂ such as tetragonal and orthorhombic structure. The effect of the milling time (10, 20 and 30 h) on the hydrogen desorption property of MgH₂ has been investigated and found that the milling time of 20 h has excellent dehydrogenation properties, which can release 3.3 wt% H₂ within 60 min at 300 ^oC, which indicates that the kinetics of hydrogen desorption of MgH₂-TiO₂ composite has been greatly enhanced compared to the pure MgH₂. Moreover, hydrogen absorption kinetics of the sample milled 20 h has been studied and the hydrogen content is 0.7, 0.8 and 1.2 wt% H₂ at 250, 280 and 300 ^oC within 60 min, respectively.

Abstract: Ingots of alloys Al-Cu-Fe were obtained by casting in a graphite mold. Mechanical milling of alloy particles in the as-cast state and after homogenization annealing was carried out in planetary ball mill Retsch PM400 in an argon atmosphere. As a result of mechanical milling granules with an average size of 35-40 μm and fine internal microstructure are formed. The size of coherent scattering regions in quasicrystalline phase after mechanical milling was about 10-15 nm. Mechanical milling after homogenization heat treatment allows much refines the quasicrystalline phase than in the case of mechanical milling of cast alloy.

Abstract: Transportation always plays a significant role in national economy, of which roads and bridges are the most important facilities. The conditions of the road surface have an overwhelming influence on the traffic flow and road safety, especially in winter and the regions with frequent snowfall, thus closely affected the development of the urbanization and economic prosperity. In view of this, this article has analyzed the characteristics of the existed de-icing technologies, coming up with a new synthetically de-icing method which consists of thermal water-jet cutting with mechanical milling technology. These two kinds of de-icing methods are combined as a set of multi-functional system, in which the thermal water-jet cuts the ice layer into separated sections, and the mechanical milling unit then easily removes the remaining sections out of the road. To obtain the prime influential factors in this new de-icing device, repeated indoor experiments were conducted, which provides some theoretical references to further advance the de-icing efficiency and make structural optimization.

Abstract: This work involves the preparation of Cu-Al-SiC composite powders by a high-energy milling process and the study of their densification behavior by cold compaction. The goal of the milling process is to get embedded the ceramic particles in the metal matrix to enhance the distribution of the metal and ceramic phases in the compacts, an important condition to derive in isotropic properties of consolidated materials. For comparison purposes, compressibility tests of a Cu-5Al matrix prepared by high-energy milling were performed; while additions of 1, 5 and 10 vol.% SiC were added to the matrix. It was found that the high-energy milling process leads to Cu-Al-SiC composite powders with a homogeneous distribution of the reinforcement in the matrix. Compressibility essays showed that densification of the powders decreased with SiC content; a densification of 73.7% was obtained for composites with 10% SiC compared to 76.0% for samples with 1% SiC at the maximum load applied. Milling time reduced the plastic deformation capacity of the matrix leading to fracture of the cold welded aggregates; the fracture process was accelerated by the addition of the hard reinforcement particles. Thus, morphology of the powders changed from laminar, to fine fragments and coarse aggregates, affecting the compaction behavior.

Abstract: Water vapor in the air affects aluminum based intermetallic compounds to form hydrogen. We take advantage of this fact to explore the amount of hydrogen obtained from CuAl₂ intermetallic after its mechanical attrition activation. For this propose, CuAl₂ intermetallic alloy was first produced by conventional casting methods and then subjected to mechanical milling processing. After the mechanical activation of the CuAl₂ powders, a chemical reaction between them and water was carried out at room temperature including additives such as CaO, Al and NaCl. The amount of hydrogen release was correlated with other phases produced after the chemical reaction. X-ray diffraction patterns and scanning electron microscopy studies indicates that these phases were aluminum hydroxides and cupper oxides. According to these studies, a significant presence of oxide and hydroxide products occurred in the samples with NaCl additions, indicating best capability for hydrogen generation.

Abstract: Nanocrystalline composite powders were prepared by mechanical alloying of pure Cu, Fe and Co as metallic major part and Al₂O₃ or Fe₂O₃ or SiO₂ as ceramic reinforcement in a high-energy ball mill. Alloys of the copper-iron-cobalt system are promising for the development of new materials and applications. Cu-Fe-Co is used in different applications depending on the properties required. These can be related for example to toughness when used as rock cutting tool, to magnetic and electric properties for microelectronics or to chemical behaviour when used as catalysts in bioalcohol production industry. The objective of the present study is to contribute to understanding how and to which amount the ceramic reinforcement affects the properties for which this Cu-Fe-Co system is used as well as to envisage other less frequently uses for the composite powders. Structural and magnetic transformations occurring in the material during milling were studied with the use of X-ray diffraction, scanning quantum induction device (SQUID) and magnetic force microscopy (MFM). In mechanical alloying the transformations depend upon milling time. The results showed that milling the elemental powders of Cu-Fe-Co in the mass proportion of 50:25:25 respectively for times up to 10h leads to the progressive dissolution of Fe and Co atoms into FCC Cu and the final product of the MA process was the nanocrystalline Cu containing Fe and Co with a mean crystallite size (from coherent crystal size determination by diffraction) of 20 nm aprox. When ceramic particles are milled together with the metals (at proportions of the oxides between 1-10%) this mechanism is retarded. On the other hand, the lowest mean crystallite size is reached without ceramic particles in the milling process. However the composite powder produced in all the cases stabilized similar lowest crystallite size between 45-50 nm. Mechanically alloyed metallic-ceramic composite powder showed lower saturation magnetization than the metallic system but enhanced coercive field (significantly for hematite reinforcement). All the studied systems are intermediate ferromagnetics (H_c≈104 A/m). Milling time significantly affects the structure, composition and properties for both metallic and composite systems.