1,721,096 research outputs found
Production and evaluation of spray formed MMCs
No full textSpray forming has been proposed some 30 years ago as a method for the production of thick coatings and semi-manufactured workpieces. Despite the high costs involved in the process, increasing interest has grown regarding the utilisation of spray forming as a means for the production of huge billets of stainless steels and even superalloys, even for industrial applications. The application of such a technique for the production of spray formed thick composite specimens is considered, with particular concern to the possibility of spray forming pre-composite melt-reinforcement mixtures. Aluminium-base alloys were adopted as matrices and SiCp the reinforcement in atomisation and spray forming tests carried out in a laboratory scale gas atomizer, overcoming problems concerning the high viscosity of the mixture. Microstructural examinations and mechanical tests were carried out in order to evaluate the composite material in terms of structural homogeneity, elastic properties and strength
Nitrogen versus helium: effects of the choice of the atomizing gas on the structures of Fe50Ni30Si10B10 and Fe32Ni36Ta7Si8B17 powders
No full textGas atomization can produce, besides a possible significant degree of undercooling, high cooling rates, whose extent depends on the size of the droplets, on their velocity with respect to the surrounding medium, on the thermo-physical properties of both the alloy and the gas, and of course on the operating conditions such as melt overheating and gas-to-metal flow ratio.
In this respect it is well-known that the atomizing gas can play a significant role in determining both the powder size distribution and the kind and mix of phases which result from the solidification and cooling processes.
The microstructures and solidification morphologies of powders obtained from nitrogen and helium sonic gas atomization of two iron–nickel base glass forming alloys, Fe50Ni30Si10B10 and Fe32Ni36Ta7Si8B17, were investigated by means of light microscopy, X-ray diffraction (XRD) and differential thermal analysis (DTA).
The Fe32Ni36Ta7Si8B17 alloy exhibits a higher proneness to the development of amorphous phase than the Fe50Ni30Si10B10 alloy, while the effect of the higher speed attainable by the stream of helium with respect to that of nitrogen, affords not only to obtain a larger amount of particles in the finer size ranges, but also to affect the relative amounts of phases within the different size fractions
Wear behaviour of a spontaneously liquid infiltrated aluminum alloy matrix composite in pin-on-disk high velocity dry sliding wear tests
No full textSpontaneous liquid infiltration of alumina preforms is a well known method for the production of disks to be used in brakes for motorcycles, automobiles and even rail transportation. The pin on disk wear testing is a mean to evaluate the suitability of the Alumina-Aluminum alloy composite for such applications and moreover allows to evaluate the stability of the surface of the rotating composite disk in contact with a stationary counterpart. A series of high velocity dry sliding wear tests were carried out on a properly modified horizontal lathe, against metallic and brake lining counterparts. Friction coefficients as well as the wear rates were determined during pin-on-disk and flat-on-disk tests carried out at different sliding speeds, up to 15 m/s. The sliding speed heavily affects the stability of the composite which showed increasing degradation rate with increasing speed. The microstructural examinations carried out by means of light and scanning electron microscopy showed the important role played by the third body, which enhances delamination, especially during the tests carried out at the highest sliding speed
Studio della formabilita` di un acciaio dual-phase e correlazione con le sue caratteristiche meccaniche e microstrutturali
No full textMorphological and structural study of gas atomized Zr-Cu-based glass-forming alloys
No full textZr–Cu-based glass-forming alloys were processed in a laboratory scale gas atomizer, operated in sonic conditions with nitrogen or helium as the atomizing medium. Powders of rather wide size distributions were obtained, in the under 212 μm range, which afforded to carry out comparative phase analyses on particles which underwent quite different cooling conditions. X-ray diffraction examinations as well as light microscopy, transmission electron microscopy, differential thermal analysis (DTA) and microhardness determinations were used to investigate the powders belonging to different size ranges. Amorphous, partially crystalline and fully crystalline powders were obtained from each atomization batch. Light microscopy afforded to evaluate the morphometric details such as the secondary dendrite arm spacing in the crystalline particles, which were correlated with the estimated cooling rates computed by means of a simplified computer code. X-ray diffraction, TEM examinations and electron diffraction confirmed that conditions were established for the development of amorphous or nanocrystalline particles, in particular in the “under 38 μm” and in the 38–45 μm size ranges. Microhardness determinations showed an extremely high hardness, of the order of 1000–1100 HV0.05 in the case of fully amorphous particles, which could be encountered mainly in the smaller size ranges, while in the case of crystalline powders the hardness was around a half of such value mainly in the larger, fully crystalline ones
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