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Abstract: Fe- and steel-based powder metallurgy (P/M) products, such as steel gears, spurs, locking mechanisms, porous filters, sliding bearings and bushes, as well as other machine parts and structural elements, are mainly produced with the so-called conventional sintering technology. It is the most efficient technology for the mass production of small, complex, functional and structural parts. Therefore, it is the most convenient and popular among all of the P/M technologies. The most important end-user of sintered parts is the automotive industry. However, small, complex, sintered parts can also be frequently used in the furniture and household industries, precise mechanics, articles for recreation and sports. A fine, iron-based powder mixture or prealloyed powder is first automatically uniaxial-die compacted (ADC) into the final shape of the product with a mechanical or hydraulic press and then sintered in a protective atmosphere at approximately 1100 °C. The metal powder mixture must have the appropriate engineering properties given by the chemistry and particle morphology, enabling a fast and reliable die-compaction process. The most important are a high tap density, a good powder flowability and a low compressibility. All this gives the green compacts an appropriate final shape with a smooth surface, a relatively high and uniform green density, as well as a green strength without internal flaws and cracks. In the case of very small two-or-more-heights products, for example, spur gears with a low module, it is very difficult to obtain a uniform green density at acceptable compaction pressures. Often small cracks are formed at height crossings and big differences in the green density appear in smaller or thinner regions. In the frame of our investigation we analysed the influence of the selected prealloyed commercial iron powder’s morphology and its technological properties on automatic die compaction, as well as the sintering process in the case of small two-level sintered gear dimensions of 5/40–7/10×7mm with module m = 0.5. The original iron powder was sieved and the finest powder particle fraction (< 45 µm) was compared with the original powder mixture considering ADC and sintering process. It was found that the selection of the finer powder mixture could not contribute to the improvement in the overall ADC process, as well as a better green compact. In the present paper the results of our investigations are presented and the reasons why a finer powder mixture cannot contribute much to an improvement of the conventional sintering process.
Keywords: Fe-based alloy powders, particles, morphology, microstructure, automatic die compaction, sintering
Published in DKUM: 27.03.2017; Views: 1428; Downloads: 356
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Abstract: The use of high-strength low-alloy steels for high-performance structures, e.g., pressure vessels and pipelines, requires often high-strength consumables to produce an overmatched welded joint. This globally overmatched welded joint contains local mis-matched regions, which can affect the unstable fracture behaviour of the welded joint and the welded structure itself. If local mis-matched regions are present in the vicinity of a crack tip, then the fracture toughness of the weld metal can be significantly lower than that of the base metal. In this paper, the influence of the weld-metal microstructure on the fracture behaviour is estimated enabling an evaluation of the resistance to stable crack growth through different microstructures. The lower bound of the fracture toughness for different microstructures was evaluated using a modified Weibull distribution. The results, obtained using specimens with a through thickness crack front, indicated a low fracture toughness, caused by the strength mis-matching interaction along the crack front. In the case of through-the-thickness specimens, at least one local brittle zone (LBZ) or a local soft region is incorporated into the process zone in the vicinity of the crack tip. Hence, an unstable fracture occurred with small stable crack propagation, or without it. Despite the fact that the differences between the impact toughness of the weld metal and the base metal can be insignificant, the fracture toughness of a weld metal can be significantly lower.
Keywords: fracture mechanics, welded joint, crack-tip opening displacement, resistance curves
Published in DKUM: 16.03.2017; Views: 1500; Downloads: 100
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Abstract: Al-based alloyed powders, appropriate for the sintering procedure (powder metallurgy, P/M) contain the alloying elements with a high solid solubility in Al, enabling reaction and liquid-phase sintering. They are surface oxidised because of a high affinity of Al to oxygen. Besides, this type of powders contains a polymeric lubricant (wax), which reduces the friction on die walls during automatic die compaction into the final compact shape of a product. This lubricant has to be removed slowly during the first stage of sintering in order to prevent deformations and cracking of the product. Consequently, its sintering is very complex. Generally, these powders are sintered in pure nitrogen with a low dew point. The optimum sintering conditions are generally determined on the basis of light and scanning electron microscopy. The investigation can also be completed very successively with differential scanning calorimetry and thermo gravimetry. The first one allows an insight into the endo- and exothermic reactions, taking place during the heating and cooling of a compacted metal powder, and the second one allows an insight into the processes, connected with the mass loss (a reduction, a lubricant removal, etc.) or mass increase (an oxidation). The DSC/TG of three commercial Al-based alloyed powders was performed in the frame of our investigations. The results were compared with the theoretical thermodynamic-based calculations and the optimum sintering conditions were proposed.
Keywords: aluminium powders, sintering, differential scanning calorimetry and thermo gravimetry
Published in DKUM: 15.03.2017; Views: 1384; Downloads: 134
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Abstract: Hitro strjene trakove zlitin Cu -0,5% Zr in Cu - 1% Zr smo izdelali s postopkom litja taline na hitro se vrteči boben pri različnih procesnih parametrih. Mikrostrukturo trakov smo raziskali v odvisnosti od debeline oziroma povprečne ohlajevalne hitrosti in od sestave zlitin. V prečnem prerezuje mikrostruktura tankih trakov (50-55▫$mu$▫m) sestavljena iz cone transkristalnih zrn, ki sega do zgornje proste površine in v kateri ločimo tri področja z različno substrukturo: področje brez mikrosegregacij, področje trakaste substrukture in področje s celično/dendritno substrukturo. Trakovi z debelino 85-90▫$mu$▫m vsebujejo v prečnem prerezu poleg transkristalne cone še cono globularnih zrn z dendritno morfologijo. Velikost posamezne cone ter substrukturnih področij znotraj transkristalne cone je pri isti debelini trakov odvisna od kemijske sestave zlitin.
Keywords: hitro strjevanje, mikrostruktura, trakovi, Cu - Zr zlitina
Published in DKUM: 10.07.2015; Views: 1600; Downloads: 57
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Keywords: kovinski prahovi, morfologija, mikrostruktura
Published in DKUM: 01.07.2015; Views: 2116; Downloads: 100
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