Materials and Technologies

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Authors: Magdalena Lungu, Stefania Gavriliu, Delia Patroi, Mariana Lucaci
Abstract: The paper presents some considerations concerning the obtaining and characterisation of some new electrical contact pieces of Ag-SnO2-MeO type, where the MeO is an additive chose from the series of metallic oxides WO3, Bi2O3 and CuO. These have a SnO2 content of 9.5 wt. %, a MeO content of 0.5 wt. %, the balance being Ag. The electrical contact materials were manufactured by powder metallurgy techniques using a new type of powder mixtures obtained from SnO2 and MeO powders which were wet mixed, simultaneously with “in situ” Ag synthesis. These present a very uniform and fine dispersion, which in turn leads to improved functional characteristics of the final electrical contact pieces in comparison to the dry mechanical powders mixtures. The new products were designed for low voltage power engineering switching devices working in air at In = 40, 125 and 200 A.
Authors: Lidia Matrici, Şerban Domşa, Liviu Brânduşan
Abstract: Ceramic Injection Molding (CIM) can provide design engineers with economic solution to otherwise apparently insoluble part production problems. CIM combine attributes of plastic injection molding with the higher properties of ceramics. The aim of the paper is examine the rheology of binder systems with independent parameters (shear rate, temperature, and binder content). The study also establish that the feedstocks used for CIM it must be assured an extremely low viscosity at the injection temperature. The experimental results show that an increasing of the wax contain with 5% leads to a more pronounced decrease of the shear stress than the increasing with 10 oC of the temperature. In all situations the temperature’s increasing determines a decrease of the feedstock’s viscosity and shear stress.
Authors: Radu Mureşan, Codruta Pavel, Dana Salomie
Abstract: The goal of present work was to study the possibility to produce bars from heavy alloys based tungsten through rotary forging. For the experiment there were selected two compositions from the system W-Ni-Fe (90, respectively 93 %W/wt., Ni/Fe ratio = 7/3). From the two compositions there were prepared samples by conventional route of liquid phase sintering, followed by thermal treatment in a neutral atmosphere (N2, Ar) and by rotary forging with different reduction degree.
Authors: Liviu Brânduşan, George Arghir
Abstract: The analysis of a large range of sintered parts shows the fact, they are strained at contact fatigue, in most of the cases. The Hertzian pressure, at which materials in contact are subjected, establishes the appearance of variable stress, which records a maximum at some depth under the contact surface. In the case of sintered materials this stress variation conducts to a faster degradation of strained surface than in the case of compact materials. This is because the stress variation establishes a critical state at the level of stress concentrators, generated by pores, standing in material structure, leading to formation of some cracks which may propagate at the level intergranular necks, producing removal for some particles of material. The accomplished research on sintered materials, having different chemical compositions, revealed the fact, at the beginning, that the materials undergo a cold hardening, indicated by a hardness increasing, followed by some cracks generating, accompanied by their extension and material removal. This process develops in the same mode for all investigated materials, but with different intensity.
Authors: Cristina Teișanu, Stefan Gheorghe, Ion Ciupitu
Abstract: The most important features of the self-lubricating bearings are the antifriction properties such as friction coefficient and wear resistence and some mechanical properties such as hardness, tensile strength and radial crushing strength. In order to improve these properties new antifriction materials based on iron-copper powders with several additional components (tin, lead and molybdenum disulphide) have been developed by PM techniques. To find the optimal relationship between chemical compositions, antifriction and mechanical properties, in this paper a mathematical model of the sintering process is developed, which highlighted the accordance of the model with data by regression analysis. For the statistical processing of the experimental data the VH5 hardness values of the studied materials were considered. The development of mathematical model includes the enunciation of the model, the establishment of the performance function (optimization) and the establishment of the model equations and verifying. The accordance of the model with experimental data has been highlighted by regression analysis
Authors: Radu L. Orban, Mariana Lucaci
Abstract: This paper investigates the effect of Fe, Cr and B additions, in small proportions, as alloying elements in Ni3Al with the purpose to reduce its intrinsic fragility and extrinsic embrittlement and to enhance, in the same time, its mechanical properties. It represents a development of some previous research works of the authors, proving that Ni3Al-Fe-Cr-B alloys obtained by reactive synthesis (SHS) starting from Mechanically Alloyed powder mixtures have superior both room temperature tensile strength and ductility, and compression ones at temperatures up to 800 °C, than pure Ni3Al. These create premises for their using as superalloys substitutes.
Authors: Victoria Chifor, Radu L. Orban, Nicolae Jumate
Abstract: The influence of cooper powder as reinforcing phase on the main mechanical and physical properties of recycled polyethylene matrix composites has been investigated. The obtained elastic modulus increases up to the upper adopted reinforcing level (∼10 [vol. %]), while elongation and fracture energy decrease. The UTS values remain quite constant within the reinforcing content interval. Electrical resistivity proved to depend, to a great extent, on the reinforcing particle distribution inside the polymer matrix. It has a sudden drop when a continuous electrical conductive path is formed at a critical volume fraction of metal particles. At a lower content, instead, it slowly decreases as reinforcing powder content increases. Both the mechanical properties and values for electrical resistivity are similar to those published for polymers obtained directly from monomers.
Authors: Luisa N. Mîtcă, Radu L. Orban
Abstract: There is investigated the influence of Al2O3 content on the density and mechanical properties of Cu-Al2O3 (2.0 ÷ 10.0 [vol.%] Al2O3) cermets processed by classical routes of powder metallurgy. The obtained results show that the cermets relative density remains quite constant, at a value of over 95 [%], up to a content of ~4 [vol.%] Al2O3. Together with reinforcing effect of the hard Al2O3 particles, proved by the apparition of an elastic zone on the Stress-Strain curve of tensile test, determine a notable increasing of ultimate tensile strength (UTS) and hardness with a small variation of elongation. Over about 4÷5 [vol.%] Al2O3, a decrease in UTS and also in toughness, proved by the Fracture energy decreasing, occurs. Consequently, from the considered points of view, is not recommended to adopt higher reinforcing levels. For electrical applications, the relative density of the processed cermets can be further increased with 3 ÷ 4 [%] by die cold repressing.
Authors: Cornelia Mátyás, Şerban Domşa, Liviu Brânduşan
Abstract: The mechanical properties of sintered steels are mainly dependent on the material microstructure determined by the matrix composition. The paper presents the mechanical properties of some sintered steels obtained from mixtures of Distaloy AB, Astaloy Mo and graphite powders. The aim of this paper is to investigate the performances increasing of the materials obtained by sintering some mixed compositions of Distaloy AB and Astaloy Mo steel powders with graphite additions.
Authors: Liviu Brânduşan
Abstract: Forming process developed by injection is influenced by rheological characteristics of the feedstock. Because the feedstock is a mixture formed by a powder and a binder, in the performed research it was pursued establishing of binder composition influence. Rheological characteristics establishing was realised with the help of a rotational rheometer RHEOTEST 2, which can be used at the establishing of some properties of Newtonian and non-Newtonian fluids in different temperature circumstances. Utilised binders were formed of different proportions of wax, low density polyethylene, and stearic acid. Obtained results show that wax has a great influence on rheological characteristics of the binder. As it contains increases the dynamic viscosity decreases pronouncedly.

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