Papers by Author: Liviu Brânduşan

Abstract: The possibilities of Cu-Al2O3 particulate reinforced composites, of competitive functional properties, processing by the classical powder metallurgy route have been investigated taking into consideration its known technical and economical advantages in respect to the known worldwide investigated technological routes of their processing. The adopted compositions, of (5.0÷20.0) [vol.%] Al2O3, were selected in agreement with published data for a large range of applications. Pharmaceutical homogenization method applied for powder mixtures preparation proved to assure a high homogeneity, evidenced by SEM and EDS analyses. Their determined compressibility has shown that, for all compositions, the obtainable compactness is very close to that of pure Cu (even over 94 %). Cold uniaxial compaction at 500 and 700 MPa, and subsequent sintering in argon of high purity at 800 °C for 45 and 60 min have been adopted for composites realization. The performed analysis of the compacting pressure and sintering time influence on the composite compactness proved that, beside the above specified values obtaining for 700 MPa and 60 minute processing parameters, high enough values, acceptable for numerous applications, can be also obtained at 500 MPa and 60 or even 45 minutes. Finally, microstructural analysis highlighted that, by the adopted processing conditions, a high uniformity of Al2O3 particles distribution in the Cu matrix can been assured, both creating premises for obtaining good functional properties of Cu-Al2O3 composites, proving the competitiveness of the investigated PM route for their elaboration.

Abstract: The behaviour of alloyed and unalloyed sintered steels in static and dynamic loading is influenced by their macro and microstructure. The characteristics of the used powder, their chemical composition, the conditions of shaping by pressing, and the parameters of the sintering process are some factors determining the structure of the steels [1, 2]. Under the same technological conditions, sintered materials were produced using iron powder of different particle size or in different powder mixtures for establishing the structure influence on some mechanical properties, The results obtained shows that the sintered materials present a higher sensitivity towards the dynamic and variable loads [2]. In the same time, it was shown that a material made from a fine powder fraction behaves, at mechanical loads, almost in the same way as an elaborated material from the same powder but with a wider particle size distribution or particles size. Using of the different powder mixtures lead to some microstructures with better mechanical properties at the intergranulars necks. Using the copper powder or the low or high alloyed iron powder in a mixture with the basic iron powder determines an improvement of the mechanical properties by changing the materials microstructure. It was found that the addition of high alloyed steel powder has the strongest effect on the fatigue resistance.

Abstract: For obtaining small parts with complex shape, for which a finishing operation cannot be applied easily, injection forming can be used. For a successful applying of this procedure not only the conditions of the injection process deployment must be taken into consideration, but also the conditions of the binder removal. These conditions impose, in a big measure, the feedstock characteristics, especially the ones of the binder used. The characteristics of the binder mixed with powder influence the injection process, the debinding process and the quality of the injected and sintered parts. Regarding that the binders used are formed from many components, it is very important to know the measure that each of them determines the rheological characteristics of the mixture. In the research made, a binder was used formed from polyethylene, paraffin and stearic acid, a binder which is often used to obtain mixtures for injection. It was watched how these components influence the binder characteristics, the injection process deployment and the effect they have on the quality of the debinded parts.

Abstract: Nb additions to NiTi smart alloys are known to lead to the PTT-hysteresis broadening and transformation temperatures raising – required in numerous applications. As Nb has a high affinity for oxygen, NiTi-Nb alloys processing by powder metallurgy, via SHS, from elemental Ni-Ti-Nb powder mixtures, seems to be more advantageous and cost-effective than by classical one. However, its application encounters difficulties determined by the NiTi higher Gibbs Free Energy of Formation than of Ni3Ti and NiTi2, possible Ni-Nb compounds formation, Nb acting as diluent in SHS. This research proved the possibility to overcome these difficulties and of NiTi-Nb alloys obtaining by SHS using energetically activated powder mixture by controlled Mechanical Alloying. Also, it was proved the possibility to reduce Nb content from the common one of 9 at.% to 5 at.% without a significant effect of transformation temperatures and hysteresis decreasing.

Abstract: Silver free Ti-43at.%Cu (Ti-Cu) eutectic alloy have been developed for the brazing of Be and Cu99.32Cr0.6Zr0.08 (CuCrZr). The structural and microstructural properties of the as obtained alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The analysis have revealed that the Cu-Ti eutectic alloy have a fine and homogeneous eutectic structure with the phase composition corresponding to the Ti-Cu phase diagram. These observations are consistent with the thermal properties of the alloys determined by DTA-TG analysis. The role of Mn on the eutectic structure, as well on the wetability properties was also studied. The effect of Mn is to produce a less coarse eutectic structure and to decrease the wetability angle. The mechanical testing of CuCrZr/Ti-Cu/CuCrZr junctions have demonstrated that these alloys are suitable for the brazing of Be and CuCrZr. The shear strength of the CuCrZr/TiCuMn/ CuCrZr increases with Mn concentration reaching the value of about 280 MPa for 2at% Mn.

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.

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.

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.

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.

Abstract: Powders-based on the Fe-Cu-Ni-Mo system are well known in the P/M industry for combining good compressibility and dimensional stability with a potential for high strength. The typical heterogeneous microstructure of these materials has proven to provide favourable mechanical properties. However, in applications where strength and hardness become critical variables, faster cooling rates after sintering are required to meet application requirements. Test specimens containing various graphite contents to achieve 0.20, 0.35 and 0.55% combined carbon were pressed to 7.0 and 7.2 g/cm3, sintered in a furnace at 1150°C and cooled directly from sintering temperature with either normal or rapid cooling. Test results are discussed in terms of tensile properties, apparent hardness and microstructure. The formation of bainite and martensite by raising carbon content and cooling rate increased tensile strength and apparent hardness, while a higher green density maintained satisfactory ductility. It was possible to achieve ultimate tensile and yield strengths as 920 and 660 MPa respectively with apparent hardness 30 HRC.