Papers by Author: Florin Popa

Abstract: The band saw steel must present a good balance between strength, toughness and materials elasticity in the weld joint, respectively in the HAZ. Those areas are affected by the heat welding cycle that is changing the material microstructure and consequently the overall mechanical properties of the saw blade. Minor changes in the chemical compositions of the saw blade material, in welding parameters or in the post-welding treatment can cause semnificative changes in the saw blade durability and performance. A SMEs from Cluj-Napoca had encountered a problem after the welding operation of the 51CrV4 QT steels, bought from two different European manufacturers. It was observed a different welding behavior for the same steel bought from one of the steel manufacturer compared to the other one. The SMEs requested a study in order to find out what went wrong during welding operation, because the bending test has shown an insufficient toughness in the welded joint. As result, we have conducted a research in the quest of the causes for the embrittlement phenomena observed in the welded joint. Chemical analysis, mechanical testing, SEM and EDX and XRD analysis have been used to investigate the material properties in the parent material, weld and HAZ.

Abstract: Fe-Si alloy with a large Si content of 6.5 wt. % is obtained in nanocrystalline state by mechanical alloying of elemental iron and silicon powders. The mechanical alloying process was carried out using a high energy ball mill in argon atmosphere. Samples were collected after 0.5, 1, 2, 4, 6 and 8 hours of ball milling. The X-ray diffraction (XRD) studies indicate that after 4 hours of milling the Fe-Si alloy is formed. The powder magnetisation decreases upon increasing the milling time up to 4 hours as a consequence of the Fe-Si alloy formation. Upon heating, the DSC studies show the Fe₃Si compound formation in the samples milled for milling times lower than 6 hours. Also, the Curie temperature of the alloy was evidenced.

Abstract: The evolution of the Al₂O₃/Ni (25% vol. Ni) composite powders, during the milling and the stability of the composite phases were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX). SEM images show a high level of homogenization of the Ni and Al₂O₃ phases for milling times larger than 120 minutes. The X-ray study indicates no reaction between the two phases. The crystallite grain size decreases with the milling time for both phases.

Abstract: The aim of the study is to identify the microstructure and voids evolution during the aluminium AA6016-T4 sheet deformation up to 93 % in order to perform deformation simulation for future applications. The deformed samples were cut in the rolling direction and to an angle of 45 and respectively 90 degrees to the rolling direction. The low deformed sample exhibit elliptic voids that are flattening out and for high deformed samples a new generation of rounded voids are generated. The microstructure evolution during cold work was studied by scanning electron microscopy and the distortion of the grain is discussed. The grain size evolution and the shape of grains related to the forming degree is analyzed.

Abstract: Fe/Fe₂O₃ composite powders were obtained by mechanical milling of iron and hematite up to 120 minutes in a high energy planetary ball mill. The particles size decreases by mechanical milling upon the formation of the Fe/Fe₂O₃ composite particles. After 120 minutes of milling the median particles size is at 7.2 μm. The Fe/Fe₃O₄ type composite were obtained by reactive sintering in argon atmosphere at 1100 °C of the Fe/Fe₂O₃ composite powders milled for 60 and 120 minutes. After sintering a FeO-wüstite residual phase is formed and this phase is eliminated by applying a subsequent annealing at a temperature of 550 °C. The sintered compact before and after annealing is composed by a quasi-continuous iron matrix in which are embedded iron oxides clusters (Fe₃O₄ and FeO before annealing and Fe₃O₄ after annealing). The iron oxide clusters are analogous with the Widmanstatten structure observed in steels before and after annealing. The materials have been investigated using laser particle size analysis, optical microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction.

Abstract: This work aims to investigate the influence of surface conditioning of porous Ti for enhancing its biological activity, as assessed by in vitro stem cell testing. Porous Ti samples with an average porosity of 32% were processed by Powder Metallurgy with dextrin as a space holder. The samples were subjected to H₂O₂ treatment to form an enhanced TiO₂ film, followed by a heat treatment at 400°C and 600°C aiming to the crystallization of the as-formed amorphous titanium oxide. Samples characterization was performed by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and X-Ray Diffraction (XRD). The treated surfaces revealed to be made of both anatase and rutile TiO₂, with groove–shaped structure and cracks on the surface of the TiO₂ film. The intrinsic biocompatibility of the chemically modified porous Ti surfaces was assessed in vitro. In our cell culture tests, stem cells were found to attach and proliferate better on the chemically treated Ti surfaces compared to the control untreated Ti surfaces.

Abstract: Fe-Si alloy with a Si content of 10 wt. % was obtained in nanocrystalline state by mechanical alloying of elemental iron and silicon powders. The mechanical alloying process was carried out in a high energy ball mill (Fritsch, Pulverisette 4) in argon atmosphere. The X-ray diffraction (XRD) studies indicated that after 4 hours of milling the Fe-Si alloy is formed. The mean crystallites size decreases down to 7 nm after 8 hours of milling. The particles morphology investigated by scanning electron microscopy (SEM) showed an evolution during milling process from two different kinds of particles to a one kind of particles with irregular shape. The magnetisation of powders decreases upon increasing the milling time up to 4 hours as a consequence of the Fe-Si alloy formation.

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: Wet mechanical alloying (MA) were used to prepare amorphous soft magnetic Fe₇₅Si₂₀B₅ (at.%) powders starting from elemental powders of Fe, Si and B. The structural, morphological and magnetic properties of the powders were investigated. It was found that wet MA leads to the amorphisation of the alloy after 40 hours of wet milling using benzene (C₆H₆) as process control agent (PCA). The influence of the wet MA process on the saturation magnetization of the powders was investigated. Amorphous powder of Fe₇₅Si₂₀B₅ (at.%) obtained by wet MA route was used to prepare compacts by spark plasma sintering (SPS). The chosen sintering temperature was 800, 850 and 900 oC. Toroidal samples of Fe₇₅Si₂₀B₅ (at.%) were investigated in DC and AC magnetization regime and their magnetic properties were correlated with sintering parameters, compacts density and phases evolution during sintering.