Papers by Author: Mauricio David Martins das Neves

Abstract: The spacer grids are part of the Fuel Element (FE) set of the PWR (Pressurized Water Reactor) type reactor. These grids maintain the position of the fuel rods within the arrangement of the FE, conserving among them the spacing necessary for the operation of the reactor. The grids are manufactured from the union of the intersecting points of stamped strips of Base Material (BM) Inconel 718, by a joint process called brazing. The addition metal (AM) used consists of a brazing paste based on Ni-Cr-P (nickel-chromium-phosphorus), which is added dropwise in the intersection of grids with a clearance of 0.025 mm. For this purpose, Inconel 718 smooth strips of 0.35 mm thick nickel plated samples were prepared, the AM was added and the vacuum oven was 10^-3 mbar, in different time and temperatures. The samples were prepared by metallography and characterized using optical microscopy (OM), scanning electron microscopy (SEM) and mechanical microhardness test. The purpose of this work is to characterize the brazing joint by means evaluate the size of precipitates and the different compounds formed in the joint. It was found different precipitates composed mainly of titanium and niobium. Phosphorus rich phases were found in the brazed region. The mean hardness of the BM was 469 ± 12 HV and in the joint region hardness of 1345 ± 34 HV was found in the lighter phases.

Abstract: This work evaluates the microstructure and the yield strength under compression at room temperature and at 800°C of specimens prepared with AISI 310 stainless steel powder (D50 = 10 μm), manufactured by gelcasting. Parts were vacuum sintered in a single batch at 1280°C. At room temperature, specimens presented average yield strength of 270 MPa, and at 800°C, 105 MPa. Microstructure analysis involved the measurement of grain size along the vertical axis of cylindrical specimens, with special attention to the effect of particles settling, and was conducted using scanning electron and optical microscopy, and X-ray diffraction. Settling effect was assessed considering the position where the specimen was taken and was negligible: both density and yield strength did not vary significantly along the vertical axis.

Abstract: Fractures resulting from wear and fatigue process have been identified as the main causes of failure in biomaterials, especially in implants that act in place of bone or other hard tissue, as they are subject to conditions involving severe cyclic loadings. In biomaterialscase, the types of failures mentioned above must also be evaluated under the effect of degradation or corrosion, due to the direct contact with body fluids. The present research analyzed the fatigue induced by corrosion fracture of an orthopaedic implant for total knee replacementproduced with an austenitic ASTM F138 stainless steel. The morphology, compositions of the interfaces and subsequent corrosive pitting were characterized by stereoscopy and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Stress concentration, inclusions and high carbon levels were the main reasons of failure.

Abstract: Advances in processes using the powder metallurgy techniques are making this technology competitive compared to the other traditional manufacturing processes, especially in medicine area. The additive rapid prototyping technique – selective laser melting (SLM) was applied in a biomaterial of CoCrMoFe alloy (ASTM F75), to study the mechanical properties and microstructural characterization in comparison between the conventional technique – casting. The gas atomized powder was investigated by their physical (as apparent density, bulk density and flow rate) and the chemical properties. The powder was analyzed using scanning electron microscope with energy-dispersed X-ray spectroscopy (SEM-EDS) and X-ray fluorescence. Specimens of standard samples were manufactured using these techniques to evaluate the mechanical properties as uniaxial tensile (yield strength, rupture tensile and elongation), transverse rupture strength and the micro hardness. The mechanical properties showed higher values in the SLM specimens than the casting specimens. Before the mechanical tests the specimens were examined using optical microscope (OM) and SEM-EDS. The micrographs revealed a microstructure with finer morphology in the SLM technique and the dendrites in the casting technique.

Abstract: The aim of this study is the consolidation of Cobalt-Chromium (CoCr) alloy powder using the additive manufacturing - selective laser melting (SLM) and the investment casting techniques. The research of this study has been applied to their biomaterial applied to development of prosthesis and dental implants. The gas atomized powder are spherical (mean diameter equal to 42,74 μm) and was analyzed by their physical and chemical properties. The microstructure of the powder and specimens was evaluated using optical microscope (OM) and scanning electron microscope with energy-dispersed X-ray spectroscopy (SEM-EDS). The mechanical properties were evaluated of standard samples using a tensile (yield strength, maximum tensile, rupture tensile and elongation), three point bending (transverse rupture strength) and micro hardness tests. The mechanical results indicate higher values for the SLM than casting specimens. The micrographs revealed a characteristic morphology of laser been used in the SLM technique and the dendrites in the casting technique. The microstructure of samples made by SLM is thinner than the samples obtained in the cast.

Abstract: The development of materials with a porous titanium surface has been widely studied in the field of biomaterials due to the excellent biocompatibility, high corrosion resistance and combination of high strength with low density. Another relevant fact is that porosity allows bone tissue growth. However, the high reactivity in liquid state ends up hindering titanium fusion, so an alternative is the powder metallurgy (PM).The aim of this work was to produce porous titanium samples by conventional PM. Porous samples was characterized by porosity and microstructure (optical microscopy - OP and scanning electron microscopy SEM), crystaline phase (X-ray diffraction –XRD), mechanical properties (three point bending test) and cytotoxic test. The results showed the presence of alpha phase, a decrease in the elasticity modulus, increase in average pore size and samples exhibited no toxic effects.

Abstract: Titanium is an attractive material for dental and biomedical applications, because of high corrosion resistance, excellent biocompatibility and high mechanical strength combined with low density. However, the high reactivity of titanium in the liquid phase make it difficult to produce it by fusion, so a alternative is powder metallurgy (P/M) method. Powder Metallurgy has been used to manufacture porous implants. The presence of a porous surface is desirable because it improves the osteointegration increases the adhesion between the bone tissue and the implant, being favorable for transporting body fluid. This paper proposes to characterize the commercial pure titanium powder obtained by process of hydride-dehydride, obtain samples with adequate porosity by uniaxial pressing and vacuum sintering and evaluate the corrosion behavior of sintered titanium in Hank ́s solution. The results showed that the titanium powder of angular shape after uniaxial pressing of 400 MPa and sintered in vacuum at 1150 ° C, allowed obtaining samples with adequate surface porosity of around 17%. In potentiodynamic polarization curves revealed no typical behavior of passive metals but show low current density, that increasing corrosion resistance. Keywords: titanium implants, powder metallurgy, porosity and electrochemical behavior.

Abstract: The influence of adding 6 wt% (NbC) niobium carbide on the sintering temperature and microstructure of high speed steel - AISI M₂ (0.87% C, 5.00% Mo, 6.00% W, 4,00% Cr, 2.00% V and Fe bal.) powder was studied. The starting material was obtained by vacuum melting followed by atomization in water. The samples were axially cold compacted in a cylindrical matrix and then vacuum sintered at 1250 and 1350 °C. Dilatometry and differential scanning calorimetry measurements indicated an increase in sintering temperature with addition of niobium to the AISI M₂ steel. Optical and scanning electron microscope observations revealed a microstructure with uniformly distributed niobium carbides.

Abstract: Stainless steel (SS) powders are used in the preparation of sintered SS products. One of the applications of sintered SS products is as filters in the petrochemical and food processing industries. In these industries, the SS filters are subject to severe conditions associated with the removal of solid particles from the fluid. Hence, SS filters should have adequate mechanical strength and high corrosion resistance. Welding can be used to manufacture SS filters. In this study, sintered AISI 316L specimens were welded using the TIG (Tungsten Inert Gas) process. The weld joints were examined by optical microscopy and by scanning electron microscopy. Electrochemical polarization measurements were carried out to evaluate the influence of welding on the corrosion resistance of sintered filters.

Abstract: The processing of a molybdenum AISI M2 high speed steel with the addition of NbC (6% in mass) by a Powder Metallurgy technique of Mechanical Alloying is the aim of this work. Mechanical Alloying (MA) has been used primarily for particle size reduction, to its present status as an important method for the preparation of either materials with enhanced physical and mechanical properties or, indeed, new phases, or new engineering materials. In this work, niobium carbide (NbC) was added to the AISI M2 HSS powders by Mechanical Alloying technique in two different types of attritor mills and the materials which resulted were characterized by means of SEM plus EDS. The powders were processed in a horizontal attritor Zoz mill and in a vertical attritor mill developed in our laboratory. The parameters of milling were distinct and the results of the processing were compared.