Materials Science Forum Vol. 1012

Abstract: The demand for lighter materials with suitable mechanical properties and a high resistance to corrosion has been increasing in the industries. Therefore, aluminum appears as an alternative due to its set of properties. The aim of this work was to evaluate residual stress levels and mechanical properties of welded joints of Aluminum-Magnesium alloy AA 5083-O using the Friction Stir Welding process. For mechanical characterization were performed a uniaxial tensile test, Vickers hardness, bending test and, finally, the determination of residual stresses. It was concluded that welding by FSW process with an angle of inclination of the tool at 3o, established better results due to better mixing of materials. The best results of tensile strength and a lower level of residual stresses were obtained using a tool rotation speed of 340 RPM with welding advance speed of 180 mm/min and 70 mm/min.

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: Equal Channel Angular (ECAP) pressing has been showed as an attractive route to produce fine and ultrafine-grained metals and alloys with high strength and fracture toughness. ECAP is a simple process for applying severe plastic deformation (SPD) to metals that can be done with common laboratory equipments (mechanical tests machines) and an adequate die. In the present work, an eutectoid steel was processed by ECAP in a 120° die. Mechanical behavior of samples deformed by ECAP was compared to the same material processed by rolling. The hardness level obtained after a single pass of ECAP was comparable to an 84% reduction by rolling in a single pass. The hardness level obtained after 1 ECAP pass on a patented steel was higher than 4 ECAP passes on the same steel without patenting. The metallographic analysis showed intense alterations on the microstructure by the ECAP processing.

Abstract: High hardness armor (HHA) steels, when subjected to high deformation rates, are prone to adiabatic shear bands formation. These heterogeneities, formed in a narrow zone, are the result of an intense plastic deformation in which the rate of heat dissipation is low. The generated shear bands can lead to a decrease in ballistic performance and impose to an armor a catastrophic failures. The appearance of these bands may be related to the microstructure or the deformation rate to which the material is subjected. Therefore, this work aims to analyze the influence of the microstructure of an HHA steel, tempered at 310, 425 and 610° C for 2 h, after high deformation rates, in the appearance of adiabatic shear bands. Specimens were dynamic tested in a split Hopkinson pressure bar. It was shown that tempering temperature at 310 ° C for 2h, which produced bainite and martensite with high hardness, was the most susceptible to the adiabatic shear bands appearance.

Abstract: The aim of this work was to verify the mechanical behavior of a Gas Inert Tungsten (TIG) welded joint of a 6061 aluminum alloy, after aging and super-aging thermal treatments. The characteristics of aluminum allow diversified applications, which contributes as one of the most used metals in the world. Being a lightweight, durable and sturdy material, it shows excellent performance and superior properties in most applications. Brazil is the eleventh primary aluminum producer, preceded by China, Russia, Canada, the United Arab Emirates, India, Australia, the United States, Norway, Bahrain and Saudi Arabia. The specific objectives were to obtain welded joints in aluminum by TIG process, performing the thermal treatments of aging and super-aging, metallography/macrography tests of welded joints, microhardness test of treated and untreated joints, and analysis of the results. The methodology used in the analysis of the samples was by the techniques of Metalografia and Microhardness.

Abstract: The development of high entropy alloys led to a great scientific interest owing to the fact that these alloys are responsible for presenting previously unexplored possibilities. Not only the breakdown of the classical paradigm of metal alloying development, but also the infinite possibilities on compositions and the unique properties exhibited, attracted the attention of a number of researchers since the "discovery" of high entropy alloys in 2004. Recently, it was suggested that some of these alloys could be suitable for ballistic application. This works aims to study the behavior of a CoCrFeMnNi high entropy alloy against a 271 m/s lean projectile. It is shown for the first time that high entropy alloys might provide a substantial protection against this ammunition threat. Furthermore, the results suggested that this specific alloy could be strengthen by shock-wave propagation and that the main ballistic energy absorption mechanism was the spalling of surfaces.

Abstract: Carbon steel is widely used in the industry due to its mechanical properties and low cost, but in contrast it resists poorly to corrosion, leading to economic losses and mechanical issues. The use of surface treatment is essential to extend the life of the metallic material. In this context, niobium is being studied for its great corrosion resistance properties. The aim of this paper was to produce and evaluate the corrosion protection of a niobium-based coating produced by the Pechini Method. The resin was applied in the metallic surface by dip-coating and then calcinated at 450 oC for 1 hour. The coated material was analyzed electrochemically by open circuit potential and potentiodynamic polarization, and morphologically by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The electrochemical analyses showed that the deposition of the coating increased the corrosion resistance and the morphological analyses indicated a homogenous coating with the presence of phases of NbO and NbO₂.

Abstract: Carbon steel is one of the most commonly used alloys in industrial applications due to its physicochemical properties and low cost. However, the use of this metal material may become limited due to its vulnerability to corrosion. Thus, it is necessary to use methods that inhibit corrosion. Organic compounds with heteroatoms possess the characteristic of inhibiting corrosion by forming a protective film. The corrosion protection of SAE 1020 carbon steel, promoted by the aqueous extract of Persea pyrifolia (PP) bark, was evaluated in this work at extract concentrations of 5% and 10% v/v, in order to replace an inhibitor of synthetic origin with an ecologically benign inhibitor. Plant extracts are generally inexpensive and can be obtained through simple extraction processes. The objective of this work was to study the use of PP peel extract as a carbon steel corrosion inhibitor (SAE 1020). The electrochemical response was determined by measurements including electrochemical impedance spectroscopy (EIS) and anodic potentiodynamic polarization (PPA) in a 0.5 M sodium chloride medium. The samples were characterized by optical microscopy to evaluate the type of corrosion.

Abstract: In the present work, samples of a binary intermetallic alloy (Fe₃Al) with 26at.%Al were submitted to electrochemical corrosion evaluation in a 0.5M H₂SO₄ solution containing naturally dissolved oxygen. The corrosion resistance was evaluated by applying linear polarization, electrochemical impedance spectroscopy and potentiodynamic polarization at 22 and 35°C. The results obtained revealed that in both conditions the material exhibits active-passive behavior. Heating to 35°C did not alter the passivity characteristics of the alloy (passivation range and passive current density), but elevated the corrosion current density and the critical current density. The polarization resistance of the samples was also decreased with temperature, following the trend observed for the corrosion current density.

Abstract: In the present work, the friction surfacing process was applied to manufacture aluminum alloy (AA6351) coatings on low carbon steel (AISI 1020) substrates. After friction surfacing the AA6351 deposited coatings were submitted to two finishing process in order to adjust surface roughness: milling and milling followed by sanding. The corrosion behavior of the two finishing process was compared with the as-deposited condition in order to determine the influence of surface roughness on the corrosion resistance of friction surfacing coatings. The corrosion behavior was examined by electrochemical impedance spectroscopy and potentiodynamic polarization in a 3.5wt.%NaCl solution containing naturally dissolved O₂. The results obtained indicated that the elevated surface roughness observed in the as-deposited condition led to relatively lower corrosion resistance in comparison, with lower values for polarization resistance and more anodic corrosion potential.