Abstract: Two fractured pins of impact wrench were evaluated by fractography using scanning electron microscopy. This pneumatic wrench is used to the fabrication of automotive components being characterized by its power and tightening capacity. The important part of the wrench is the pin clutch impact mechanism used to generate high torque. An original pin manufactured with AISI E52100 steel adjusted 2,580 nuts before fracture and a substitute pin of E52100 modified steel with S adjusted 7,095 nuts before failure. Fractographic analysis in both fracture surfaces indicated a ductile-brittle mixed fracture mode. Original pin surface exhibited a granular appearance while substitute pin surface showed fibrous and rough morphology. Precipitation of nearly rounded, elliptical and elongate second – phase particles containing Cr, C and Mn measured as intergranular and transgranular particle density was observed, promoting some brittle failure zones and ductile fracture measured as the volume fraction of grain boundaries and microvoids, respectively. A larger amount of intergranular medium precipitates was found on the original pin which favored the brittle failure among the grain boundaries compared to that of the substitute pin. The ductile fracture by larger transgranular fine particle density which acted as nucleation sites of higher volume fraction of microvoids was found in the substitute pin.
Abstract: Among the different surface treatments used to improve the wear resistance of metallic materials, plasma transferred arc (PTA) is an attractive alternative to conventional techniques due to the intrinsic properties of its higher deposition rate, lower heat input and especially for the wide applicability of materials. The wide range of materials makes it possible to produce metallurgical bonding between the hardfacing layer and substrate material with very low dilution and distortion. Weld deposits are characterized by less level of inclusions, oxides, discontinuities and wear resistance. Metal-mechanic industry continuously requires recovering tool steel components subjected to severe wear. In this research Fe-based filler metal was deposited on D2 steel by using plasma transferred arc (PTA) process. The influence of Cr and Nb on Fe-based filler metal microstructure was investigated using scanning electron microscopy (SEM). In order to evaluate the mechanical properties were performed wear and hardness tests. The wear resistance and hardness values were compared with the results of a weld bead using nickel-based filler metal.
Abstract: This study deals with the production of some Al-SiO2 composites and the evaluation of milling intensity over the distribution of silica particles into the Al matrix. Samples for mechanical characterization were prepared from powders by compaction and sintering using a solid-state route complemented with mechanical milling. The mechanical response was modified as a direct function of the milling intensity, but an adverse effect was observed with prolonged milling times. Electron microscopy studies reveal a homogeneous dispersion of insoluble particles into the Al matrix, which is associated with the high grain refinement in the synthetized composites giving an important improvement on the composites strength. Also, the silica spheroidal structure is not altered nor destroyed (mechanically and/or chemically) during the composite synthesis.
Abstract: In this investigation the Al64Cu24Fe12 alloy was melted in an induction furnace and solidified under normal casting conditions. The as-cast sample was subject to a heat treatment at 700 oC under argon atmosphere in order to obtain the icosahedral quasicrystalline phase in a monophase region. Subsequently, the icosahedral phase was milled for different times and water added conditions. The pre-alloyed and milled powders were characterized using scanning electron microscopy, X-Ray diffraction, and transmission electron microscopy. The experimental results showed that the icosahedral phase is sensitive to the reaction between water and aluminum of the quasicrystalline alloy to generate hydrogen. As the milling time and the amount of water are increased, the embrittlement reaction of the alloy is accentuated releasing more hydrogen.
Abstract: Al based intermetallic materials are commonly susceptible to hydrogen embrittlement reaction. Water vapor in the air reacts with the aluminum in the alloy and releasing hydrogen. Thus, the aid of this work is to know how much hydrogen can be released when the embrittlement reaction is induced inside the milling container. For this purpose the CuAl2 intermetallic material was made by conventional castings methods and then subjected to high-energy ball milling in water. The samples were characterized by X-ray diffraction pattern, attenuated total reflectance spectroscopy and scanning electron microscopy (SEM). After the milling process, the amount of hydrogen released was correlated with the other reaction products obtained during the reaction. These products were primarily aluminum hydroxides. The amount of hydrogen that can be released is similar to the theoretical amount possible that can be released.
Abstract: This work involves the preparation of Cu-Al-SiC composite powders by a high-energy milling process and the study of their densification behavior by cold compaction. The goal of the milling process is to get embedded the ceramic particles in the metal matrix to enhance the distribution of the metal and ceramic phases in the compacts, an important condition to derive in isotropic properties of consolidated materials. For comparison purposes, compressibility tests of a Cu-5Al matrix prepared by high-energy milling were performed; while additions of 1, 5 and 10 vol.% SiC were added to the matrix. It was found that the high-energy milling process leads to Cu-Al-SiC composite powders with a homogeneous distribution of the reinforcement in the matrix. Compressibility essays showed that densification of the powders decreased with SiC content; a densification of 73.7% was obtained for composites with 10% SiC compared to 76.0% for samples with 1% SiC at the maximum load applied. Milling time reduced the plastic deformation capacity of the matrix leading to fracture of the cold welded aggregates; the fracture process was accelerated by the addition of the hard reinforcement particles. Thus, morphology of the powders changed from laminar, to fine fragments and coarse aggregates, affecting the compaction behavior.
Abstract: Carbon nanotubes (CNT) based gas sensors have attracted interest due to their excellent properties. Several studies have reported changes in the CNT’s electrical properties when functionalized with platinum (Pt) nanoparticles. In this investigation, the vapor phase impregnation decomposition (VPID) method was employed to incorporate Pt nanoparticles on CNT. Both, Pt nanoparticles and CNT were characterized by high resolution transmission electron microscopy (HR-TEM). The sensitivity of sensors based on CNT doped with Pt, was evaluated with ozone molecules. TEM images showed low and heterogeneous distribution on the surface of carbon nanotubes. The gas evaluation of CNT-Pt sensor presents good and quick response to ozone molecules at different concentrations and temperatures. The best response was found to be at 120 °C.
Abstract: This paper presents experimental research work on the combination of grinding and wet welding techniques to repair T-welded connections employed in the construction of offshore structures. A longitudinal rectangular grinding profile was performed at the weld toe of T-welded connections for localized cracking material removal. Two different grinding depths of 6 mm and 10 mm were performed in the welded connections to eliminate two different level of damage depth. Subsequent wet welding was applied in the grinded region to repair the grinded material. The wet welding was performed in a hyperbaric chamber simulating three different water depths: 50 m, 70 m and 100 m (shallow water). Once the combined repair techniques were performed, further experimental work was done to characterize the mechanical behavior of the repaired structures. The mechanical characterization was done with tensile, Charpy tests and Vickers Hardness tests. The region of interest from the structures was the weld toes the grinded-wet welding repair of the T-welded connections. Subsequent scanning electron microscopy (SEM) was also performed to examine the developed microstructures in the T-welded connection. The results showed that the combination of the repair techniques can restore the mechanical properties of the damaged structures. This was demonstrated by the measurement of the ultimate tensile strength, which were similar to those measured with no repair applied techniques. But the Charpy energy values were quite lower to those previously measured. This behavior was attributed to the level of porosity formed by the high level of gases created during the welding process for the simulated water depths, which were more severe at the higher water depth resulting in pore formation
Abstract: This paper presents an electrochemical characterization of X60 steel corrosion using polarization curves. In order to get a surface analysis, a Scanning Electron Microscopy (SEM) was used. Analyses by Energy Dispersive Spectroscopy (EDS) were done to characterize the corrosion products films formed on surface of the steel sample at the different overvoltages studied. The electrolyte was a synthetic soil solution and all electrochemical parameter was measured at room temperature, atmospheric pressure and 24 hours of the total exposure time. It is important to point out that these parameters were measured in the protection potential (versus saturated calomel electrode, SCE) and at different overvoltages. The results of the polarization curves show that the corrosion rate increased as the overvoltage also increased and this behaviour was corroborated by the superficial analysis obtained by SEM; where the corrosion morphology was localized and the aggresivity increased with the overvoltage. In addition, in all overvoltages, the anodic reaction was limited by a mass transfer process. A localized corrosion form was found in all tests.