Resumo em Inglês:

Abstract The aim of this work is to develop a numerical subroutine for the commercial finite element method software QFORM VX 8.1.4 capable of predicting, simultaneously, regions of closed dies which are prone to thermal fatigue as well as the number of cycles required for thermal fatigue cracking. The numerical subroutine was based on Manson (thermal fatigue) and Coffin-Manson (number of cycles) equations. Hot forging of AISI 1045 grinding balls using AISI H13 closed dies was performed and the numerical and experimental results were compared. The findings indicated that after forging 66 grinding balls, the numerical model achieved steady state for the temperature inside the die. After forging 600 grinding balls, both the experimental and numerical results showed that the regions of the dies subjected to the highest thermal gradient were prone to thermal fatigue. In addition to that, the numerically predicted values for the onset of thermal cracking were consistent with the experimental results: 517 and 600 cycles, respectively.

Resumo em Inglês:

Abstract Additive manufacturing processes have been developed over the last decades, especially vat photopolymerization (VP) processes, due to its simplicity and speed. The objective of this paper is to characterize commercial VP resins widely used for technical applications. Thus, test specimens were printed by Digital Light Processing and subjected to tensile, compression, flexural, hardness, and inorganic composition analyses. The resin with the highest resistance and hardness (containing 0.6 vol% of inorganics load) reached 53 MPa in tension, 110 MPa in compression, 79 MPa in bending, and 82.3 Shore D, which is comparable to injected polymers. A case study was made, replacing the injected gears of a reducer by printed ones and comparing the finite element analysis with resin properties. The characterization and case study results encourage the expansion of VP processes in the manufacturing of products in several industries and service sectors, as well as the development of new composite resins.

Resumo em Inglês:

Abstract Chatter causes poor surface quality and damage to tools in machining. Therefore, strategies for chatter monitoring and reduction are constantly under investigation. Among these strategies, the use of piezoelectric layers embedded in the tool-holder as sensors/actuators for control strategies had been proposed for improving the stability limit reducing the system vibration. Nevertheless, actuators implemented using a single piezoelectric layer may fail to suppress chatter due to its limited actuation power. A method for assembling a series multilayer piezoelectric actuator is then proposed, which can provide a better performance and ultimately suppress chatter in turning operations. This actuator is used with passive and active control and the system response is investigated using frequency response functions. Two fixation conditions are tested to analyze the robustness of the control strategies. The results show that the use of a multilayer piezoelectric actuator made it possible for a greater reduction in vibration amplitude in the chatter frequency.

Resumo em Inglês:

Abstract This work presents a study on effect of cutting parameters in force, texture and chips morphology obtained during the turning of a self-lubricating sintered composite. The tests were performed varying the cutting speed (vc = 100 and 200 m/min), feed (f = 0.1 and 0.2 mm), depth of cut (ap = 0.5 and 1 mm), as well as the materials manufacturing route in single pressing (SP) and double pressing (DP). The variation in the cutting parameters influenced the components of the machining force. The cutting, feed and passive force were approximately 10%, 10% e 25% higher, respectively, for DP materials. The turning process generated surfaces with values of maximum height (Sz) between 28.3 to 10.8 μm, Kurtosis (Sku) > 3 and Skewness (Ssk) < 0.3. The chip type obtained was segmented and the chip thickness ratio was influenced by the variation of the cutting parameters and the materials manufacturing route.

Resumo em Inglês:

Abstract It was verified that SAE 4130 steel plates crack when laser beam welded at room temperature (RT). To overcome this problem, this work proposes a high temperature (HT) laser welding in order to reduce the residual stresses and create a bainitic structure instead of a martensitic one. A conventional post-welding heat treatment (PWHT) had been used as a comparison for HT. The centerline crack disappeared after a heat treatment of both in-situ (HT) or after inserting in a furnace (PWHT) at 500 °C for 10 minutes. The finite element analyses indicated a residual stress reduction from 163.70 to 3.72 MPa in the fusion zone (FZ) of the welds from RT to HT. The hardness obtained in FZ depends on the thermal cycle induced microstructure of the welds as 400 HV, 340 HV and 250 HV, for martensite (RT), tempered martensite (PWHT) and bainite (HT) micro-constituents. The proposed in-situ high-temperature laser beam welding method proved its usefulness to solve the center crack issue in SAE 4130 joints.

Resumo em Inglês:

Abstract Understanding chemical reactions in bacterial cellulose (BC) films is important to control the insertion of biomolecules that can improve their biomedical properties. Diphenyltetrazole molecules are able to react with tiols under UV irradiation and has been useful for immobilize proteins on different materials. In this work, we show the functionalization of a BC film with diphenyltetrazole and subsequent attempts of conjugation of the bovine serum albumin (BSA) to it. Diphenyltetrazole was synthesized in two steps, and functionalization of it with BC was carried out under ultrasound irradiation. Spectroscopic analysis of the film by UV–Vis and FTIR confirmed the success of the reaction. Attempts to conjugate BSA to the modified cellulose film by photoclick reaction failed under these conditions. Although this methodology is important in the conjugation of biomolecules, we understand that its application in cellulosic materials in heterogeneous phase is disadvantaged due to the half-life of the unstable intermediate and the biomolecule's access to it.

Resumo em Inglês:

Abstract Hypoperitectic Cu − 20 wt.% Sn alloy was solidified under different cooling rates and solidification growths using directional solidification system. The effects of cooling rate and solidification growth on the microsegregation profiles and tertiary dendritic arm spacing (λ3) were experimentally investigated along the casting. A mathematical model known as phase-field was applied to simulate microstructure and microsegregation during solidification in system Cu-Sn liquid. In this paper the applicability of the phase-field model to the solidification problem in a real alloy system was systematically explored. Microsegregation profiles and realistic dendritic structures were obtained using the phase-field model. The results calculated by phase-field model show various solidification features consistent with our experiment. The calculated tertiary dendritic arm spacing (λ3) and microsegregation profiles were compared with experimental values from directional solidification system. Since the calculated microsegregation profiles using the equilibrium partition coefficient (keq) can yielded discrepancies from the experimental results, an effective partition coefficient (kef) as a function of solidification growth, is proposed in phase-field model, showing a good agreement with the experimental data for any case examined.

Resumo em Inglês:

Abstract Semisolid slurry of ZA27 alloy was prepared by Self-inoculation Method (SIM), the effects of isothermal holding parameters on microstructures of rheo-diecasting were researched, and the mechanical properties of ZA27 rheo-diecastings were tested. The results indicate that the dendritic microstructure of ZA27 alloy formed by permanent mold casting can be significantly modified by RDC with SIM, and obtain fine spherical microstructures. After comprehensive consideration, the suitable melt treatment temperature for the ZA27 alloy semisolid processing is 550~560°C. The isothermal holding process of ZA27 alloy slurry has great effect on primary α-Al particles (α1), while has little effect on the microstructure of secondary solidification in the process of thin-walled rheo-diecasting, and the suitable isothermal holding time of semisolid slurry for rheo-diecasting is 3 min. Compared with HPDC, the RDC process can increase the tensile strength of ZA27 alloy by more than 6.9%, with a maximum increase of about 12.5%. The best performance die casting can be obtained when the slurry holding time is 3-5 min.

Resumo em Inglês:

Abstract The solidification of metals and alloys and the resulting microstructures, which as a function of thermal and solutal parameters can evolve as planar, cellular and dendritic, are important from a practical point of view, since they strongly influence the properties and quality of the final product. In many practical situations it is impracticable to develop analytical solutions permitting reliable predictions of microstructural growth during unsteady-state solidification conditions. The Phase Field method has become very popular and effective in modeling complex solid/liquid interfaces due to its ability to simulate the interface kinetics and the formation and evolution of different morphologies along the solidification process. In this work, a numerical analysis of the microstructural evolution during the transient solidification of dilute alloys of the Al-Cu-Si system is developed, which uses a phase-field approach for the simulation of ternary alloys. The phase-field, energy and solute concentration equations were numerically solved for the correspondent ternary system, varying the mesh parameters, temperature and alloy composition. The analysis performed were confronted with existing theoretical models and the results obtained are in agreement with the solidification theory.

Resumo em Inglês:

In the search for new solid fuels that can mitigate emissions of greenhouse gases and reduce municipal solid waste, it is proposed to produce a solid fuel from elephant grass charcoal (EGC) and blend it with the following binding recyclable materials, polyethylene terephthalate (PET) and high-density polyethylene (HDPE) with the aim of increasing their mechanical strength. Immediate analysis results indicate that there was an increase in volatile material content from 21.18% to 28.02% and a reduction in fixed carbon from 65.00% to 58.40% with the addition of binding agents. The higher heating value of pure charcoal was 5924.16 kcal/kg and there was no significant alteration by adding HDPE, however, with the addition of PET, there was an average reduction of 4.82%. According to the elemental analysis of charcoal, there were no significant amounts of sulphur, but silicon and potassium oxides were predominantly composed followed by aluminium, titanium, magnesium and iron according to the analysis of ashes. The addition of thermoplastic binders allowed producing pellets and it was found that those produced by using HDPE are stronger than those produced with PET.

Resumo em Inglês:

Abstract Dental materials should be stable over time. This study investigated volumetric change of the root canal sealers AH Plus, Fill Canal, and Sealapex; and the root-end filling materials Biodentine, IRM, and MTA after different periods of immersion in distilled water. Resin models were manufactured with cavities 3 mm deep. The cavities were filled with the materials and scanned by microcomputed tomography after setting, and after 7 and 30 days immersed in distilled water. Percentages of volumetric changes after 7 and 30 days of immersion were compared by t-test. ANOVA/Tukey tests were used to compare endodontic materials (5% significance level). All root canal sealers had an increase in their volumetric change after 30 days. Among root-end filling materials, only IRM showed difference between the periods. Biodentine showed more volumetric changes than MTA, but both materials kept their volume from 7 to 30 days. In conclusion, the immersion period of endodontic materials may affect their stability.

Resumo em Inglês:

Abstract Squeeze casting is the most industrially advantageous process for producing zamak 5 parts. However, this process generates porosity defects in parts, compromising their corrosion resistance. Therefore, this work aims to minimize defects and improve zamak 5 corrosive performance through the anodizing process. For this purpose, zamak 5 was anodized in 300 mol.m-3 oxalic acid and analyzed by ATR-FTIR, SEM, wettability and EIS techniques. The anodizing process promoted the formation of oxalate and oxide layers of Zn and Al, thereby reducing porosity defects. Although the anodized layer has fragile behavior, being porous and permeable, the immersion time of anodized zamak 5 in NaCl forms corrosion products in the alloy that exhibit better protection.

Resumo em Inglês:

Superaustenitic stainless steel with high nickel percentage, chromium, molybdenum and nitrogen, has replaced austenitic AISI 304 and 316, mainly in the petrochemical industries, where high toughness, stress corrosion resistance and pitting resistance is suitable. Due to high mechanical stress in the cold deformation manufacturing, residual stresses in the pipes are common. In this case, the possibility of the corrosion process increases. Considering the area reduction after tensile tests, the objective of this study was to evaluate the effects of cold deformation on a superaustenitic steel BS EN 10283 No 14587. The steel was elaborated in an electric induction furnace and the liquid metal was poured out in sand molds. Afterward, samples cut from a specimen submitted to the tensile test were prepared for X-ray diffraction, Vickers hardness measurements, linear polarization and impedance electrochemical tests. The corrosion tests were performed in solution with 3.5% NaCl. The results show that there is a linear increase in hardness and also a decrease in corrosion resistance of the material, with increasing cold deformation. In addition, the cold work reduction was not sufficient to promote strain-induced martensite.

Resumo em Inglês:

Abstract Zr-based bulk metallic glasses (BMGs) are widely used as mechanical components due to their excellent properties and high glass-forming ability. However, there is a lack of research concerning the tribological behavior of BMG versus BMG. The sliding characteristics of Vitreloy-105 (Zr52.2Cu17.9Ni14.6Al10Ti5 at.%) were studied in air using a pin-on-disk apparatus equipped for continuous measurement of friction force and without lubrication. The counterface material was the same BMG. The structure, thermal stability, and wear morphology were examined by X-ray diffraction, differential scanning calorimetry, non-contact profilometer, and scanning electron microscopy. For a normal load applied (10.0, 20.4 and 38.2 N) at constant 233 rpm the coefficient of friction ranged from 0.30 to 0.36 at steady state. Average wear rate was more pronounced on the pins under higher load and the predominant wear mechanism observed was adhesive, with presence of peeling-off and micro-cracks accompanied by some grooves and abrasive wear at lower loads.

Resumo em Inglês:

Abstract The solidification control is of extreme importance, because it strongly affects the final casting quality sanity. The structure obtained is generally not homogeneous and gives rise to great variations in composition, with position at small and large scales, which is known as segregation. An understanding of the way segregation occurs in continuous casting is of great importance for steels and in designing post-casting processes. As-cast structures are responsible the reduction in both scale and extension of segregation, because mass transport is dependent on the time required to diffuse a solute over a characteristic distance, e.g., the dendrite spacing that the characterizes the solidification structure. In this work, the effect of pouring temperature in steel slabs on the continuous casting processes are systematically investigated. Relationships between pouring temperature (PT) and center macrosegregation was qualitatively examined. Photomicrographs of specimen taken from transverse sections of steel slabs, shows that macrosegregation is strongly affected by pouring temperature (PT). For solute of carbon, phosphorus and sulfur, has been shown that the pouring temperature (PT) has a significant role on the resulting macrosegregation profiles, while that the elements as such silicon, manganese and aluminum, the said thermal parameter seems not able to affect its macrosegregation profiles. This is due to the fact that the solutes with lower partition coefficients favors segregation during the continuous casting process. It is shown for considered steels, the pouring temperature (PT) influences the position of the columnar to equiaxed transition (CET). Experimental results show that the end of the columnar region is abbreviated when lower pouring temperatures is used in continuous casting process. One can observe that as the pouring temperature (PT) increase in continuous casting process, the secondary dendritic arm spacing (λ2) increase, i.e., the dendritic morphology became coarsen.

Resumo em Inglês:

This work analyzes the effect of Cu additions on the microstructure of quaternary alloys Al-6Si-7Mg-xCu (x= 3, 5 and 7 wt.%) produced by conventional metallurgy. Microstructural modifications were studied using Optical Microscopy (OM), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), focusing on second phases identification and quantification. Conventional image analyses were developed measuring the total percentage of second phases, and individually for each phase determining its equivalent diameter, shape factor, roundness and aspect ratio; while fractal analyses were carried out complementarily through fractal dimension measurements for the whole microstructure and for individual phases. Compression tests were also carried out. SEM and XRD results revealed that second phases were eutectic Si, Mg2Si, Al2Cu and Al5Cu2Mg8Si6 (Q). The increase in Cu content led to higher quantities of these phases, mainly Al2Cu, which was barely present for the alloy with the lowest Cu content. Morphologies of second phases depended on Cu content, being present as eutectic, block like, finely dispersed or primary particles. The combination of conventional and fractal parameters provided a useful tool for comparatively analyze microstructural modifications and second phases features. It was also found that the compressive behavior of the alloys depended on the resulting microstructures.

Resumo em Inglês:

Abstract The aim of this study was to investigate the influence of the cutting parameters on monocrystalline silicon cut by diamond wire sawing. The sawn surface was analyzed in terms of surface morphology, surface roughness, material removal mechanism and residual stress (by Raman spectroscopy). The surface morphology exhibited evidence of both material removal mechanisms: the brittle mode and the ductile mode. The surface roughness increased with a high vf, which promoted the formation of craters on the sawn surface. On applying a higher vc, the surface roughness reduced, since this favored the formation of damage-free grooves. The Raman spectrum showed evidence of different residual crystalline phases on the sawn surface, which confirms the material removal mechanisms. An increase in vf, for the same vc, caused at reduction in the compressive stress, since the brittle mode predominated as the material removal mechanism. Maintaining vf constant and increasing vc results in higher compressive stress, caused by plastic deformation of the silicon during chip formation.

Resumo em Inglês:

Severe plastic deformation (SPD) of metals leads to their strengthening and grain refinement, but to low uniform elongations. Low Strain Amplitude Multi Directional Forging (LSA-MDF) is an important method for increasing this low uniform elongation. The application of workpieces of SPD-processed materials requires that their distributions of mechanical properties, microstructures and deformation be as homogeneous as possible. Analyses of these distributions after LSA-MDF have not been found in the literature, and are presented in the current investigation utilizing microhardness measurements, optical and electronic microscopy and finite element simulations. LSA-MDF causes higher strains, mcrohardeness and structural distortions in the central regions of the specimens than at their edges and corners. In addition, LSA-MDF utilizing confined compressions seems to be the preferred processing route, due to its ease and to the more homogeneous microhardness, microstructure and strain distributions in relation to other experimental procedures.

Resumo em Inglês:

Resonant vibration residual stress relief (R-VSR) is an alternative method to post-weld heat treatments (PWHT) which is said to lead to less distortion and lower costs. The method of superposition of cyclic stresses and residual stresses, which can lead to localized yielding of the material, dislocation movement and subsequent stress relief. This article aims at investigating the efficiency of resonant vibration in the relief of residual stresses in welded joints of HSLA Domex700 steel sheets. Mechanical stress relief was compared to a common PWHT, and stress levels were then quantified through X-ray diffraction. Samples were also characterized by CTOD fracture toughness tests, fractographic analysis and tensile tests. Results indicate that the mechanical method was significantly less effective in relieving stresses in comparison with the PWHT, but led to apparent improvements in fracture toughness and in tensile tests. FWHM values indicated significant distortion for PWHT and less distortion for R-VSR.

Resumo em Inglês:

Multiphasic bioceramic scaffolds has been enhanced for dental and orthopedic applications. In this perspective, the laser surface texturing of metallic surfaces combined to bioactive calcium phosphate coatings have shown to be promising and economically feasible for biomaterial clinical applications. Ti-15Mo alloy samples were irradiated by pulsed Yb: YAG laser beam. The formation of HA and other calcium phosphates phases by biomimetic method should occur in the presence of Ca2+, PO43-, Mg2+, HCO3-, K+ and Na+. The modified surfaces were submitted to thermal treatment at 380 and 580°C. The results showed the processes of fusion and fast solidification from the laser beam irradiation, inducing the formation of stoichiometric α-Ti, TiO2 and non-stoichiometric titanium oxides, Ti3O and Ti6O with different oxide percentages depending on applied fluency (fluency of 0.023, 0.033, 0.040 and 0.048 J/mm2). The morphological and physicochemical properties have indicated the formation of a multiphase bioceramic coatings. It was observed the formation of amorphous calcium phosphate (ACP), octacalcium phosphate (OCP), and magnesium phosphate (Mg3(PO4)2) phases at 380°C, whereas β-TCP (tricalcium phosphate), OCP, and substituted β-TCP with Ca2,589Mg0,41(PO4)2 were obtained at 580°C. Therefore, the multiphasic bioceramic modified Ti-15Mo surface could enhance osteointegration for bone regeneration.

Resumo em Inglês:

A considerable number of engineering materials is polycrystalline. Cahn proposed analytical expressions for transformations nucleated at the grain faces, triple junctions, and quadruple junctions. Those places are usually the places in which a new phase takes place. Cahn assumed that the new grain boundary nucleated phase grew as spheres. Nonetheless, the austenite grain boundary nucleated ferrite does not always grow as spheres. Bradley et al. demonstrated in a series of papers that a grain boundary nucleated ferrite allotriomorph is best described by an oblate ellipsoid. The reason for ellipsoidal growth is that the growth along the grain boundary plane is faster than the thickening into the austenite. Because of this, it is of interest to have analytical solutions and computer simulations of grain boundary nucleated transformations not only for spherical growth but also for ellipsoidal growth. In this work, we present a computer simulation of grain boundary nucleated transformations that grow as ellipsoids. The computer simulation results were compared with a new exact analytical expression obtained by Villa and Rios.

Resumo em Inglês:

In the present work, the microstructure, electrochemical behavior and localized corrosion of the AA2198-T851 Al-Cu-Li alloy were studied. The microstructure was correlated with corrosion results obtained by immersion, gel visualization and scanning electrochemical microscopy (SECM) tests. Immersion and gel visualization tests showed high kinetics of corrosion attack during the first hours of immersion. SECM analyzes by means of surface generation/tip collection (SG/TC) mode detected hydrogen evolution generated during spontaneous corrosion from severe localized corrosion (SLC) sites on the metal surface. SECM results revealed sites of intense hydrogen evolution after 2 h of immersion and increased amounts of corrosion products after 4 h of immersion. Hydrogen evolution sites detected by SECM were associated with severe localized corrosion (SLC).

Resumo em Inglês:

The purpose of this study is to analyze the use plastic deformation (ECAP - Equal Channel Angular Pressing) in the preparation of the 355 aluminum for posterior use in Thixoforming. For this purpose, the 355 alloy, in the as cast state and after a pass of ECAP was heated to temperatures of 575 and 595 °C, enough to produce 45 and 55% of liquid fraction, for times of 0, 30, 60 e 90s. In these conditions, it is possible to obtain a refined globular structure with the size of a grain, close to 75 µm, globule size of 55 µm and high sphericity (around 0.66), parameters which are excellent for this kind of processing. Hot compression tests at the same temperatures and holding times determine the rheological behavior. Maximum processing strain was obtained around 1 MPa for the sample in ECAped condition against 4 MPa of the as cast condition, which corresponds to maximum viscosity around 105 Pa.s, (molten glass, clay), inferior to the as cast condition (11.5 * 105 Pa.s). Therefore, the ECAPed 355 alloy is an easily conformable material very likely to be applied to semisolid processing.

Resumo em Inglês:

The use of mineral fillers in rubber industry has been increasingly focused. Sepiolite is considered as one of the mineral fillers that exploits unique characteristics. This is due to its unique needle-like fibrous and a tunnel-like micro-pore channel structures. Therefore, adding this filler in natural rubber (NR) matrix would bring tremendous advantage. The study was carried out in the form of curing characteristics, tensile properties and morphology together with the dynamic properties. Filling sepiolite to NR has brought to a noticeable enhancement of curing characteristics, tensile properties and morphology. This is contributed to the distinctive structure of sepiolite itself, it provides a better dispersion to the NR. Surface roughness of the composites is visibly found which is responsible for the mechanical properties. Further verification was also monitored through the dynamic properties of the composites. The obtained profiles are in good agreement to the tensile properties and morphological characteristics observed.

Resumo em Inglês:

Abstract Chromite fines (defined as particles smaller than 6 mm) account for a significant fraction of mined South African chromite ore. These fines are pelletized to allow safe furnace smelting conditions. However, pelletization is an energy- and cost-intensive process that requires significant capital investment and has high operational costs as continuous curing temperatures of 1300 – 1500 °C are required. In this study, various cold-bonded pelletization processes were investigated to identify a process to prepare cold-bonded chromite pellets. A feasible process was identified and developed to produce mechanically strong chromite pellets using 3wt% sodium silicate-3 wt% SiO2 as the binder and KOH as an alkaline activator. The binding mechanism is based on the combined effects of sodium silicate and geopolymerization. It was determined that pellets cured at 75 °C had compressive strengths of 2.41 – 3.12 kN. The mechanical strength of these cold-bonded pellets where similar to, or exceed that of pellets currently used in the FeCr industry.

Resumo em Inglês:

Abstract Iron(III) niobate, FeNbO4, was synthesized for the first time by microwave-assisted combustion reaction between Fe(NO3)3.9H2O and NH4[NbO(C2O4)2(H2O)2](H2O) in the presence of NH4NO3 and Urea {O=C(NH2)2}. Experiments of X-ray powder diffraction revealed a small crystallite size for this material, 16 nm, and the results of MEV showed that it forms agglomerates with diameters between 80 and 180 nm with spherical shapes. HRTEM images revealed that the material is formed by clusters of particles with a diameter near 20 nm. A surface area of 40 m2g-1 was determined by experiments of specific surface area (SSA). The FeNbO4 induced the decomposition of H2O2, forming radicals that, in turn, discoloured Indigo dye Carmine (IC) a solution. The results showed that this niobate was able to degrade 55% of the initial solution in the presence of H2O2 after 360 minutes of reaction time.

Resumo em Inglês:

This study analyzed the average temperature value of the metallic droplets and weldpool of the Pulsed GMAW welding process using 309L-Si electrode as the addition metal. The study was based on the formulation and evaluation of pulsed waves, starting from the equation of energy constant (or detachment constant), and waveforms from two commercial welding sources, that were evaluated and reproduced. The technique of filming by high speed infrared radiation was used for evaluation. The results point to a higher droplet temperature for a lower intensity of pulse current, due to longer dwell time at this level, and also revealing that a longer time in the base phase may be responsible for a further heating of the fusion pool.