Efeito da moagem de alta energia na microestrutura e nas propriedades magnéticas do compósito wc-10%p.Co

This work a studied the high energy milling effect in microstructure and magnetic properties of the WC-10wt.%Co composite. The composite powders were prepared by mechanical mixed and milled at 2 hours, 100 hours, 200 hours and 300 hours in planetary milling. After this process the composite were compacted in stainless steel die with cylindrical county of 10 mm of diameter, at pressure 200 Mpa and sintered in a resistive furnace in argon atmosphere at 1400 oC for 5 min. The sintered composite were cutted, inlaid, sandpapered, and polished. The microestrutural parameters of the composite was analyzed by X-ray diffraction, scanning electronic microscopy, optical microscopy, hardness, magnetic propriety and Rietveld method analyze. The results shows, with milling time increase the particle size decrease, it possibility minor temperature of sintering. The increase of milling time caused allotropic transformation in cobalt phase and cold welding between particles. The cold welding caused the formation of the particle composite. The X-ray diffraction pattern of composite powders shows the WC peaks intensity decrease with the milling time increase. The X-ray diffraction pattern of the composite sintered samples shows the other phases. The magnetic measurements detected a significant increase in the coercitive field and a decrease in the saturation magnetization with milling time increase. The increase coercitive field it was also verified with decrease grain size with milling time increase. For the composite powders the increase coercitive field it was verified with particle size reduction and saturation magnetization variation is relate with the variation of free cobalt. The Rietveld method analyze shows at milling time increase the mean crystalline size of WC, and Co-cfc phases in composite sintered sample are higher than in composite powders. The mean crystallite size of Co-hc phase in composite powders is higher than in composite sintered sample. The mean lattice strains of WC, Co-hc and Co-cfc phases in composite powders are higher than in composite sintered samples. The cells parameters of the composite powder decrease at milling time increase this effect came from the particle size reduction at milling time increase. In sintered composite the cells parameters is constant with milling time increase

Caracterização de compósitos Nb-20%cu obtidos por moagem de alta energia e sinterizados por fase líquida

In this work, was studied the formation of a composite of the refractory metal niobium with copper, through the process of high-energy milling and liquid phase sintering. The HEM can be used to synthesize composite powders with high homogeneity and fine size particle distribution. It may also produce the solid solubility in immiscible systems such as Nb-Cu, or extend the solubility of systems with limited solubility. Therefore, in the immiscible system Cu-Nb, the high-energy milling was successfully used to obtain the composite powder particles. Initially, the formation of composite particles during the HEM and the effect of preparation technique on the microstructure of the material was evaluated. Four loads of Nb and Cu powders containing 20%wt Cu were synthesized by MAE in a planetary type ball mill under different periods of grinding. The influence of grinding time on the metal particles is evaluated during the process by the withdrawal of samples at intermediate times of milling. After compaction under different forces, the samples were sintered in a vacuum furnace. The liquid phase sintering of these samples prepared by HEM produced a homogeneous and fine grained. The composite particles forming the sintered samples are the addition of a hard phase (Nb) with a high melting point, and a ductile phase (Cu) with low melting point and high thermal and electrical conductivities. Based on these properties, the Nb-Cu system is a potential material for many applications, such as electrical contacts, welding electrodes, coils for generating high magnetic fields, heat sinks and microwave absorbers, which are coupled to electronic devices. The characterization techniques used in this study, were laser granulometry, used to evaluate the homogeneity and particle size, and the X-ray diffraction, in the phase identification and to analyze the crystalline structure of the powders during milling. The morphology and dispersion of the phases in the composite powder particles, as well the microstructures of the sintered samples, were observed by scanning electron microscopy (SEM). Subsequently, the sintered samples are evaluated for density and densification. And finally, they were characterized by techniques of measuring the electrical conductivity and microhardness, whose properties are analyzed as a function of the parameters for obtaining the composite

Divergência genética de cultivares de arroz quanto à resistência a Tibraca limbativentris Stål (Hemiptera: Pentatomidae)

A divergência genética de 16 cultivares de arroz quanto à resistência ao percevejo-do-colmo-do-arroz, Tibraca limbativentris Stål, foi avaliada por meio de técnicas de análise multivariada. O experimento foi conduzido em casa de vegetação, em delineamento experimental de blocos ao acaso, com oito repetições. Na avaliação foram considerados oito caracteres de resistência ao ataque do inseto. A divergência genética foi avaliada por procedimentos multivariados: distância generalizada de Mahalanobis (D²), método de agrupamento de otimização de Tocher e técnica de variáveis canônicas. As cultivares mais dissimilares foram Bico Ganga e Marabá Branco, enquanto Agulha e Branco Tardão foram as mais similares. Foram formados cinco grupos pelo método de otimização de Tocher. As três primeiras variáveis canônicas explicaram 88,5% da variabilidade total disponível. Conclui-se que as técnicas de análise multivariada são eficientes para a análise da divergência genética entre as cultivares de arroz, com destaque para Marabá Branco e Bico Ganga, consideradas as mais promissoras a serem utilizadas em futuros cruzamentos para melhoramento visando resistência ao percevejo-do-colmo-do-arroz.

Um estudo da aplicação do conceito de risco na conscientização e conhecimento de estudantes de ensino técnico sobre riscos de segurança e saúde ocupacional em laboratório de soldagem

This dissertation presents a discussion on the level of risk awareness of students who have welding practice in the laboratories at CEFET/PI, both in Teresina and Floriano. Its main goal is to investigate risk factors involving students, teachers and employees that work in these laboratories. It s an empirical survey, held among a certain amount of students from the course in Mechanic Technology. For data analysis, the concept of risk was compared to the students direct measure of perception. The main results suggest the students must be better informed, more risk aware and more competent, in order to avoid accidents. They also point to a strong need for a more formal and effective performance to assure full consciousness about the risks involving the welding practice.

Estudo do acoplamento entre superfícies seletivas de frequência assimétricas em estruturas de multicamadas

This work presents the development of new microwaves structures, filters and high gain antenna, through the cascading of frequency selective surfaces, which uses fractals Dürer and Minkowski patches as elements, addition of an element obtained from the combination of the other two simple the cross dipole and the square spiral. Frequency selective surfaces (FSS) includes a large area of Telecommunications and have been widely used due to its low cost, low weight and ability to integrate with others microwaves circuits. They re especially important in several applications, such as airplane, antennas systems, radomes, rockets, missiles, etc. FSS applications in high frequency ranges have been investigated, as well as applications of cascading structures or multi-layer, and active FSS. In this work, we present results for simulated and measured transmission characteristics of cascaded structures (multilayer), aiming to investigate the behavior of the operation in terms of bandwidth, one of the major problems presented by frequency selective surfaces. Comparisons are made with simulated results, obtained using commercial software such as Ansoft DesignerTM v3 and measured results in the laboratory. Finally, some suggestions are presented for future works on this subject

Análise do espalhamento espectral em superfícies de estruturas complexas para comunicações móveis

In this work, the transmission line method is explored on the study of the propagation phenomenon in nonhomogeneous walls with finite thickness. It is evaluated the efficiency and applicability of the method, considering materials like gypsum, wood and brick, found in the composition of the structures of walls in question. The results obtained in this work are compared to those available in the literature, for several particular cases. A good agreement is observed, showing that the performed analysis is accurate and efficient in modeling, for instance, the wave propagation through building walls and integrated circuit layers in mobile communication and radar system applications. Later, simulations of resistive sheets devices such as Salisbury screens and Jaumann absorbers and of transmission lines made of metal-insulator-semiconductor (MIS) are made. Thereafter, it is described a study on frequency surface selective structures (FSS). It is proposed the development of devices and microwave integrated circuits (MIC) of such structures, for the accomplishment of experiments. Finally, future works are suggested, for instance, on the development of reflectarrays, frequency selective surfaces with dissimilar elements, and coupled frequency selective surfaces with elements located on different layers

Desenvolvimento de um sistema para análise de integridade estrutural de equipamentos estáticos baseado em cad vps micro cracks e ultrassom

In general, the designs of equipment takes into account the effects and processes of deterioration it will undergo and arrives at an approximate useful life. However, changes in operational processes and parameters, the action of external agents, the kind of maintenance conducted, the means of monitoring, and natural and accidental occurrences completely modify the desired performance of the equipment. The discontinuities that occur in anisotropic materials often and due to different factors evolve from being subcritical to critical acquiring the status of defect and compromising the physical integrity of the equipment. Increasingly sophisticated technological means of detection, monitoring and assessment of these discontinuities are required to respond ever more rapidly to the requirements of industry. This paper therefore presents a VPS (Virtual Pipe System) computational tool which uses the results of ultrasonic tests on equipment, plotting the discontinuities found in models created in the CAD and CAE systems, and then simulates the behavior of these defects in the structure to give an instantaneous view of the final behavior. This paper also presents an alternative method of conventional ultrasonic testing which correlates the integrity of an overlay (carbon steel and stainless steel attached by welding) and the reflection of ultrasonic waves coming from the interface between the two metals, thus making it possible to identify cracks in the casing and a shift of the overlay

Influência da corrente de pulso, do tempo de pulso e diâmetro de gota sobre a estabilidade da transferência metálica no processo MIG-P

To obtain a process stability and a quality weld bead it is necessary an adequate parameters set: base current and time, pulse current and pulse time, because these influence the mode of metal transfer and the weld quality in the MIG-P, sometimes requiring special sources with synergistic modes with external control for this stability. This work aims to analyze and compare the effects of pulse parameters and droplet size in arc stability in MIG-P, four packets of pulse parameters were analysed: Ip = 160 A, tp = 5.7 ms; Ip = 300 A and tp = 2 ms, Ip = 350 A, tp = 1.2 ms and Ip = 350 A, tp = 0.8 ms. Each was analyzed with three different drop diameters: drop with the same diameter of the wire electrode; droplet diameter larger drop smaller than the diameter of the wire electrode. For purposes of comparison the same was determined relation between the average current and welding speed was determined generating a constant (Im / Vs = K) for all parameters. Welding in flat plate by simple deposition for the MIG-P with a distance beak contact number (DBCP) constant was perfomed subsequently making up welding in flat plate by simple deposition with an inclination of 10 degrees to vary the DBCP, where by assessment on how the MIG-P behaved in such a situation was possible, in addition to evaluating the MIG-P with adaptive control, in order to maintain a constant arc stability. Also high speed recording synchronized with acquiring current x voltage (oscillogram) was executed for better interpretation of the transfer mechanism and better evaluation in regard to the study of the stability of the process. It is concluded that parameters 3 and 4 exhibited greater versatility; diameters drop equal to or slightly less than the diameter of the wire exhibited better stability due to their higher frequency of detachment, and the detachment of the drop base does not harm the maintenance the height of the arc

Strains of Xylella fastidiosa rapidly distinguished by arbitrarily primed-PCR

Genomic DNAs isolated from strains of Xylella fastidiosa that caused citrus variegated chlorosis, coffee leaf scorch, Pierce's Disease of grapevine, and plum leaf scorch were analyzed by arbitrarily primed polymerase chain reaction. Purified DNA was amplified under nonstringent conditions with single primers 21 nucleotides (nt) long. Thirty-nine amplification products were observed that were useful to distinguish among the strains and to derive a similarity matrix and construct a phenogram showing possible relationships among the strains. Strains isolated from diseased coffee and citrus in Brazil were closely related to each other (coefficient of similarity of 0.872), but only distantly related to a strain isolated from diseased grapevine in the USA (coefficient of similarity of 0.650). Strains of Xylella fastidiosa isolated from diseased plums in the USA and Brazil clustered with strains from different hosts isolated from their respective countries of origin. Thus, there may be two quite dissimilar clusters of strains of Xylella fastidiosa, one in North America and the other in South America. Each cluster contains strains that can cause disease in plum. The methods described provide a convenient and rapid method to distinguish between strains of Xylella fastidiosa that cause diseases of coffee and citrus in the same region of Brazil. This has not been possible previously. This will potentially enable the two strains to be distinguished in alternate hosts or in insect vectors.

Análises estatísticas em redes complexas: propriedades topológicas, críticas e dinâmicas

In this thesis, we address two issues of broad conceptual and practical relevance in the study of complex networks. The first is associated with the topological characterization of networks while the second relates to dynamical processes that occur on top of them. Regarding the first line of study, we initially designed a model for networks growth where preferential attachment includes: (i) connectivity and (ii) homophily (links between sites with similar characteristics are more likely). From this, we observe that the competition between these two aspects leads to a heterogeneous pattern of connections with the topological properties of the network showing quite interesting results. In particular, we emphasize that there is a region where the characteristics of sites play an important role not only for the rate at which they get links, but also for the number of connections which occur between sites with similar and dissimilar characteristics. Finally, we investigate the spread of epidemics on the network topology developed, whereas its dissemination follows the rules of the contact process. Using Monte Carlo simulations, we show that the competition between states (infected/healthy) sites, induces a transition between an active phase (presence of sick) and an inactive (no sick). In this context, we estimate the critical point of the transition phase through the cumulant Binder and ratio between moments of the order parameter. Then, using finite size scaling analysis, we determine the critical exponents associated with this transition

Influência do tipo de chanfro, tecimento e sentido de laminação na distorção angular em soldagem GMAW-P robotizada de alumínio

The industry's interest in having a greater control of the deformations caused by welding is due to the geometric and dimensional tolerances been more and more precise in the project specifications, motivating the manufacturing engineering to develop stable processes and to ensure routine production. Aiming at it, the main goal of this present work is to analyze how much routine situations used in automatic aluminum welding can influence on the angular deformations of this material. Using the alloy AA 5052 H34, and the automatic welding in pulsed GMAW process, three types of weaving were applied throughout the length of the weld, in butt joints assembled without groove and with 60 degrees single-V-groove, arranged transversely as well as longitudinally to the rolling direction of the plate. The measurement of the deformations was made by a three-dimensional equipment, before and after the welding, in three distinct regions in the specimens. The profile of the weld bead was the main factor for the different types of deformations found, as revealed by macrographical analysis. The 60 degrees single-V-groove had higher amplitudes of deformations as the joint without groove. The torch oscillation wasn't a variable of statistically significant influence on this amplitudes.

Experience gained and difficulties in performing dynamic load test in composite piles made with steel rails

The results of dynamic instrumentation in two construction sites that used steel rails as pile foundation are presented in this paper. The first dynamic load tests were executed in piles made by single steel rails of the type TR32 and TR37. In the second group of dynamic tests, the piles were made by a composition of two and three steel rails of the same type TR37. The difficulties in placing the sensors, the effect of hammer blow eccentricity and the influence of the non-uniform welding along the pile length are presented in detail and discussed.

Desenvolvimento da Tecnologia de Soldagem do Titânio pelo Processo TIG Orbital no Segmento Aeroespacial Brasileiro - Parte 2: Caracterização das Juntas Soldadas

In this work, three welding programs for orbital TIG previously developed were used, using pulsed current and increasing speed (M), constant current (#B) and pulsed current and decreasing current (#C). One of those should be used for the propulsion system of the satellite CBERS (CHINA-BRAZIL EARTH RESOURCES SATELLITE). Welded joints using tubes of commercially pure titanium were obtained with these procedures, which were characterized by means of mechanical and metallographic tests. The obtained results show that the three welding procedures produce welded joints free of defects and with adequate shape. Although small differences on mechanical properties and microstructure have been observed, the three welding programs attained compatible results with international standards used in the aerospace segment. The welding program #B, due the reduced heat input used, was considered to obtain slightly advantage over the others.

Considerations on corrosion and weld repair effects on the fatigue strength of a steel structure critical to the flight-safety

The aim of this study is to analyze the effects of corrosion and successive tungsten inert gas (TIC) welding repairs on the reverse bending fatigue strength of AISI 4130 steel used in components critical to the flight-safety. The tests were performed on hot-rolled steel plate specimens, 1.10 mm and 1.60 mm thick, by means of a SCHENK PWS equipment, with load ratio R = -1, constant amplitude, 30 Hz frequency and room temperature. It was observed that the reverse bending fatigue strength of AISI 4130 steel decreases due to the corrosion and the TIC welding and re-welding processes. (C) 2010 Elsevier Ltd. All rights reserved.

Effects of several TIG weld repairs on the axial fatigue strength of AISI 4130 aeronautical steel-welded joints

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)