RESUMO O polimento e desbaste de rochas ornamentais ou revestimentos cerâmicos como o porcelanato, tem por finalidade promover, por meio de ferramentas abrasivas, melhor acabamento superficial nestes produtos. Neste contexto, verifica-se o uso de matrizes magnesianas para produção de compósitos à base de carbeto de silício (SiC) que podem ser usados pela indústria em rebolos ou coroas abrasivas. Outra opção, são as matrizes poliméricas que fazem uso do diamante sintético como abrasivo. Neste sentido, apesar das matrizes magnesianas serem de baixo custo comparativamente às matrizes poliméricas, observa-se a necessidade de aprimoramento desta tecnologia por meio do uso de adições, como a sílica da casca do arroz (SCA) estudada neste trabalho, em vista de melhorias no polimento e maior durabilidade destes compósitos. No Brasil, o emprego desta sílica pode representar uma alternativa de menor custo e, assim, colaborar no desenvolvimento de matrizes abrasivas à base de óxido de magnésio. Nesta pesquisa foi resgatada a tecnologia do cimento magnesiano e buscou-se implementar a adição da SCA para atendimento não somente dos requisitos técnicos para sua aplicação em rebolos abrasivos, mas, também, colaborar em soluções mais sustentáveis pelo emprego de uma sílica proveniente de fontes renováveis. Dessa forma, as matrizes desenvolvidas foram analisadas aos esforços de compressão, tração na flexão, dureza Mohs e estabilidade dimensional, para verificação de seu desempenho mecânico e durabilidade. Adicionalmente, análises de Difratometria de Raios-X (DRX) e Microscopia Eletrônica de Varredura (MEV) complementaram o estudo para identificação das fases formadas e verificação da viabilidade de aplicação destes compostos na produção de matrizes para rebolos abrasivos.

ABSTRACT Magnesia cements are frequently used for the production of abrasive composites based on silicon carbide (SiC) to polishing ornamental rocks or porcelain stoneware tiles, but, currently, it has lost market share for polymer matrices that use synthetic diamond as abrasive. In this sense, despite the low cost of the magnesian matrices, there is a need to improve this technology through of materials, such as the addition of the rice husk ash (RHA) showed in this study. The addition of silica in the production of abrasive materials is an alternative for improving the polishing effect and durability of these composites. In Brazil, the production of amorphous silica from the controlled burning of rice husks may represent a lower cost alternative and, thus, collaborate in the development of abrasive matrices based on magnesium oxide. In this research, magnesia cement technology was recovered together the addition of (RHA) to meet not only the technical requirements for its application in abrasive tools, but also to collaborate in sustainable solutions by using a silica from renewable sources. Thus, the developed matrices were analyzed for compressive and flexural tests, Mohs hardness and dimensional stability, to verify their mechanical performance and durability. Additionally, X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM) analyzes complemented the study to identify the formed phases and verify the feasibility of applying these compounds in the production of abrasive materials.

The prediction of microstructure morphology is fundamental for the manufacture of metallic components, since the expected levels of mechanical properties will be associated with the final aspects of the microstructure. In this work, an Al-7wt.%Si-3wt.%Mg alloy was directionally solidified in unsteady state conditions in order to investigate the influence of the addition of 3wt.%Mg to an Al-7wt.%Si alloy on the solidification evolution. The microstructure of the examined alloy is shown to be characterized by a more complex arrangement of phases, as compared to that of the Al-7wt.%Si alloy, which includes the binary (α-Al+Mg2Si) and refined ternary (α-Al+Si+Mg2Si+Fe-rich IMC) eutectic mixtures. A higher Vickers hardness profile is shown to be associated with a more refined microstructural arrangement. However, for cooling rates lower than 2K/s the microhardness is shown to increase with the increase in the microstructural spacing, which is shown to occur caused by Si macrosegregation and higher content of free Mg.

This study aims to analyze the influence of macrosegregation on microstructure evolution, and of microstructure length scale on the corrosion resistance of a Zn-5.0wt.%Mg alloy casting. The analyzed samples were taken along the length of castings unidirectionally solidified in unsteady state conditions of heat extraction. Microstructure characterization, microhardness, linear polarization and electrochemical impedance spectroscopy (EIS) tests were performed. A unique type of microstructure was observed, characterized essentially by a morphology typified by idiomorphic MgZn2 crystals in a “complex eutectic mixture” [coexistence of stable (Zn+Mg2Zn11) and metastable (Zn+MgZn2) eutectics]. The correlation between thermal and microstructural parameters, permitted experimental growth laws, correlating the evolution of the lamellar eutectic spacing with the cooling rate to be established. Vickers microhardness and electrochemical corrosion tests showed that more refined microstructures associated with higher experimental cooling rates, are related to a better set of higher hardness and corrosion resistance.

The present research work examines the microstructural arrangements formed during the transient solidification of eutectic Sn-0.2wt.%Ni and hypereutectic Sn-0.5wt.%Ni alloys. Also, it examines their respective correlations with solidification thermal parameters: eutectic growth rate (VE) and eutectic cooling rate (ṪE); length scales of matrix and eutectic phases: microstructural spacings and the corresponding tensile properties: ductility and strength. Both alloys were directionally solidified upwards under unsteady-state regime, and characterized by optical and scanning electron microscopy. Concerning the hypereutectic Sn-0.5wt.%Ni, the increase in Ni content is shown to influence both thermal behavior and cellular spacing (λC). The NiSn4 intermetallics is present in the eutectic mixture of both alloys, whilst in the Sn-0.5wt.%Ni alloy the primary phase has been identified by SEM-EDS as the Ni3Sn4 intermetallics. A β-Sn morphological cellular/dendritic transition occurs in the 0.2wt.%Ni eutectic alloy for ṪE> 1.2K/s. Despite that, regular cells in the hypereutectic alloy (0.5wt.%Ni) turns into plate-like cells for ṪE> 1.4K/s. If considered a reference cellular spacing about 20μm (i.e.,λ(c-1/2=0.22), the samples associated with the Sn-0.5wt.%Ni alloy are shown to be associated with higher tensile strengths, but much lower ductility as compared with the corresponding results of the eutectic alloy.

RESUMO Diversas aplicações industriais, tais como nos setores automotivo e aeronáutico, que necessitam de resistência e tenacidade, utilizam ligas do sistema Al-Cu. As ligas do sistema ternário Al-Cu-Ag são empregadas como modelo para o estudo de ligas eutéticas ternárias, além do uso em certo número de aplicações técnicas. Estudos relacionados aos efeitos da adição de Ag em ligas do sistema Al-Cu solidificadas em regime transitório, sob aspectos microestruturais e de microdureza, não são encontrados na literatura. Nesse sentido, o objetivo principal deste trabalho consiste na investigação dos efeitos da adição de 4%Ag na liga Al-4%Cu solidificada unidirecionalmente no sentido vertical ascendente em regime transitório, abordando análises de microestrutura, microdureza e parâmetros de solidificação dados pela velocidade de crescimento (VL) e pela taxa de resfriamento (Ṫ). São apresentadas leis experimentais para as ligas Al-4%Cu e Al-4%Cu-4%Ag que correlacionam os espaçamentos dendríticos primário (λ1) e secundário (λ2) com VL e Ṫ. Foram realizadas análises de Difração de Raios-X (DRX) para a caraterização das fases cristalinas dos compostos presentes na microestrutura das ligas. O difratograma da liga binária mostrou a presença dos intermetálicos θ-Al2Cu, Al-Cu, Al4Cu9 e Al6Fe e, para o caso da liga ternária, adicionalmente foi detectado o intermetálico µ-Ag3Al. Composições químicas de regiões do contorno da dendrita, assim como de seu interior, foram obtidas com o auxílio de um microscópio eletrônico de varredura (MEV) acoplado a um sistema de análise por energia dispersiva (EDS). Diversas posições ao longo do lingote foram analisadas através da técnica de Fluorescência de Raios-X (FRX) com objetivo de se detectar a existência de macrossegregação. Constatou-se que a microdureza não é influenciada pelas duas formas de espaçamentos da matriz rica em Al. Os intermetálicos ρ-Ag2Al e µ-Ag3Al são responsáveis pelo aumento de dureza devido a sua interação com o espaçamento dendrítico terciário e outros intermetálicos.

ABSTRACT Al-Cu alloys are used in several industrial applications, such as automotive and aerospace manufacturing, which demand mechanical strength and toughness performances. The Al-Cu-Ag ternary alloy system has been investigated to obtain a better understanding on ternary eutectic alloys besides its inherent applications. However, studies concerning the effects on microstructure and hardness due to Ag addition on Al-Cu alloys solidified under transient conditions, are not found in literature. In this sense, the main objective of this work consists in investigating the solidification parameters such as growth (VL) and cooling (Ṫ) rate, microstructure and microhardness regarding the effects of adding 4wt% Ag on a Al-4wt%Cu alloy unidirectionally solidified vertically upwards under unsteady-state solidification conditions. Experimental growth laws correlating the primary (λ1) and secondary (λ2) dendritic spacings with growth and cooling rate are proposed for Al-4wt%Cu and Al-4wt%Cu-4wt%Ag alloys. X-Ray Diffraction (XRD) analyses were performed in order to determine the crystalline phases of the compounds present in the alloys microstructure. The diffractogram of the binary alloy showed the presence of θ-Al2Cu, Al-Cu, Al4Cu9 and Al6Fe intermetallics and, for the case of the ternary alloy, the µ-Ag3Al intermetallic was detected. Chemical compositions at the boundary regions of the dendrite as also inside the dendrite were obtained using scanning electron microscope (SEM) connected to an Energy-Dispersive Spectrometer (EDS) system. Several positions along the ingot were analyzed through the X-Ray Fluorescence (XRF) technique permitting the detection of macrosegregation occurrence. It is shown that the microhardness is not influenced by both spacing patterns of the Al-rich matrix. The ρ-Ag2Al and µ-Ag3Al intermetallics are responsible for the increase in hardness due to the interaction with ternary dendrite arm spacing and other intermetallics.

In this work, the simulation of a filling process for a mold casting was accomplished by the finite elements method. In this study, a Cu-5%Zn alloy solidified in a sand mold was used for numerical modeling. The mold filling process was simulated in atmospheric conditions under gravity and the liquid metal was poured into the mold through a mold channel. The analysis was conducted in the regions of the mold channel, in the mold and at the exit area of the mold, concerning velocity, pressure, temperature, solid fraction and the turbulent kinetic energy of the fluid. The advantage of simulations is to analyze several phenomena present in the mold filling process which are not trivial to be experimentally visualized. The simulation is a powerful tool that permits the analysis and control different parameters, in order to achieve an optimized design to attend the required quality of the cast product.

Neste trabalho foi realizada a simulação numérica do processo de enchimento de um molde pelo método de elementos finitos. Para esta finalidade foi usada uma liga Cu-5%Zn solidificada em um molde de areia. A simulação foi realizada em condições atmosféricas, sob a influência da gravidade e o enchimento do molde pelo metal líquido foi realizado através do canal de alimentação. A análise do resultado do campo de velocidade, pressão, temperatura, fração sólida e energia cinética de turbulência do fluido foi realizada nas regiões do canal de alimentação, no molde e na área externa do molde. A simulação numérica apresenta a vantagem de analisar diversos fenômenos presentes no processo de enchimento o que não é trivial pela visualização experimental. A simulação é uma poderosa ferramenta que permite analisar e controlar os diversos parâmetros deste processo com a finalidade de alcançar um projeto otimizado que atenda a qualidade desejada do produto fundido.

The aim of this work is to develop a mathematical model to predict the depth of laser treated zone in the LTH process. The Fourier equation of heat conduction is solved by using the Finite Difference Method in cylindrical coordinates in order to study the temperature distribution produced in a workpiece and hence to obtain the depth to which hardening occurs. The theoretical simulations are compared with results produced experimentally by a CO2 laser operating in continuous wave, showing good agreement.