RESUMO Neste trabalho estão mostrados o projeto, os principais componentes e os resultados obtidos com o protótipo de um torcímetro digital que foi construído para medir a variação do ângulo de torção em ensaios mecânicos de materiais. Este dispositivo foi construído utilizando um encoder magnético, modelo AS5048A, com 14 bits de resolução e para o processamento dos dados foi utilizada uma placa microcontroladora Arduino Mega 2560. Além do baixo custo dos componentes eletrônicos e mecânicos, outra vantagem do sistema de medição desenvolvido é a grande quantidade de aplicativos gratuitos disponíveis para o processamento e aquisição dos dados com a placa microcontroladora utilizada. Foram realizados testes para verificar a exatidão nas leituras da variação angular e ensaios de torção com corpos de prova tubulares de alumínio (liga AA6351T6) para avaliar o desempenho do sistema de medição em ensaios monotônicos. A relação de Ramberg-Osgood foi utilizada para descrever o comportamento do material e as tensões de cisalhamento foram avaliadas empregando relações não lineares propostas por diferentes autores. Os dados obtidos indicam que o torcímetro digital apresentou resultados satisfatórios para a obtenção do ângulo de torção e as principais vantagens deste dispositivo incluem a medição do ângulo de torção sem contato, linearidade, boa acurácia, faixa de medição ilimitada, tamanho reduzido e baixo custo para confecção.

ABSTRACT In this work, the project, the main components and the results obtained with the prototype of a digital torsiometer that was built to measure the variation of the torsion angle in mechanical tests of materials are shown. A magnetic encoder model AS5048A was used in this device. This encoder has 14 bits of resolution and an Arduino Mega 2560 microcontroller board was used for data processing. In addition to the low cost of electronic and mechanical components, another advantage of the measurement system developed is the large amount of free applications available for processing and acquiring data with the microcontroller board used. Tests were carried out to verify the accuracy in the angular variation readings and torsion tests with aluminum tubular specimens (AA6351T6 alloy) to evaluate the performance of the measurement system in monotonic tests. The Ramberg-Osgood relationship was used to describe the behavior of the material and the shear stresses were evaluated using nonlinear relationships proposed by different authors. The results obtained indicate that the digital torsiometer presented satisfactory results for obtaining the torsion angle and the main advantages of this device include the measurement of the non-contact torsion angle, linearity, good accuracy over the entire measurement range, unlimited measurement range, reduced size and low cost for confection.

Abstract The exact calculations of the stress and strain distributions based on the controlling equations for a forming process with large deformation are often difficult. To circumvent such difficulties, some analytical methods such upper-bound analysis and slip-line field theory have been established by making a number of simplifying assumptions regarding the material properties and deformation modes. In this work an analytical model based on the upper-bound theory was successfully developed to predict material flow pattern and maximum process loads for an Equal Channel Angular Pressing (ECAP) die with circular cross-section and an intersecting channel angle of 120°. Based on the model, the power dissipated on all frictional and velocity discontinuity surfaces were determined and optimized in order predict the maximum process force as function of the channel geometry and the material plastic behavior. To validate the developed model, the ECAP die were produced and used to determine experimental load-displacement curves of AA6061-T6 specimens with different lengths. A good correlation between theoretical and experimental results was observed. In addition, the constant friction factor demonstrated to have a strong effect on the relative extrusion pressure.

Abstract: The GMAW torch orientation, whether pulling or pushing, influences both arc welding and hybrid processes. In hybrid laser-arc welding, for example, when the torch is pulled, a greater bead penetration is obtained. To promote greater penetration, the literature also indicates the use of a buried arc in GMAW, although it was initially developed to only operate with a vertically-positioned torch. Therefore, this work aims to investigate the influence of the push and pull techniques on the behavior of buried-arc GMAW at high welding speeds. Welds were performed with the push and pull techniques under the following conditions: buried and unburied (long) arc with welding speeds of 1.0, 1.5 and 2.0 m/min and current ranging from 450 to 470 A. The process tends to be more stable when pulling than when pushing (buried or long arc). Evidence of instability was only identified for the pushed buried arc, due to material accumulation at the front region of the molten pool, for the higher welding speeds. Only the 1.0 m/min buried-arc processes resulted in beads with an appropriate surface finish.

In this study, a tailored die quenching experimental method using low closing pressures and an aluminum flat die was developed in order to evaluate the effects of the gap size on the cooling rate in the hot stamping process of a 22MnB5 steel grade. Initially, the necking behavior in hot stamping was investigated by deforming the specimens to different strain levels and subjecting them to quenching in order to assess this effect on the gap profile formed between the die surface and the sample. Next, a controlled gap was promoted in the central region of the aluminum cooler using aluminum spacers (simulating the gap) with different thickness (0.06/0.15/0.40/0.70mm). Then, its effects on the cooling rate and on the hardness after quenching were evaluated. The results showed that the cooling rate is very susceptible to gap size, that the ideal situation occurred when there was no gap, good heat transfer and higher average cooling rates of 120ºC/s were achieved, and that when the gap was wider than 0.20mm the average cooling rates were lower than 20ºC/s, consequently below the critical 22MnB5 steel cooling for quenching, which is 28ºC/s.

RESUMO Neste trabalho, foram investigados os efeitos dos tratamentos térmicos de solubilização e envelhecimento artificial (T6) sobre a microestrutura da liga 356 (Al7Si0,3Mg). Para a caracterização dos resultados, foram utilizados ensaios de microdureza e técnicas de microscopia ótica e eletrônica. Amostras fundidas da liga A356 com diferentes percentuais em peso de Mg (0,28%, 0,38% e 1%) foram solubilizadas durante 10h a 540°C e submetidas a diferentes tempos e temperaturas de envelhecimento artificial. Os resultados mostraram que a dureza aumenta com o aumento de temperatura de envelhecimento, mas apresenta uma queda brusca em amostras expostas a temperaturas mais elevadas (215°C). Quanto ao tempo de envelhecimento, foram observados incrementos nos valores de dureza para o uso de maiores tempos. Com o intuito de identificar as alterações microestruturais associadas com estas variações nas propriedades mecânicas, bem como identificar os mecanismos envolvidos, foram realizadas análises via microscopia óptica, microscopia eletrônica de varredura e de transmissão. Constatou-se, em função das adições crescentes de magnésio, alterações significativas na microestrutura e no endurecimento da matriz da liga A356. No entanto, os fenômenos microestruturais associados ao endurecimento por precipitação somente foram detectados usando microscopia eletrônica de transmissão, que revelou a presença dos nano precipitados responsáveis pelo endurecimento por precipitação bem como dos precipitados incoerentes com a matriz que indicam a ocorrência de superenvelhecimento.

ABSTRACT The present study investigated the effects of solution heat treatment and artificial aging (T6) on the microstructure of the A356 (Al7Si0.3Mg) aluminum alloy. To characterize the results, micro hardness test and techniques of light and electron microscopy were used. Samples of the A356 alloy casting, with distinct weight percent of Mg (0.28, 0.38 and 1%Mg), were solubilized for 10h at 5400 C and subjected to different temperatures and times of artificial aging. The results showed that the hardness rises with increasing aging temperature, but exhibits a sharp drop in samples exposed to higher temperatures (2150 C). Regarding aging time, increments were observed in the hardness values for the use of higher aging times. In order to identify microstructural changes associated with modifications in mechanical properties and to identify the mechanisms involved, analyzes were performed by optical microscopy, scanning and transmission electron microscopy. Significant changes in the microstructure and the hardening of the alloy matrix were found, as a function of increasing additions of magnesium. However, the microstructural phenomena associated with precipitation hardening were only detected by using transmission electron microscopy, which showed the presence of nano precipitates responsible for precipitation hardening as well as precipitates incoherent with the matrix, thus indicating the occurrence of overaging.