Abstract Overlay welding with stainless steels has gained attention in several sectors of industry by increasing the mechanical and electrochemical properties of surfaces, in order to obtain them using lower cost substrates. However, the control of welding parameters can become complex due to the multiple control variables, being of interest to obtain deposits with greater productivity. In this context, the objective of this paper is to analyze the effect of the heat input on weld overlays of austenitic stainless steel 316L-Si obtained by the GMAW process, to determine the best welding conditions. For this, a Design of Experiments was implemented to analyze the response variables, which include heat input, reinforcement, width, dilution and hardness of the weld metal. The models were considered significant, presenting R2>0.9 for all responses, based on a 95% confidence limit. Surface graphs indicated influence of welding speed or/and voltage for all responses, in addition to the results demonstrating optimal parameters for overlays such as high ratio between reinforcement and width (0.631), and low dilution values (8.81%). The microstructural analysis indicated the presence of residual γ-austenite and δ-ferrite in all samples, and also the formation of peninsulas and islands, added to defects such as voids, for some weld deposits.

RESUMO Neste trabalho utilizou-se uma liga hipereutética Al-40wt%Cu para verificar a influência das variáveis de entrada taxa de resfriamento (°C/min) e superaquecimento (°C), na variável de saída microdureza (HV). As amostras foram vazadas em cadinhos de cerâmica e de cobre e um termopar acoplado à um sistema de aquisição de dados mediu as taxas de resfriamento que foram 20°C/min e 24°C/min, respectivamente. Avaliou-se a influência dessas taxas de resfriamento e do superaquecimento de 100°C e 200°C acima da temperatura de fusão da liga na microdureza. As medições de dureza Vickers foram realizadas na seção transversal da amostra com área de 0,8 cm2. As amostras foram caracterizadas microscopicamente para avaliar a formação microestrutural. O potencial das variáveis envolvidas no estudo foi avaliado por um planejamento fatorial completo 22. Os resultados do planejamento fatorial indicaram que as amostras solidificadas usando o superaquecimento de 200°C acima da temperatura de fusão e resfriadas a uma taxa de 24°C/min obteve maiores valores de microdureza. As ligas superaquecidas em maiores temperaturas apresentaram microestruturas mais refinadas com grãos alongados, direções de crescimento desordenadas e espaçamento dendrítico curto, provocando uma transição na microestrutura colunar para equiaxial.

ABSTRACT In this work a hypereutectic Al-40wt%Cu alloy was used to verify the influence of the input variables cooling rate (°C/min) and superheating (°C), on the output variable microhardness (HV). The samples were poured in ceramic and copper crucible and a thermocouple coupled to a data acquisition system measured the cooling rates that were 20°C/min and 24°C/min, respectively. The influence of these cooling rates and overheating of 100°C and 200°C above the melting temperature of the alloy on microhardness was evaluated. Vickers hardness measurements were performed on the sample cross section with an area of 0.8 cm2. The samples were characterized microscopically to evaluate the microstructural formation. The potential of the variables involved in the study was evaluated by a 22 full factorial design. The results of the factorial design indicated that the samples solidified using overheating to 200°C above the melting temperature and cooled at a rate of 24°C/min obtained higher microhardness values. The alloys superheated to higher temperatures showed more refined microstructures with elongated grains, disordered growth directions and short dendritic spacing, causing a transition in columnar to equiaxial microstructure.

RESUMO Um grande desafio no projeto de microssistemas é a implementação de princípios eficientes de atuação miniaturizados em microescala. Ligas com efeito memória de forma (EMF) têm sido consideradas como uma solução potencial para este problema, uma vez que estes materiais oferecem propriedades atraentes como grande deformação e capacidade para ser processada em microescala. Ligas com composição Ni-44, 8wt% Ti e Ni-45, 3wt% Ti foram produzidas usando o método melt spinning em atmosfera argônio. Fitas obtidas neste processo mostraram tamanho médio de grãos martensíticos entre 5 e 30 µm, dependendo do tipo de liga e da velocidade linear da roda do melting spinning. As temperaturas de início de transformação martensítica (MS) assim como o tamanho de grãos foram reduzidos com o aumento de velocidade linear da roda e da quantidade de titânio.

ABSTRACT One important challenge of microsystems design is the implementation of efficient principles of miniaturized actuation at the micro-scale. Shape memory alloys (SMAs) have early been considered as a potential solution to this problem as these materials offer attractive properties like a high-power to weight ratio, large deformation and the capability to be micro-scale processed. Alloys of composition Ni-44,8wt%Ti and Ni-45,3wt%Ti were produced using the melt spinning method in air atmosphere. Ribbons obtained in this process showed martensitic grain size between 5 and 30 µm, depending on the alloy composition and the linear velocity of the wheel.

Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.