In this work, the effect of the growth rate (V L ) and cooling rate (T R ), primary dendritic arm spacing ( 1 ) and Al 2 Cu intermetallic phase on the microhardness was investigated during transient horizontal directional solidification of Al-3wt%Cu and Al-8wt%Cu alloys. Microstructural characterization of the investigated alloys was performed using traditional techniques of metallography, optical and SEM microscopy and X-Ray diffraction. The microhardness evolution as a function of the thermal and microstructural parameters (V L , T R , and  1 ) was evaluated using power and Hall-Petch type experimental laws, which were compared with other laws in the literature. In order to examine the effect of the Al 2 Cu intermetallic phase, microhardness measurements were performed in interdendritic regions. Finally, a comparative analysis was performed between the experimental data of this work and theoretical models from the literature that have been proposed to predict primary dendrite arm spacing, which have been tested in numerous works considering upward directional solidification.

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Resumo IntroduçãoPela técnica "Switch-back", a tocha do processo MIG/ MAG é conduzida por um robô a oscilar na direção longitudinal do cordão. Ela realiza várias reversões no sentido contrário ao da soldagem, como apresentado na Figura 1. Inicialmente, a tocha é deslocada para frente (avanço) em um determinado comprimento e, em seguida, deslocada para trás (reversão) em um comprimento cerca da metade do anterior. Estes dois movimentos da tocha se repetem durante a soldagem e podem ser realizados com velocidades iguais ou diferentes.Pesquisas desenvolvidas recentemente com esta técnica no LAPROSOLDA (Centro para Pesquisa e Desenvolvimentos de Processos de Soldagem da Universidade Federal de Uberlândia) permitiram mostrar que ela pode promover o aumento da velocidade limite de soldagem. Aplicada ao processo MIG/ MAG Pulsado no modo automático (tocha adaptada em robô), permitiu um aumento da velocidade de soldagem em até 60%, quando usado juntas sobrepostas com chapas de aço de 3 mm de espessura, aumentando ainda mais a produtividade do processo; sem a utilização da técnica, a velocidade de soldagem máxima alcançada para a realização dos cordões com aparência e penetração adequados foi de 35 cm/min, enquanto que com a técnica a velocidade de soldagem máxima alcançada foi de 57 cm/mim [1].Outros trabalhos [2,3,4] investigaram o potencial desta técnica no controle da raiz da solda na soldagem MIG/MAG em corrente pulsada de juntas de topo sem a utilização de cobrejunta. Kaneko et al [2], na união de chapas de aço de 3,2 mm de espessura posicionadas em topo, verificaram que o uso da técnica permitiu o controle da raiz da solda em relação a soldagem MIG convencional. Este controle cresceu quando utilizou-se maior nível de velocidade de soldagem durante o avanço da tocha em relação a reversão. Yamane et al [3,4] utilizaram com sucesso a combinação da técnica Switch-back com o tecimento transversal ao cordão de solda na soldagem MIG Pulsado de juntas de topo

Este é um artigo publicado em acesso aberto (Open Access) sob a licença Creative Commons Attribution Non-Commercial, que permite uso, distribuição e reprodução em qualquer meio, sem restrições desde que sem fins comerciais e que o trabalho original seja corretamente citado.Recebido: 28 Maio, 2016 Aceito: 21 Nov., 2016 E-mail: cmota@ufpa.br (CAMM)Resumo: Este artigo apresenta um estudo das características operacionais, geométricas e microestruturais de soldas MIG e MIGAF (MIG com arame frio) aplicadas no revestimento de chapa de um aço AISI 1020, com uma superliga de níquel ER NiCrMo-4, tipo Hastelloy 276C. A soldagem foi automatizada, na posição plana, e realizada por uma fonte eletrônica em CC + com aquisição instantânea dos oscilogramas de corrente e tensão de soldagem. As variáveis de entrada foram às velocidades do arame eletrodo e do arame frio, e a velocidade de soldagem. Foram avaliadas a operacionalidade dos processos e o efeito da energia de soldagem sobre as características geométricas (reforço e largura), diluição, microestrutura e microdureza do revestimento. Os resultados indicaram um bom desempenho operacional, a ausência de defeitos nos passes isolados e nos revestimentos para as soldagens com MIGAF. Além disso, constataram-se menores níveis de diluição.Palavras-chave: Superligas de níquel; Hastelloy 276C; Soldagem dissimilar; Revestimento. Coating Welds with a Nickel Superalloy Deposited by GMAW and GMAW Cold WireAbstract: This paper presents a study about the operational, geometrical and microstructural characteristics of GMAW and GMAW-CW (GMAW Cold Wire) welds applied on a coating of steel AISI 1020 plate, with a nickel superalloy ER NiCrMo-4, Hastelloy 276C type. The welding was automated in flat position, and executed with an electronic source on DC+ and having instantaneous acquisition of the welding voltage and current oscillograms. The input variables were the speeds of the electrode and cold wires and the welding speed. It was evaluated the operational processes and the effect of heat input on the geometrical characteristics (reinforcement and width), dilution, microstructure and microhardness of the coating. The results indicated a good operating performance, the absence of defects in single passes and in the GMAW-CW coatings. Furthermore, it has been found the lowest level of dilution. Key-words:Nickel superalloys; Hastelloy 276C; Dissimilar welding; Coating welding. IntroduçãoA degradação corrosiva em superfícies de equipamentos é um dos principais problemas das indústrias de petróleo e gás natural. Ocorre principalmente devido à contaminação por materiais corrosivos e/ou abrasivos que afetam às linhas de destilação e processamento de petróleo, derivados e gás natural [1][2][3][4][5][6][7]. Pode ocorrer pela ação isolada ou combinada da erosão, da corrosão e/ou do atrito com influências da temperatura, da composição química e das características de resistência do material na superfície em exposição. A aplicação de camadas de revestimento por materiais especiais e resistentes a corrosão...

The present work evaluates the mechanical and metallurgical behavior of ASTM A182 F22 steel heat-affected zone (HAZ) through the double-layer weld deposition technique, with different welding energies between the first and second layers. This methodology leads to the reduction of hardness levels in the HAZ and can exempt the need for post-weld heat treatment on low-alloy steel after buttering welding. The welding was performed by the automated GMAW process in flat position. As the filler metal, the AWS ER309L, ER 80S-G and ER NiCrMo-3 (Inconel 625) alloys were applied on ASTM A182 F22 steel. The analysis of the refining and/or tempering in coarse-grained HAZ (CGHAZ) of the first layer was performed by metallographic tests and microhardness measurements. The tempering of the first-layer CGHAZ is dependent on the correct overlap of the weld layers, weld bead geometry and the highest imposed heat of the second layer. The results revealed that the Higuchi technique is effective for obtaining welding parameters for the second-layer deposition that promotes thermal treating of the hard region of the first-layer HAZ. However, this technique was not able to reduce the hardness values at the 250 HV level required by NACE 0175 for the test conditions in this work.

Switchback Gas Metal Arc Welding (GMAW) consists of a forward and backward periodic oscillation of the welding torch in the longitudinal direction of the joint. The present work has two objectives, to evaluate the use of a simple and practical device proposed for the switchback manipulation of the torch and to analyze the effect of the switchback parameters on the operational characteristic of the process. Two series of bead-on-plate depositions were planned, using the GMAW process with or without the switchback technique. To find an operational envelope, two levels of equivalent welding speeds were used for covering ranges of oscillation frequency and amplitude. A Ni superalloy was employed as deposition material, aiming at simulating weld overlays. Wire feeding speed, set voltage and contact tip-to-work distance (CTWD) were kept constant. The proposed device was evaluated and proved to be able to make the overlays with all combinations of planned parameters. It was demonstrated that if the backward length (B) is larger than half of the forward's (F), the torch passes more often over the same point of the deposit, and the oftener the torch passes over the same point, the lower the incidence of intermittent narrowing of the bead. By evaluating the surface aspect of the beads, the greater the F, the higher the likelihood of this irregularity. Penetration is shallower with switchback (reason demonstrated based on the fraction of stroke length that the arc is over the subtract), yet the bead surface presents more ripples. Another incident non-geometric conformity is the "dragon back" aspect of the bead, which is favored by faster equivalent welding speeds and high values of forward and backward speeds. The found operational envelope for the GMAW process with switchback was stablished with low-values of speeds and lengths of forward and backward strokes.

This work aims to propose and assess a methodology for parameterization for WAAM of thin walls based on a previously existing working envelope built for a basic material (parameter transferability). This work also aimed at investigating whether the working envelope approach can be used to optimize the parameterization for a target wall width in terms of arc energy (which governs microstructure and microhardness), surface finish and active deposition time. To reach the main objective, first, a reference working envelope was developed through a series of deposited walls with a plain C-Mn steel wire. Wire feed speed (WFS) and travel speed (TS) were treated as independent variables, while the geometric wall features were considered dependent variables. After validation, three combinations of WFS and TS capable of achieving the same effective wall width were deposited with a 2.25Cr-1Mo steel wire. To evaluate the parameter transferability between the two materials, the geometric features of these walls were measured and compared with the predicted values. The results showed minor deviations between the predicted and measured values. As a result, WAAM parameter selection for another material showed to be feasible after only fewer experiments (shorter time and lower resource consumption) from a working envelope previously developed. The usage of the approach to optimize parameterization was also demonstrated. For this case, lower values of WFS and TS were capable of achieving a better surface finish. However, higher WFS and TS are advantageous in terms of production time. As long as the same wall width is maintained, variations in WFS and TS do not significantly affect microstructure and microhardness.

The development of electronic power sources for welding has allowed the study of innovative processes, generally with the objective of improving the capacity of production linked to low levels of thermal support. The existing processes are based on the metal transfer mode and on the development of wave forms that improve control of the process. The metal inert gas-variable polarity pulsed process is a derivative of the conventional metal inert gas process that combines the advantages of the use of positive polarity, such as good arc stability and cathode cleaning, with those given by negative polarity, principally the high fusion rate of the electrode and low thermal support to the base metal. However, this process has only found limited use due to the scarcity of technical and scientific publications. As a result, this work sets out a proposal for a methodology to determine welding parameters in four different wave formats and varying the negative electrode rate across three levels in a way that provided arc and metal transfer stability, the evaluation of the process was carried out by filming the arc and analysing the geometric characteristics of the weld bead. The method was shown to be sufficiently efficient with satisfactory stability results.