Abstract:The effect of torch weaving on the microstructure, tensile strength, impact resistance and fracturing in robotic welded joints using varing welding speeds, voltages and currents was evaluated through fractography using scanning electron microscopy, tension, impact and micro-hardness tests and optic microscopy. Results indicated that linear, sinusoidal and circular weavings favored an increase of the width of the HAZ as well as a slight increase in yield resistance accompanied by hardening in comparison with a triangular weave. The latter favored larger impact energy in the HAZ with less width, containing coarse-grained ferrite due to lower tensile strength and Vickers hardness. A circular weave generated the highest level of hardening and the lowest energy absorbed in the HAZ as a consequence of an increase in yield strength related to the fine needles of acicular ferrite. A linear weave favored the greatest width of the HAZ compared with other weld weavings due to heat accumulation along the fusion line of the welded joint. Hardening and loss of toughness were evaluated through fractographic analysis showing mixed fractures mainly composed of brittle fracturing made by transgranular cleavages with facets containing well defined river patterns.Key-words: HAZ; Cleavage fracture; Robotic welding; Torch weaving; ASTM A36 steel. Efeito do Movimento da Tocha na Microestrutura, Resistência à Tração e ao Impacto, e Fratura da ZAC e Cordão de Solda por Processo GMAW Robotizado em Aço ASTM A36Resumo: O efeito do movimento da tocha sobre a microestrutura, resistência à tração e impacto e fratura em juntas soldadas por processo robotizado, variando velocidade, voltagem e corrente de soldagem foi avaliado através de fratografia, usando microscopia eletrônica de varredura, provas de tração e impacto e testes de microdureza e microscopia óptica. Os resultados indicaram que os movimentos lineares, senoidais e circulares favorecem um aumento da largura da ZAC, assim como, um ligeiro aumento do limite elástico acompanhado de endurecimento em comparação com o movimento triangular. A energia de impacto é mais favorecida na ZAC com menor largura, contendo grãos grosseiros de ferrita devido à menor resistência à tração e dureza Vickers. Um movimento circular produziu o nível mais elevado de endurecimento e a menor energia absorvida na ZAC como consequência de um aumento na limite elástico relacionado com as agulhas finas de ferrita acicular. Um movimento linear favoreceu maior largura da ZAC em comparação com outros movimentos da tocha, devido à acumulação de calor ao longo da linha de fusão da junta soldada. Endurecimento e perda de tenacidade foram avaliados através de análise fratográfica mostrando fraturas mistas compostas principalmente de fratura frágil por clivagem transgranular com facetas contendo marcas de rio bem definidas.Palavras-chave: ZAC; Fratura por clivagem; Soldagem robotizada; Movimento de tocha; Aço ASTM A36.
Aluminum alloys are very promising for structural applications in aerospace, military and transportation industries due to their light weight, high strength-to-weight ratio and excellent resistance to corrosion. In comparison to unreinforced aluminum alloys, aluminum/aluminum alloy matrix composites reinforced with ceramic phases exhibit higher strength and hardness, improved tribological characteristics. A novel surface modifying technique, friction stir processing (FSP), has been developed for fabrication of surface composite with an improved performance. The effect of FSP parameters such as number of passes, direction of each pass, sealed or unsealed groove on microstructure was investigated. In this work, nano-particles of TiC (2% in weight) were added to aluminum alloy AA7075-T651 to produce a functional surface. Fixed parameters for this AA7075 alloy were used; rotation speed of 1000 rpm, travel speed of 300 mm/min and pin penetration of 2.8 mm. Optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to study the microstructure of the fabricated surface composites. The results indicated that the selected FSP parameters influenced the area of surface composite, distribution of TiC particles and micro-hardness of the surface composites. Finally, in order to evaluate rate wear the pin on disk test was carried out. Key-words:Friction Stir Processing (FSP); Nano-particles; Aluminum alloy 7075; Surface composite; Wear. Efeito dos Parâmetros do Proceso de Fricção Agitação para Reforçar Uma Placa de Aluminio Al7075-T651 com TiCResumo: As ligas de alumínio são promissoras para aplicações estruturais nas indústrias aeroespaciais e de transporte devido ao seu elevada relação resistência-peso e excelente resistência à corrosão. Em comparação com as ligas de alumínio não reforçadas e alumínio/compósitos de matriz de alumínio reforçados com fases cerâmicas, exibem maior resistência e características tribológicas melhoradas. Foi desenvolvida uma técnica de modificação superficial, friction stir processing, para a fabricação de superfície de compósitos. Foi investigado o efeito dos parâmetros FSP como número de passes, direção de cada passe e ranhura selado ou não selada. Neste trabalho, nanopartículas de TiC (2% em peso) foram adicionadas à liga de alumínio AA7075-T651 para produzir uma superfície funcional. Foram utilizados parâmetros fixos para a liga AA7075: velocidade de rotação de 1000 rpm, velocidade de deslocamento de 300 mm/min e penetração do pino de 2,8 mm. Microscopia óptica (MO), microscopia eletrônica de varredura (MEV) e microscopia de força atômica (AFM) foram utilizadas para estudar a microestrutura. Os resultados indicaram que os parâmetros FSP selecionados influenciaram na área da superfície do compósito, na distribuição de partículas de TiC e na microdureza da superfície. Finalmente, a fim de avaliar a taxa de desgaste o teste de pino sobre disco foi realizado. Palavras-chave:Processamento por Fricção Agitação (FSP); Nanopartículas; Liga de ...
A characterization study was done to analyze how microstructural regions affect the mechanical properties, corrosion and fractography of the Heat Affected Zone (HAZ), weld bead and base metal for pipe naturally aged for 21 years at 30 °C. Results showed that microstructures exhibited damage and consequently decrease in properties, resulting in over-aged due to service. SEM analysis showed that base metal presented coarse ferrite grain. Tensile test indicated that microstructures showed discontinuous yield. Higher tensile strength was obtained for weld bead, which exhibited a lower impact energy in comparison to that of HAZ and base metal associated with brittle fracture by trans-granular cleavage. The degradation of properties was associated with the coarsening of nano-carbides observed through TEM images analysis, which was confirmed by SEM fractography of tensile and impact fracture surfaces. The weld bead reached the largest void density and highest susceptibility to corrosion in H 2 S media when compared to those of the HAZ and base metal. RESUMEN:Efecto del envejecimiento natural sobre la microestructura, propiedades mecánicas, resistencia a corrosión y fractura en uniones soldadas de acero para tubería API5L X52. Se realizó un estudio de caracterización para analizar cómo la microestructura afecta a las propiedades mecánicas, corrosión y fractura de la zona afectada por calor (ZAC), soldadura y metal base para tubería envejecida naturalmente durante 21 años a 30 °C. Los resultados indicaron que las microestructuras presentaron daño y consecuentemente reducción en propiedades mecánicas, como consecuencia del envejecimiento por servicio. El estudio mediante MEB mostró que el metal base presenta grano ferrítico grueso. La prueba de tensión indicó que las microestructuras mostraron fluencia discontinua. La mayor resistencia a la tracción se presentó en la soldadura, la cual alcanzó menor energía de impacto en comparación con la ZAC y metal base asociado con fractura frágil por clivaje transgranular. La degradación de las propriedades está en relación con el engrosamiento de nanocarburos observados a través del análisis de imágenes por MET, lo que se confirmó mediante fractografía por MEB de superfícies de fractura bajo tensión e impacto. La soldadura alcanzó la mayor densidad de microhuecos y la mayor susceptibilidad a la corrosión en un medio conteniendo H 2 S que cuando se compara con la ZAC y metal base.
Plasma Transferred Arc (PTA) process is increasingly used in applications where enhancement of wear, corrosion and heat resistance of metals surface is required. The shape of weld bead geometry affected by the PTA welding process parameters is an indication of the quality of the weld. PTA is a versatile method of depositing high-quality metallurgically fused deposits on relatively low cost surfaces. The overlay deposited is an alloy that is hard and more corrosion resistant than counterparts laid down by Gas Tungsten Arc Welding (GTAW) or Oxy Fuel Welding (OFW) processes. Weld deposits are characterized by very low levels of inclusions, oxides, and discontinuities. This process produces smooth deposits that significantly reduce the amount of post weld machining required. Metal-Mechanic industry continuously requires recovering tool steel components subjected to severe wear. The steel known as D2 is considered to be a high carbon, high chromium cold work tool steel. In this research, weld beads were deposited on D2 steel by using PTA process with different parameters as welding current and travel speed using base nickel filler metal. In order to evaluate the metallurgical features on the weld beads/substrate interface a microstructural characterization was performed by using Scanning Electron Microscopy (SEM) and to evaluate the mechanical properties was conducted the wear test.
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