José Antônio Esmerio Mazzaferro

Preliminary study on the mechanical behavior of friction spot welds

O processo de Soldagem a Ponto por Friccao (FSpW - Friction Spot Welding) caracteriza-se por possibilitar a uniao no estado solido de duas ou mais chapas de material sobrepostas sem deixar o furo residual caracteristico do processo FSW. O presente trabalho apresenta os primeiros resultados de estudos visando determinar a resistencia mecânica de juntas em ligas de aluminio soldadas por FSpW. Recentemente diversos estudos sobre o modelamento numerico de soldas realizadas por processos de soldagem baseados em friccao tem sido realizados, entretanto a maioria destes estudos esta relacionada a fisica do processo. Alguns trabalhos estabelecem a relacao entre as variaveis de processo e as propriedades mecânicas da junta resultante, mas e muito dificil encontrar na literatura resultados quantitativos que possam ser usados diretamente para projeto destas unioes. O presente trabalho objetiva desenvolver um processo de analise baseado nas caracteristicas do processo que permita avaliar como a geometria e microestrutura das regioes da solda afeta o comportamento mecânico das juntas resultantes. Com este objetivo, os resultados de ensaios mecânicos realizados em juntas de aluminio AA2024-T3 foram usados para validar e calibrar um modelo numerico desenvolvido para prever o modo de falha da uniao. O modelo numerico reproduz as dimensoes e carregamento utilizados nos ensaios de cisalhamento e tracao. As dimensoes e propriedades das regioes principais da junta (ZM - zona de mistura, ZTMA - zona termo-mecânicamente afetada, ZAC - zona afetada pelo calor e MB - metal base) foram definidas com base na medicao da dureza e analises macrograficas de corpos de prova soldados com diferentes parâmetros. Pode-se comprovar uma otima correlacao entre os resultados obtidos nas simulacoes e os dados experimentais. O modelamento numerico das juntas pernite a previsao das propriedades mecânicas da junta e auxilia na compreensao da influencia da geometria e propriedades de cada regiao sobre o seu desempenho em servico. Baseado nos resultados obtidos, o procedimento de analise pode ser facilmente adaptado para o processo de Soldagem a Ponto por Friccao e Mistura Mecânica (FSSW - Friction Stir Spot Welding).

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.

Microstructure evaluation and mechanical properties of a friction stir spot welded TRIP 800 steel

A TRIP 800 steel was friction stir spot welded using three different tool rotational speeds, 1600, 2000 and 2400 rpm, and the dwell time was kept constant at 2 s. The resultant microstructures formed in each weld zone were analysed as well as their hardness. Higher hardness values were observed for the lowest rotational speed, 1600 rpm, where the heat input in theory was lower and, therefore, the cooling rate was faster. However, for this rotational speed, allotriomorphic ferrite was also observed in the stir zone. In the lap shear tests, samples welded at 1600 and 2400 rpm did not reach the minimum value recommended by the AWS D8.1M standard, which was attributed to the lower bonding ligament length and also lower distance between the keyhole left by the pin and the end of the zinc line, which is formed in the stir zone. The fracture of the samples occurred along this line. As a result, the influence of the microstructure on the failure process could only be inferred when the zinc line disappeared.

Avaliação da geometria de ferramenta e parâmetros do processo FSW na soldagem da liga de alumínio AA 5052

Welding of aluminum alloys with no considerable degradation of the properties of the base metal is a problem to be overcomed by industry manufacturing processes. In the aeronautical industry, no-melt joining processes such as adhesive bonding or riveting are often considered when designing aluminum connections. Alternatively, a welding process named Friction Stir Welding (FSW) is receiving crescent attention for its potential applications where heat input shall be minimized or when dissimilar metals must be joined. In this process a high strength rotating tool with a special profile is introduced at the interface of the materials to be joined and translated along the joint at controlled speeds. Heat generated softens the material and allows the tool to stir while traveling along the joint. This work aimed to product welds on AA 5052-H34 plates, 6.35mm (0,25 inches) thickness, using a conventional milling machine. In order to do that, three tool geometries were designed, manufactured and tested so as to define which welding parameters could generate the best results. Once these parameters were chosen, each tool produced three welds and their performance was evaluated. Transversal bending, tensile tests, micro-hardness measurements along the weld cross-section and macrographical analysis were carried out in order to assess weld properties. In addition, MIG welds were produced and subjected to the same test conditions. Considering yield stress as an efficiency parameter, all the tested tools presented similar results (around 80% of the base metal yield stress). However one of these tools showed inferior performance when considering elongation or transversal bending test as a comparison parameter due to the presence of a longitudinal groove, as observed in macrographical analysis. FS welded samples have not showed considerable variation along the different microstructure zones in micro-hardness measurements, while MIG welds presented well defined zones as characteristic to electric-arc processes.

Influência da Vazão e Geometria da Região de Entrada Sobre a Eficiência Térmica Medida por um Calorímetro de Fluxo Contínuo de Água

The main aim of the present work is to analyze the influence of water flow rate and inlet geometry on the arc thermal efficiency of a continuous water flow calorimeter. The experimental procedure consists of varying water flow rate and testing three different calorimeter inlet seal geometries: straight seal, conical diffuser seal and seal with water flux obstacle. The experiments were designed and the results evaluated based in a one-factor statistical analysis of variance, in this case the inlet calorimeter water flow. The welding beads were deposited on low carbon steel pipes by Gas Metal Arc Welding – GMAW process, using the same parameters and welding time. The highest average thermal efficiency is 80.5% to water flow of 4 l/min, with a low statistical error, using the conical diffuser seal inlet geometry, whereas for smaller or higher flow rates the measured efficiency values were lower. The inlet with straight seal model shown all the arc thermal efficiency values with slightly lower numerical values compared with conical diffuser, while the seal with flux obstacle exhibited high statistical error.

Efeitos da Energia de Soldagem e Consumível sobre a Resistência à Flexão Simples de Juntas Soldadas em Perfis Tubulares de Aço TMCP

The introduction of steels made by thermomechanical controlled processing (TMCP) demanded higher mechanical and metallurgical properties of the welded joints produced on them. Given these factors, the heat input is strictly limited, and to avoid any reduction in the mechanical strength of the heat affected zone (HAZ), it is required to test welding procedures close to those that will be used in the actual structure. Therefore, the aim of this study is to obtain detailed data on the mechanical efficiency of welded joints by GMAW process and different heat inputs, on square tubular profiles in TMCP steel, disposed as a column/beam weldment. So, six different heat inputs and two filler metals were used (AWS ER80S-G and AWS ER120S-G), with welds located around the profile contour. The twelve welded structures were instrumented and subjected to simple bending. Comparing welded joints produced with same heat input, and despite similar effects on the heat affected zone (HAZ), those made with consumable AWS ER120S-G have shown higher mechanical strength.

Efeitos da Taxa de Resfriamento na Soldagem do Aço API 5L-X80

One of the main reasons for the development of the API 5L-X80 steel was its utilization in Polar Areas where a relatively high toughness in low temperatures is needed. The present work aimed at studying the welding cooling rate effects on the mechanical and metallurgical properties of this type of steel. These various cooling rates also simulate various heat input’s. These different cooling rates were obtained through five distinct pre-heatings. The interpass temperature was the same of the pre-heating temperature presented by each one of the double V joints with root face; being the robotics weldings itself performed through gas metal arc welding (GMAW), using solid wire with matching mechanical resistance, and as a shielding gas a mixture of 75% Ar and 25% CO2. In order to analyze the welded joints, light microscope and scanning electron microscope (SEM) were utilized, as well as mechanical tests, such as tension, bending, impact (Charpy-V) and hardness (Vickers). The results of such mechanical tests were satisfactory and suitable correlated with the metallurgical analyses. The joint presented a high Charpy-V impact resistance in testing temperatures up to –40 °C.

Avaliação Microestrutural e Propriedades Mecânicas de um Aço TRIP Soldado a Ponto por Fricção e Mistura Mecânica (Microstructural Evaluation and Mechanical Properties of a Friction Stir Spot Welded TRIP 800 Steel)

A TRIP 800 steel was friction stir spot welded using three different tool rotational speeds, 1600, 2000 and 2400 rpm, and the dwell time was kept constant in 2s. The resultant microstructures formed in each weld zone were analyzed, as well as their hardness. Higher hardness values were observed for the lowest rotational speed, 1600 rpm, where the heat input in theory was lower and, therefore, the cooling rate was faster. However, for this rotational speed allotriomorphic ferrite was also observed in the stir zone. In the lap-shear tests, samples welded at 1600 rpm and 2400 rpm did not reach the minimum value recommended by the AWS D8.1M standard, which was attributed to the lower bonding ligament length and also lower distance between the keyhole left by the pin and the end of the zinc line, which is formed in the stir zone. The fracture of the samples occurred along this line. As a result the influence of the microstructure on the failure process could only be inferred when the zinc line disappeared.