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  • Influence of softening mechanisms on base materials plastic behaviour and defects formation in friction stir lap welding
    Publication . S, SREE; Galvão, Ivan; Leitao, Carlos; Rodrigues, Dulce
    The AA6082-T6 and AA5754-H22 aluminium alloys were selected as the base materials to fabricate similar and dissimilar friction stir lap welds. Three lap configurations, AA6082/AA5754, AA5754/AA6082 and AA6082/AA6082, were produced using three pin profiles and tested to analyse the role of the plastic behaviours of the base materials on the welding conditions. The macrostructural characterisation was carried out to understand the material flow response and hook defect formation. The mechanical characterisation of the joints was done by microhardness and lap tensile shear testing. The finite element analysis and phase simulation were conducted to predict the phase dissolution temperatures and the softening kinetics. The welding torque and axial forces registered were analysed to quantify differences in the alloy’s flowability during welding. The analysis of the welding machine outputs enabled to conclude that higher axial forces were registered when the AA5754 alloy was placed at the top of the dissimilar lap joint, showing that the non-heat-treatable alloy has lower flowability than the heat-treatable alloy. These results were associated with the flow-softening of the AA6082 alloy in plastic deformation at high temperatures. The coupled experimental and numerical analysis revealed that the plastic behaviour of the base materials strongly influenced the material flow and, in this way, the hook defect formation and the shear tensile properties of the welds.
  • New mandrel design for ring hoop tensile testing
    Publication . KHALFALLAH, ALI; KTARI, Zied; Leitao, Carlos; Fernandes, José Valdemar
    The determination of mechanical and fracture properties of anisotropic tubular materials along hoop direction needs the use of the ring hoop tensile test, for which, the obtained results are deceived by the effect of friction between the ring sample and the Dshaped block mandrel. Commonly, lubricants are applied to reduce the friction, which are inefficient in some specific cases; despite of that, it was noticed that scarce works have focused on the development of new mechanical mandrel designs or trying to improve the current ones to resolve the friction concern. The aim of this research is to correctly address the friction issue between the ring sample and the fixture mandrel to significantly reduce its effect on the ring hoop tensile test results without using any kind of lubricants. New mechanical design of D-shaped block mandrels are developed to carry out ring hoop tensile tests to simply characterize the mechanical behaviour of tubular materials. New mechanical D-shaped block mandrels were designed, manufactured and used to carry out experimental ring hoop tensile tests. An inverse identification method based on an artificial neural network trained by finite element simulation responses, was developed to efficiently segregate the flow stress curve from the influence of the friction, inherent in the global force-displacement curve for the classical ring hoop tensile test. The experimental force – displacement curves using five mandrel-types are established and quantitatively compared on the base of their ability to reduce the friction issue. The analysis of the finite element simulations, related to the investigation of the influence of the friction on the ring hoop tensile test results, shows that one of the new developed mandrels reduces the friction coefficient by about 10 times compared to that identified using the classical D-shaped block mandrel. It has been found that, the finite element simulation of ring hoop tensile test using the identified material parameters matches the experimental results. This investigation provides a useful fixture mandrel, which is able to drastically reduce the friction without resort to any lubricants to just determine the material flow stress curve using ring hoop tensile test, regardless the friction level between the sample and mandrel.
  • Tensile properties of S355 butt welds after exposure to high temperatures
    Publication . Rodrigues, D. M.; Leitao, Carlos; Balakrishnan, M.; Craveiro, Hélder D.; Santiago, A.
    The influence of the exposure to high temperatures on the tensile properties and failure mode of butt-welded connections in 5355 J2 steel was assessed. With this aim, welds were produced using GMAW and FCAW semiautomatic processes. Transverse tensile specimens were extracted from the welded coupons, heated to high temperatures (300, 600 and 900 degrees C), cooled to room temperature and then loaded to failure. Microstructural characterization and hardness measurements were performed to explain the tensile behaviour of the specimens after exposure to high temperatures. Tests at ambient temperature were also conducted for benchmark comparison. From the analyses, it was possible to conclude that, in the absence of welding defects, the residual properties of the 5355 joints, after heat exposure, are very similar to that of the base material. However, the presence of welding defects, depending on its severity and typology, may conduct to rupture in the welds and low residual strength, after heat exposure. This happens even if those defects not affect the tensile properties of the connections in the as-welded condition. The residual capacity of steel structures will be only severely reduced after heat exposure to temperatures in the eutectoid range, i.e., between 700 and 900 degrees C. The critical exposure temperature may vary according to the chemical composition of the steels/welds and to the duration of the heat exposure. The strength loss and the ductility of the BM and of the connections are dependent of the duration of the heat exposure. Very long exposure conditions conduct to a maximum decrease in residual yield and ultimate strength of 30 % and 20%, respectively, in S355 J2 steels and welds.
  • Thermo-mechanical modelling of the Friction Stir Spot Welding process: Effect of the friction models on the heat generation mechanisms
    Publication . Hannachi, Nasra; KHALFALLAH, ALI; Leitao, Carlos; Rodrigues, Dulce
    Friction Stir Spot Welding involves complex physical phenomena, which are very difficult to probe experimentally. In this regard, the numerical simulation may play a key role to gain insight into this complex thermo-mechanical process. It is often used to mimic specific experimental conditions to forecast outputs that may be substantial to analyse and elucidate the mechanisms behind the Friction Stir Spot Welding process. This welding technique uses frictional heat generated by a rotating tool to join materials. The heat generation mechanisms are governed by a combination of sliding and sticking contact conditions. In the numerical simulation, these contact conditions are thoroughly dependent on the used friction model. Hence, a successful prediction of the process relies on the appropriate selection of the contact model and parameters. This work aims to identify the pros and cons of different friction models in modelling combined sliding-sticking conditions. A three-dimensional coupled thermo-mechanical FE model, based on a Coupled Eulerian-Lagrangian formulation, was developed. Different friction models are adopted to simulate the Friction Stir Spot Welding of the AA6082-T6 aluminium alloy. For these friction models, the temperature evolution, the heat generation, and the plastic deformation were analysed and compared with experimental results. It was realized that numerical analysis of Friction Stir Spot Welding can be effective and reliable as long as the interfacial friction characteristics are properly modelled. This approach may be used to guide the contact modelling strategy for the simulation of the Friction Stir Spot Welding process and its derivatives.
  • Influence of the structure and phase composition of the bond interface on aluminium-copper lap welds strength
    Publication . Andrade, David; Galvão, Ivan; Verdera, David; Leitao, Carlos; Rodrigues, Dulce
    The structure and phase composition of the bond interface of aluminium-copper lap welds produced by friction stir welding and tool-assisted friction welding were analysed. Microstructural analysis proved that no through-interface material flow took place in tool-assisted friction welding and that aluminium-copper joining resulted from the formation of a thin and continuous intermetallic layer at the lap interface. For the welds produced by friction stir welding, evidences of through-interface material flow were found, promoting mechanical interlocking of both base materials, at the lap interface, and formation of discontinuous intermetallic layers. Mechanical testing showed that the tool-assisted friction welds, with excellent surface finishing, had low strength, contrary to the friction stir welds, which displayed excellent bond strength. The comparison of the mechanical and microstructural results, for both weld types, pointed to the ineffectiveness of the continuous intermetallic layer in providing high strength bonding.
  • Mechanical design of ring tensile specimen via surrogate modelling for inverse material parameter identification
    Publication . KTARI, Zied; Leitao, Carlos; Prates, Pedro; KHALFALLAH, ALI
    The mechanical characterization of anisotropic thin walled-tubes along hoop direction is not a trivial task. It is necessary to develop experimental techniques, numerical methods and design test samples, which enable to determine the real tube properties along hoop direction without any external influences. In this study, first we propose a surrogate based-model for the mechanical design of the ring hoop tensile test (RHTT) specimen, in order to obtain the effective homogeneous stress and strain distribution of the uniaxial tensile test along hoop direction. Second, the optimized sample is used to carry out RHTT and to obtain the actual flow stress curve and the anisotropy coefficients of AA6063-O extruded tube. However, the experimental curve measured from RHTT (force –displacement) is a degenerate response, since it suffers from intermixture effects of the effective material behaviour with the friction between the sample and the sample-holding tool. Hence, we developed an inverse parameter identification method, which uses design of experiments, finite element analysis and artificial neural network to separate out the tubular material parameters from the friction coefficient. The assessment of the developed method is achieved by comparing the predicted material parameters and the identified flow stress curve obtained by artificial neural network algorithm. The finite element simulation results corroborate the obtained findings.
  • Shoulder related temperature thresholds in FSSW of aluminium alloys
    Publication . Andrade, David G.; S, SREE; Leitao, Carlos; Rodrigues, Dulce
    Friction Stir Spot Welding (FSSW) is assumed as an environment-friendly technique, suitable for the spot welding of several materials. Nevertheless, it is consensual that the temperature control during the process is not feasible, since the exact heat generation mechanisms are still unknown. In current work, the heat generation in FSSW of aluminium alloys, was assessed by producing bead-on-plate spot welds using pinless tools. Coated and uncoated tools, with varied diameters and rotational speeds, were tested. Heat treatable (AA2017, AA6082 and AA7075) and non-heat treatable (AA5083) aluminium alloys were welded to assess any possible influence of the base material properties on heat generation. A parametric analysis enabled to establish a relationship between the process parameters and the heat generation. It was found that for rotational speeds higher than 600 rpm, the main process parameter governing the heat generation is the tool diameter. For each tool diameter, a threshold in the welding temperature was identified, which is independent of the rotational speed and of the aluminium alloy being welded. It is demonstrated that, for aluminium alloys, the temperature in FSSW may be controlled using a suitable combination of rotational speed and tool dimensions. The temperature evolution with process parameters was modelled and the model predictions were found to fit satisfactorily the experimental results.
  • Influence of the galvanized coating thickness and process parameters on heat generation and strength of steel spot welds
    Publication . Andrade, David; Sabari, S. Sree; Leitao, Carlos; Rodrigues, Dulce
    The influence of galvanized coating thickness, tool diameter and rotational speed, on the thermal cycles, in spot welding of steels produced by Tool Assisted Friction Welding (TAFW), a Friction Stir Spot Welding (FSSW) related technique, is analysed. To study the influence of the galvanized coating thickness on the thermal cycles, thin steel plates commonly used in steel construction and automotive industry with galvanized coatings of varied thicknesses were welded. Numerical simulation of the welding process was conducted to understand some of the physical phenomena observed experimentally. Numerical and experimental results were compared and discussed. The influence of the above described parameters, as well as of the dwell time, on welds strength was also characterized. The results showed that steel spot welds with very good mechanical strength can be obtained in very short process cycle times. It was also determined that the welds strength was much higher than the minimum strength recommended for resistance spot welds (RSW).
  • Copper/stainless steel friction stir spot welds: feasibility and microstructural analysis
    Publication . Taborda, Diogo; Leal, Rui; Morgado, Teresa; Leitao, Carlos; Galvão, Ivan
    The possibility of using solid-state joining technologies, such as friction stir welding (FSW) and its variants, to perform dissimilar joints is one of the well know advantages of this class of processes, namely because they are impossible to be produced by other conventional welding processes due to the evident differences in physical and chemical properties of both materials. Relevant advances have been made over the last 20 years in this field. The material pairs that are mostly addressed in the literature are based on systems involving aluminum alloys and other metallic and non-metallic materials. Indeed, with the upgraded interest in this technology concerning specific engineering applications, some specific material combinations such as aluminum-to-copper (Al-Cu) and aluminumto-ferrous alloys (Al-Fe) have become relevant. However, the research about some material pairs is still scarce or inexistent.
  • Lateral-torsional buckling of high strength steel beams: experimental resistance
    Publication . Tankova, Trayana; Rodrigues, Filipe; Leitao, Carlos; Martins, Cláudio; Silva, Luis Alberto Proença Simões da
    EN 1993-1-1 gives stability design rules for columns, beams and beam & ndash;columns up to S460, whereas EN 19931-12 gives additional guidance for S500 up to S700. Recent studies show that high strength steel members may be designed using improved buckling curves, where the enhanced behaviour is usually attributed to the improved material properties but mainly due to the more favourable residual stress distribution. The behaviour of unrestrained beams in HSS has not been widely studied. At present in EN 1993-1-1, the design rules for lateral-torsional buckling of beams are not dependent on the steel grade, meaning that the code does not distinguish between beams in conventional strength steel or HSS. In pursuit of an answer to the mentioned shortcomings, the present research is based on the experimental programme covering 12 full-scale tests, residual stress measurements, advanced numerical models and analytical derivations. The experiments cover different steel grades up to S690, welded and hot-rolled sections, homogeneous and hybrid (flanges in HSS and web in mild steel), double and mono-symmetric sections as well as variations in the cross-section class. This paper provides an overview of the experimental programme, discusses the results for lateral-torsional buckling of beams, and presents an advanced numerical model that was calibrated to the experimental results including the measured residual stress distribution and geometrical properties of the members. The numerical model was explored to assess various assumptions for the member imperfections, and these are further compared with code recommendations.