Institute for Sustainability and Innovation in Structural Engineering

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Publications

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.

2021-08-05Journal article

Open access

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.

2021-10-04Journal article

Restricted access