Issue 47

P. Ferro et al., Frattura ed Integrità Strutturale, 47 (2019) 221-230; DOI: 10.3221/IGF-ESIS.47.17 222 welded joints compared to parent material, is due to different reasons, the most important of which is the stress concentration at the weld toe or weld root. For this reason, some techniques, aimed to improve the fatigue strength of welded joints, were developed [1]. They act mainly on two aspects: 1) the geometrical variation of the weld bead and 2) the reduction of the stress concentration factors. Among these techniques, the TIG-dressing is probably the most used and consists in the re-melting of the weld toe by means of the TIG heat source (without filler metal) [2-4]. Such operation promotes a smoother transition between the plate and the weld crown and thus a reduction of the stress concentration factor as well as a residual stress redistribution. Furthermore it doesn’t affect significantly the angular distortion of the joint because of the reduced dimensions of the remelted zone. Especially the reduction of weld flaws and inclusions in combination with the increase in weld toe radius is assumed to create the beneficial behavior of TIG-dressed specimens when compared with as-welded specimens. Evidence of beneficial aspects of TIG-dressing against the fatigue strength of welded joints comes from literature. Dahle [5] found an increase in fatigue strength at two millions cycles ranging from 10% to 90% according to the steel grade and being the largest increment obtained with high strength steels. Similar results came from the work of Pedersen et al. who reported an increase in fatigue strength of 70% at two millions cycles [6]. Further evidences about the positive effect of TIG-dressing on fatigue strength of welded joints were published by Haagensen et al. [7] who observed an average increase of 44% in the fatigue strength of a fillet weld joint. Despite the great number of fatigue tests performed in the past in order to assess the positive effects of TIG-dressing on welded joints, the residual stress assessment obtained either by means of experimental technique or numerical models is still lacking in literature. Compared to the numerical models of welding processes developed in the last years [8-14], the numerical simulation of TIG-dressing process is particularly complex because it requires a high coupling between thermo- metallurgical-fluid analysis and mechanical analysis. The weld bead geometrical variation induced by re-melting is influenced by the weldment distortions during TIG-dressing and vice-versa. For this reason, it was recently published in literature a numerical model of the TIG-dressing process that simplifies a lot the computation by using the activation- deactivation function of elements [15]. In this way, it is possible to avoid the fluid analysis, but the weld toe geometrical variation has to be ‘a priori’ known by means of welding and TIG-dressing trials. Such model is applied in this work to a real weldment and the results in terms of metallurgical and mechanical properties are compared and discussed. M ATERIALS AND M ETHODS ig. 1 shows a schematic of the welded joint cross section. Two 16 mm thick plates were fillet welded with a 12 mm thick plate by means arc welding. The chemical composition of plates and fusion zone (FZ) was measured by the quantometer Foundry-Master Pro and results are summarized in Tab. 1 where it is observed the greater amount of alloy elements in the 12 thick plate compared to that of the 6 mm thick one. Figure 1 : Schematic of the transversal section of the analyzed joints Fe C Si Mn P S Ni Al Cu Parent Metal (plates 12 mm thick) Bal. 0.1223 0.0101 1.3467 0.0168 0.0161 0.0253 0.0378 0.0502 Parent Metal (plates 6 mm thick) Bal. 0.0756 0.0128 1.1575 0.0143 0 .0073 0.0056 0.0285 0 .0087 Fusion Zone Bal. 0.0892 0.4453 1.3800 0.0186 0.0142 0.0181 0.0083 0.0549 Table 1 : Chemical composition of the plates (wt%). F 6 mm 12 mm