Effect of multipass welds and pores on residual stresses in butt-welded joints

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Welding is a widely used process in various industries for joining materials, but it can introduce several complexities that affect the mechanical properties of the welded components. In particular, the presence of pores within the welded material can have significant implications for fatigue resistance, residual stresses, and overall structural integrity. The primary objective of this thesis is to develop and analyze thermal and mechanical models to understand how various parameters, including geometry, mesh, material properties, boundary conditions, and heat input, influence the behavior of a welded thick plate containing pores. The simulations are conducted with different pore configurations and multiple welding passes to comprehensively explore their impact. Two Finite Element (FE) models were designed. In the first model (Model 1), the objective was to replicate the experimental setup detailed by Qiang et al. in their work "Through-thickness welding residual stress and its effect on stress intensity factors for semi-elliptical surface cracks in a buttwelded steel plate" [41]. This endeavor was accomplished utilizing Abaqus as software. A Dflux subroutine, written in Fortran, was incorporated to effectively emulate the welding procedure. The model was constructed to account for multiple welding passes (up to 7). By extrapolating transverse and longitudinal stresses, a comparative analysis was conducted across five distinct models and the experimental data [41]. The intention was to elucidate variations and similarities in the results. Subsequently, the second model (Model 2) was conceived to exploit the more favorable boundary conditions present in Zhang’s model as described in "Effect of Welding Sequence and Constraint on the Residual Stress and Deformation of Thick Welded Butt Joint Made of Q345qD Steel" [58], when contrasted with the initial experiment. Following a similar procedure, the FE model was developed, with the added feature of an increased number of welding passes (up to 12). Distortions along three distinct paths and transverse and longitudinal residual stresses within two specific regions were extrapolated and subsequently compared. The investigation then progressed to the introduction of pores into Model 2, in the simulation characterized by the lowest number of passes. The aim was to assess the impact of these pores on temperatures, distortions, and residual stresses. By comparing the outcomes of the models both with and without defects, valuable insights were gained into the effect of pores during welding processes. The main findings of this research can be summarized as follows: • Thermal data extracted from a welding experiment conducted at four distinct positions within a 250mm x 200mm x 32mm plate, as documented in Zhang et al.’s publication [58], exhibited inconsistencies when compared to the results obtained through Abaqus simulations in this thesis. These disparities predominantly stem from the absence of accurately specified thermocouple placements in the reference paper [58]. Despite conscientious endeavors to pinpoint the precise thermocouple coordinates, discernible variations of 100°C and a maximum divergence of 25% between the outcomes of the simulations and the reference findings were identified. • Residual stresses obtained in Abaqus (Model 1) trying to replicate Qiang et al. paper [41] in a 700mm x 400mm x30 mm showed good agreement with the reference simulation but deviated from the experimental data. The obtained numerical values peaked around 150 MPa, while the experimental values reached more than double that magnitude (305 MPa), for specific simulations. The number of welding passes significantly influenced the compression at the sides of the plate, with an increasing trend as the number of passes increased. • The simulations (Model 2) created to replicate Zhang et al. experiments [58] demonstrated better agreement with the experimental data for both longitudinal and transverse residual stresses. The number of passes played a crucial role in the results, with the eight to twelve passes simulation showing the closest match to the experimental data. • The presence of the studied configuration of pores in the material did not affect significantly the shrinkage, distortions, and residual stresses in the simulations. Closer to the pores, higher values for residual stresses were found. Based on the findings of this thesis, several recommendations for future research can be made. Laboratory experiments should be conducted to validate the numerical results obtained in this study. Sensitivity analysis considering modifications in the dimensions, boundary conditions, and pores locations of the model can provide further insights. Additionally, incorporating thermally independent properties can reduce the computational time of the simulations. Overall, this thesis contributes to understanding the impact of pores on the thermal and mechanical behavior of thin and thick plates and provides a foundation for future research in this area.