Title: The Influence of Anisotropy due to Crystallographic Texture on Residual Stresses in Welds
Authors: DECROOS Koenraad
Publisher: University of Ghent
Publication Year: 2011
JRC N°: JRC65471
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC65471
Type: PhD Theses
Abstract: Welding as a fabrication technique is commonly used to join components in many industries, among which the aircraft and automotive industries, ship building, and construction industry. After the welding process the joined component is inevitably internally stressed, even when the component is not externally loaded. These stresses are called residual stresses. The residual stresses combine with applied loads and can lead to unexpected failure of the component and the entire structure if the magnitude and the spatial distribution of them is unknown. In the study of the structural integrity of a welded component, it is essential to know the residual stresses caused by the welding process. They can be determined experimentally and simulated computationally, both on various ways. The most widely used modelling technique is that of the thermo-mechanical Finite Element (FE) models where the coupled thermo-mechanical differential equations describing the problem in the welded component are solved. For an accurate, physically based model, it is important to understand and quantify the related physical phenomena that occur in welding. The objective of this thesis was to study the prediction of welding stresses by macroscopic FE modelling. The innovative part is that the microstructural effect of crystallographic texture development in the fusion zone of the weld after solidification, has been investigated and incorporated in the model. The texture in welds has been described quantitatively by EBSD measurements, which allowed to obtain numerical results for the material properties. These results have been used to define an anisotropic weld puddle material after solidification, and to investigate the influence of this anisotropy on deformations and residual stresses. To incorporate the texture in a FE model, 3 steps have to be followed. Firstly, the material properties in the fusion zone of welds on grain size level , namely the crystallographic orientations, have to be quantified. Secondly, the effect of these microscopic properties on the macroscopic properties, the anisotropy on component size level, have to be calculated. Thirdly, the resulting macroscopic anisotropy has to be introduced in the FE model to predict the welding stresses. It is shown that the thermal history of a weld can be modelled accurately by a thermal heat input model. Modelled results have been compared with experiments by comparing thermocouple measurements with temperature predictions at the thermocouple positions, as well as comparing the observed fusion boundary profile with the predicted one. In this work, the fusion zone has experimentally been determined by means of micrographs, and by means of the difference in microstructure between the melted fusion zone and the base material. A good agreement has been found between modelled temperatures and thermocouple measurements, and between predicted and observed fusion zone dimensions. It is shown that upon solidification of the fusion zone of austenitic welds, a strong crystallographic texture arises, with a preferential <100> growth in the solidification direction. A model is presented to predict the orientation of the grains in the fusion zone of a single bead austenitic weld. The orientations are determined based on predictions of the macroscopic thermal history. ...
JRC Directorate:Nuclear Safety and Security

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