ODS steels (Oxide Dispersion Strengthened) are candidate materials for fuel cladding in Sodium Fast Reactors (SFR), one of the concepts at study for the forth generation of nuclear power plant. These materials have good mechanical properties at high temperature due to a dispersion of nanometer-sized oxides into the matrix. Previous studies have shown that melting can induce a decrease of the mechanical properties at high temperatures due to modifications of the nanometer-sized oxide dispersion. Therefore the fusion welding techniques are not recommended and the solid state bonding has to be evaluated. This study is focused on resistance upset welding.Welding experiments and numerical simulations of the process are coupled in this thesis. All the trials (experimental and numerical) are built using the experimental design method in order to evaluate the effects of the process parameters on the welding and on the weld. A 20Cr ODS steel is used in order to conduct the study.The first part is dedicated to the study of the influence of the process parameters on the welding. The simulation shows that the welding steps can be divided in three stages. First, the temperature of the contact between pieces increases. Second, the process is driven by the pieces geometry and especially the current constriction due to the thinness of the clad compare to the massive plug. Therefore, the heat generation is mainly located in the clad part out of the electrode leading to its collapse which is the third stage of the welding step. The evaluation of the process parameters influences on the physical phenomena (thermal, mechanical ...) occurring during the welding step allow to adjust them in order to influence the thermal and mechanical solicitation undergone by the pieces during the welding process.The second part is dedicated to the study of the influence of the physical phenomena on the welds. In the process parameter range, some welds exhibit compactness defects or a modification of the microstructure and of the oxide dispersion. Compactness defects are related to thermal and mechanical phenomena occurring at the contact between pieces. The modification of the microstructure is related to dynamical recrystallization or to a local fusion. The dynamical recrystallization occurring in the clad due to high deformation and high temperature is linked to modification of the oxide dispersion.Using the effects of the process parameters on the welding and on the weld, it is possible to adjust the temperature and the deformation in order to avoid the compactness defects and the modification of the oxide dispersion. All these results are then apply to the welding of a 9Cr ODS steel which is a candidate alloy for the SFR fuel cladding. The effects of the material properties on the welding and the weld are then discussed by comparing the two alloy with a different chromium content but also by comparing results on the 20Cr-ODS with a material of similar chemical composition but without the oxide dispersion.
