The aim of the OPTIMUM project is to study both experimentally and numerically metal joints produced by linear friction welding (LFW). An original and comprehensive approach which links the welding process parameters, their effects on the microstructure evolution and the consequences on the final use properties of the welded components, will be developed. The focus will be on joining new grades of titanium based or nickel based alloys and joining dissimilar materials. Multi-scale characterization techniques will be used in order to deeply analyze the effects of the process parameters on the microstructure evolution in the welded zone (i.e. the ability to interpret the mechanisms behind the different observed microstructure zones). Using finite element method, a simulation tool of the LFW process will be developed in order to study the evolutions of the thermal, the kinematic and the stress fields during the welding process. The numerical tool will describe the different steps of the LFW process and will be validated based on several experimental data (e.g. thermal fields, force-displacement…). This numerical modeling will help in interpreting the physical phenomena responsible of the microstructure evolution (e.g. the temperature level reached and the microstructure transformation, the stress fields and the refining of the microstructure). The OPTIMUM project has four main work-packages: The first work-package is devoted to welding of different pairs of titanium-based or nickel-based materials. Tests will be conducted for various process parameters (forging pressure, frequency and amplitude of oscillations) and for different geometrical configurations. The second work-package is dedicated to the physical-chemical analysis and the microstructure analysis of the weld zone supplemented by measurements of residual stresses and by hardness profiles performed by an instrumented nanoindentation technique. The third work-package is dedicated to the development of a thermo-mechanical simulation tool of the LFW process in the Forge ® software environment. The quality of the developed numerical tool will be evaluated through the comparison of experimental and numerical data such as the fields of local thermal gradients , the geometry of the welded joint (e.g. size and geometry of the burr ) and the material consumption (i.e. the reduction in the size of the slug of materials after assembly ). This tool will be subsequently used to help in the interpretation of the observed microstructure evolution (local mechanical fields, cooling rates ...). The fourth work-package of the project deals with the study of service life of welds by using 3D non-destructive experimental techniques such as X-ray tomography and laminography for the analysis of defects induced by the process (e.g. porosity). Some in situ tests with a monitoring of the damage evolution and/or crack propagation will be realized by sequential mechanical tests or in situ sub synchrotron beam.