In this study, Al2024-T3 aluminium alloy and Ti-6Al-4V titanium alloy were lap jointed by friction stir welding (FSW). New insights are given into the multiscale effects of the pin tip shape and length on both the interface microstructure and the tensile-shear properties of dissimilar joints. The correlation between the stress distribution at the joint interface, governed by the pin shape, and the phase formation sequence was notably studied. The microstructure of the joints was examined from macro-to-nanoscale by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while their mechanical properties were investigated with tensile-shear tests, coupled with digital image correlation (DIC). It was found that with a semi-spherical pin tip, the joint interface was sharp and semi-coherent. Al and Ti solid solutions were identified by HRTEM analysis over a distance of 10 nm on both sides of this interface. Conversely, a longer flat pin tip generated a 15 μm thick multi-layered interface containing Ti3Al, TiAl and TiAl3 intermetallic compounds. The effect of the welding conditions on the reaction kinetics during friction stir welding of Al/Ti alloys has been investigated. It was found that the intermetallic compounds were formed by atomic diffusion favoured by both temperature and plastic deformation. The interfacial joint microstructures were explained by the different stress and strain fields generated by the distinct pin tip shapes and by their different penetration depths in the Ti-6Al-4V bottom plate. Although these microstructural features at the joints interface very likely governed the tensile-shear properties of safe joints, they were, in this case, less critical than the hooks and the deformed Alclad layer which led to the joint fracture. Stronger and more ductile joints were obtained with the semi-spherical tip shorter pin. A maximum joint coefficient of 50 % compared to Al2024 base material was found. Some ways of optimization were finally proposed.