The main objective of the DAHLIAS project is to optimise hybrid joining (Refill Friction Stir Spot Welding, RFSSW, with an adhesive sealant bond) for application in aircraft structures. RFSSW is a solid-state joining technology especially applicable to lightweight alloys in similar and dissimilar configurations. The process has been successfully applied to difficult-to-weld and non-weldable alloys and is considered a potential replacement of mechanical fastening. Overlap joints in aircraft structures require the use of sealants for corrosion protection reasons. Using sealants with an adhesive function would complement the high quality joints produced by RFSSW by not only protecting against corrosion but also contributing to the overall mechanical performance of the structure. To achieve the main goal of this project a work plan consisting of eight work packages has been devised. Two work packages are focused on the RFSSW process development using standard samples with and without adhesive sealant. In parallel the adhesive sealant technology, including surface pre-treatment will be developed. The hybrid joining solution developed in this project will be compared to conventional joining technologies to define its potential. Hybrid joining brings a number of advantages to production and performance of aircraft structures. The change from differential to integral design introduces new challenges in the analysis of mechanical performance. Hence, the damage tolerance behaviour of hybrid joints is also investigated in this project. Quality control methods will be investigated by selecting NDT methods and by process control based on online monitoring of parameters. In the last phase of this project technology demonstrators will be produced to evaluate the capabilities of the proposed technology in actual aircraft structures. The consortium (3 R&D and 2 industrial partners) has the exact fitting, prooven expertise and high excellence. HZG (coordinator) holds key patents.