Initially focus was placed on process development for similar AA2024-T3 and AA7075-T6 and dissimilar AA7075-T6/AA2024-T3 joints without sealant, followed by quality assessment. The main results achieved in the in this phase were:
- Superior fatigue behaviour of all investigated joints, Figure 2.
- Corrosion testing of similar joints has revealed that localized attack which leads to intergranular corrosion.
- Corrosion testing of dissimilar joints has revealed a very selective attack along the joint line (Figure 3).
The development of the adhesive sealant indicated that the most suitable variant for application with RFSSW is the high viscosity type. It guarantees corrosion protection, adhesive function and does not contaminate the weld zone.
Once the adequate adhesive sealant was selected, RFSSW process parameter optimization of joints with adhesive sealant was conducted. The results have shown that:
- Forces applied during pre-clamping and RFSSW can effectively squeeze the adhesive sealant from the area to be welded avoiding contamination in the welds.
- The adhesive sealant (combined with RFSSW), improves the maximum load in lap shear, cross tensile and fatigue testing (Figure 4).
A comparative study in terms of mechanical behaviour between RFSSW and conventional processes used in aircraft assembly have shown that:
- RFSSW is the superior process compared to RSW and riveting regarding the lap shear strength of the joints (Figure 5).
- Riveting offers the best fatigue properties (Figure 6).
Table 1 presents a summary of the technical assessment of NDT methods and techniques to detect flaws encountered in RFSSW (with and without sealant).
Additionally, Acoustic Emission (AE) has been investigated as technique for online process monitoring. The following observations have been made:
- The feasibility of quality assessment of spot welds, using AE monitoring, was demonstrated.
- It was proven that AE can detect defects in RFSSW, with a high confidence index.
- The method can also be used to predict when the tool must be cleaned.
In the final phase of the project two demonstrators were assembled: a mock-up fuselage section and a cabin separation wall. Initially the demonstrators have been re-designed to explore all the advantages of the RFSSW process, followed by the design and manufacturing of clamping fixtures. Figures 7-8 display two of these fixtures, which allow for variable positioning around the welding machine and a flexible manufacturing operation.
Both demonstrators were subjected to the same surface pre-treatment procedure BEFORE ASSEMBLY and the same painting procedure AFTER ASSEMBLY. Figures 9 and 10 present the assembled mock-up fuselage section and Figures 11 and 12 the assembled cabin separation wall.
In the course of the DAHLIAS project 2 manuscripts have been published in peer reviewed scientific journals. Three documents have been presented at international conference. Three additional documents have published in industrial journals. Results of the project, have been reported as part of the University lectures at the Leuphana University Lüneburg and Technical University Ilmenau. The DAHLIAS Project has been selected as highlight presentation in the "2021 TechTalks" by the BASF Group and was also presented at the Chemetall Global Technology Colloquium.
The results of DAHLIAS have additionally been presented to industrial partners in bilateral workshops. As a result, 3 German national research projects and one European project further exploiting the technology developed in the DAHLIAS have been approved for funding.
