Periodic Reporting for period 3 - DAHLIAS (Development and application of hybrid joining in lightweight integral aircraft structures)
Période du rapport: 2021-04-01 au 2021-09-30
Mechanical fastening is the most employed technology to assemble aircraft structures. It brings a weight penalty and employs expensive consumables and has high operational costs. In the DAHLIAS project, the refill Friction Stir Spot Welding (RFSSW) process has been applied to optimize the manufacturing of aircraft structures. The RFSSW process produces defect-free joints with superior performance. The adoption of this process is estimated to result in weight and costs savings of about 15% compared with conventional fastening. Assembly time and cost will also be reduced due to fewer assembly operations. In RFSSW a non-consumable tool consisting of two rotating parts, a probe and a sleeve, as well as a stationary clamping ring is used to join two or more similar/dissimilar materials in lap configuration (see Figure 1). DAHLIAS has also developed a sealant with an adhesive promoter, adding therefore a new function to conventional sealants. The combination of the RFSSW process with the adhesive sealant technology constitutes the main body of the scientific challenge of the project.
The main benefits for the society originating from the DAHLIAS Project are:
- The implementation of an environmental and operator-friendly, low-energy technology in aircraft manufacturing
- Reduction of non-recurring assembly costs and activities in the assembly lines;
- Reduction of assembly time and improvement of quality by automation.
The global aims of DAHLIAS were to close the knowledge gaps regarding the development of a new production technology (RFSSW) coupled with a new adhesive sealant (hybrid joining) addressing questions regarding quality control and process monitoring.
The main benefits for the society originating from the DAHLIAS Project are:
- The implementation of an environmental and operator-friendly, low-energy technology in aircraft manufacturing
- Reduction of non-recurring assembly costs and activities in the assembly lines;
- Reduction of assembly time and improvement of quality by automation.
The global aims of DAHLIAS were to close the knowledge gaps regarding the development of a new production technology (RFSSW) coupled with a new adhesive sealant (hybrid joining) addressing questions regarding quality control and process monitoring.
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.
- 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.
A comprehensive database correlating process parameters with properties and the presence of defects for the alloys AA2024-T4 and AA7050-T6 has been developed. This database will support further development of this technology for the aircraft industry as well as for other industrial branches. These results can be successfully extrapolated to other alloys using machine learning and other data management technologies.
The new sealant with adhesive function developed in the DAHLIAS project ensures corrosion protection and also substantially increases the mechanical performance of the joints. The sealant EP-80-19 C-4 is a new European product, resulting from the DAHLIAS project, within the Naftoseal product line of CM, specific for use in conjunction with joining processes in aircraft structures. Such a product was so far not available on the market worldwide.
The simplicity and modularity of the clamping fixtures developed to produce the cabin separation wall (measuring 2,1m x 2,0m) illustrate how complex Aluminium structures can be easily joined by the process technology developed in this project.
The welding time for the alloys investigated is below 4 seconds and is one order of magnitude shorted than the time estimated for riveting. Furthermore, the RFSSW process does not require any additional consumable. In summary, the cost reduction achieved with the hybrid process developed in DAHLIAS is a result of an extremely short production cycle and reduced material expenditure.
The rivets used for the benchmarking study in this project weight 0.169 gr. Considering that a medium size aircraft has in average 2 million rivets it accounts for approximately 340 kg weight saved by using the DAHLIAS joining technology.
The RFSSW process is classified as a “clean manufacturing” technology. The process does not generate fumes, or any form of radiation and it features a high energy efficiency. It does not require any filler or added material. The process is considered “operator” friendly.
The new sealant with adhesive function developed in the DAHLIAS project ensures corrosion protection and also substantially increases the mechanical performance of the joints. The sealant EP-80-19 C-4 is a new European product, resulting from the DAHLIAS project, within the Naftoseal product line of CM, specific for use in conjunction with joining processes in aircraft structures. Such a product was so far not available on the market worldwide.
The simplicity and modularity of the clamping fixtures developed to produce the cabin separation wall (measuring 2,1m x 2,0m) illustrate how complex Aluminium structures can be easily joined by the process technology developed in this project.
The welding time for the alloys investigated is below 4 seconds and is one order of magnitude shorted than the time estimated for riveting. Furthermore, the RFSSW process does not require any additional consumable. In summary, the cost reduction achieved with the hybrid process developed in DAHLIAS is a result of an extremely short production cycle and reduced material expenditure.
The rivets used for the benchmarking study in this project weight 0.169 gr. Considering that a medium size aircraft has in average 2 million rivets it accounts for approximately 340 kg weight saved by using the DAHLIAS joining technology.
The RFSSW process is classified as a “clean manufacturing” technology. The process does not generate fumes, or any form of radiation and it features a high energy efficiency. It does not require any filler or added material. The process is considered “operator” friendly.