Bobbin Tool Friction Stir Welding of Al-Li alloy 2198 as Approach to Higher Demisability in Propellant Tanks

Within the scope of the green space initiative and the raising awareness of orbit pollution, demisability has been brought to the attention of industry and academia alike. With up to 20 % propellant tanks represent a major mass proportion in all space missions and being built of high melting temperature metals, show low demisability. Some of the present satellite tanks survive re-entry and pose a possible hazard to the system earth. Changing the material to advanced aluminium alloys is considered as an opportunity to gain demisability and mass reduction while maintaining the required properties.

Aluminum lithium alloys have been considered as suitable candidates to replace current titanium in propellant tanks, providing comparable physical, mechanical and chemical properties. A major drawback is the lack of weldability. Friction Stir Welding (FSW) was chosen to positively affect the resulting properties. Friction Stir Welding (FSW) has proven to weld age hardenable aluminum alloys with superior mechanical performance. Bobbin Tool Friction Stir Welding is a process variant that uses two shoulders to increase flexibility. This process variant is particularly suitable to weld hollow structures like pressurized tanks.

 

In this study, aluminum-lithium alloy 2198 was butt welded using semi stationary bobbin tool friction stir welding (SSuBT-FSW). This process variant is introduced for the first time using high strength aluminum alloys. It provides a lower heat input and excellent surface finish. The 3mm thick welded panels are defect free and the joints show high strength efficiency above 80% efficiency. Thermal cycle, microstructure formation and material flow behavior during welding were taken into account in the interpretation of the obtained results.