Stress Corrosion Cracking Aspects of Al-Li Alloys Welded by Bobbin Tool Friction Stir Welding as Approach to Higher Demisability of Space Structures

The continuously increasing number of missions in space emphasizes the need for an international removal strategy in order to avoid debris incidents in space and on earth. Therefore, the green space initiative of ESA follows the NASA approach to design space structures that comply with demise criteria guaranteeing structural burn-up after the mission. Titanium does not demise to a satisfactory level during atmospheric re-entry and, for example, is used in the manufacturing of propellant tanks. By changing the material to advanced aluminum alloys, there is an opportunity to gain demisability without incurring a significant mass penalty.

Aluminum lithium alloys are under consideration as candidate materials for demisable propellant tanks as they provide comparable physical, mechanical and chemical properties. A drawback is the lack in weldability, as Al-Li alloys can be difficult to weld using conventional welding techniques. The Bobbin Tool Friction Stir Welding (BT-FSW) technique has already been proven as a method of welding aluminum alloys and is particularly suitable
for joining hollow structures. However, the final assembly must still exhibit mechanical and corrosion performance that match the properties of currently used materials such as titanium. Stress corrosion cracking is known as a critical aspect when heat treatable Al alloys are processed under heat impact.

In this work, the aluminum alloys AA2060-T8 and AA2196-T8 were welded employing stationary shoulder and standard BT-FSW. The 3 mm thick welded panels were defect-free showing high weld efficiencies. Welded coupons were exposed to SCC testing where different mechanisms in the welding zones were observed. Whilst pitting occurs in the weld center and selective grain dissolution in the base material, the heat-affected material suffers from intergranular corrosion. The mechanical and stress corrosion cracking observations were correlated with the experienced thermal cycle and resulting microstructural evolution, revealing the guiding mechanisms for the inhomogeneous material’s property distribution.