Microstructural and Corrosion Characterization of Bobbin Tool Friction Stir Welded Al-Cu-Li Alloy for Manufacturing of a Demisable Spacecraft Propellant Tank

Increasing space activities produce a high number of objects in space and lead to increasing collision risks which urges leading industries to develop future removal strategies. Therefore, modern spacecraft design must comply with demise criteria guaranteeing structural burn-up after the mission. Titanium, used for example to manufacture liquid propellant tanks for satellites, does not demise to a satisfactory level during atmospheric re-entry. Modern aluminum lithium alloys are under consideration to replace high-temperature melting materials like titanium. For these new alloys, friction stir welding was proposed as suitable joining technology in order to avoid high heat inputs during manufacturing.

In this work, we demonstrate the microstructural evolution and the resulting corrosion response of modern Al-Cu-Li alloy AA 2060 T8 welded using the stationary shoulder variant of bobbin tool friction stir welding which is especially suitable for hollow structures. The 3 mm thick welded panels were defect-free and reach high welding efficiencies. Welded coupons were analyzed using DSC, TEM and XRD as well as SCC testing. Different mechanisms in the welding zones could be observed. Among those, stress corrosion phenomena were discovered to occur within the heat-affected zone during post-welding exposure to 3.5 % NaCl solution. The mechanisms leading to intergranular corrosion initiate on grain boundary precipitates and correlate with the microstructural history formed during the welding process.

Additionally, the study led to a successful manufacturing of a full size technology demonstrator employing bobbin tool friction stir welding and Al-Cu-Li alloys. Detailed analysis of the process forces allowed the construction of a suitable clamping device.