Investigation of grain boundary chemistry in Al-Li 2195 welds using Auger electron spectroscopy

Abstract

Al-Li alloy 2195 is a low-density material with high fracture toughness that is particularly well-suited for aerospace systems. It will replace Al-Cu alloy 2219 in the Super Light Weight Tank (SLWT), a modified version of the external tank being developed for the Space Shuttle to support Space Station deployment. Recent efforts have focused on joining 2195 with variable polarity plasma arc welding, as well as repairing 2195 welds with tungsten inert gas techniques. During this study, Auger electron spectroscopy (AES) was used to examine grain boundary chemistry in 2195 welds. Results indicated that weld integrity depends on whether (and how much) the grain boundaries are covered with thin films comprised of a mixture of discontinuous Al 2O 3 in Al (Al/Al 2O 3), which form during weld solidification. O was probably introduced as a contaminant in the shielding gases, occurring at low levels considered negligible for Al alloys that do not contain Li. However, oxidation kinetics in 2195 are increased by Li enrichment of small quantities of Al 2O 3, further enhancing thin film formation at the grain boundaries. Al 2O 3 can ultimately occupy sufficient grain boundary area to degrade the material's mechanical properties, producing negative effects that are compounded by the cumulative heat input of multi-pass repair welding.