Investigation of Microstructure in Solid State Welded Al-Cu-Li alloy

Al-Li alloys have been extensively used in aerospace vehicle structure since the presence of lithium increases the modulus and reduce the density of the alloy. Especially the third generation Al-Cu-Li alloy shows enhanced fracture toughness at cryogenic temperatures so that the alloy has been used on the fuel tank of space launchers, like Super Lightweight External Tank of the Space Shuttle. Since the commercial size of the plate cannot accommodate the large tank size of the launcher, joining several pieces is required. However, lithium is highly reactive and its compounds can decompose with heat from conventional fusion welding and form different types of gases which result in formation of defects. In this study, the microstructure change is investigated after solid state welding process to join the Al-Cu-Li sheets with optical and transmission electron microscopic analysis of precipitates.