Mechanical properties, corrosion and microstructure distribution of a 2195-T8 Al[sbnd]Li alloy TIG welded joint

Abstract

2195 AlLi alloy is being used in the manufacturing of ultra-lightweight external fuel tanks for launch vehicles, but the correlation between the local properties and microstructure of its tungsten inert gas (TIG) welded joint has not been thoroughly investigated. In this work, softening and hardening behaviors of the welded joint are detected through microhardness and tensile tests. Due to the coarse dendritic structure and severe elemental segregation at the grain boundaries, the weld nugget zone (WNZ) is the softest position among the whole joint. A local hardening in the heat affected zone (HAZ) is attributed to the optimal combination of solid solution strengthening brought by the full redissolution of aging precipitates and subsequent precipitation strengthening of the Guinier-Preston (GP) zone. Furthermore, intergranular corrosion (IGC) test shows that severe IGC and sub-grain boundary corrosion (SGC) occurs in the base material (BM), while slight IGC and SGC develops in the HAZ and some pits are left in the WNZ. The corrosion mechanism is proposed to be: In the BM and HAZ, the T1 precipitates at the grain boundaries and sub-grain boundaries degrades the IGC and SGC resistance. In the WNZ, large numbers of Cu-rich secondary phases and the anodic dissolution of surrounded Cu-poor α-Al matrix leads to the pitting of the secondary phases.