Microstructure evolution and strengthening mechanism of Al–Cu–Li–Mg–Ag VPPA welded joint during local three-stage and natural aging treatment

Abstract

Local three-stage (LTA) and natural aging (NA) treatments were proposed to address the challenges associated with the aging treatment of Al–Li alloy fuel tanks and improve the mechanical properties of VPPA welded joints. The microstructure evolution and mechanical properties of aging welded joints were investigated, followed by a discussion of strengthening mechanisms. The results demonstrated that LTA treatment significantly improved the mechanical properties of VPPA welded joints, with an elongation of 6.8 % and a tensile strength of 501 MPa, representing 88.6 % of the base metal. Following the local double-stage solution (LDS) treatment, the eutectic phases in the welded joints dissolved. During the LTA treatment, there was a consistent decrease in elongation. Initially, the θ'' phase precipitated, followed by the T1(Al2CuLi), σ(Al6Cu5Mg2), and S′(Al2CuMg) phases. With increasing aging time, some θ'' phases transformed into θ′(Al2Cu) phases, while others combined with Mg and Ag to form Ω phases. The precipitated phases continued to grow, increasing the precipitation-free zone (PFZ) width. After 30 h of aging, three types of <110> twinned dendrites were observed in the weld zone (WZ) with (111) twinned planes. The improvement in performance was credited to the elimination of eutectic segregation, as well as the enhanced solid solution and precipitation-strengthening effects. The increase in strength of the LDS state welded joints was primarily due to solid solution strengthening. The strength of the precipitated phase was mainly achieved through the bypass mechanism. The eutectic phases did not dissolve after NA treatment, which resulted in limited strength increment and unchanged elongation (EL). Numerous T1 phases precipitated contributed to the precipitation-strengthening effect.