Microstructural evolution of Al-Cu-Li alloys with different Li contents by coupling of near-rapid solidification and two-stage homogenization treatment

Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density. In this regard, the Al-Cu-Li alloy castings with high Li content from 1.5wt.% to 4.5wt.% were prepared by near-rapid solidification, followed by two-stage homogenization treatment (490 °C/16 h and 530 °C/16 h). The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software. The results indicate that with the increase of Li content, the grain sizes decrease, the solution ability of Cu in the matrix α-Al phase increases, while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification. Additionally, by the coupling of near-rapid solidification and two-stage homogenization, the metastable precipitated phases (Al₇Cu₄Li and AlCu₃) can be dissolved effectively in the alloys with Li content of 1.5wt.%–2.5wt.%; moreover, the stable precipitated phases (Al₆CuLi₃ and Al₂CuLi) uniformly distribute at the grain boundaries in the alloys with Li content of 3.5wt.%–4.5wt.%. As a result, the refined and homogenized microstructure can be obtained.