Microstructure evolution and mechanical properties of the wire + arc additive manufacturing Al-Cu alloy

Understanding and clarifying the formation and evolution mechanism of the microstructure in wire + arc additive manufacturing (WAAM) Al-Cu alloy is primary and essential for achieving the higher and more isotropic mechanical properties. In this study, the evolution of grain structure, precipitated phases and corresponding mechanical properties of the as-deposited WAAM processed 2219 Al-Cu alloy are investigated. The microstructure exhibits the heterogeneous band characteristics between the fine equiaxed α-Al grains in inter-layer zone (ITZ) and the columnar α-Al grains in inner-layer zone (INZ). Based on the re-melting experiment and the time-dependent nucleation theory, the occurrence of equiaxed grains is attributed to the heterogeneous nucleation of α-Al caused by the prior precipitation of Al₃Zr particles at the molten pool boundaries. The relationship between the solidification conditions and the number of the prior Al₃Zr nucleation sites is established. A large amount of spherical θ″-Al₂Cu phases precipitate in Cu-rich regions around the α-Al+θ-Al₂Cu eutectics and they agglomerate and transform to flaky-like θ′ phases under the in-situ thermal effect during WAAM deposition. The hardness of the ITZ (81 ± 2.7HV) is approximately 12% higher than that of the INZ (74 ± 3.6HV) which is mainly due to the finer grains and the more dispersive precipitates in ITZ.