Characterising the Microstructure of an Additively Built Al-Cu-Li Alloy

Al-Cu-Li alloys are famous for their high strength, ductility and weight-saving properties, and have for many years been the aerospace alloy of choice. Depending on the alloy composition, this multi-phase system may give rise to several phases, including the major strengthening T₁ (Al₂CuLi) phase. Microstructure investigations have extensively been reported for conventionally processed alloys with little focus on their Additive Manufacturing (AM) characterised microstructures. In this work, the Laser Powder Bed Fusion (LPBF) built microstructures of an AA2099 Al-Cu-Li alloy are characterised in the as-built (no preheating) and preheat-treated (320 °C, 500 °C) conditions using various analytical techniques, including Synchrotron High-Energy X-ray Diffraction (S-HEXRD). The observed dislocations in the AM as-built condition with no detected T₁ precipitates confirm the conventional view of the difficulty of T₁ to nucleate on dislocations without appropriate heat treatments. Two main phases, T₁ (Al₂CuLi) and T_B (Al_7.5Cu₄Li), were detected using S-HEXRD at both preheat-treated temperatures. Higher volume fraction of T₁ measured in the 500 °C (75.2 HV_0.1) sample resulted in a higher microhardness compared to the 320 °C (58.7 HV_0.1) sample. Higher T_B volume fraction measured in the 320 °C sample had a minimal strength effect.