Abstract
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 TB (Al7.5Cu4Li), were detected using S-HEXRD at both preheat-treated temperatures. Higher volume fraction of T1 measured in the 500 °C (75.2 HV0.1) sample resulted in a higher microhardness compared to the 320 °C (58.7 HV0.1) sample. Higher TB volume fraction measured in the 320 °C sample had a minimal strength effect.
Highlights
The aerospace industry‘s interest in Al-Li alloys dates back to the 19500 s, and this is attributed to their high strength and weight-saving potential, based on the unique property of the Al-Li system where there is a density reduction of 3% and an increase in the elastic modulus between 5% to 6% with each wt.% of Li added [1]
Several options are available for minimizing residual stresses in Laser Powder Bed Fusion (LPBF) such as the adoption of in-situ heat treatment, post-fabrication stress-relief mechanisms or the lowering of the highthe residual stress-induced cracks are clearly seen in the as-built sample
It stands to reason that, dislocations induced by the large amounts of residual stress in the as-built condition, which could have acted as potent T1 nucleation sites, T1 nucleation and growth could not possibly have taken place without sufficient time and appropriate temperature
Summary
The aerospace industry‘s interest in Al-Li alloys dates back to the 19500 s, and this is attributed to their high strength and weight-saving potential, based on the unique property of the Al-Li system where there is a density reduction of 3% and an increase in the elastic modulus between 5% to 6% with each wt.% of Li added [1]. In work done in Reference [8] on an AA2198 Al-Cu-Li alloy, the authors reported that through a combination of various degrees of pre-deformation and artificial ageing, mean thickness and diameter of 1.3 and 40 nm at 155 ◦ C ageing temperature respectively, increased to 2 and 55 nm at 190 ◦ C. A 5–10 nm diameter of T1 occurring on the {111} planes was reported by the authors of Reference [14], who investigated microstructural evolution of an AA2198 Al-Cu-Li alloy using Atom Probe Tomography (APT) Phases such as S and θ’ were observed at various ageing temperatures. The influence of preheat treatment as opposed to any form of post-heat treatment on the phase precipitation of the precipitation hardenable AA 2099 alloy is preferred in this investigation
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