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

Area of Science:

• Metallurgy and Materials Science
• Additive Manufacturing
• Aerospace Materials

Background:

• Aluminum-Copper-Lithium (Al-Cu-Li) alloys are critical aerospace materials due to their high strength-to-weight ratio.
• The T1 (Al2CuLi) phase is the primary strengthening precipitate in these alloys.
• Limited research exists on the microstructural characterization of Additive Manufacturing (AM) processed Al-Cu-Li alloys.

Purpose of the Study:

• To characterize the microstructure of AA2099 Al-Cu-Li alloy fabricated via Laser Powder Bed Fusion (LPBF).
• To investigate the effect of post-processing heat treatments (320 °C and 500 °C) on phase formation and microhardness.
• To compare AM-built microstructures with conventionally processed alloys.

Main Methods:

• Laser Powder Bed Fusion (LPBF) for sample fabrication.
• Synchrotron High-Energy X-ray Diffraction (S-HEXRD) for phase identification.
• Microhardness testing (HV0.1) to assess mechanical properties.

Main Results:

• The as-built condition showed dislocations but no T1 precipitates, indicating nucleation difficulty without heat treatment.
• Both T1 (Al2CuLi) and TB (Al7.5Cu4Li) phases were detected after heat treatments at 320 °C and 500 °C.
• The 500 °C heat-treated sample exhibited a higher T1 volume fraction and significantly greater microhardness (75.2 HV0.1) compared to the 320 °C sample (58.7 HV0.1).

Conclusions:

• Heat treatment is essential for nucleating the strengthening T1 phase in LPBF-processed Al-Cu-Li alloys.
• Higher processing temperatures (500 °C) favor T1 phase formation, leading to improved microhardness.
• The TB phase, while present, had a minimal impact on the alloy's strength.