Investigation of the Lithium-Containing Aluminum Copper Alloy (AA2099) for the Laser Powder Bed Fusion Process [L-PBF]: Effects of Process Parameters on Cracks, Porosity, and Microhardness

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

“…Several options are available for minimizing residual stresses in LPBF such as the adoption of in-situ heat treatment, post-fabrication stress-relief mechanisms (post-heat treatment) or the lowering of the high-temperature gradient through variation of scanning strategy between the scan tracks [ 27 ]. Post-heat-treated as-built samples at various temperatures proved to be detrimental to the mechanical properties (hardness), as opposed to in-situ heat treatment [ 20 ].…”

“…Through appropriate LPBF preheat treatment (in combination with dissociated dislocation spots which act as nucleation sites), T 1 phase formation can be achieved through the LPBF process. This puts LPBF in a competitive advantage compared to conventional processing of Al-Cu-Li alloys through the elimination of prior-deformation by cold working with subsequent artificial ageing treatments [ 20 ].…”

“…T 1 formation temperature in a ternary Al-Cu-Li equilibrium system is reported to be greater than 635 °C [ 3 ]. However, in an equilibrium ThermoCalc simulation of an Al-Cu-Li alloy of similar composition to the alloy under investigation [ 20 ], T 1 formation temperature was predicted to approximately range between 240 and 480 °C and that of Scheil-Gulliver between 460 and 520 °C. The authors concluded that LPBF processing using a preheat treatment temperature of 520 °C therefore created favourable conditions for the nucleation and growth of the T 1 phase, which was suspected to be responsible for improved microhardness over post-heat-treated samples.…”

“…The highest recorded microhardness for the 500 °C preheated samples was 75.2 HV0.1 compared to 58.7 HV0.1 for 320 °C and 67.6 HV0.1 for as-built samples, respectively. The microhardness of 72 HV0.1 for 520 °C preheat-treated samples in our earlier work [ 20 ], shows that it is possible to achieve a relatively higher microhardness with a lower preheat treatment temperature (500 °C). The build-up of residual stresses in the LPBF-built part, typically in the as-built condition, is due to the high-temperature gradient, thermal contraction and expansion or the lack of uniformity in plastic deformation as a result of heating and cooling cycles, and is known to boost hardness properties [ 46 ].…”

“…In our earlier work [ 20 ], we first successfully reported the LPBF processability of the AA2099 Al-Cu-Li alloy with relative density of 98.8% and a micro-Vickers hardness (HV) of 72 HV 0.1 using a preheat treatment temperature of 520 °C. The alloy’s recorded microhardness was suspected to be due to the presence of the T 1 phase, although no phase characterisation was performed.…”