Evaluation of Strengthening Mechanisms of Nanoparticle Aluminum DED Builds

High strength aluminum (2XXX, 6XXX, 7XXX series) alloys are widely used in the aerospace industry based on their high strength to weight property, and availability of extruded shapes for airframes. However, these Al alloys are not used in welded structures due to their solidification cracking susceptibility and loss strength. There is an emerging need for high strength Al alloys that could be used for Directed Energy Deposition (DED) Additive Manufacturing (AM) to improve the design and affordability of advanced structures. New consumables have been recently developed that use ceramic inoculants, also called nanoparticles, to subdue solidification cracking by nucleating fine, equiaxed grain during deposit solidification. The use of inoculants has also shown a rise in ultimate tensile strength and yield strength of DED builds after post-build heat treatment. In this study, inoculated Al DED builds were evaluated to understand the influence of nanoparticles on strengthening mechanisms for 6061, 2024 and 2319. A range of GMA waveform experiments were used to build single-pass per layer walls. Both pulse and reciprocating wire feed waveforms were explored to evaluate the effects of metal transfer on grain refinement in constant deposit area builds. For 6061, half of each build underwent a T6 condition post-build heat treatment and the other half was evaluated as-deposited. Characterization was done using optical microscopy, energy dispersive X-ray spectrometry (EDS), SEM, and TEM. Micro-tensile testing and hardness testing were performed to measure the impact on mechanical properties. Results to date demonstrate the ability to use these new consumables to make crack-free deposits, and achieve good high strength properties with equivalent T6 heat treatment. Ongoing work will be presented that evaluate the effects of these inoculated consumables for alloys 2024 and 2319 and develop strengthening mechanism relationships for DED of these alloys.