Characterization of Structural Evolution as a Function of Alloy Composition, Strain Rate, and Heat Input for Precipitation Strengthened Aluminum Alloys Processed via Additive Friction Stir Deposition

Abstract

Electronic Thesis or DissertationPrecipitation strengthened aluminum alloys have been identified for their use in a wide range of aerospace and structural applications due to the excellent strength-to-weight ratio these alloys exhibit. However, these alloys experience significant challenges due to cracking caused by solidification shrinkage when they are processed with fusion based manufacturing methods. This issue severely limits their use case as fusion additive manufacturing (AM) begins to become more common in the commercial manufacturing space. Recent advances in solid-state additive manufacturing offer the opportunity to overcome these issues for the applications of near net shape additive manufacturing and material repair. Additive friction stir deposition (AFSD) provides fully dense material deposition without the concerns caused by the re-solidification of the material during the manufacturing process. This dissertation investigates the effect of variations in the fundamental components of the AFSD process and the composition of high strength aluminum alloys has on the structure and mechanical properties of 7000-series aluminum alloys. Modifications in total heat input due to parameter driven operating temperatures and total number of thermal cycles lead to changes in the secondary phase precipitate size and distribution in as-deposited AA7050. These changes correspond to observable gradients in mechanical properties recorded across regions with different total heat inputs. Using AA7020, AA7050, and AA7075, the compositional variations in the Al-Mg-Zn-Cu alloy system have on the secondary phase composition and kinetics of material processed via the AFSD process. In situ neutron diffraction allowed for the observation of phase kinetics during the dissolution of the η-phase in as-deposited and feedstock samples as well as the how the modification of alloy composition leads to quantifiable changes in the amount of secondary strengthening phases in both as-deposited and feedstock samples. Finally, the impact of the fundamental components of the AFSD process have on AA7050 both in isolation and coupled together was observed. In situ heating during electron microscopy allowed for the observation of the effect of subsequent thermal cycling on the as-deposited AA7050 nanostructure and the quantification of the precipitate growth coefficient in the deposition