Understanding the Process-Structure-Property Relationships of High Strength Aerospace Alloys Processed Via Additive Friction Stir-Deposition

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
University of Alabama Libraries

Additive manufacturing has emerged as the leading forefront alternative technology for fabricating and repairing complex geometry aerospace components. However, a majority of the additive processes are fusion-based, which can create underachieving mechanical responses from materials that are susceptible to hot cracking and phase transformations. A solid-state severe deformation-based additive manufacturing process, Additive Friction Stir-Deposition (AFS-D), offers an innovative solution and a new path to fabricate or repair components to achieve fully-dense depositions with wrought-like mechanical performance. In this work, the process-structure-property relationships will be quantified, through extensive characterization of the microstructural evolution and mechanical response of IN625, a fabricated free-standing deposition of AA7075, and lastly, repaired AA7075 plate additively repaired through the AFS-D process. To quantify the fatigue behavior of the as-deposited IN625, stress-life experiments were conducted, where improved fatigue resistance was observed compared to the feedstock. Post-mortem analysis of the as-deposited IN625 revealed a similar fatigue nucleation and growth mechanism to the feedstock for most of the specimens. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate structure-property fatigue damage mechanisms. The microstructural characterization of the as-deposited AA7075 employed optical, scanning electron microscope, and electron backscatter diffraction. The as-deposited AA7075 exhibited a refinement of the constituent particles and grains within the microstructure. Additionally, to quantify the fatigue behavior of the as-deposited AA7075, strain-life experiments were conducted, where a reduction in fatigue resistance was observed compared to the heat-treated feedstock. Post-mortem analysis of the as-deposited AA7075 revealed a change in the fatigue nucleation and growth mechanisms compared to the control feedstock. Lastly, a microstructure-sensitive fatigue life model was employed to capture the fatigue life for the first time in AFS-D aluminum alloys. In this work, we quantify the fatigue performance of repaired AA7075. Simulated crack repair was carried out by machining a rounded groove into a plate, which was then additively repaired using the AFS-D process. An extensive microstructural characterization of as-deposited and heat-treated conditions was conducted to elucidate the microstructural evolution of the repaired plate. Additionally, the mechanical performance of the heat-treated repair was then quantified, as well as the fatigue performance, and fatigue crack initiation mechanisms.

Electronic Thesis or Dissertation
Additive Friction Stir-Deposition, Additive Manufacturing, EBSD, Fatigue, Fatigue Modeling, Fractography