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

Abstract

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.