Microstructure Influence on the Strain-Rate and Stress-State Dependence of Additive Friction Stir Deposition Aluminum Alloy 7075
By Paul Allison (Univ. Alabama), Brian Jordon (Univ. Alabama), C. Taylor Mason (Univ. Alabama), Dustin Avery (Univ. Alabama), Brandon Phillips (Univ. Alabama)

The Solid-State Additive Manufacturing (SSAM) process referred to as Additive Friction Stir - Deposition (AFS-Deposition) provides a new path for coating, joining and additively manufacturing a variety of aluminum alloys. This additive manufacturing process differs from traditional friction stir welding since metal powder or solid rod is fed through a non-consumable rotating cylindrical tool generating heat and plastically deforming the feedstock material through controlled pressure from the tool as successive layers are built upon a substrate. In this research, the dynamic recrystallization and grain refinement is characterized for the successive layers in as-deposited Aluminum Alloy 7075 samples using Electron Backscattered Diffraction (EBSD). The EBSD results depict grain structures formed by dynamic recrystallization (DRX) with even finer grains observed in the as-deposited material than the wrought filler material. Additionally, fully dense equiaxed grain morphology is observed in the as-deposited structure. Tensile behavior of these fine-grained structures is also characterized at both quasi-static (0.001/s) strain rates using a servohydraulic load frame and high strain rates (1500/s) using a direct tension-Kolsky bar and a split-Hopkinson pressure bar. Fractography of the quasi-static and high strain rate tensile specimens using a Tescan Lyra FIB-FESEM showed traditional ductile fracture morphology with microvoids and dimples on the fracture surface.