Processing Mechanics of Additive Friction Stir Deposition

Additive friction stir deposition (AFSD) is a newly developed solid-state metal additive manufacturing (AM) technology that adds a material feeding mechanism to the friction stir principle (Yu et al.., 2018). As a newly developed process, the development of a sound understanding of the process mechanics is necessary and may shed light on both limiting factors and new opportunities. This work explores the fundamental modes of deformation through an analytical decomposition of three flow components: 1) radial spreading, 2) rotating, and 3) traversing shear flow. The analytical models provide 'back-of-the-envelope' estimates of mechanical requirements (machine torque, for example), and straightforward algebraic equations for estimating the peak strain rate associated with deformation and the expected residence time of material underneath the AFSD tool head. A more complex, but preliminary, numerical modeling approach is then presented to models the steady state material flow as a fully coupled non-Newtonian fluid with rate and temperature dependent properties. Additionally, a transient thermal model is presented which captures the thermal history of the material along a dynamic printing trajectory. The numerical models provide insight into the pressure distribution underneath the AFSD tool, which impacts deformation bonding conditions at the interface, and suggest that temperature differences under the tool may be as high as 70℃. Several interface fracture experiments reveal a well-bonded center region, with high ductility and energy dissipation, and a poorly bonded outer edge region. Novel characterization work has been presented showing evidence of a nearly uniform 50μm thick shear layer on the top surface of a deposit. Analysis of the Prandtl number suggests that this shear layer is a consequence of a thin thermal boundary layer, which in the presence of frictional shear stress, becomes a thermo-mechanical boundary layer with a distinct flow regime from the bulk. Further characterization shows viscous mixing patterns in the wake of tool pins, and incomplete bonding at the edges of the deposition track. An additional application is presented for AFSD – selective area cladding of thin sheet metal. Substrates as thin as 1.4mm were clad without localized deformation, which is dependent on the clamping configuration of the substrate. Cladding quality, interface integrity, and certain failure modes are identified for thin cladding operations. In-situ monitoring and ex-situ laser scanning shows the slow evolution of thermal distortion during cooling of the cladding-on-sheet system. Finally, residual stress and strain estimates are made using curvature methods for bi-layer specimens extracted from the cladding.