Initial Characterization of the Layer Interface for Graphite-Free Additive Friction Stir Deposition of AA7075

Additive friction stir deposition (AFSD) is a novel friction stir technology. It is one of the most prolific solid-state metal deposition processes. In recent years, the aerospace and defense industries have increased their investment in the deposition of 7xxx aluminum alloys. This has allowed AFSDs of 7xxx aluminum to move from a laboratory environment to being tested in an industrial setting. This work strives to help move the AFSD of AA7075 toward an effective production environment by providing an initial characterization of the graphite-free layer interface. To the authors’ knowledge, this is the first graphite-free study to utilize both knub–scroll and scroll tools in AA7075. It is also the first study to compare how flat, knub, knub–scroll, and scroll influence layer mixing in graphite-free AA7075. The condition of the layer interface is particularly important to build direction properties. As many end users of AFSD desire isotropic properties, improving build direction properties is extremely important. This work looks at how external tool geometries and layer height impact the layer interface. The objective is to not only better characterize the layer interface but also to determine if a specific external geometry and or layer height could help facilitate a stronger layer interface. It was found that depositions made by the knub tool at a 2.5 mm layer height generated the most visually consolidated layer interface at an optical and SEM level. Under EDS analysis, the knub tool only saw a 12% variation between peak and background oxygen counts. EBSD scans also revealed a more consistent grain size distribution.