This paper examines the impact of oxide coatings on the surfaces of feedstock material used for Additive Friction Stir-Deposition (AFS-D). The AFS-D is a solid-state additive manufacturing process that uses severe plastic deformation and frictional heating to build bulk depositions from either metallic rod or powder feedstock. Since aluminum alloys naturally form an oxide layer, it is important to determine the influence of the feedstock surface oxide layer on the resultant as-deposited microstructure and mechanical properties. In this study, three AA6061 square-rod feedstock materials were used, each with a different thickness of aluminum oxide coating: non-anodized, 10-micron thick, and 68-micron thick. Macroscale depositions were produced with these feedstock rods using the AFS-D process. Optical and electron microscopy showed that the two oxide coatings applied through anodization were efficiently dispersed during the AFS-D process, with oxide particles distributed throughout the microstructure. These oxide particles had median sizes of 1.8 and 3 μm², respectively. The yield and tensile strengths of these materials were not measurably impacted by the thickness of the starting oxide coating. While all three feedstock material variations failed by ductile rupture, the elongation-to-failure did decrease from 68% to 55% in the longitudinal direction and from 60% to 43% in the build direction for the thickest initial oxide coating, 68 microns.

In this research, we explore the preliminary effects of processing conditions using a novel additive manufacturing (AM) process, known as additive friction stir deposition (AFSD), on resulting build direction (BD) mechanical performance. Using the AFSD process, a feasibility study of three AM builds of identical size are created using differentiating processing parameters. A relationship referred to as the deposition pitch, exhibiting similarities to weld pitch, is determined to be a simple but effective predictor of the interlayer bonding in AFSD processing of AA7020. The deposition pitch directly correlates the necessary temperature, time, and pressure required for effective solid-state bonding. Using this correlation, increased mechanical performance in the BD is achieved through an increase in deposition pitch. A reduction in the deposition pitch from 4.46 rev/mm to 1.08 rev/mm resulted in a significant decrease in failure strain from 24.4% to 0.82%, with the failure mechanism shifting from a ductile failure to brittle failure. The inverse relationship between grain refinement and BD failure strain at high deposition pitches suggests deposition pitch and heat input are the dominant factors in the resulting BD mechanical properties.