Graphite-free fabrication of fully dense crack-free AA7075 aluminum alloy using additive friction stir deposition

Abstract Additive Friction-Stir Deposition is a form of solid-state additive manufacturing that has shown potential for producing large-scale components. Often, AFSD utilized a graphite lubricant during the manufacture of components from 7xxx-series aluminum alloys. In this work, AFSD was utilized without lubricant to print high-strength AA7075 aluminum alloy, which will reduce the risk of contamination to the feed material, ensuring that the deposited material retains mechanical properties akin to wrought or forged AA7075 aluminum alloy. The fabricated material underwent mechanical testing and characterization to better understand the mechanical properties and microstructure associated with AFSD-printed AA7075 alloy when graphite is removed. Optical imagery showed that the as-deposited material exhibited a fully dense and crack-free build. The microstructure was found to be heavily refined and equiaxed, which is consistent with most AFSD results, although different grain orientations were observed in each plane. The mechanical properties of the as-deposited material were not homogeneous, and mechanical properties were found to vary with height. T6 heat treatment resulted in significant grain growth, along with grain elongation. Remarkably, the heat treatment significantly minimized the anisotropic mechanical properties of the samples, enhancing them to reach 95% of the performance typically seen in wrought AA7075-T6 alloy, with yield strength and ultimate tensile strength averaging 480 MPa and 533 MPa respectively, and elongation averaging 9%. Post heat treatment, hardness also exhibited a significant increase, even for thicker samples, with average values of at least 169.9 HV.