Elucidating the influence of high deposition rate via layer height on process structure property relationships of lubricant free twin-rod additive friction stir deposition of 6061 aluminum alloy

This study investigates for the first time the effects of layer height on the microstructure and mechanical properties of AA6061 processed with the lubricant-free twin-rod additive friction stir deposition (TR-AFSD) approach. TR-AFSD is a solid-state metal additive manufacturing method that uses frictional heat and severe deformation to deposit material layer-by-layer with two round feedstock rods simultaneously through a hollow spinning tool without the need for lubricant. In this study, two separate AA6061 builds were produced using different deposition layers: H = 4 mm layer height; H = 1 mm layer height. In addition, a standard post-deposition heat treatment (PDHT) restored T6 tensile properties to the as-deposited (AD) material in both builds, even though the builds exhibited variations of abnormal grain growth after PDHT compared to the refined microstructure in the AD material. Results showed that layer height as an independent variable does not have a major effect on mechanical performance in the longitudinal direction, so that increased build times can be achieved via increased layer height.