Effects of post-deposition heat treatment on an Al-Mg-Si alloy produced by friction screw extrusion additive manufacturing

This study explores the microstructural evolution of AA6060 builds that were fabricated via the recently developed Friction Screw Extrusion Additive Manufacturing (FSEAM) approach, and the role of a subsequent post-manufacturing ageing heat treatment on the mechanical properties. FSEAM is a solid-state approach based on Friction Stir Welding (FSW) principles that offers a promising solution for addressing critical challenges in fusion-based additive manufacturing of high-strength aluminium alloys. Wall-like builds were created using FSEAM with a range of deposition velocities from 100 to 300 mm/min.
Sound, defect-free builds were consistently achieved when the deposition temperature and normal force reached sufficiently high levels to create proper bonding and intermixing between deposited layers. Microstructural assessments revealed a strongly refined, homogeneous and equiaxed microstructure for all builds, featuring an average grain size smaller than 5 μm. The mechanical properties of the as-printed builds, such as hardness, were reduced as compared to the feedstock material.
Interestingly, a post-manufacturing heat treatment comprising ageing at 170°C for 20 hours was already sufficient to significantly enhance the mechanical properties of the builds with a deposition temperature above the solvus temperature of the alloy. In fact, the hardness of the top layers came close to the values of the feedstock material where the top layer, that was deposited last, showed a value that even exceeded that of the feedstock material. Furthermore, following the post-deposition heat treatment, no abnormal grain growth (AGG) was occurred within the microstructure.
Overall, FSEAM proves to be a promising method for solid-state additive manufacturing of aluminium alloys, offering defect-free builds, microstructural uniformity, and notable enhancements in hardness, when coupled with tailored post-deposition ageing heat treatments and further improvements in temperature control during build fabrication.