Development of a novel hybrid powder bed friction stir additive manufacturing-friction stir forging process for fabrication of AA6061 spur gear: process development, thermomechanical modeling, and process-structure-property correlation

Existing manufacturing processes often struggle to fabricate lightweight gears with reliable mechanical properties while simultaneously achieving full densification, microstructural homogeneity, and near-net-shape geometric complexity. In the present work, a novel hybrid solid-state manufacturing route combining powder bed friction stir (PBFS) additive manufacturing and friction stir forging (FSF) was developed to fabricate AA6061 spur gears from powder feedstock by sequentially integrating both processes within a single manufacturing setup. The process involved layer-wise deposition of AA6061 powders via friction-stir-assisted severe plastic deformation to form a consolidated billet, followed by friction stir forging of the billet into the final gear geometry. A fully coupled thermo-mechanical numerical model based on the coupled Eulerian-Lagrangian (CEL) framework was developed to analyze densification behavior, cavity filling, and local deformation conditions during the process. The simulation predicted complete densification of the initially less-dense powder bed and identified a billet height of ~30 mm as sufficient for near-complete cavity filling with minimal residual voids. The predicted thermo-mechanical fields revealed temperatures of ~470 to 501 °C, accumulated strain of ~0.064 to 17.5, strain rates of ~0.04 to 2 s −1 , and the resultant material flow velocity of ~1.6 × 10 −4 m s −1 to ~2.4 × 10 −3 m s −1 across the gear tooth, establishing spatially varying deformation history at the end of the FSF process. EBSD analysis showed that the fabricated gear developed a predominantly equiaxed recrystallized microstructure throughout the tooth profile, with grain size remaining nearly uniform along the tooth height, while exhibiting slight variation in the transverse and build directions. The finest grain structure (~2.86 ± 1.48 μm) formed under intermediate deformation conditions where deformation-driven grain subdivision and continuous dynamic recrystallization remained dominant, whereas regions subjected to continued thermo-mechanical exposure exhibited slight grain coarsening (~4.21 ± 1.45 μm) due to recovery-assisted grain boundary migration. The fabricated gear exhibited a relatively narrow, homogeneous hardness range of ~49–59 HV across the gear tooth, lower than that of the AA6061-T6 substrate, consistent with the substantial softening associated with the PBFS-FSF thermal cycle.