Role of Porosity in Mechanical Performance of 6061 Al Alloy Fabricated via Wire Arc Additive Manufacturing

In this work, two distinct microstructural configurations were designed for wire arc additive manufacturing (WAAM) of 6061 Al alloy to elucidate the role of porosity in governing mechanical performance. The two samples exhibited contrasting porosity levels—an area fraction of 1.3% with an average pore size of 26.0 μm for one sample versus 3.4% and 29.8 μm for the other—while the low-porosity sample possessed coarser grains, higher dislocation density, and more densely distributed grain-boundary secondary phases, conventionally regarded as detrimental. Despite these unfavorable characteristics, the low-porosity sample had not only a higher ultimate tensile strength (183–190 MPa vs. 170–177 MPa) but also a substantially larger elongation (18.6–19.2% vs. 6.2–10.7%) and a drastically lower tensile anisotropy (3.2% vs. 42.1%). The elevated porosity of the high-porosity configuration introduced abundant stress concentration sites and provided preferential pathways for crack initiation and propagation, triggering premature fracture and severely curtailing the uniform plastic deformation stage. In contrast, the reduced porosity effectively suppressed crack nucleation, prolonged the uniform deformation stage, and fully compensated for the adverse effects of grain coarsening and secondary-phase coarsening. These results reveal a competing mechanism in as-deposited 6061 alloy, in which the ductility benefit of porosity suppression fully over-rides the classical detrimental effects of grain and secondary-phase coarsening, indicating that pore mitigation, rather than grain refinement, should be assigned the highest priority in the process optimization of WAAM Al alloys.