On the constitutive relationship between solidification cells and the fatigue behaviour of IN718 fabricated by laser powder bed fusion

IN718 combines excellent mechanical properties with a good weldability and is therefore an ideal alloy for laser powder bed fusion (LPBF). Knowledge of the relationship between its as-built microstructure, particularly solidification cells, and its fatigue properties is needed to better utilise additively manufactured microstructures and guide their further optimisation. This study presents a comprehensive investigation of the as-built microstructure and the associated monotonic and fatigue properties of LPBF IN718 aimed at highlighting the influential effect of solidification cells on monotonic and cyclic plasticity. In monotonic tension, cells induced pronounced strain hardening and good ductility by acting as strong yet not impenetrable obstacles to dislocation slip. In fatigue loading, cyclic hardening followed by cyclic softening was linked to the stability of the as-built solidification cells, the high initial dislocation densities and the subsequent rearrangements of such dislocations during cyclic loading using the similitude relation and the evolution of friction and back stresses. By thoroughly investigating the evolution of the cyclic response of samples printed using two different scanning patterns, the relationship between process (scanning line length and thus local substrate temperature), microstructure (dislocation cell size and their spatial arrangement) and mechanical properties (cyclic hardening and softening responses) was comprehensively discussed.