Crystal plasticity model for cyclic hardening-softening of 316L steel produced by laser-powder bed fusion including the role of sub-grain structures

Recent literature has emphasised the need to assess the process-structure-properties of additively manufactured (AM) metals to exploit their full capability. Many studies focused on the static mechanical properties of AM materials and their relationship with their microstructure, whereas the cyclic elastoplastic response was rarely addressed. In the present work, a crystal plasticity (CP) model is proposed to model the macroscopic stress response during cyclic strain in a 316L steel produced by laser-powder bed fusion (L-PBF). To accurately capture cyclic hardening-softening of the material, the proposed model includes essential microstructural features – such as crystallographic texture, AM-induced intragranular cellular structure and persistent slip bands (PSBs) – in a dislocation-based framework. The model parameters are estimated from microstructural observations by the authors' experimental investigation and literature. After experimental validation of the model's cyclic response at 0.4 % strain amplitude, parametric analyses elucidate interrelationships between microstructure and cyclic response. Simulations, supported by experiments, suggest that the AM cellular structure drives the dislocation evolution. The accumulation of dislocations rules the initial moderate cyclic hardening, while the depletion of dislocations dominates the following early cyclic softening due to the formation of PSBs. The proposed model can also serve as a tool to tune the cyclic response by adjusting the AM cell size and initial dislocation density, thus controlling the cyclic hardening and hardening-softening transition. Given the relationship between cyclic softening, PSBs and fatigue crack nucleation, the model can be extended to assess the fatigue damage during the early cyclic response of AM metals.