Design concepts of additive manufacturing require an in-depth understanding of the linkage of process, microstructure, and mechanical properties. Laser Powder Bed Fusion (L-PBF) 316L Stainless Steel (SS) can achieve comparable or better mechanical performance than its counterparts manufactured by casting and forging [1]. This is due to the formation of a cellular structure that strengthens the material in the interior subgrains during the rapid solidification process. Thus, the microstructural heterogeneity is dependent on the cellular structure. It was shown [2] that the critical resolved shear stress (CRSS) can be used as a first indicator of a material's fatigue strength and, thus, the material potential for applications exposed to fatigue. This study focuses on determining a relationship between the CRSS and the apparent cellular structure by conducting and correlating micropillar tests of L-PBF 316L SS. Multiple pillars were site-specifically prepared from the selected grains with known orientation, cell morphology and size, and local subgrain misorientation. A variation of the cell size from 320 nm up to 783 nm was observed resulting in variations of the CRSS between 215 MPa and 294 MPa. Weak correlations were observed between the CRSS of the equiaxed and longitudinal cells at the top and the cell wall distance along the activated slip plane indicated in the literature [3]. The Hall-Petch strengthening effect of the effective cell size and the Taylor strengthening effect of the dislocation density were observed in the CRSS of the equiaxed cells. Considering the distribution of more equiaxed cells perpendicular to the building direction, smaller cells are suggested for the structural design under a load along the building direction due to the increased CRSS.

References

[1] L. Cui; F. Jiang; R.L. Peng; R.T. Mousavian; Z. Yang; J. Moverare International Journal of Plasticity, 2022, 149, 103172.

[2] M. Mlikota; S. Schmauder Metals, 2018, 8, 883.

[3] M. Wang; M.H. Fan; S. Cruchley; Y.L. Chiu International Conference on Processing and Manufacturing of Advanced Materials Processing, Fabrication, Properties, Applications, 2021, Austria, 1576-1584.