Influence of anisotropy on the deformation behaviour in microtensile 316L steel specimens fabricated by laser powder bed fusion (PBF-LB/M)

Abstract

Laser powder bed fusion of metals (PBF LB/M) allows fabrication of intricate structures without any need of tools and moulds. In this process the building orientation becomes a significant issue for the complex structures. Here we fabricated small size 316L steel tensile specimens aligned with their loading axis perpendicular and parallel to the building direction by PBF LB/M. Microstructure and mechanical properties were characterized by colour etching, in-situ tensile testing in a scanning electron microscope, nanoindentation and digital image correlation. The orientation of the melt pools has a significant impact on the deformation behaviour of the PBF LB/M specimens. In parallel specimens (polar angle = 90o) the semi-circular melt pools are aligned perpendicular to the loading axis resulting in an accommodation of larger amount of plasticity as compared to the perpendicular specimens (polar angle = 0o). While strain is localized in the case of perpendicular specimens because primary slip lines are aligned perpendicular to the loading axis, strain is homogenously distributed along the longitudinal axis of the parallel specimens because primary slip lines are aligned parallel to the loading axis. Twin-induced-plasticity and transformation induced plasticity are the deformation mechanisms observed on the fractured specimens. Strain hardening rate in stage IV extended to a higher stress value for the perpendicular specimens as compared to parallel specimens because cross slip is confined in the fusion boundaries. The building direction has shown significant influence on the deformation mechanisms of the PBF LB/M steels which may be a significant issue to overcome for the complex shape specimens.