Creep Crack Growth Testing and Analysis of Laser Powder Bed Fusion 316L Stainless Steel

Abstract

Additive manufacturing, specifically laser powder bed fusion, is a novel technique which could become key to manufacturing net-shaped metal components with complex geometries. By manufacturing components in successive layers, restrictions on geometric complexity as well as better material economy, reduced manufacturing variability and a reduced manufacturing footprint could be achieved. The current issues with this technique are that very high residual stresses, variations in microstructure and significant pores can be developed. In this work the creep crack growth (CCG) resistance of LPBF 316LSS has been examined by performing tests on compact tension, C(T), samples. The C(T) samples were manufactured in three orthogonal orientations to understand the anisotropic nature of LPBF. The creep crack paths were examined by interrupting tests at the cusp of sample failure, sectioning and preparing the samples for metallographic analysis. Lack of fusion porosity, which forms along layer boundaries, was found to be the dominant factor in creep crack growth rate behaviour, with samples loaded normal to build layers initiation and growing multiple creep cracks. Due to the atypical CCG behaviour of many of the LPBF samples, classical CCG theory could not be applied to analyse the results. It was concluded that for the cases examined, samples which are loaded perpendicular to the build direction but have the crack growing in the build direction i.e. through the build layers sequentially, have the highest CCG resistance due to the relatively lower stress concentration factor of lack of fusion porosity, despite the higher creep strain rate expected in this orientation compared to loading a sample along the build direction.