Nondestructive characterization of laser powder bed fusion parts with neutron Bragg edge imaging

Abstract

Laser powder bed fusion is an efficient technique for additive manufacturing of metallic materials. The quality of the material produced depends on the optimization of a large range of build parameters and the complex thermo-mechanical build process is prone to inducing detrimental material features such as porosity and residual stresses negatively affecting fatigue resistance and lifetime. Here we apply neutron Bragg edge radiography in a parametric study on printing 316L steel. The parameters concerned are the laser scanning speed and strategy as well as the optional use of support structures. Analyses of the full field single shot wavelength-resolved Bragg edge radiography data enables to characterize local density inhomogeneities, as well as cracks, based on the long wavelength tail of the spectrum and variations of the stress field but also textural features based on the Bragg edge pattern. It is found that in the performed study not only respective differences in the residual stresses due to parameter variation are manifesting but also systematic irregularities due to machine imperfections (e.g. issues with the powder coater) are observed in the printed samples. The study supports the use of the parallel scanning strategy without supports and with the lower utilized scanning speed.