On the determination of residual stresses in additively manufactured lattice structures.

Tobias Fritsch,Romeo Saliwan-Neumann,Alexander Evans,Giovanni Bruno,Lena Farahbod-Sternahl,Michael Hofmann,Maximilian Sprengel

doi:10.1107/s1600576720015344

Abstract

The determination of residual stresses becomes more complicated with increasing complexity of the structures investigated. Additive manufacturing techniques generally allow the production of 'lattice structures' without any additional manufacturing step. These lattice structures consist of thin struts and are thus susceptible to internal stress-induced distortion and even cracks. In most cases, internal stresses remain locked in the structures as residual stress. The determination of the residual stress in lattice structures through nondestructive neutron diffraction is described in this work. It is shown how two difficulties can be overcome: (a) the correct alignment of the lattice structures within the neutron beam and (b) the correct determination of the residual stress field in a representative part of the structure. The magnitude and the direction of residual stress are discussed. The residual stress in the strut was found to be uniaxial and to follow the orientation of the strut, while the residual stress in the knots was more hydro-static. Additionally, it is shown that strain measurements in at least seven independent directions are necessary for the estimation of the principal stress directions. The measurement directions should be chosen according to the sample geometry and an informed choice on the possible strain field. If the most prominent direction is not measured, the error in the calculated stress magnitude increases considerably.

Highlights

Additive manufacturing (AM) technologies promise dramatic advances in many industrial aspects including part design, production flexibility, and reductions of time to market and scrap
We showed that the alignment of the lattice structure in the neutron beam needs prior X-ray computed tomography scans to acquire the exact internal geometry of the part
Because of the filigree structure, mirror scans during the neutron diffraction experiment are needed to avoid pseudo-strains

Summary

Introduction

Additive manufacturing (AM) technologies promise dramatic advances in many industrial aspects including part design, production flexibility, and reductions of time to market and scrap. For the laser powder bed fusion (L-PBF) AM technique, microstructures are often unconventional and residual stress (RS) is always present after production (Serrano-Munoz et al, 2020; Mishurova et al, 2019; Thiede et al, 2018; Wang et al, 2017). Today the characterization of RS in AM parts is mainly performed by destructive or semi-destructive techniques [e.g. the contour method (Ahmad et al, 2018; Moat et al, 2011; Vrancken et al, 2014), hole drilling (Casavola et al, 2009) and bridge curvature measurements (Kruth et al, 2010; Bagg et al, 2016; Mishurova et al, 2017)]. Many examples of nondestructive RS investigations are focused on surface investigations by means of laboratory X-ray diffraction (XRD) (Mercelis & Kruth, 2006; Vrancken et al, 2013).

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