Anisotropy in Metals Processed by Laser Powder Bed Fusion: Elastic and plastic anisotropy in static and dynamic mechanical loading

Abstract

This thesis is addressed to engineers and researchers working with powder bed fusion (PBF) additive manufacturing (AM) and provides insight into anisotropic mechanical properties for metals subjected to static and dynamic loading. Three common AM materials are investigated; Inconel 718, AlSi10Mg, and maraging steel grade 300. The thesis highlights the need to account for anisotropy when performing finite element analysis and provides methodology for determining anisotropic material models. The focus is on PBF metals subjected to appropriate heat treatments commonly used in the industry. In the case of static loading the focus is on anisotropy caused by either preferred crystallographic orientation or alignment of inclusions, defects, or particles in the microstructure. In the case of dynamic loading, the focus has been on the contribution of surface roughness on fatigue life reduction. The objective is to determine robust anisotropic material models in an efficient way. Digital image correlation (DIC) in combination with tensile testing is used to investigate the strain fields on rectangular tensile specimens in uniaxial tension. This is complimented with microstructural evaluation with scanning electron microscopy and fractography with both optical and electron microscopy. For dynamic loading, a combination of experimental results and numerical results are analysed to benchmark build orientations in an effort to maximise fatigue life. The main scientific contributions are presented in the form of six peer-reviewed articles published in international journals. The main contributions are summarized as: • A method for determining anisotropic elastic constants using DIC and an optimisation algorithm in Matlab. • A significant reduction of anisotropy in AlSi10Mg by elevating the process temperature in the PBF process. • A description of how the melt pool boundaries causes anisotropy, and how this can be reduced by proper heat treatment. • A methodology for benchmarking the fatigue life of AM components based on simple surface roughness measurements and numerical analysis. • A methodology and software suite that can optimise part orientation in the PBF build chamber to maximise fatigue life. • A novel fatigue test specimen that experimentally captures the effect of asbuilt surface roughness on fatigue life. The contributions in this thesis can be applied by researchers and engineers when designing parts for manufacturing with AM. Anisotropic elastic constants can be used directly in finite element analysis to optimise design for AM. Future work can be done to improve the models for fatigue life estimation. The methodology for benchmarking orientations based on surface roughness are only valid when fatigue initiation stems from the as-built surface. Experimental fatigue data gathered with the novel test specimen can be used to further improve this methodology.