Finite element modelling for component-level additive manufacturing applied to residual stress-deformation mitigation and fatigue crack initiation in welded connections
Zhou, Jinbiao
Zhou, Jinbiao
Loading...
Publication Date
2023-08-21
Type
Thesis
Downloads
Citation
Abstract
Additive manufacturing (AM) has attracted significant attention in many applications due to its capability of fabricating complex and customized metal parts. However, the potential for high inherent residual stresses that produce distortion in AM components and have detrimental effects on fatigue life, prevents more widespread application of the AM technique. Efficient and accurate prediction of residual stress and distortion at component level (macro-scale) is a complex task. Nowadays, the petrochemical and energy industries are evolving towards higher temperature and pressure operating conditions to improve efficiency and thereby reduce emissions, and thus help to reduce the “greenhouse effect”, as well as increased operational load cycling, to facilitate increased renewable uptake. Such increased temperature-pressure and load cycling conditions inevitably poses significant new challenges for safe design and life analysis of key high temperature components, requiring materials and structures, such as welded connections, which are resistant to thermal fatigue, creep-fatigue and thermo-mechanical fatigue. The heat affected zone (HAZ) of welded connections is particularly susceptible to fatigue crack initiation (FCI) due to the increased thermo-mechanical fatigue. This thesis presents the development of computational methods for addressing two specific challenges in relation to additive manufacturing and welding of metals, respectively. As a first step towards an efficient three-dimensional finite element (FE) methodology for thermo-mechanical simulation of additive manufacturing processes for realistic full-scale engineering components, the directed energy deposition (DED) manufacture of a realistic Ti-6Al-4V component is investigated using a recently developed AM capability of the nonlinear FE code, Abaqus. The method essentially combines the ‘element birth’ method with a layer-scaling approach for highly efficient simulation of AM processes. It is shown that the method can be implemented to achieve highly-efficient and highly accurate simulation of DED of the complex, large-scale Ti-6Al-4V component with respect to (i) thermal histories for selected sample locations, to facilitate microstructure prediction, for example, and hence, mechanical properties, and (ii) residual stresses, as required for accurate assessment and design for structural integrity, such as fatigue. The predicted results are successfully validated against published experimental and numerical data. The effects of different scanning strategies on temperature histories and residual stresses are investigated as a basis for identification of optimal manufacturing protocols. Finally, fatigue life predictions of the Ti-6Al-4V component have been considered based on the Basquin-Goodman equation with the effect of residual stress taken into account. The new Abaqus-based method is implemented for simulation method with detailed validation for powder bed fusion (PBF) manufacture of a complex 3D Inconel 625 benchmark bridge component (macro-scale) to predict residual stress and distortion at component-level (macro-scale) efficiently and accurately. It is shown that the new Abaqus-based method can achieve very good agreement with the published benchmark experimental measurements from neutron diffraction, X-ray diffraction (XRD), contour method and coordinate measurement machine (CMM) by the National Institute of Standards and Technology (NIST) laboratory. The key advantages of this method are the significant improvement in computational efficiency, on the one hand, and the ease of implementation, on the other hand. Both of these will facilitate industrial application of this technique, which will, in turn, foster more widespread use of AM itself. The new modelling method has been applied to identify optimal preheating conditions for mitigation of residual stresses and distortions and, thus, inevitable increase in fatigue life. A methodology is presented for physically-based prediction of high temperature fatigue crack initiation in 9Cr steels, with specific application to welding-induced material inhomogeneity due to thermally-induced metallurgical transformations. A modified form of the Tanaka-Mura model for slip band formation under cyclically-softening conditions is implemented in conjunction with a physically-based unified cyclic viscoplasticity constitutive model. The physically-based constitutive model accounts for the key strengthening mechanisms, including precipitate hardening and hierarchical grain boundary strengthening, successfully predicting cyclic softening in 9Cr steels. A five material, finite element model of a P91 cross-weld test specimen, calibrated using the physically-based yield strength and constitutive models, successfully predicts the measured detrimental effect of welding on high temperature low-cycle fatigue crack initiation for P91 cross weld tests, via the modified Tanaka-Mura model. A key finding here is the requirement to adopt an energy-based Tanaka-Mura method to account for cyclic softening in 9Cr steels, with packet size as the critical length-scale for slip band formation. The developed AM process (DED and PBF) simulation models and mechanisms-based FCI model are key building blocks towards a pragmatic process-structure-property performance (PSPP) design tool for industry, which not only can guide the selection of optimal manufacturing protocols, but also facilitate integration of computational modelling for industrial application to complex geometries. The ultimate aim of the work presented here is to directly contribute to this PSPP tool for fatigue of complex geometry AM components including residual stress effects, e.g. conformally-cooled injection moulding dies.
Funder
Publisher
NUI Galway