Finite element modelling for component-level additive manufacturing applied to residual stress-deformation mitigation and fatigue crack initiation in welded connections
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2023-08-21Author
Zhou, Jinbiao
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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.