<p>There
has been a long-standing need in the marketplace for the economic production of
small lots of components that have complex geometry. A potential solution is
additive manufacturing (AM). AM is a manufacturing process that adds material
bottom-up. It has the distinct advantages of low preparation cost and high geometric
creation capability. Components fabricated via AM are now being selectively
used for less-demanding applications in motor vehicles, consumer products,
medical products, aerospace devices, and even some military projects.</p><p><br></p>
<p>For engineering
applications, high value-added components require consistency in the fatigue
properties. However, components fabricated by AM have large variation in the
fatigue properties compared to those by conventional manufacturing processes. To
alleviate unpredictable catastrophic failures of components, it is essential to
study and predict fatigue life. Previous study reported that fatigue crack
initiation process accounts for a large portion of fatigue life, especially for
low loading amplitude and high cycle fatigue. However, this major portion of
fatigue life prediction is mostly ignored by main stream researchers working on
fatigue modeling. For industrial applications, engineers often specify a lower
stress condition to obtain a higher safety factor. Under these circumstances,
fatigue crack initiation becomes even more important, so it is essential to further
study of crack initiation.</p><p><br></p>
<p>The
objective of this research is to develop a fatigue crack initiation model for
metal components produced by AM. To improve life prediction accuracy, the model
must incorporate the effect of different microstructures, which are typically
produced by AM due to a large number of repetitive cycles of re-heating and re-cooling
processes. To fulfill this objective, the tasks are separated into three studies:
(1) developing a temperature model to simulate temperature history, (2) modeling
the component’s microstructure for the potential crack initiation zone, and (3)
developing a fatigue crack initiation model for life estimation. A summary of
each task is provided in the following.</p>
<p>First,
the role of temperature model is to understand the mechanism that leads to the
variation of microstructures. The existing temperature models are
computationally expensive to obtain an accurate prediction of the temperature
history due to repetitive heating and cooling. The main reason is that these
models considered entire boundary conditions of all the material points. In
this section, we proposed and employed the concept of effective computation zone,
which can save the computational time significantly for AM process. </p><p><br></p>
<p>Second,
it is critical to include the effect of microstructure in the fatigue life
model since the microstructure variation at different locations within the real
AM component is large. The grain size variation is modeled by using representative
volume element, which is defined as a volume of heterogeneous material that is
sufficiently large to be statistically representative of the real component’s
microstructure. Regarding phase transformation, a continuous cooling
transformation (CCT) diagram is a useful tool that can be used with a thermal
model for microstructure design and manufacturing process control. However,
traditional CCT diagrams are developed based on slow and monotonic cooling
processes such as furnace cooling and air cooling, which are greatly different
from the repetitive heating and cooling processes in AM. In this study, a new
general methodology is presented to create CCT diagrams for materials
fabricated by AM. We showed that the effect of the segmented duration within
the critical temperature range, which induced precipitate formation, could be
cumulative. </p><p><br></p>
<p>Third, the
existing fatigue crack initiation life model has poor accuracy. One of the reasons
for the poor accuracy is the coefficients change due to the variation in
microstructure is not accounted for. In this section, a semi-empirical fatigue
crack initiation model is presented. The important coefficients include maximum
persistent slipband width, energy efficiency coefficient, resolved shear stress
and plastic slip rate per cycle. These coefficients are modeled and determined
as a function of microstructure, which can improve the accuracy of life
estimation.</p><p><br></p>
<p>The contribution
of this study is to provide a new engineering tool for designing the melting AM
process based on scientific research. With this tool, the fundamental mechanism
contributing to a large variation of the fatigue life of the metal components
made by AM process can be understood, attributed, predicted and improved. The seemly
‘stochastic’ nature of fatigue life of the AM components can be changed to be
more deterministic and predictable. This approach represents a major advance in
fatigue research on AM materials. The model
developed is considered as a tool for research, design, and control for
laser-based AM process applications. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12233465 |
| Date | 12 October 2021 |
| Creators | Chun-Yu Ou (8791262) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Modeling_Material_Microstructure_and_Fatigue_Life_of_Metal_Components_Produced_by_Laser_Melting_Additive_Process/12233465 |
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