<p>Aeronautical
products additively manufactured by Selective Laser Melting (SLM), are known to
have fatigue properties which are negatively impacted by porosity defects,
microstructural features and residual stresses. Little research is available
studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency
problem, the large focus being on the long FCG. This thesis seeks to add useful
knowledge to the understanding of the mechanisms for short crack growth variability
in SLM manufactured Ti-6Al-4V, with the two variables for the process
conditions and build directions investigated. An in-situ FCG investigation
using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually
observe and quantify the short crack path evolution. Crack growth, deflections
and porosity interactions were noted and discussed in relation to
microstructure, build layer thickness and build layer orientation. A novel use
of in-situ energy dispersive x-ray diffraction (EDD) was able to show the
lattice strains evolving as a propagating crack moved through a small region of
interest. The results presented show the ability to reliably obtain all six elastic
strain tensor components, and interpret useful knowledge from a small region of
interest. </p>
<p> </p>
<p>There
are conflicting views in literature with respect to the damage tolerance
behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et
al., the more prominent studies were considered with Leuders et al. showing the
highest long FCG rates for cracks parallel to the build layer and Cain et al.
showing cracks propagating through successive build layers as highest [1]–[3].
Cain et al. and Vilaro et al. report significant anisotropy in long FCG for
different build orientations whereas Edwards
and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized
build parameters without any (detectable) pores, the building direction does
not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported
martensitic microstructures and the presence
of prior
grain structures for as built SLM Ti-6Al-4V.</p>
<p> </p>
<p>No
studies to the authors knowledge have considered the short FCG of SLM
manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance
behaviors reported in literature [1]. In this work small cross-sectional area (1.5
x 1.5
) samples in
two different build conditions of as built SLM Ti-6Al‑4V are studied. The short
FCG rate of three different build directions was considered with cracks
parallel to the build layers shown to be the most damaging. The microstructure
and build layer are shown to be the likely dominant factors in the short FCG
rate of as built Ti-6Al-4V. In terms of porosity, little impact to the
propagating short crack was seen although there is local elastoplastic behavior
around these defects which could cause toughening in the non-optimized build
parameter samples tested. The fracture surfaces were examined using a Scanning
Electron Microscope (SEM) with the results showing significant differences in
the behavior of the two build conditions. From the microindentation hardness
testing undertaken, the smooth fracture surface of the optimized sample
correlated with a higher Vickers Hardness (VH) result and therefore higher
strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH
result and therefore strength. Furthering the knowledge of short FCG in SLM
manufactured Ti-6Al-4V will have positive implications to accurately life and
therefore certify additive manufactured aeronautical products.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7418339 |
| Date | 17 January 2019 |
| Creators | Michael C. Waddell (5930921) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/INVESTIGATION_OF_SHORT_FATIGUE_CRACK_GROWTH_AND_DAMAGE_TOLERANCE_IN_ADDITIVE_MANUFACTURED_Ti-6Al-4V/7418339 |
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