Electron beam additive manufacturing (EBAM) is one of powder-bed-fusion additive manufacturing processes that are capable of making full density metallic components. EBAM has a great potential in various high-value, small-batch productions in biomedical and aerospace industries. In EBAM, because a build part is immersed in the powder bed, ideally the process would not require support structures for overhang geometry. However, in practice, support structures are indeed needed for an overhang; without it, the overhang area will have defects such as warping, which is due to the complex thermomechanical process in EBAM. In this study, a thermomechanical finite element model has been developed to simulate temperature and stress fields when building a simple overhang in order to examine the root cause of overhang warping. It is found that the poor thermal conductivity of Ti- 6Al-4V powder results in higher temperatures, also slower heat dissipation, in an overhang area, in EBAM builds. The retained higher temperatures in the area above the powder substrate result in higher residual stresses in an overhang area, and lower powder porosity may reduce the residual stresses associated with building an overhang.

1. Proceedings of the ASME 2014 International Manufacturing Science and Engineering Conference
MSEC2014
June 9-13, 2014, Detroit, Michigan, USA
MSEC2014-4063
THERMOMECHANICAL INVESTIGATION OF OVERHANG FABRICATIONS IN ELECTRON BEAM
ADDITIVE MANUFACTURING
Bo Cheng, Ping Lu and Kevin Chou
Mechanical Engineering Department
The University of Alabama
Tuscaloosa, AL 35487, USA
KEYWORDS
Electron beam additive manufacturing (EBAM), Finite element analysis, Overhang geometry, Thermomechanical modeling;
Simulations
ABSTRACT
Electron beam additive manufacturing (EBAM) is one of
powder-bed-fusion additive manufacturing processes that are
capable of making full density metallic components. EBAM
has a great potential in various high-value, small-batch
productions in biomedical and aerospace industries. In EBAM,
because a build part is immersed in the powder bed, ideally the
process would not require support structures for overhang
geometry. However, in practice, support structures are indeed
needed for an overhang; without it, the overhang area will have
defects such as warping, which is due to the complex
thermomechanical process in EBAM. In this study, a
thermomechanical finite element model has been developed to
simulate temperature and stress fields when building a simple
overhang in order to examine the root cause of overhang
warping. It is found that the poor thermal conductivity of Ti-
6Al-4V powder results in higher temperatures, also slower heat
dissipation, in an overhang area, in EBAM builds. The retained
higher temperatures in the area above the powder substrate
result in higher residual stresses in an overhang area, and lower
powder porosity may reduce the residual stresses associated
with building an overhang.
INTRODUCTION
Additive manufacturing (AM) based on “layer-adding”
fabrications is a group of technologies, by which a physical
solid part is produced directly from digital data of the part
geometric model. This group of technologies offers many
design and manufacturing advantages such as short lead time,
design freedom in geometry, and tooling-free productions.
Powder-based electron beam additive manufacturing (EBAM)
is a relatively new AM technology [1]; it utilizes a high-energy
electron beam, as a moving heat source, to melt and fuse metal
powder and produce a solid part in a layer-by-layer fashion. In
building each layer, the processes involve powder spreading,
pre-heating, contour melting and hatch melting. In powder
spreading, a metal rake is utilized to uniformly distribute one
layer of powder, supplied from the hoppers. Then, pre-heating
is applied using a single beam at a high speed (e.g., 14 m/s),
with multi-pass scans, to reach a high temperature across the
entire powder-bed surface. The purpose of pre-heating is to
slightly sinter the powder so to prevent them from expelling
when bombarded by high-energy electrons. Thus, pre-heated
powder will possess a certain level of porosity. Following pre-
heating, contour-melting and hatch-melting stages take place,
during which an electron beam, either multiple split or single,
moves across the powder-layer surface tracing the cross-section
contour of the model and then raster-scan throughout the inside
of the contour at a lower scanning speed (e.g., around 0.5 m/s).
Once all layers are completely built, the system is cooled down,
remained in vacuum, until it is close to the room temperature
before the build chamber can be opened. Then, the entire
powder bed is removed from the machine for cleaning: sand
blasting to clean off loose sintered-powder from the build part.
In post-processing, support structures, if any, will be removed,
1 Copyright © 2014 by ASME

2. mostly by a mechanical means. Recently, Oak Ridge National
Laboratory published a video animation of the EBAM process
[2] that offers good illustrations of the process details. EBAM
is one of a few AM technologies capable of making full-density
metallic parts, which drastically broaden AM applications in a
wide variety of industries [1,3].
Because of the high energy density, EBAM has the
potential to work with many material classes, e.g., aluminum
alloys [4], tool steel [5], cobalt-based superalloys [6], Cu [7],
and Inconel alloys [8], etc. One titanium alloy, Ti-6Al-4V, was
the first material extensively researched [9,10], also widely
used, in EBAM technologies for aircraft parts and medical
implants. Moreover, Intermetallic groups such as titanium
aluminide have also been studied for EBAM applications, but
the process development was considered very time consuming
[11]. Another intriguing EBAM capability is to fabricate
complex geometries and structures (e.g., meshed, porous,
cellular). Schwerdtfeger et al. applied EBAM to fabricate a
unique structure that exhibits the “auxetic behavior,” a negative
Poisson’s ratio, good for high impact and shear resistances [12].
Despite of an intense interest in attainable part accuracy by
EBAM [19], few studies have been emphasized on the
geometric aspects in EBAM. Cooke and Soon [20] studied the
geometric accuracy of metallic test parts manufactured by
EBAM and other powder-based metal AM processes. The
authors reported that overall, the observed errors of EBAM
parts are significantly larger, at least an order of magnitude,
than those of typical machined parts. The errors seem to be
repeatable, providing opportunities for compensation strategies.
In research conducted by Koike et al. [21], an enlarged view of
the gauge section of the EBAM Ti-6Al-4V specimen shows a
rough exterior appearance comparable to as-cast counterparts.
The surface of typical EBAM specimens generally shows a
feature of rippled layers.
EBAM is a complicated process and process physics
modeling of EBAM is only sparsely found in literature. Zah
and Lutzman developed a simplified heat transfer model of
EBAM, based on a finite element (FE) method, to study the
melt-pool geometry [22]. Shen and Chou recently initiated a
study to investigate the thermal phenomenon of the EBAM
process. An FE heat transfer simulation incorporating Gaussian
heat flux distribution, fusion latent heat, temperature-dependent
thermal properties was developed to investigate the thermal
response of metal powder in EBAM [23]. Moreover, process
measurements such as temperatures have not been frequently
reported for EBAM either. Zah and Lutzman applied
thermocouples, attached to the build plate, to evaluate
temperature response during EBAM to evaluate the model
developed by this research group [22]. Dinwiddie et al. [24]
and Rodriguez et al. [25] applied different thermal imaging
systems to attempt for in-situ process monitoring and for
feedback controls, respectively. On the other hand, Price et al.
reported of using a near-infrared thermal imager for process
temperature measurements in EBAM [26]. The result indicates
that using an infrared thermography, with a correct spectral
range, for temperature measurements in EBAM is feasible.
One of the advantages in AM technologies is freedom in
part geometry designs, virtually no limit (except resolution).
However, for some AM processes, e.g., the material extrusion
type AM, also known as fused deposition modeling (FDM),
part designs with overhang (or undercut) geometry are
considered not favorable. In such an AM process, a part
overhang will require the so-called “support structure” to carry
the weight of the overhang portion. However, the support
structure has to be removed during post-processing, which may
be time consuming and add additional production cost. Hence,
overhang geometry may be considered as undesired design
geometries. Some approaches have been used to tackle support
structure removals, e.g., designing break-away structures or
using water solvable materials in FDM. On the other hand,
powder-bed-fusion AM processes including EBAM are
considered not requiring a support for an overhang because the
powder bed itself is able to bear the weight of a built overhang.
However, in common practices, still, the general rule is to
arrange support structures underneath an overhang, or defects
such as warping may occur. Defects such as warping associated
with overhang geometry problems in EBAM have not been
frequently studied in literature. Recently, Vora et al. attempted
to benchmark the problems with overhangs and reported cases
that overhang warping occurs [27].
For laser-based powder-bed AM processes such as
selective laser sintering or melting (SLS or SLM), there are a
few studies related to support structures in literature. The focus
was, however, on the design and fabrication of support
structures. Jihabvala et al. reported using pulse laser, instead of
a continuous mode, to fabricate support structures and claimed
the fabricated supports are much easier to be removed [28].
Yan et al. studied “cellular lattice” structures for support and
investigated the effect of cellular geometry parameters. The
evaluation criterion was the ease of removal and access. The
authors also intended to use cellular-type support to minimize
the material volume needed for support [29]. In general, the
above mentioned studies have not comprehensively addressed
the aspect of the need of supports for an overhang and for
suitable support types.
EBAM has a potential to be a cost-effective alternative to
conventional discrete component manufacture for high-value,
small-batch, and custom-designed metallic parts. However, one
of the challenges in part designs is inevitable overhang
geometries. The overhang area will require support structures,
or there will be defects around the overhang area, shown in
Figure 1, including size inaccuracy and, most noticeably,
distortions.
Figure 1. An example of overhang model and associated
warping defect.
2 Copyright © 2014 by ASME

3. Furthermore, post-processing for support removal can be
of a high cost. Figure 2 below shows an example of support
patterns from a complex build part (condyle of a
temporomandibular joint prosthesis pair). The part has such
geometry inevitably requiring support structures in certain
areas, which was obtained by default from commercial
software. Figure 2b displays the area of the support showing
contact (solid) with the build part. Though a mechanical tool
may be used to remove the support structures, small piece by
piece (Figure 2c), it is labor intense and time consuming, with a
risk of damaging the part surface.
Figure 2. (a) and (b) EBAM build condyle (front and back
sides) showing support structures, and (c) same part showing
partial support removed.
There are commercial software packages, e.g., [30], which
can generate support structures with a variety of options
(patterns/size); however, there is no clear guideline of support
design, relying on trial and error and experiences. Moreover, it
needs to be pointed out that those support designs were
developed more for the weight-carrying purpose, to avoid
overhang area deformation due to gravity. It does not address
the powder bed conditions for the process like EBAM.
Moreover, additional questions are: what is the root cause of
overhang defects, and what type of “support” configurations
will function to minimize warping defects, and yet, without
burdening post-processing?
To effectively design necessary supports, it is essential to
understand the source of defects associated with overhang
geometries in EBAM. It is argued that for EBAM, heat load,
instead of gravity load, is more the source of the problems.
Because the powder bed (with sintered powder) has a far poor
thermal conductivity compared to a solid, and thus, heat
dissipation around the overhang area may be less efficient
compared to an area with a solid substrate. The assumption of
overhang defect causes, therefore, is that in the powder area
beneath an overhang, the ineffective heat dissipation and
repeated thermal gradient cycles result in greater thermal
stresses, leading to warping defects. Hence, the research needs
would be a fundamental thermomechanical study of EBAM for
temperature and stress evaluations. The knowledge obtained
may be applied to facilitate investigations of different overhang
configurations, thermal and mechanical responses, and to
enable effective designs for heat load supports and
simultaneously for ease of support structure removals. The
objective of this research is to understand the thermal and
mechanical characteristics in fabricating overhang layers during
the EBAM process. A three-dimensional (3D) finite element
(FE) model was developed to simulate the thermal and
mechanical phenomena in EBAM in a raster-scanning pattern
across an overhang area.
MODELING APPROACH AND SIMULATION
METHODOLOGY
A 3D FE thermomechanical model was developed by using
ABAQUS to simulate the process temperature history, residual
stress distributions and deformation in a multi-layer raster
scanning and deposition on an overhang model. Figure 3(a)
shows the geometry of the overhang model, which has a
substrate dimension of 8 × 8 × 10 mm (length, width, and
height). A smaller model was used in order to keep a
reasonable computational time. The thickness of the substrate is
small comparing to the actual size of common parts; therefore,
only the top section of the actual substrate was modeled.
During simulations, powder layers were added to the substrate
sequentially. The model volume was composed of a powder
substrate on one side and a solid substrate on the other side
(half and half) to represent an overhang fabrication (on top of
the powder substrate), and the powder layers added were Ti-
6Al-4V particles with a given levels of porosity, with each
layer of 70 µm thickness.
The FE simulations consisted of a preheat phase, a melting
phase, and a cooling phase. The preheating phase (also called
initial phase) was simplified and considered as the initial
thermal conditions. During the multi-layer deposition process,
the layers to be deposited were not activated until the melting
phase on that particular layer started. An 8-node coupled
thermal-displacement type of C3D8T was used to mesh the
substrate and the powder layer, and the element size in the
scanning area was 0.2 × 0.2 × 0.05 mm. The total element
number for the double layer deposition model was 6084. Figure
3(b) displays the model after meshing.
(a)
(b)
Support
Support
(c)
3 Copyright © 2014 by ASME

4. Figure 3. (a) Part geometry, and (b) element meshing of the
EBAM overhang model.
The material properties used, including both thermal and
mechanical, temperature dependent, were from previous
studies, [23,31] and [32], respectively. In particular, the thermal
conductivity of Ti-6Al-4V solid and powder were measured at
different temperatures, up to 750 °C. It is worth to point out
that the thermal conductivity of the Ti-6AL-4V powder is much
smaller than the solid counterpart, e.g., 0.63 W/m·K vs. 6.2
W/m·K at 25 °C.
During the melting and cooling phase, the bottom surface
of the model was fully constrained as the boundary condition,
Figure 4(a). The contact between the substrate material and the
powder layers was assumed to be rigid. Convection between
the powder layer and environment was not included due to the
vacuum environment. Hence, only the radiation was considered
in the heat transfer between the powder/part and surroundings.
The substrate and the powder layer were assigned with a
uniform temperature distribution of Tpreheat as the initial thermal
condition. The temperature of the substrate bottom was
confined with a constant temperature of Tpreheat as the thermal
boundary condition. For the multi-layer raster scan, two layers
of powder were simulated; the raster scan is shown in Figure
4(b). The left side of the dashed line is above top of a powder
substrate (the overhang area), while the right side is on top of a
solid substrate. During the simulation, the electron beam
traveled along the top surface of the powder and solid
substrates horizontally with a raster length of 3.2 mm. The
raster scanning area is 3.2 × 3.2 mm2
and the width of the raster
is the same as the beam diameter, assumed as 0.4 mm. There
were total 9 scanning paths along the x direction (longitudinal)
on each layer. The stress and temperature characteristics along
these scanning paths were analyzed from the simulation results.
Since each scan (or layer) was considered as an intermediate
scan (or layer) in a continuous multi-layer part building, the
previous layer was considered to be transformed into solid bulk
materials, meaning solidified from the powder in the previous
melting scan.
Figure 4. (a) Boudary conditions, and (b) raster scan pattern
applied in multi-layer analysis (the left side of the dashed line is
above of a powder substrate, while the right side is on top of a
solid substrate).
After the melting phase, the simulation of the cooling
phase was followed. In the multi-layer raster scan, there were
two cooling steps; the first cooling phase was the part
temperature self-balancing corresponding to a few seconds
interval before a new powder-layer spreading, and the second
one was the final cooling step to the room temperature. The
time for the first cooling step was assumed as 3 sec which is
approximately the duration for the next powder layer spreading
during the EBAM process. The thermal boundary conditions
were the same as those in the melting step. The last cooling step
was realized using general static analysis method in which the
temperature of the EBAM part dropped to room temperature of
25 °C.
Results and Discussion
FE analysis was first performed for the overhang model
with a powder porosity of 50%. Figure 5 shows the temperature
(a)
(b)
(a)
(b)
4 Copyright © 2014 by ASME

5. distribution at the top surface in the model at the end of a two-
layer deposition. It can be seen that the maximum temperature
occurs at the end of the scanning with the magnitude around
2900 °C. In addition, the longer lasting of high temperatures in
the scanned area of the left part (powder substrate) is
significantly higher than the right side of the part (solid
substrate), which is due to the low thermal conductivity of the
powder substrate.
Figure 5. Temperature contour (top view) at the end of two-
layer deposition.
Figure 6 displays temperature history of 2 nodes (A and B
in Figure 4) above the powder and solid substrates. Nodes A
and B are on the scanning path 4, with the same distance to the
center of the EBAM part model (also the interface between the
powder and solid substrates). Node A is above the powder
substrate, while B above the solid substrate. The result shows
that the peak temperature of Node A is about 200 °C higher
than Node B, also the cooling rate of Node A is noticeably
slower than that of Node B.
Figure 6. (a) Temperature history, and (b) magnified area
around the peak temperature of the 2 nodes above powder and
solid substrates.
Figure 7 shows the temperature profile around the electron
beam center location (approximately at 0 mm in the distance
axis) when the beam traveled above the powder and solid
substrates during the scanning path 2 and 7. Both Figure 7(a)
and 7(b) show that a higher maximum temperature occurred
when the beam traveled above the powder substrate; this is
again attributed to the significant lower thermal conductivity of
the powder substrate.
Figure 7. Temperature profiles around electron beam center (0
mm) above solid and powder substrates for path 2 and path 7.
(a)
(b)
(b)
(a)
5 Copyright © 2014 by ASME

6. Figure 8 illustrates the melt pool size shape when the
electron beam traveled above the powder and solid substrates at
the initial (P1) and final (P9) scanning paths. The jagged melt-
pool shape was due to element meshing, not fine enough. Table
1 further lists the dimensions of the melt pools. It can be noted
when the beam traveled from the powder substrate to the solid
substrate area, the melt pool sizes became smaller due to a
higher thermal conductivity of the solid substrate.
Figure 8. Melt pool size and shape in the powder and solid
substrates at initial and final scanning paths.
Table 1. Melt pool size dimension (unit: µm).
Melt pool Path 1 Path 9
Powder Solid Powder Solid
Length 950 900 1180 1100
Width 380 375 400 380
Depth 146 144 160 150
Figure 9 illustrates the von Mises stress distribution in the
overhang EBAM model after the final cooling stage. It can be
found that the maximum von Mises stress occurs at the end of
electron beam scanning location with the magnitude of about
1250 MPa, which exceeds the yield strength of Ti-6Al-4V alloy,
and thus, plastic deformation will occur in this region after the
EBAM process.
Figure 9. von Mises stress contour after final cooling (MPa).
Figure 10(a) displays the von Mises stress evolution of the
nodes (Node A and B, same nodes as in Figure 6) above the
solid and powder substrates. The results indicate that the
maximum von Mises stress at Node A is around 190 MPa
higher than that of Node B after the final cooling process. It has
been indicated earlier that the maximum temperature of Node A
(corresponding to the overhang area) is higher than Node B
during the scanning process. The residual stress after the final
cooling is resulted from the temperature at the end of the
second layer deposition, which is consistent with the
observation from the temperature results of the two nodes
(Figure 6).
Figure 10. (a). von Mises stress evolution, and (b) magnified
time zone around the peak stress of 2 nodes above solid and
powder substrates.
Figure 11 shows the von Mises stress profile at different
scanning paths on one-layer deposition after final cooling
process. It can be noted that the average residual stress on the
scanning path increases with the scanning path number. The
trend of residual stress on each path is relatively uniform along
the longitudinal direction in the individual left or right sides,
except the 7th
scanning path, which is close to the edge of the
scanning area. Moreover, the stress on the left part (powder
substrate) is greater than that on the right part (solid substrate).
(b)
(a)
6 Copyright © 2014 by ASME

7. Figure 11. von Mises stress at different paths from one-layer
deposition after final cooling.
Figure 12 displays the von Mises stresses at the top surface
after the cooling process comparing the single and double-layer
depositions. The results indicate that the average residual
stresses are slightly different between single vs. double-layer
depositions. For example, the average residual stress on the 2nd
scanning path for the single layer deposition model is about 480
MPa after final cooling, and the average residual stress
decreases to about 470 MPa for the double layer deposition. It
is also noted that the residual stress on the left half
(corresponding to the overhang part) is consistently greater than
that in the right half (above solid substrate) for all scanning
paths.
Figure 12. von Mises stress after final cooling deposited with
single and double layers.
Simulations of cases with different porosity levels, 50% vs.
35%, were conducted to examine the porosity effects. Figure 13
shows the temperature profiles on the paths for the EBAM
model with different porosity levels. It can be noted that the
average temperature on the scanning path increases slightly
with the scanning paths. It is also clearly evident that for the
powder with a larger porosity, the temperatures above the
overhang area are much higher, over 300 °C in average, so is
the difference in temperatures in the areas between above the
powder and above the substrate.
Figure 13. Temperature profiles on the scanning paths for two
different powder porosity levels.
Figure 14 shows the von Mises stress contours for the
EBAM model with porosity of 35% and 50%. It can be noted
that with the increase of powder porosity, the residual stress in
the left half (above powder substrate) of the model becomes
larger, 1252 MPa vs. 1074 MPa, comparing to the lower
porosity model, which is due to the smaller thermal
conductivity of the powder with a higher porosity level.
Figure 14. von Mises stress contours after final cooling for
different powder porosity levels: (a) 50%, and (b) 35%
Figure 15 displays the von Mises stress on the three paths
(2nd
, 5th
, and 7th
) of the top layer after the final cooling process
for the EBAM model with different powder porosity levels. It
can be noted that the average von Mises stress in the overhang
area increases, on all scanning path, when the powder porosity
is larger; for example, the average von Mises stress on the 2nd
scanning path (powder substrate side) increases from about 350
MPa to about 500 MPa when the powder porosity increases
from 35% to 50%.
(a)
(b)
Powder
Solid
7 Copyright © 2014 by ASME

8. Figure 15. von Mises stress on different scanning paths of the
top layer after final cooling for the models with different
porosity levels.
CONCLUSIONS
In this study, a 3D FE thermomechanical model was
developed for temperature and stress simulations in EBAM,
particularly, applied to investigate temperatures and stresses
when fabricating an overhang feature, which by nature is above
a powder substrate. Ti-6Al-4V powder materials with two
different porosity levels, resulted from different preheating
conditions, were studied. The major findings can be
summarized as follows.
1. The poor thermal conductivity of Ti-6Al-4V powder results
in higher temperatures, also slower heat dissipations, in
overhang areas during an EBAM build,
2. The retained higher temperatures in the area above the
powder substrate, when building an overhang feature in
EBAM, result in higher residual stresses after final cooling,
and
3. A smaller porosity (e.g., 35% vs. 50%) may noticeably
reduce the residual stresses, also process temperatures,
associated with building an overhang.
Future work will investigate stress and defect severity of
different overhang patterns and study various configurations for
designs of effective functional support structures for any
specific types of overhang.
ACKNOWLEDGEMENTS
The materials presented in this paper are supported by NSF
(Award No. 1335481) and NASA (Award No.
NNX11AM11A). This research is in collaboration with
Marshall Space Flight Center (MSFC), Advanced
Manufacturing Team. Helpful discussions with K. Cooper, J.
Lydon and M. Babai at MSFC are acknowledged.
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