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Review on the Progress of Porosity Detection and
its Effect on Mechanical Properties of Additive
Manufacturing Components
Mohammad Rashid Mohammad Shoaib Instructor Dr. Leonard Bond
Aerospace Engineering
Iowa State University

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ABSTRACT
Additive Manufacturing (AM) has seen a risen interest, both in the consumer sector as well as in
industrial applications. However, AM industry is also facing several technical challenges. This report
provides a brief review on the defects in Additive Manufactured components, factors affecting these
defects, their measurements techniques using in-situ NDE and mechanical properties. The aim of this
review is to identify and summarize the factors affecting the fabrication of AM parts and its allowables.

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Table of Contents
1. Introduction ……………………………………………………………………………………. 1
2. Defects In Additive Manufacturing ……………………………………………………… 1
Porosity ……………………………………………………………………………………. 1
Gas Porosity ……………………………………………………………………………. 1
Lack of fusion ……………………………………………………………………………. 2
3. Factors ……………………………………………………………………………………………... 4
4. Measurements and Properties …………………………………………………………… 5
5. Conclusion ………………………………………………………………………………………. 19

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1. Introduction
Additive manufacturing (AM) is a process of fabricating parts or objects by joining and building
up material layer by layer from a computerized three dimensional model data. The AM process
always starts with a CAD model, which then is converted into a STL file. Here it is sliced into a certain
number of layers depending on desired precision and then exported to the AM machine. AM makes
effective and efficient use of raw materials, producing less waste than its counterpart, subtractive
manufacturing, while producing satisfactory accuracy in finished parts. AM has ability to quickly
produce parts with complex geometry which is difficult to achieve using conventional
manufacturing. It is also considered a sustainable technology due to its low energy consumption
during the manufacturing process. Some of the advantages of additive manufacturing when
compared to conventional manufacturing are a shorter time to market, use of inexpensive
materials, high production rate, versatility, accuracy and an ability to produce unique features.
2. Defects in Additive Manufacturing
In Additive Manufacturing, powder is melted within the melt pool and solidified to form a deposited
track. These deposited tracks may contain porosities or cracks which affect the functionality of the part.
When these defects go undetected, they may cause failure of the part. These defects act as stress
concentrators that compromise the components quality. Common defects in AM components are gas
porosity, lack of fusion and cracking. It is important to understand the effects of these defects on the
components to facilitate a better understanding of manufacturing and repair of AM parts.
Porosity
Porosity is the most common defect found in components produced through AM. It can be
divided into two categories: gas porosity and lack of fusion (LOF). Liujinhui et al (2011) discussed that
the porosity is a result of fluid, fusion, welding, separation and balling of metal in the melt pool and its
creation is complex. Gu et al (2013) also pointed out two types of porosity formation mechanisms: the
balling phenomenon and high thermal stresses.
Gas Porosity
Gas Porosity is porosity caused by gasses that get entrapped in the AM part and typically have
smooth spherical morphology. Bauereiß et al (2014) defined gas porosity as a result of gas entrapped
within gas atomized powder particle. These are common in AM parts and growth over the layer has
been experimentally observed. The porosity as seen in Figure1 and 2 can be decreased by choosing
proper process parameters and material parameters which will be discussed in later sections but could
not be eliminated.
GKL Ng et al (2009) found that gas porosity is mostly due to an overly high powder flow rate
which traps the shielding gas within the melt pool and also lowers the specific energy of the melt pool.
In AM mostly gas atomized powder is used, this increasing the probability of entrapped gas within the
powder particles.

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Lack of Fusion
Lack of fusion (LOF) is a material discontinuity at the melt layer boundary in which two
faces become separated and form porosity due to lack of fusion. Lack of fusion are irregularly
shaped and often contain trapped and un-melted powder particles.
GKL Ng et al (2009) used gas atomized Inconel 718 powder and concluded that porosity
due to lack of fusion is because of the inability of the melt pool to melt the powder particles due
to low specific energy.
Olakanmi et al (2015) informed that lack of fusion defects are usually found along the
layer boundaries and are irregularly shaped, often containing un-melted powder. They
attributed lack of fusion to insufficient dissipation of the laser energy density into the powder
layer thickness.
Liu Qianchu et al (2014) explained a similar reason for lack of fusion i.e. insufficient
energy to the melt pool. They divided lack of fusion into three categories (a) Separated surface
with un-melted powder, (b) Separated surface without un-melted powder and (c) Narrow and
long shaped with un-melted powder. Type (a) and type (b) were present in vertical built
specimen and type (c) was observed in horizontal built specimen. Bauereiß et al (2014) found
similar defects to LOF and they concluded that it was possible to eliminate these defects by
increasing the power of the beam. According to the study of Liu Qianchu et al (2014) LOF defects
have an influence on the fatigue life of the specimen. Defects closer to the surface affected
fatigue life less compared to the defect that were deeper or far from the surfaces.
LOF can be corrected by adjusting external factors such as scan speed, laser power, layer
thickness etc.

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Bauereiß et al. (2014) explained the formation of large cavities in the AM products. According to
their study combining of the melt pool is much faster than the melting of single powder particle, which
means that the powder particle starts blending even before it is melted. Due to this behavior, powder
particles combine with the first liquid or solid surface they come into contact with and form isolated
drops of liquid. The direction of this liquid is not downward instead its lateral and is in spherical shape.
Because of this, the energy from the beam does not reach the previous layer and particles cannot fuse
and this causes large cavities for several layers as shown in Figure 4.

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3. Influencing Factors
The quality of the AM manufactured part is highly dependent on many interrelated factors such as
process parameters, powder characteristics and surrounding conditions. The factors affecting the AM
process can be divided into two categories process parameters and material parameters. Process
parameters are the inputs and primarily determine the rate of energy delivered to the surface of the
powder as well as how energy interacts with material. Process parameters be controlled to achieve
variations in the properties of these components. Rombouts et al (2006) named following as process
parameters: laser power, scan speed, layer thickness, spacing of scan lines and powder feed rate.
Material parameters such as powder size and distribution are decided at the beginning of the
manufacturing process.
Simchi (2006) pointed out that sintered density of AM component depends on powder
characteristics, fabrication parameters, layer thickness and scan line spacing. Higher laser power results
in higher density and increasing the thickness of layer decreases the density. They concluded that
density seems to be linearly proportional to the ratio of laser to the scan rate on a semi log scale. They
also analyzed the influences on the porosity from oxygen content, shape, size and its distribution. The
conclusion is that high density is obtained when the powder particles are fine and oxygen content is low,
assuming proper parameters.
Ng et al (2009) concluded that LOF and gas porosity were affected by different factors. LOF was
largely governed by specific energy provided to the melt pool whereas gas porosity was believed to
originate from gas entrapped in the powder. Gas porosity was harder to eliminate than the LOF and can
be as high as 0.7%. LOF was affected by powder feed rate, traverse speed and track overlap. LOF
reduced with the increase in transverse speed. This was an unexpected result because they thought this
should have increased the LOF, as increasing the transverse speed decreases the specific energy and
therefore increases LOF. LOF increased as the powder feed rate was increased. Increasing the track
overlap did not have a significant effect. Gas porosity was found to be dependent on process
parameters and pool dynamics. The porosity measured average around 0.2%. It was deduced that gas
porosity increases with powder feed rate and shielding gas. At low powder feed rate and low laser
power, gas porosity increased significantly.
Meier and Haberland (2008) concluded that an increase in laser power
results in a decrease in porosity. With effective laser power of 90 Watts, a
relative density of 99% is realistic. (Figure 5p). With decreasing scan speed and
hatch distances (i.e., increasing input energy) the relative density increases.
This means that a higher density is achieved by the input of higher energy. They
pointed out that the pores appeared lattice like when the hatch distance was
too large and when the scanning speed was large, the pores were irregular.
They also investigated the effect of scanning speed on the horizontal surface
and vertical walls and concluded that an increase in scanning speed initiated
fragmentation first in vertical walls, and then horizontal.

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4. Measurements Techniques And Properties
There is a real need to monitor manufacturing of AM parts layer by layer while they are being
manufactured to detect defects and reduce cost. This can be done by different measurement tools and
techniques. These techniques are useful in giving data needed for optimizing the process and learning
which parameter needs to be controlled and monitor to ensure high quality of the component.
Bond et al (2014) provided an overview of current non-destructive evaluation (NDE) tools and new
approach to total quality management for the characterization of materials from metal powder to
finished parts. They concluded that micro computed tomography (CT) is a very rapid and cost effective
way to obtain structural information at very early stages. From a CT scan, size distribution, shapes and
internal features of the particle such as porosity can be determined quickly. They also applied a high
resolution digital X-ray radiography (Figure 6) to metal powder in order to inspect porosity and grain size
distribution. According to them radiography can be used with image processing to assess uniformity of
particle’s shape, size and distribution. Ultrasonic imaging was another tool Bond et al (2014) explained
that can be used to measure porosity, elastic moduli and density of a part.
O’Brien and James concluded that the direct current resistivity technique appears to be capable
of not only detecting cracks but also measuring hardness and density. Eddy current testing can be used
for surface crack but it is not suited for internal cracks. Computed Tomography and real X-ray imaging
have potential for detecting defects but have high cost. They said Ultrasonic testing can be applied to
sintered parts to detect defects.

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Slotwinski and EJ Garboczi (2014) applied three different techniques to monitor the porosity in
AM built parts. They used Archimedes’ method, X-Ray computed Tomography (XRCT) and Mass/Volume
measurements to determine the porosity. An image taken by XRCT (Figure 7) shows several localized
areas of porosity as well as cracks. They compared the results of these three methods and the outcomes
were as follows:
Figure 8 compares the results for the individual cylinders as measured by Archimedes and Mass/Volume
methods. There is a good agreement between each method but there are some instances of discrepancy
perhaps due to effects of water infiltration during Archimedes measurement.
Figure 9 compares the composite disk porosity results using Archimedes, the Mass/Volume of the small
cylinders and the mass/volume of the whole disks. The measured values are very close to each other.
Figure 10 compares mass/volume, XRCT and Archimedes for 14 individual cylinders, one from each disk.
The results of these methods were very similar.

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All three methods for determining porosity were generally in agreement, but each method has
its positive and negative aspect. The ultrasonic velocity measurements reported here demonstrates
sufficient sensitivity to detect small changes (~ 0.5%) in total porosity and should be sensitive enough to
detect process change in material porosity. The samples have local variations in porosity, both in build
direction and in the plane of each build. This was concluded from the images taken by XRCT and
measurements of the individual cylinders.
Meier and Haberland (2008) performed tensile tests of stainless steel and cobalt-chromium
parts. They said material exhibit anisotropic behavior. They used
* *
eff
v
P
E
v h d
= to calculate energy
density. Where
3
[J/ mm ]
[ ]
[ / ]
d [ ]
v
eff
E energy density
P effective laser power W
v scanning speed mm s
layer thickness mm
=
=
=
=
h hatch= distance [ ]mm
They concluded that a value of energy density between 40 and 90 J/mm3
(shaded region in
Figure 11) gives nearly 100 % density with the best surface quality.

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The tensile tests performed by Meier & Haberland indicated that the selective laser melting
(SLM) fabricated specimen showed lower ductility and lower elongation when compared to the parts
manufactured by a conventional method. By comparing the vertical and horizontal build specimens with
a reference material, they concluded that horizontally built specimens have higher tensile strength and
elongation compared to the vertically built. When a thickness of 75 mµ was investigated, it was found
that the vertically built part failed dramatically. This can be seen in Figure 12 showing brittle fracture
with cracks starting at inner defects. It was reasoned that this failure was due to the induced residual
stress which caused plastic deformation. The residual stresses are present as a result of high heating up
rates and low heat conduction.
When they compared 50 mµ thick layer built in different orientations. They concluded that less
obtuse the angle between the built direction and direction of load, the lower the strength. They also
found out that the tensile strength of horizontally generated parts i.e. orientation (b) and (c) as shown in
Figure 13 is higher than the tensile strength of conventionally fabricated parts.
0.2
5
limit
mR tensile strength
technical elastic
A Elongation after fracture
ρρ
=
=
=

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Meier and Haberland (2008) concluded that tensile strength, elastic limit and elongation after
fracture depends on the direction of build. If the load applied is perpendicular to the direction of
manufacture the structural properties are lower but reduction in layer thickness enhances the
properties. As pointed out earlier the tensile strength of the horizontally fabricated parts were higher
than those of conventionally manufactured. This hinted presence of residual stress in the parts.
Ng et al (2009) achieved the minimum lack of fusion and gas porosity processing conditions by
using numerical optimization. A laser power of 640 W, speed =620 mm/min, powder feed rate= 4g/min
shielding gas = 30 1/min and overlap of 45% was optimal for achieving minimum defects.
Barua et al (2014) performed vision based test. They used Red, Blue and Green (RGB) calibrated
values and radiant surface temperature to approximate the temperature of each pixel in the image.
Heat is lost by material being deposited by the processes of convection, conduction and radiation. They
explained that a defect free deposit should show gradual decrease in temperature through which the
reference cooling curve can be obtained using standard deposition parameters. Defects such as porosity
or cracks will lead to an increase in the temperature around the defective region because of interruption
of heat flow, which leads to deviation from the reference cooling curve, thus indicating the presence of
defect.
Gu et al (2013) explained the influences of energy density on porosity and microstructures of
4PH stainless steel. They applied two sets of process parameters: (a) different energy densities were
obtained by changing the scan speed and keeping laser power constant and (b) changing laser power
and scan speed to keep the energy density constant. Nearly 100% dense parts were built using 195W
laser power, 800 mm/s. Density of 7.857 g/mm was selected as threshold value for density. By varying
the process parameters the pores were studied and are presented in Figures 16, 17 and 18. For high
laser power and scan speed (195W & 1200 mm/s) pores are small compared to pores observed in Figure
17 (95W & 389 mm/s) and Figure 18 (70W & 287 mm/s). It is also noticeable that there are less un-
melted particles for 195W and 1200mm/s compared to the other two energy densities. Using
Archimedes method and image analysis they found similar results in level of porosity. They also looked
into the microstructural features and concluded that austenitic grains showed increasing diameter when
the scan speed was decreased.

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Cerniglia et al (2015) concluded that the laser ultrasonic technique can be used to inspect flaws
in laser deposited components during manufacturing. This technique can be used to inspect the
component as soon as the layer is solidified while manufacturing with a sensitivity of flaw detection of
above 100 mµ . Defects less than 100 mµ can be detected only if they are surface breaking. Defects
deeper than 300 mµ were easily detected. Flaws deeper than 800 mµ could only be detected if high
reflected waves were produced.
Abd-Elghany and Bourell (2012) evaluated 304L stainless steel and fabricated 24 different
samples with three different layer thickness (30 mµ 50 mµ and 70 mµ ) and laser scanning speeds of 70
mm/s & 90mm/s by Selective laser melting. They said that small particles of the powder exhibit better
compaction and lower tendency for porosity, thus higher density (experimental values tabulated in
Table 1). At lower scanning speeds, density increases because this powder has more time to melt due to
increased applied energy, thus allowing atoms to diffuse and fill up the voids. When the scan speed is
decreased, the surface tension of the melt pool is reduced as well. This contributes to formation of
porosity. These conclusions are similar to Rombouts et al (2006).

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The surface roughness of the parts increased with the use of larger particles inside the thick
layer (Table 2). Abd-Elghany and Bourell (2012) examined the microstructures of the specimens using
optical microscopy, SEM and XRD analysis. It was found that a layer thickness of 50 mµ and scanning
speed of 70 mm/s allow complete melting, whereas a layer thickness of 70 mµ shows increased porosity
and cracks formation due to presence of large powder
particles.

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Figure 21 shows that low scanning speed and 30 mµ layer
fabricated parts are 80% in strength compared to base line Yield
Strength of 170 MPa and Ultimate Tensile Strength of 480 MPa. But
70 mµ samples were brittle in nature because of the presence of
porosity. Yield strength and ultimate tensile strength were
measured to be 180 and 393 MPa and surface hardness was
recorded as high as 190 HV (Vickers hardness).
Ahsan et al (2011) analyzed the trend of porosity generation in Ti-6Al-4V using SEM and Micro
Computed Tomography of Gas Atomized (GA) and Plasma Rotating Electrode Powders (PREP) using the
same parameters. They found that the gas atomized deposited sample had three times more interlayer
porosities than the PREP deposition sample. Shown in Figure 23 (a) and (b).

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Figure 24 shows the trend of porosity in laser powers of 800W and 1000W. It was concluded by
Ahsan et al that the highest porosity percentage is achieved at a mass flow rate of 0.033 gs-1
. Porosity
decreases with decreasing mass flow rate until 0.066 gs-1
and then starts increasing again. PREP powder
shows lower porosity compared to GA powder. GA powder deposited samples show higher intralayer
porosity at all mass flow rates and laser powers than the PREP powder deposition samples.

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Figure 25 and 26 conclude that the gas atomized powder has pores of larger diameter compared
to PREP powder at both the laser powers of 800W and 1000 W and at different mass flow rates. There is
less porosity with diameter less than 40 micrometers in all cases. Ahsan et al concluded that PREP
powder should be used to obtain minimum intralayer porosity. This experiment showed a reduction in
porosity level to 0.0025% using PREP powder at 1000 W, 0.066 gs-1
and 5 mms-1
.
Shunmugavel et al (2015) compared the microstructure and mechanical properties of wrought
Ti-6Al-4V cylindrical rod with selective laser melting Ti-6Al-4V cylindrical rod. They found that SLM rods
have higher yield strength and ultimate tensile strength compared to wrought sample but low ductility.
The difference in these properties were agreed to be due to the difference in the microstructures of
these two sample. SLM sample exhibited trans-granular fracture as a result of their brittle nature
whereas wrought rod showed deep dimple fracture revealing ductile behavior (Figure XX). Stress- Strain
curve of the samples is shown in Fgure XXX and the values are tabulated in table 3.

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Qianchu et al revealed that fatigue crack initiated from the LOF defects in SLM Ti-64 specimens
for both vertically and horizontally built directions. LOF defect had more detrimental influence on
fatigue life due to its morphology. It was concluded that elimination of LOF defect will increase the
fatigue life significantly.

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5. Conclusion
Porosity is one of the major defects present in AM parts. Gas porosity is a result of gas trapped in
the parts and is hard to eliminate whereas lack of fusion defects occur due to insufficient density and
are mainly responsible for fatigue crack initiation.
There are so many different process parameters such as laser power, scan speed, layer thickness
that it’s hard to control the manufacturing. If these process can be controlled with accuracy, parts with
high density but not defect free can be achieved. The main trends found are a reduction in porosity with
increased laser power. Material parameters such as metal powder type, size and shape also affect the
mechanical properties of the component. Metal powders prepared by other techniques other than gas
atomization such as plasma rotating electrode can be used to overcome small percentage of porosity.
Several NDE tools have been identified that can be used to inspect these parts its quality such as
X-ray radiography, micro computed tomography, IR tomography, Ultrasonic imaging, Eddy current,
electrical resistivity etc. But there is a need to monitor the manufacturing of these parts layer by layer
while they are being produced to eliminate defects and reduce cost. This can be done by observing the
variation in radiance temperature of the deposit and IR temperature source measurement.
More research and investigations need to be conducted in order to improve and ensure the
functional integrity of the AM components.
Acknowledgement
The author of this report would like to thank Dr. Leonard Bond for his support and guidance.

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