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Charpy impact testing of metallic selective laser

melting parts
a a a b b
Evren Yasa , Jan Deckers , Jean-Pierre Kruth , Marleen Rombouts & Jan Luyten
a
Mechanical Engineering , KU Leuven, Celestijnenlaan 300B, Heverlee, 3001, Belgium
b
VITO NV , Boeretang 200, Mol 2400, Belgium
Published online: 16 Jun 2010.

To cite this article: Evren Yasa , Jan Deckers , Jean-Pierre Kruth , Marleen Rombouts & Jan Luyten (2010)
Charpy impact testing of metallic selective laser melting parts, Virtual and Physical Prototyping, 5:2, 89-98, DOI:
10.1080/17452751003703894

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 Virtual and Physical Prototyping, Vol. 5, No. 2, June 2010, 89
98

Charpy impact testing of metallic selective laser melting parts

Evren Yasa1*, Jan Deckers1, Jean-Pierre Kruth1, Marleen Rombouts2 and Jan Luyten2
1
Mechanical Engineering, KU Leuven, Celestijnenlaan 300B, Heverlee 3001, Belgium
2
VITO NV, Boeretang 200, Mol 2400, Belgium

Selective laser melting (SLM) is a layer-additive manufacturing technology which makes

it possible to create fully functional parts directly from standard metals without any
intermediate binders or any additional post-processing steps. During SLM, a high-
intensity laser beam selectively scans a powder bed according to the CAD data of the part
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to be produced and powder particles are completely melted. SLM is capable of producing
near full density parts with an almost infinite geometric freedom. However, the
mechanical properties obtained with SLM may differ from the ones of bulk material.
In this study, Charpy impact tests are applied on the samples produced by SLM from
different materials; Ti-6Al-4V, AISI 316L (X2CrNiMo18-14-3) and maraging steel 300
(X3CoMoTi18-9-5). The influence of the building axis as well as of various heat
treatments applied on the samples after SLM is investigated. The evolution of the
microstructures of the sample parts is also studied.

Keywords: mechanical testing; selective laser melting; toughness

1. Introduction on the already solidified layer. Successive scanning and

Selective laser melting (SLM) technology allows to obtain lowering the building platform continues until the part is
fully functional, three-dimensional objects by selectively completely made. A typical SLM machine is shown
consolidating successive layers of powdered metal material schematically in Figure 1 with its main components.
on top of each other without using any intermediate binders The mechanical properties obtained with SLM might be
or any additional post-processing steps (Kruth et al. 2005). different from those of bulk material produced by conven-
Nowadays, selective laser sintering/melting technologies are tional techniques for several reasons. Laser processing of
widely used in various industries, such as in medical, materials generally results in high cooling rates due to the
automotive and aerospace applications, offering a range of short laser/material interaction time because of high scan-
advantages compared to conventional manufacturing tech- ning speeds and high thermal gradients. This might lead to
niques: shorter time to market, mass customization, geome- the formation of non-equilibrium phases such as glasses,
trical freedom and ability to produce more functionality in quasi-crystalline phases and new crystalline phases with
the parts with unique design and intrinsic engineered extended composition ranges (Rombouts 2006). Finer
features (Kruth et al. 2007; Yadroitsev et al. 2007a). structures may be observed in the microstructure at
During the SLM process, a powder layer is deposited sufficiently high cooling rates compared to conventional
onto a base plate attached to the building platform of the manufacturing methods. Moreover, gas bubbles can become
machine. The laser beam scans the powder bed according to entrapped in the material during solidification due to
the slice data of the CAD model, and the powder being various causes such as decrease in the solubility of the
fully molten forms the first layer on the base plate. Then, dissolved elements in the molten pool during cooling and
the building platform is lowered with an amount equal to solidification, chemical reaction or trapped gas (Rombouts
the layer thickness and a fresh layer of powder is deposited 2006). Many material properties, such as yield strength,

*Corresponding author. Email: evren.yasa@mech.kuleuven.be

Virtual and Physical Prototyping

ISSN 1745-2759 print/ISSN 1745-2767 online # 2010 Taylor & Francis
http://www.tandf.co.uk/journals
DOI: 10.1080/17452751003703894
 90 E. Yasa et al.

strengths are low because of porosity, and oxygen pick-up

although the tensile strength tests showed results comparable
to the wrought material. Some researchers focused on the
influence of residual stresses formed during SLM on the
fatigue behavior of metallic parts (Osakada and Shiomi
2006). There are also some investigations for other layer
manufacturing processes, such as stereolithography (Saleh
et al. 2002) and selective laser sintering (Miani et al. 2001;
Hague et al. 2002). In the field of rapid prototyping, a
computer tool was also developed to simulate the mechanical
properties of scaffolds for tissue engineering as a function of
the pore size and selected material (Miani et al. 2001;
Almeida et al. 2008).
In this research, Charpy impact tests were applied to
samples that were produced by SLM from three different
metallic powders; a titanium alloy (Ti-6Al-4V) which is
Figure 1. A typical SLM machine layout. commonly used for medical applications, AISI 316L stain-
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less steel (X2CrNiMo18-14-3) and maraging steel 300

thermal conductivity or ductility highly depend on micro-
(X3CoMoTi18-9-5). The influence of various heat treat-
structural properties. One of the most striking examples of ments applied to the samples after the SLM process was
a structure-sensitive property is the fracture toughness investigated, as well as the effect of the building axis.
which measures the ability of a structural material to Hardness measurements were also conducted and the
inhibit crack propagation. Very small changes in the microstructures of the specimens were analyzed.
chemical composition and highly localized grain boundary
segregation may cause a catastrophic loss of ductility
(Brandon and Kaplan 2001). Thus, the mechanical proper-
ties of SLM parts do not only depend on the material 2. Experimental procedure
composition, but also on the microstructures obtained and
Each of three materials was processed on a different
the presence of defects in the final product that are
machine. AISI 316L stainless steel is processed on a
determined by the process parameters and manufacturing
Concept Laser M3 Linear machine which employs an
strategy (Simchi 2006; Yadroitsev et al. 2007b). Nd:YAG laser with a wavelength of 1064 nm and a
The mechanical properties obtained with SLM and other maximum laser output power of approximately 100 W
layer manufacturing processes are widely studied by many measured in continuous mode. The specimens from mara-
research groups throughout the world. At the University of ging steel 300 were produced on an EOSINT M 270
Leuven, the mechanical properties (hardness, tensile and machine. It employs a Yb-fibre laser with a maximum
bending properties) of SLM samples from Ti-6Al-4V power of 195 W. The third powder material, titanium alloy
materials were studied, concluding that the obtained Ti6Al4V, was processed on a self-made SLM machine at the
mechanical properties of SLM samples are comparable to University of Leuven (see Table 1). All specimens were
those of bulk material (Vandenbroucke and Kruth 2007). manufactured with the selection of optimized parameters
Paul et al. reported an investigation of laser rapid manu- for density and roughness at the University of Leuven and
facturing of Inconel-625 components using the Taguchi LayerWise.
method. They used tensile and impact tests in order to The Charpy impact test was used to determine material
study the effects of different processing parameters, such as toughness by hitting a test specimen with a hammer
powder feed rate, scan speed and laser power (Paul et al. mounted at the end of a pendulum (ASTM, E 23-96) (see
2007). In the field of laser-engineered net shaping Figure 2a). The specimen is broken by a single blow from
(LENSTM), mechanical properties of Ti-6Al-4V were in- a pendulum that strikes the middle of the specimen on the
vestigated with tension, fatigue and crack-growth tests. The un-notched side. The height of rise subtracted from the
tests indicated that the static tensile strength and ductility,
fatigue strength and fracture toughness of hot isostatic Table 1. Materials and machines used in this study.
pressurized (HIP) parts produced via LENSTM compared
Powder Maraging Steel AISI 316 L Ti-6AI-4V
favorably to those of wrought products (Kobryn and
Semiatin 2001). The mechanical properties of pure titanium Machine EOSINT m270 Concept Laser M3 Self-made LM machine
models processed by SLM were also investigated (Santos used Yb-Fibre laser Linear Nd: YAG Yb:UAG fibre laser
195 W laser 100 W 300 W
et al. 2002) showing that the impact and torsional fatigue
 Virtual and Physical Prototyping 91
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Figure 2. (a) Charpy impact test setup (ASM Handbook 1997); (b) produced specimens by SLM in three building axes.
 92 E. Yasa et al.

height of fall gives the amount of energy absorption The experimental layout for all the batches produced is
involved in deforming and breaking the specimen given in Table 2, where the number of produced samples
(ASM Handbook 1997). A V-shaped notch is generally from each material and specimen design is shown. Three
used in the impact specimen in order to control the batches of specimens from two materials (Ti alloy and
fracture process by concentrating stress in the area of the maraging steel 300) were produced in order to investigate
minimum cross-section. In this experimental study, different factors that may influence the toughness of SLM
Charpy tests were done according to ASTM E23 standard parts. Only one batch was produced in AISI 316L. In the
(ASTM, E 23-96). The size of the standard specimen was first batch, two-part designs were used to study whether high
101055 mm, with a notch as defined in the roughness values encountered in SLM cause any notch-
same standard. effect influencing toughness results. Different part designs
In this study, the experimental procedure was the same used in the experiments are shown in Figure 3a. A part
for all specimens. First, the samples were made by SLM
processing (See Figure 2b) and then they were cut off the
base plate by electro-discharge machining (EDM). Due to
the process, loose powder that should stay un-molten
around the scanned contours sometimes melts and sticks
to the walls. In order to remove these loosely sticking
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powder particles, all produced samples were sand blasted.

Afterwards, the densities of the parts were measured using
Archimedes’ method. The next step was to apply the impact
test at room temperature if no heat treatment had been
applied. Otherwise, the parts were first treated in a furnace
with an argon atmosphere according to a certain heat
treatment cycle. After the impact test, the broken surfaces
and the parts’ insides were analyzed for their fracture
surfaces and microstructures. Vickers hardness measure-
ments were also conducted.

Table 2. The experimental layout.

Maraging
AISI 316 L Ti-6AI-4V Steel

BATCH 1
SPECIMEN

Standard specimen 3 3 3
DESIGN

Non notch specimen 3 3 3

EDM-notch specimen no no no

Building axis x-axis x-axis x-axis

Heat Treatment none none none
Sand Blasting yes yes yes
Number of replicates 3 3 3
BATCH 2
SPECIMEN

Standard specimen 3 3
DESIGN

Non notch specimen 3 3

EDM-notch specimen 3 3

Building axis x,y and z x,y and z

Heat Treatment none yes
Sand Blasting yes yes
Number of replicates 1 1
BATCH 3
SPECIMEN

Standard specimen 6 6
DESIGN

Non notch specimen no no

EDM-notch specimen no no

Building axis x-axis x-axis Figure 3. (a) Three-part designs produced by SLM to be
Heat Treatment 2-types 2-types used in Charpy tests; (b) Orientation of a standard part
Sand Blasting Yes yes
Number of replicates 3 3
manufactured with the building axis coinciding with the
x-direction.
 Virtual and Physical Prototyping 93
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Figure 4. Micrographs of etched SLM parts.

 94 E. Yasa et al.

design without a notch (design: ‘no notch’) but with an

equal cross-section area was utilized as well as a standard
Charpy test specimen (design: ‘standard’). In the second
batch, the influence of the building axis was taken under
investigation with three-part designs: the coordinate system
attached to the part is shown in Figure 3b. In addition to the
two designs explained above, a bar without any groove or
notch was made, in which the notch defined by the standard
was made afterwards by EDM (design: ‘notch to be made by
EDM’). Finally, in the third batch, standard specimens were
produced along the x-axis to test how different heat
treatments may influence the toughness of SLM materials.
For each material, different heating cycles were applied. The
details of the heating cycles are explained in Section 3.3.
Figure 5. The density measurement results of the first batch
specimens by the Archimedes method.
3. Experimental results
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3.1 First batch specimen designs follow the same trend for three materials.
The maraging steel 300 and AISI 316L stainless steel show
As given in Table 2, none of the specimens were heat treated more or less similar results, whereas Ti alloy has much less
after the first batch. In order to check the repeatability of toughness than steel for both designs.
the process, three replicates for each specimen design and The stainless steel specimens have a waved fracture
material were produced with a building direction parallel to surface. Figure 7 shows that the waviness is a result of
the x-axis of the specimen. aligned pores along which the fracture has occurred. Pores
The cross-section of the parts is shown in Figure 4. AISI containing incompletely molten particles are present on all
316L stainless steel part consists mainly of an austenitic fracture surfaces, as illustrated in Figure 8.
phase, as revealed by X-ray diffraction. The cellular micro- Fracture of the Ti-6Al-4V parts is mainly brittle as
structure looks similar to the structure obtained after casting indicated by the minor deformation of the cross-section
of austenitic stainless steel: delta ferrite in an austenitic at the position of fracture (see Figure 9). The maraging steel
matrix. The micro-Vickers hardness (for a load of 0.5 kg) is parts behave in a more ductile behavior as indicated by the
23595: The Ti-6Al-4V parts consist of large grains oriented higher deformation and presence of larger shear lips.
along the building direction. The elongated grains are the
result of epitaxial solidification and extend over multiple
layers. Inside the grains a needle-like martensitic phase, 3.2 Second batch
which is formed as a result of the rapid solidification, can be
As shown in Table 2, the second batch was performed to
distinguished instead of the two-phase hcpa and bccb
investigate the effect of the building axis on the Charpy
structure that would be present in equilibrium conditions.
test results for SLM parts. In order to determine the
The micro-Vickers hardness is 36995: The structure of the
influence for different notch making methods, all three
maraging steel parts is mainly a low carbon bcc lath
martensitic structure. The borders of the melt pools are
revealed after etching. The micro-Vickers hardness is 37695:
The density measurements and Charpy test results are
presented in Figures 5 and 6, respectively. The results shown
in all figures are given with a 95% level of confidence. The
theoretical densities for AISI 316L, Ti6Al4V and the
maraging steel 300 are taken as 8.0, 4.2 and 8.1 g/cm3,
respectively. Figure 5 depicts that all specimens have relative
densities of more than 98.5%.
The Charpy test results show that the specimens of the
same material with and without a notch absorb quite
different energy values before breakage. For all materials,
the specimens with a notch have less resistance to breakage
which means that the high roughness of the SLM process Figure 6. The Charpy test results for SLM produced parts
does not behave like stress-concentrating notches. Both for the first batch.
 Virtual and Physical Prototyping 95

Figure 7. Cross-section of stainless steel AISI 316L perpendicular to the building direction after Charpy testing.
Downloaded by [New York University] at 02:47 28 November 2013

specimen designs were produced. Only one specimen was seems to be the x-axis for the specimens without a notch. It
manufactured per each case. No heat treatment was can be concluded that in the case of a good connection
applied for Ti alloy, whereas solution annealing followed between successive layers without any pores, the building
by aging was applied to maraging steel 300 specimens axis does not play a significant role in the toughness results.
since it is a material which is always used with a heat As also observed from the macro pictures of the broken
treatment. surfaces, all specimens made along the x-, y- and z-axes
The results derived with the two materials exhibited have similar brittle fractures. The reason for the low
similar results to those obtained with the specimens toughness of the z-specimens made from the Ti alloy can
produced in the first batch when the same building axis be attributed to porosity caused by the accidental reduction
and same design geometries were considered. For the Ti of laser power (about 5%) during the build of the
alloy, although the specimens built along the y-axis showed z-specimens. The protection glass that is located between
a slightly higher resistance to breakage, the effect of the the process chamber and the lens became dirty as the
building axis seemed negligible, as seen in Figure 10. The build height increased, and this caused the undesired
same holds true for the maraging steel, the results of which reduction in the power leaving an extra porosity of 2% in
are given in Figure 11, where the weakest building direction the samples.

Figure 8. SEM investigation of fracture surface.

 96 E. Yasa et al.
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Figure 9. Top (left) and side (right) view of SLM parts after Charpy testing.

3.3 Third Batch Charpy energy after heat treatment at 5958C may be
attributed to the higher amount of less ductile a phase
The influence of various heat treatments on the toughness
present than after heat treatment at 7358C.
of the two materials was investigated in this batch. All
For the maraging steel, the applied two heat treatments
specimens with a standard design were produced along the
decreased the resistance to breakage significantly, as de-
x-axis with three replicates for each case.
picted in Figure 13. It can also be observed that the amount
For the Ti alloy, two different heat treatments were
of plastic deformation is significantly lower than without
applied. The first one was the full annealing conducted at
heat treatment. The first heat treatment was solution
7358C for 2 h in a BIP argon furnace. The second heat
annealing at 8158C for 1 h and cooling to room temperature,
treatment was stress relieving at 5958C for 3 h in an
followed by aging conducted at 4808C for 3 h. The second
argon atmosphere. The first heat treatment did not
was only aging at 4808C for 3 h. The results also show that
improve or worsen the toughness of this material, the
solution annealing is not necessary to homogenize the
hardness was slightly increased (36299): The second heat
microstructure after the SLM process since the results with
treatment decreased the toughness of the Ti alloy
and without solution annealing were almost the same. The
slightly and significantly increased the hardness to 38695
hardness increases to 57297 as a result of aging. The
(see Figure 12). During heat treatment the martensitic
hardening during aging has been attributed in the literature
structure transforms into a mixture of hexagonal close
packed a and body centered cubic b phases. The lower

Figure 10. The Charpy results of the Ti alloy in the second Figure 11. The Charpy results of the maraging steel 300 in
batch. the second batch.
 Virtual and Physical Prototyping 97

than 130 J (ASM Handbook 1991). For Ti-6Al-4V, cast

specimens exhibit an impact energy of 15 J for V-notched
samples (Alloy Data Sheet), whereas SLM parts reach up
to 11.5 J without any heat treatment in the present work.
After full annealing or stress relieving, the toughness does
not change or slightly decreases. Bulk maraging steel 300,
i.e. a pre-alloyed high strength and hardness steel, has an
impact energy of 18 J at room temperature after aging
(Alloy Data Sheet). Our experiments showed an impact
energy of approximately 10 J for this material when heat
treated in the same way. Without any heat treatment, the
impact energy is found to be between 30 and 40 J, thus
yielding a higher toughness but sacrificing the strength
and hardness normally obtained through aging. The
comparison between test results of SLM and conventional
Figure 12. The Charpy results of Ti alloy for full annealing manufacturing processes is summarized in Table 3. The
and stress relieving compared to parts which were not heat reason for the lower toughness with SLM can be
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treated (third batch). attributed to the presence of defects like pores, pick-up
of impurities like oxygen and nitrogen (especially for
to short-range ordering in the cobalt-bearing solid solution titanium alloys) and the presence of more brittle non-
and the precipitation of nickel-rich intermetallic compounds equilibrium phases.
in the lath martensitic structure (ASM Handbook 1991). Tests not presented in this paper revealed that the porosity
X-ray diffraction has revealed that after aging a fcc of SLM parts is of high importance since it might cause a
phase appears, which was not present without heat treat- significant reduction in toughness. Controlling the process in
ment. In the literature, it is reported that during aging of terms of density is hence most critical: a slight drop of density
18 Ni maraging steel, 300 austenite can precipitate and due to unexpected loss of laser energy (e.g., dirt on optics)
adversely affect the toughness of the material (ASM may substantially reduce the toughness (typically 20% for
Handbook 1991). less than 1% reduction in density measured with optical
microscopy picture analysis). Testing density should best be
done with different methods since Archimedes’ method may
overestimate the relative density if the pores still contain un-
4. Comparison of toughness of SLM parts and bulk material
molten powder particles. In terms of stainless steel, a slight
The toughness of bulk materials is higher than that of change of alloying element compositions may significantly
SLM parts. For bulk annealed stainless steels, the impact reduce the toughness due to undesired phases such as high
Charpy energy for V-notched parts is generally greater temperature delta ferrites in the austenite phase. Thus the
material composition should also be strictly determined to
ensure a good repeatability.

5. Conclusions
One of the important conclusions from the experiments
conducted in this study is the fact that the roughness of

Table 3. Comparison of SLM and conventional processes in

terms of Charpy V-notch toughness.

SLM Conventional

Ti-6AI-4V 11,590,5 J (as built) 15 J for investment casting

10,190,5 J (full annealed)

Maraging Steel 300 36,394,8 J (as built) 18 J after aging

10, 191,4 J (after aging)
Figure 13. The Charpy results of the maraging steel for
Stainless Steel 316 L 59,293,9 J (as built) 160 J for cast CF-3M
solution annealing followed by aging and only aging after annealing
compared to not heat treated parts (third batch).
 98 E. Yasa et al.

SLM parts does not behave like stress-concentrating Kruth, J.-P., Vandenbroucke, B., Van Vaerenbergh, J. and Mercelis, P., 2005.
Benchmarking of different SLS/SLM processes as rapid manufacturing
notches. The parts without a notch showed significantly
techniques. In: International Conference Polymers and Moulds Innovations
higher impact energies compared to specimens with a
(PMI), 20
23 April 2005, Gent, Belgium [CD ROM].
notch, either made during the SLM process or after SLM Miani, F., Kuljanic, E. and Sortino, M., 2001. Modelling the mechanical
by EDM. This also concludes that the method of properties of direct metal selectively laser sintered parts. In: Proceedings
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results. Additionally, the specimens made in the second Erlangen, Germany, 383
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products by selective laser melting of powder. International Journal of
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