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Journal ArticleDOI

Optimization of SLM Process Parameters for Ti6Al4V Medical Implants

08 Apr 2019-Rapid Prototyping Journal (Emerald)-Vol. 25, Iss: 3, pp 433-447

Abstract: Ti6Al4V alloy has received a great deal of attention in medical applications due to its biomechanical compatibility. However, the human bone stiffness is between 10 and 30 GPa while solid Ti6Al4V is several times stiffer, which would cause stress shielding with the surrounding bone, which can lead to implant and/or the surrounding bone’s failure.,In this work, the effect of selective laser melting (SLM) process parameters on the characteristics of Ti6Al4V samples, such as porosity level, surface roughness, elastic modulus and compressive strength (UCS), has been investigated using response surface method. The examined ranges of process parameters were 35-50 W for laser power, 100-400 mm/s for scan speed and 35-120 µm for hatch spacing. The process parameters have been optimized to obtain structures with properties very close to that in human bones.,The results showed that the porosity percentage of a SLM component could be increased by reducing the laser power and/or increasing the scan speed and hatch spacing. It was also shown that there was a reverse relationship between the porosity level and both the modulus of elasticity and UCS of the SLM part. In addition, the increased laser power was resulted into a substantial decrease of the surface roughness of SLM parts. Results from the optimization study revealed that the interaction between laser process parameters (i.e. laser power, laser speed, and the laser spacing) have the most significant influence on the mechanical properties of fabricated samples. The optimized values for the manufacturing of medical implants were 49 W, 400 mm/s and 99 µm for the laser power, laser speed and laser spacing, respectively. The corresponding porosity, surface roughness, modulus of elasticity and UCS were 23.62 per cent, 8.68 µm, 30 GPa and 522 MPa, respectively.,Previous investigations related to additive manufacturing of Ti alloys have focused on producing fully dense and high-integrity structures. There is a clear gap in literature regarding the simultaneous enhancement and adjustment of pore fraction, surface and mechanical properties of Ti6Al4V SLM components toward biomedical implants. This was the objective of the current study.
Topics: Laser power scaling (59%), Selective laser melting (56%), Surface roughness (55%), Young's modulus (52%), Elastic modulus (52%)

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University of Birmingham
Optimization of SLM Process Parameters for
Ti6Al4V Medical Implants
El-Sayed, Mahmoud; Ghazy, Mootaz; Yehia, Youssef; Essa, Khamis
DOI:
10.1108/RPJ-05-2018-0112
License:
Other (please specify with Rights Statement)
Document Version
Peer reviewed version
Citation for published version (Harvard):
El-Sayed, M, Ghazy, M, Yehia, Y & Essa, K 2018, 'Optimization of SLM Process Parameters for Ti6Al4V
Medical Implants', Rapid Prototyping Journal. https://doi.org/10.1108/RPJ-05-2018-0112
Link to publication on Research at Birmingham portal
Publisher Rights Statement:
This is the Accepted Author's Manuscript for the following article: Mahmoud Elsayed, Mootaz Ghazy, Yehia Youssef, Khamis Essa, (2018)
"Optimization of SLM process parameters for Ti6Al4V medical implants", Rapid Prototyping Journal, https://doi.org/10.1108/RPJ-05-2018-
0112
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Download date: 10. Aug. 2022

Optimization of SLM Process Parameters for Ti6Al4V Medical Implants
Abstract
Ti6Al4V alloy has received a great deal of attention in medical applications due to its
biomechanical compatibility. However, the human bone stiffness is between 10 and 30 GPa
while solid Ti6Al4V is significantly stiffer, which would cause stress shielding with the
surrounding bone which can lead to implant and/or the surrounding bone’s failure. In this
work, the effect of SLM process parameters on the characteristics of Ti6Al4V samples, such
as porosity level, surface roughness, elastic modulus and compressive strength (UCS), has
been investigated using Response Surface Method (RSM). The examined ranges of process
parameters were 35-50 W for laser power, 100-400 mm/s for scan speed and 35-120 µm for
hatch spacing. The results showed that the porosity % of a SLM component could be
increased by reducing the laser power and/or increasing the scan speed and hatch spacing. It
was also shown that there was a reverse relationship between the porosity level and both the
modulus of elasticity and UCS of the SLM part. In addition, the increased laser power
resulted in a substantial decrease of the surface roughness of SLM parts. The process
parameters have been optimized to obtain structures with properties very close to that in
human bones. Results from the optimization study revealed that the interaction between laser
process parameters (i.e. laser power, laser speed, and the laser spacing) have the most
significant influence on the mechanical properties of fabricated samples. The optimized
values for the manufacturing of medical implants were 49 W, 400 mm/s and 99 m for the
laser power, laser speed and laser spacing, respectively. The corresponding porosity, surface
roughness, modulus of elasticity and UCS were 23.62%, 8.68 µm, 30 GPa and 522 MPa,
respectively.
Keywords: Selective laser melting (SLM); Design of Experiment; Ti-6Al-4V; Medical
Implants
1. Introduction
Selective laser melting (SLM) is an additive manufacturing technique that produces near
fully dense metal parts directly from a CAD design by adding layer upon layer [1-4]. The
main concept is based on a laser beam that passes over a thin layer of powder and diffuses it
selectively to the desired shape. Next, a new layer of powder is spread, the platform is
lowered according to the required layer thickness and then the melting process is repeated
until the full part is obtained [5,6]. SLM has many advantages such as producing complex
shapes that are difficult to fabricate via conventional methods, short time from design to
market, and near net shape production which minimizes waste of materials [7,8]. For these
reasons, the SLM process is used in aerospace and biomedical applications such as implants
and prostheses [9,10]. Examples of metal powder used in SLM processes are: titanium alloys,
steels, cobalt, chromium and aluminum alloys [11]. On the other hand, SLM has some
limitations that include the stair step effect which increases surface roughness, and balling
phenomenon which increases both the surface roughness and the porosity of SLM parts [12].

T
h
and m
o
affect
s
defect
s
Ψ, wh
i
Wher
e
thickn
e
sampl
e
identi
f
[15].
O
desig
n
Analy
s
Comp
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desig
n
know
n
param
varied
Fig 1
proces
qualit
y
A
t
micro
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contro
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Ti6Al
4
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Furth
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a
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een
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orphology
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and poros
i
t
tar et al. [1
s
tructure a
n
lled manne
r
f
acturing co
m
4
V alloy i
s
s
e of its bio
c
orrosion re
s
e
rmore, stre
n
a
l applicati
o
o
ng et al.
c
teristics of
s
uccessfull
y
f the SLM
f
o
f the pow
d
e
of consoli
e approach
e
e
expresse
d
a
ser power,
researcher
s
heat input,
timum ene
r
e
r hand sev
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m
ents (Do
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a
nce (AN
O
n
(CCD). I
n
b
ination of
o
ints) and c
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e
ls (- α, -1,
0
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se techniq
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[23] hav
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Ti6Al4V
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[14] appl
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gy densit
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ral studie
s
E
) techniqu
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O
VA). One
n
this desig
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two-level
f
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ntre point
s
t
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o
0
, 1 and α) [
u
es were s
u
a
ser power,
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n Selectiv
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ntral comp
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most co
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nature [20
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studied
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16]. Desig
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22]. It has
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ery compat
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ts from bi
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d Ti mate
r
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ow densit
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ted that f
u
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o
r is the la
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well as th
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s [13]:
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ch spacing
orrelate th
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te fully so
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ials for i
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and t is t
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lysis by m
e
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a
gns is the
s noted as "
,
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a
and each
fa
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ctors are s
h
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the influ
e
h
e resulting
p
rocesses [
1
6].
L
M techniq
u
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an be gra
d
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lications i
n
e
metals is
m
plant appl
i
hanical pro
x
imately 1
1
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were suit
a
arameters
i
6Al4V pa
r
l
owing par
a
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he size
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of any
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nction
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ents by
content
e
ans of
a
nd the
Central
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". The
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ctor is
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e
nce of
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17,18].
u
e. The
ed in a
n
which
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rucial.
i
cations
perties,
0 GPa.
a
ble for
on the
r
ts have
a
meters

(laser power = 110 W, scan speed = 400 mm/s, scan spacing = 40 µm, and layer thickness =
50 µm). Sun et al. [24] used the Taguchi method to optimize four process parameters: layer
thickness, linear energy density, hatch spacing and scanning strategy. They reported that 80
W laser power, 200 mm/s scan speed, 60 μm hatch spacing, a 20 μm layer thickness and X-Y
inter-layer for scanning strategy was sufficient to achieve fully dense, good quality Ti6Al4V
components. In another study Murr et al. [25] have produced Ti6Al4V parts via SLM for
biomedical implants. It was indicated that SLM was capable of producing good quality parts
with mechanical properties better than wrought and cast Ti6Al4V parts. Vandenbroucke and
Kruth [26] also produced medical and dental parts fromTi6Al4V alloy and tested their
mechanical and chemical properties. The Ti6Al4V produced had achieved 99.98 % density.
However, it should be noted that in the earlier studies such as those by Murr [25] and
Vandenbroucke and Kruth [26], the objective was mainly to produce SLM parts with
minimum porosity in order to achieve mechanical properties that could reach, or even
exceed, those of bulk material. In the work reported by Vandenbroucke and Kruth [26], a
tensile young's modulus of about 94 GPa was obtained. Nevertheless, the elastic modulus of
bones in human body ranges from 10 to 30 GPa. The large difference in moduli between
titanium implants and bones, known as stiffness mismatch, can result in stress shielding,
which has been held responsible for implant loosening and consequently could cause the
patients to require a revision surgery. Two solutions were found to this problem: the first one
was developing new types of titanium alloys that have modulus closer to bones and the
second one was developing porous structure instead of solid structures which reduces
material modulus [27-30].Titanium alloys that have 30% volume porosity can have modulus
similar to human bones. One problem of porous structures is that it decreases toughness and
creates stress concentration around the pores [31].
Furthermore, a medical implant should have high compressive strength to prevent
fractures and improve functional stability. High strength is also required to impede
spring-back both during and after the operation procedure [32,33]. Finally, an implant should
have sufficient surface roughness to improve the ingrowth of the human tissues into it.
Compared to smooth surfaces, textured implants surfaces exhibit more surface area for
integrating with bone via osseointegration process. It was suggested that a surface roughness
in the range from 1 to 10 microns would be required to enhance both the osteoconduction
(in-migration of new bone), and osteoinduction (new bone differentiation) processes [34-36].
Previous investigations related to additive manufacturing of Ti alloys have focused on
producing fully dense and high integrity structures. There is a clear gap in literature regarding
the simultaneous enhancement and adjustment of pore fraction, surface and mechanical
properties of Ti6Al4V SLM components towards biomedical implants. In the present work,
artificial pores have been created in Ti6Al4V parts fabricated via SLM by controlling the
process parameters to achieve surface and mechanical properties suitable for biomedical
applications. The influence of processing parameters by means of laser power, scan speed
and hatch spacing on the surface roughness, porosity content and mechanical properties of
Ti6Al4V components produced by SLM will be investigated. Statistical analysis by means of
Design of Experiments (DoE) and Analysis of Variance (ANOVA) will be adopted to
optimise the SLM process parameters and fabricate custom parts with elastic modulus, UCS
and surface roughness sufficiently close to that of human bones.

2. Experimental Methods
2.1 Materials
Ti6Al4V gas atomized alloy powder was supplied by LPW Technology. Most of the
powder particles had a size range between 19-45m as measured using a laser diffraction
analyzer (Microtrac) following the ASTM B822 standard. The size distribution of powder
used is shown in Table 1.
Table 1. Ti6Al4V powder size distribution
Particle size
(m)
<16 16-22 22-31 31-44 >45
Percentage
(%)
5 10 28 46 11
2.2. Statistical design of experiment (DoE) using response surface
In this study the design of experiment RSM was carried out to generate an experimental
plan with minimum possible trials. ANOVA was utilized to find a relationship between the
input and output parameters, identify the most significant parameters, and find the optimal
setting of those parameters that can achieve the intended objective function. The response
surface “Y” can be expressed by a second order polynomial (regression) equation as shown
in Equation 2:
Yb
b
x
b

x
b

x
x
(2)
where x
i
are the factors input parameters. The terms b
0,
b
i
, b
ii
, and b
ij
are the model
coefficients that depend on the main and interaction effects of the process parameters.
Method of least squares is used to determine the constant coefficients. To perform the design
of experiment, Design-Expert Software Version 7.0.0 (Stat-Ease Inc., Minneapolis, USA)
was used.
The procedure adopted in this study was as the following:
1. Identification of the key process parameters, and setting the upper and lower bound for
each.
2. Selection of the output response.
3. Developing the experimental design matrix.
4. Carrying out the experiments according to the design matrix, and recording the output
response.
5. Developing a mathematical model to correlate the process parameters to the output
response.
6. Optimizing that model using genetic algorithm.
In the current study three factors (process parameters) were considered which are the laser
power, scan speed and hatch spacing. According to the central composite design, and as
described above, each parameter was varied over 5 levels (-α, -1, 0, 1 and α). See Fig 1. In
this work α was considered to be 2 in order to change each factor over five equal levels. Table
2 shows the levels of each factor in this investigation. As shown -α and α represent the
minimum and maximum levels respectively, of each factor. Also, three center points (at the 0
level (middle) of all factors, see Fig1) were considered. The center points are used to provide

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References
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Book
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Abstract: Comprehensive datasheets on more than 60 titanium alloys More than 200 pages on metallurgy and fabrication procedures Input from more than 50 contributors from several countries Careful editorial review for accuracy and usefulness Materials Properties Handbook: Titanium Alloys provides a data base for information on titanium and its alloys, and the selection of specific alloys for specific applications The most comprehensive titanium data package ever assembled provides extensive information on applications, physical properties, corrosion, mechanical properties (including design allowances where available), fatigue, fracture properties, and elevated temperature properties The appropriate specifications for each alloy are included This international effort has provided a broad information base that has been compiled and reviewed by leading experts within the titanium industry, from several countries, encompassing numerous technology areas Inputs have been obtained from the titanium industry, fabricators, users, government and academia This up-to-date package covers information from almost the inception of the titanium industry, in the 1950s, to mid-1992 The information, organized by alloy, makes this exhaustive collection an easy-to-use data base at your fingertips, which generally includes all the product forms for each alloy The 60-plus data sheets supply not only extensive graphical and tabular information on properties, but the datasheets also describe or illustrate important factors which would aid in the selection of the proper alloy or heat treatment The datasheets are further supplemented with back-ground information on the metallurgy and fabrication characteristics of titanium alloys An especially extensive coverage of properties, processing and metallurgy is provided in the datasheet for the workhorse of the titanium industry, Ti-6Al-4V This compendium includes the newest alloys made public even those still under development In many cases, key references are included for further information on a given subject Comprehensive datasheets provide extensive information on: Applications, Specifications, Corrosion, Mechanical Design Properties, Fatigue and Fracture

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01 Jul 2007-Dental Materials
TL;DR: The local release of bone stimulating or resorptive drugs in the peri-implant region may also respond to difficult clinical situations with poor bone quality and quantity, which should ultimately enhance the osseointegration process of dental implants for their immediate loading and long-term success.
Abstract: The osseointegration rate of titanium dental implants is related to their composition and surface roughness. Rough-surfaced implants favor both bone anchoring and biomechanical stability. Osteoconductive calcium phosphate coatings promote bone healing and apposition, leading to the rapid biological fixation of implants. The different methods used for increasing surface roughness or applying osteoconductive coatings to titanium dental implants are reviewed. Surface treatments, such as titanium plasma-spraying, grit-blasting, acid-etching, anodization or calcium phosphate coatings, and their corresponding surface morphologies and properties are described. Most of these surfaces are commercially available and have proven clinical efficacy (>95% over 5 years). The precise role of surface chemistry and topography on the early events in dental implant osseointegration remain poorly understood. In addition, comparative clinical studies with different implant surfaces are rarely performed. The future of dental implantology should aim to develop surfaces with controlled and standardized topography or chemistry. This approach will be the only way to understand the interactions between proteins, cells and tissues, and implant surfaces. The local release of bone stimulating or resorptive drugs in the peri-implant region may also respond to difficult clinical situations with poor bone quality and quantity. These therapeutic strategies should ultimately enhance the osseointegration process of dental implants for their immediate loading and long-term success.

1,940 citations


"Optimization of SLM Process Paramet..." refers background in this paper

  • ...It was suggested that a surface roughness in the range from 1 to 10 microns would be required to enhance both the osteoconduction (in-migration of new bone), and osteoinduction (new bone differentiation) processes [34-36]....

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  • ...However; relatively higher surface roughness may result in an increase in ionic leakage as well as peri-implantilis [36]....

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Journal ArticleDOI
Lore Thijs1, Frederik Verhaeghe1, Tom Craeghs1, Jan Van Humbeeck1  +1 moreInstitutions (1)
01 May 2010-Acta Materialia
Abstract: Selective laser melting (SLM) is an additive manufacturing technique in which functional, complex parts can be created directly by selectively melting layers of powder. This process is characterized by highly localized high heat inputs during very short interaction times and will therefore significantly affect the microstructure. In this research, the development of the microstructure of the Ti–6Al–4V alloy processed by SLM and the influence of the scanning parameters and scanning strategy on this microstructure are studied by light optical microscopy. The martensitic phase is present, and due to the occurrence of epitaxial growth, elongated grains emerge. The direction of these grains is directly related to the process parameters. At high heat inputs it was also found that the intermetallic phase Ti3Al is precipitated during the process.

1,729 citations


"Optimization of SLM Process Paramet..." refers background in this paper

  • ...SLM has many advantages such as producing complex shapes that are difficult to fabricate via conventional methods, short time from design to market, and near net shape production which minimizes waste of materials [7,8]....

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Journal ArticleDOI
Abstract: Selective laser melting (SLM) is driven by the need to process near full density objects with mechanical properties comparable to those of bulk materials. During the process the powder particles are completely molten by the laser beam. The resulting high density allows avoiding lengthy post-processing as required with selective laser sintering (SLS) of metal powders. Unlike SLS, SLM is more difficult to control. Because of the large energy input of the laser beam and the complete melting of particles problems like balling, residual stresses and deformation occur. This paper will describe SLM applied to a mixture of different types of particles (Fe, Ni, Cu and Fe3P) specially developed for SLM. The different appearing phenomenons are discussed and the process optimization is described. The latter includes an appropriate process parameter adjustment and the application of special scanning strategies. Resulting parts are characterized by their microstructure, density and mechanical properties.

1,132 citations


"Optimization of SLM Process Paramet..." refers background or methods in this paper

  • ...Selective laser melting (SLM) is an additive manufacturing technique that produces near fully dense metal parts directly from a CAD design by adding layer upon layer [1-4]....

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  • ...In addition, the increased laser power increases the energy density which improves the wettability of the melt pool, eliminating the differences in surface tension and in turn decreasing the chance of encountering the balling phenomenon which dramatically decreases the side surface roughness [2]....

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Journal ArticleDOI
Lawrence E Murr1, Stella Quinones1, Sara M. Gaytan1, M.I. Lopez1  +6 moreInstitutions (1)
TL;DR: The microstructure and mechanical behavior of simple product geometries produced by layered manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V.
Abstract: The microstructure and mechanical behavior of simple product geometries produced by layered manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of layered manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional manufacturing from Ti-6Al-4V wrought bar stock are discussed.

731 citations


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