abstract in pdf- format

Micro-CT based characterisation of the effect of
surface modification on the morphology and
roughness of selective laser melted Ti6Al4V open
porous structures
G. Pyka1, G. Kerckhofs1, S. Van Bael2, J. Schrooten1, M. Wevers1
1
Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B3001 Leuven, Belgium; [email protected], [email protected],
[email protected], [email protected]
2
Department of Mechanical Engineering, Division of Production engineering, Machine design
and Automation, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium
[email protected],
Introduction and aim
Additive manufacturing (AM) represents the most advanced method to build porous
structures with a controlled and robust internal and external geometry. However it does not
allow a high control of the surface properties at the micro-scale1-3. For that reason, an
appropriate surface modification is needed as a post-production treatment. Controlled
surface modification of three dimensional (3D) open porous structures is complicated since it
requires surface treatment both on the outer as inner surfaces of the structure. Chemical
and/or electrochemical treatment can provide a solution for this problem since acid-based
solutions can penetrate porous structures through the interconnected pores4-6. It is obvious
that by modifying the surface properties, the morphological characteristics of the porous
structure are changed. Therefore a thorough analysis of the surface morphology of porous
structures is crucial. In this study, Ti6Al4V open porous structures, produced by selective
laser melting (SLM) were treated with chemical etching and electrochemical polishing to
homogenise the surface roughness throughout the entire structure. To quantitatively assess
the quality of these treatments microfocus X-ray computed tomography (micro-CT) was used
for the morphological characterisation of the Ti6Al4V open porous structures prior to and
after each surface treatment step. The specific surface after chemical etching was used as
input to optimise the electrochemical polishing. The aim of this study was to assess the effect
of the surface treatments on the morphology of the porous structures and the strut surface
roughness and hence on the effectiveness of the surface treatments.
Materials and methods
SLM was used in this study to produce open porous structures starting from bio-inert Ti6Al4V
powder. Cylindrical porous structures were designed using Magics software [Materialise NV,
Haasrode, Belgium] with an open porous unit cell. Three different architectures with designed
strut thickness of 100, 140 and 180 µm (Strut 100, Strut 140 and Strut 180 respectively) and
a pore size of 1 mm were tested. The designed diameter and height of the porous structures
were respectively 6 mm and 12 mm. More information about the porous structure design and
production can be found in Ref. [1].
Two consecutive surface roughness modification procedures, suggested by Pyka et al.7,
were applied: (i) 10 minutes of chemical etching (CHE) to remove the entire strut surface,
including loosely sintered SLM powder remnants and (ii) 8 minutes of electrochemical
polishing (ECP) to remove me
etal ions fro
om the surrface and obtain
o
a sm
moother and
d more
nous surface
e. The disso
olution rate depends on
o the current density, w
which is governed
homogen
by the su
urface topology. In ord
der to ensurre homogeneous ECP
P reduction rates, the current
density w
was kept at 2 mA/mm² for
f all tested
d designs.
To deterrmine the current
c
to be applied
d, the surfa
ace area of
o the poroous structurre was
calculated by means of micro--CT. Additio
onally, the complete porous
p
struccture morph
hology,
d after each surface modification
m
n step, was
s evaluated
d by micro--CT-based image
prior and
analysis using the Philips HOMX 161 m
microfocus X-ray
X
system with AE
EA Tomohawk CT
el size of the imagess was 12.6 µm. Ma
anual, but consistent global
software. The pixe
ation was carried
c
out to allow q
quantificatio
on of the surface areaa, as well as the
segmenta
porosity, average po
ore and stru
ut thicknesss and their distribution
ns using CT
TAn [Skysca
an NV,
Belgium].
Kontich, B
The roug
ghness wass determine
ed based o
on 2D cross
s-sectional micro-CT images usiing the
surface p
profile line fo
or the calcu
ulations. Th e pixel size
e of the images was 1. 75 µm. As the 2D
images w
were taken with
w a high--resolution S
SkyScan 11
172 micro-C
CT system, w
which required no
special sa
ample prep
paration, the
e measurem
ments could be perform
med in a nonn-destructiv
ve way.
Compare
ed to comm
mercially av
vailable pro
rofile measuring syste
ems, the nnovel protocol for
surface rroughness measurement offers the possib
bility to qua
antitatively analyse the strut
surface m
morphologyy of complex 3D porou
us structure
e as part off the manuffacturing prrocess.
The app
plied acquissition param
meters for both the morphological characcterisation as the
p
in
n Table 1.
roughnesss measurements are presented
Table 1. Micro-CT a
acquisition parameterss used for both
b
the morphologicaal characterrisation
as the ro
oughness measureme
m
ents of the
e as-produc
ced and su
urface treatted Ti6Al4V
V SLM
structuress.
CT system
s
Philipss HOMX
1
161
SkyyScan
1172
Voltage
V
Current
90 kV
390 µA
100 kV
100 µA
Filter
F
material
1 mm
aluminium
0.5 mm Cu
0.5 mm Al
Voxel sizee
12.6 µm
m
1.75 µm
m
Results
s and disc
cussion
Surfa
ace roughn
ness meas
surements o
of the Ti6A
AlV open po
orous strucctures
The untre
eated SLM Ti6Al4V po
orous structtures (i.e. as-produced
a
d structuress) for the different
strut thicknesses sh
howed a no
on-uniform roughness (Fig. 1a ,b
b and c) priimarily caused by
d, non-meltted powderr grains. Also a higheer designed strut
spatial differences in attached
n a larger am
mount of no
on-melted grains on the
e strut surfaace.
thicknesss resulted in
Figure 1:. Represe
entative sca
anning electtron micros
scope (SEM) images off a single sttrut of
the as-pro
oduced Ti6A
Al4V porouss structures with design
ned strut thiickness:
a) 100 µm, b) 140 µm
µ and c) 180
1 µm (the
e y-direction
n is the build
ding directioon, T-strut top,
t
Bstrrut bottom).
The results of the ovverall strut roughness measureme
ents of the top and boottom of the struts,
d in Figure 2a, did nott show sign
nificant diffe
erences bettween desiggns Strut 10
00 and
presented
Strut 140
0 as well ass between designs Strrut 140 and
d Strut 180 for the as--produced porous
structuress and afte
er ECP. Ho
owever, takking into account
a
the
e visible sppatial differrences,
roughnesss analysis performed separatelyy for the strut top and
d bottom reevealed sign
nificant
difference
es between
n top and bottom. Mo
oreover, that differenc
ce increaseed for the porous
structuress produced
d with thick
ker struts, w
which confirmed visua
al inspectio n of the sa
amples
(Fig. 1), where morre powder grains atta
ached to the bottom of
o the strut were notic
ced for
d 180 comp
pared to dessign Strut 100. This spatial differe nce confirm
med the
design Sttrut 140 and
need of an appropriate post-p
production surface tre
eatment tha
at can hom
mogenize the strut
oughness th
hroughout the
t complette porous sttructure for the differennt designs.
surface ro
e overall surrface roughn
ness and (b) comparis
son of roughhness of the
e top
Figure 2:. (a) The
om of the sttruts of poro
ous structurres produce
ed with different designned strut thic
ckness
and botto
prior to
o and after each
e
surfac
ce treatmen
nt step (*p>0
0.05 = not significant).
s
(c) The rela
ative
reductio
on of the strrut surface roughness
r
a
and (d) the absolute re
eduction in sstrut thickne
ess of
the porou
us structure
es produced
d with differe
ed strut thickness priorr to and afte
er each
ent designe
surface
e treatment step.
b seen tha
at the total rroughness of the struc
ctures signifficantly decreased
In Figure 2a, it can be
ested desig
gns after each appliied surface
e treatment step. Coomparison of the
for all te
roughnesss for the to
op and botto
om of the sttruts (Fig 2b) showed that the rouughness red
duction
of the strrut bottom was
w higher compared the top aftter CHE, which reduceed the top-bottom
dissimilarrities. Only for
f design Strut
S
100, th
he differenc
ce between the strut toop and botto
om was
insignifica
ant after CH
HE and com
mbined CH
HE-ECP. Fig
gure 2c sho
ows that thee effectiven
ness of
CHE dep
pended on the porou
us structure
es design and was the highesst for the porous
structuress Strut 100
0, which contained the
e least amo
ount of non
n-melted poowder grain
ns after
SLM prod
duction.
phological characterization of th
he Ti6AlV open
o
porou
us structurres
Morp
Morpholo
ogical analyysis of the as-produce
ed and surfface treated
d porous sttructures re
evealed
that the a
average stru
ut thickness
s reduction of the poro
ous structures with dessign Strut 14
40 and
180 were
e comparab
ble, but a higher
h
redu ction of the
e absolute average strrut thicknes
ss was
found forr design Strut 100. Th
his implied that the CH
HE effective
eness depeends on the
e initial
amount o
of non-meltted powder grains atta
ached to th
he bottom surface
s
rathher than the
e initial
strut thicckness. Forr porous structures
s
w
with design
n Strut 100
0, 10 minuutes of CHE was
sufficient to remove
e all grains,, but not en
nough for designs
d
Strrut 140 andd 180. In order to
ncy of the CHE,
C
the tre atment time
e should be
e optimized depending on the
increase the efficien
ut roughnesss.
initial stru
Based on
n the surfacce area of the sampless after CHE
E determine
ed by micro--CT (i.e. 9, 10, 11
cm2 for S
Strut 100, 14
40 and 180 respectivel y), the follo
owing curren
nt values weere applied during
ECP: 1.2
2, 1.6 and 1.9
1 mA for designs St rut 100, 14
40 and 180 respectivelly. Howeve
er, after
ECP a higher reducttion of the struts
s
thickn
ness was ob
bserved forr structures with thicker struts
nspection of the crosss-sectional micro-CT images of the as-pro
oduced
(Fig. 2d)). Visual in
structuress, (fig. 3) obtained
o
with an isotrropic voxel size of (12.6 µm)³, rrevealed th
hat this
limited spatial imag
ge resolutio
on makes it difficult to discriminate the nnon-melted grains
ut surface from
f
the su
urface itselff (average grain size is about 30 µm).
attached to the stru
e effect, usiing a globa
al threshold
d for segmeentation, th
he strut
Because of the parrtial volume
i
by
b including
g the non-m
melted powd
der grains aas a cohere
ent part
thicknesss might be increased
of the stru
ucture. Beccause of this
s, the surfacce area would be determined incoorrectly, hav
ving an
influence on the currrent density
y calculatio
ons. This efffect would even more be express
sed for
Strut 140 and
a
180, where a larg
ger amount of non-me
elted surfacce-attached grains
designs S
were obsserved. Beccause of the
e potential error in the
e measurem
ment of the surface are
ea, the
applied ccurrent density might not
n be equ al for all de
esigns, and
d hence migght have le
ed to a
higher re
eduction ra
ate for design Strut 1
140 and 18
80, explaining the diffferences in
n ECP
effectiven
ness. In future experiiments, hig
gher resolution micro-CT imagess will be used to
determine
e the surfacce area of th
he porous sstructures after
a
CHE as
s input for E
ECP.
Figurre 3: A repre
esentative cross-sectio
c
onal micro-C
CT image of an as-prodduced Ti6A
Al4V
porous structure with
w designe
ed strut size
e 180 µm. The isotropic
c voxel size was (12.6 µm)³.
Conclus
sion
Inhomoge
eneous rou
ughness of open poro
ous metal structures hampers itt use for different
applicatio
ons, also for the SLM Ti6Al4V
T
ope
en porous structures,
s
assessed
a
inn this study, which
contained
d a high and inhomoge
eneous stru
ut surface ro
oughness. This
T
proble m was addressed
by the inttroduction of
o an appro
opriate and robust surfface modific
cation methhod and com
mbined
assessment of the as-produce
ed porous sstructures to
t bring the
eir microscaale morpho
ological
ace topological propertties to a co ntrollable le
evel. The as
ssessment could be done by
and surfa
a thoroug
gh characte
erisation us
sing micro-C
CT, enablin
ng the quantification oof the chan
nges in
surface rroughness and the morphologic
m
cal propertties due to
o surface m
modification
n. This
quantitative characterisation of the effect of surface modification on meso- and micro-scale
morphological properties of the Ti6Al4V porous structures is a powerful tool to optimise the
surface modification protocol according to desired morphological properties and feedback
could be provided for optimisation or fine-tuning of the production technique.
It was shown that roughness inhomogenity can be reduced by the combination of CHE and
ECP. However the effectiveness of the surface treatment depends both on the design, as
well as on the applied spatial image resolution of the micro-CT images. A higher initial
design-dependent roughness after production resulted in an inadequate removal of the nonmelted powder grains by CHE, and hence makes CHE optimisation design-dependent. Also
incorrect calculation of the strut surface area caused by the partial volume effect and
segmentation errors might have influenced the ECP effectiveness and could be solved by
using high-resolution micro-CT.
References:
1. S. Van Bael G. Kerckhofs, M. Moesen, G. Pyka, J. Schrooten, J. Kruth, “Micro-CTbased improvement of geometrical and mechanical controllability of selective laser
melted Ti6Al4V porous structures”, Mater. Sci. Eng A, 528, 7423-7431, 2011.
2. J. Kruth, L. Froyen, J. Van Vaerenbergh, P. Mercelis, M, Rombouts, B. Lauwers,
“Selective laser melting of iron-based powder”, J. Mat. Proc. Tech. 149, 616-622,
2004.
3. M. Rombouts J Kruth, L Froyen, P Mercelis, “Fundamentals of Selective Laser
Melting of alloyed steel powders”, CIRP Annals – Manuf. Tech. 55, 187-192, 2006.
4. A. Bloyce, ASM Handbook, ASM International, Vol 5, pp. 835–851, 1994.
5. G. Nawrat, W. Simka. Przemysł Chemiczny, 82, 851-854, 2003.
6. A. Kuhn, “The electropolishing of titanium and its alloys”, Metal Finishing Information,
120, 80-86, 2004.
7. G. Pyka, G. Kerckhofs, S. Van Bael, M. Moesen, D. Loeckx, J. Schrooten, M.
Wevers, “Non-destructive characterisation of the influence of surface modification on
the morphology and mechanical behaviour of rapid prototyped Ti6Al4v bone tissue
engineering scaffolds”, European Conference for Non-Destructive Testing (ECNDT).
Moscow, Russia, 7-11 June, 2010