Characterization of selectively laser melted Ti-6Al-4 V micro-lattice struts.

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Procedia Engineering 10 (2011) 536–541

ICM11

Characterization of selectively laser melted Ti-6Al-4V microlattice struts
Rafidah Hasana,b, Robert Minesa*, Peter Foxa
b

a
Department of Engineering, University of Liverpool, The Quadrangle, Liverpool L69 3GH, UK
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka,
Malaysia

Abstract
This paper presents a characterization study of titanium alloy (Ti-6Al-4V) micro-struts manufactured using selective
laser melting (SLM). Previous test results from sandwich structures with titanium alloy micro-lattice cores showed
that the material experienced brittle fracture failure, although it had a reasonable specific strength. Therefore, the
microstructure present in the struts has been investigated in order to understand its influence on the mechanical
behaviour. Conclusions on the way forward for improved mechanical behaviour, by the use of subsequent heat

treatment and careful control of the manufacturing process can then be identified.

© 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11
Keywords: selective laser melting; titanium alloy; micro-lattice; sandwich structure

1. Introduction
The University of Liverpool has investigated the use of selectively laser melted (SLM) micro-lattice
structures within a number of engineering areas including medical devices and aerospace mechanical
structures, such as lightweight sandwich constructions [1, 2]. Previous work reported that the specific
strength of titanium alloy (Ti-6Al-4V) micro-lattice structures is competitive with that of aluminum
honeycomb [3] and this is especially true if the energy density is high (higher laser powers and long
exposure times) during the SLM process, with optimum results being obtained with a laser power of 200
Watts and an exposure time of 1000 —s. This combination of parameters for the Ti-6Al-4V micro-lattice
structure core resulted in a more localized impact area in the sandwich constructions as compared to
aluminum honeycomb core [4]. A more localized impact area means that a smaller area is affected by
* Corresponding author. Tel.: +44-0-151-794-4819; fax: +44-0-151-794-4819.
E-mail address: r.mines@liv.ac.uk

1877-7058 © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11
doi:10.1016/j.proeng.2011.04.090

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Rafidah Hasan et al. / Procedia Engineering 10 (2011) 536–541

im
mpact damagee, and this is preferred
p
by the
t aircraft maanufacturer [5
5]. However, it
i has been obbserved that
thhe impacted area
a
of the Ti-6Al-4V
T
miccro-lattice strructure core experienced
e
b
brittle

failure, with struts
shhowing almosst flat fracturee surfaces. Sinnce plasticity is an importaant criterion for
f energy abssorbency in
looad bearing structure perfoormance, espeecially in aeroospace applicaations [6], thee failure of theese Ti-6Al44V micro-latticce structures needs
n
to be fuurther studiedd and analyzed
d. Therefore, the
t investigatiion into the
bbehavior of thhe basic microo-strut build is an importaant part of cu
urrent researcch, as the prooperties and
fa
failure of the micro-struts
m
i thought to predict the beehavior of thee micro-latticee. An explanaation of the
is
bbrittle fracturee failure obserrved needs to be found, andd in parallel, the elastic prooperties of the structures
nneed to be deteermined by the use of a reliable micro-strrut test.
11.1. Micro-latttice strut
The micro-pplastic and -elastic propertiies of SLM Ti-6Al-4V
T

miccro-lattice struuctures can bee quantified
[1] from their basic buildinng block whicch is the micrro-strut. Fig 1(a)
1 shows a BCC Ti-6Al--4V microlaattice block with
w 2.5 mm unit
u cell size, while
w
Fig 1(bb) graphically shows a BCC
C unit cell witth 8 microsttruts protrudinng from a nodde at the centrre. When subjjected to impaact or compreession load, it was found
thhat fracture occcurred in thee end part of certain
c
struts, near the nodee region. As reported
r
by Shhen [7], the
S
SLM Ti-6Al-44V micro-lattiice block failss on a 45° diaagonal plane as shown in Fig
F 2. The faiilure of the
sttrut ends can be
b clearly obsserved in the scanning
s
electtron microscop

py (SEM) imaage in the figuure.

(a)

(b)

F
Fig. 1. (a) side vieew of 20 mm X 20
2 mm X 20 mm
m BCC Ti 6Al 4V
V micro-lattice strructure block; (b)) graphic illustrattion of a BCC
unnit cell with 8 strruts [2]

F
Fig. 2. SEM imagee of 200W X 10000—s SLM Ti-6All-4V micro-latticee block which sho
ows failure at cerrtain point in the strut, near
node area [7]

Previous work
w

on the SL
LM processinng of stainlesss steel micro-lattice structtures [1] highhlighted the
ppossibility thatt the ‘cast’ miicrostructure may
m contribute to the less ductile
d
behavioor. It has beenn suggested
thhat a similar problem
p
may occur with Tii-6Al-4V and that this issu
ue could be claarified by exaamining the

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Rafidah Hasan et al. / Procedia Engineering 10 (2011) 536–541

m
microstructuree present. Thee effect of miicrostructure on
o behavior is also impliedd in other reseearch work
[88, 9], where the strength of
o SLM Ti-6A

Al-4V produccts varied witth the microstructure form
med and the
pphases presennt, while the microstructuure varied with the SLM
M process parrameters usedd and any
suubsequent heeat treatment. In this paper, the microsstructure of th
he SLM Ti-6Al-4V micro-struts was
sttudied and repported, with an
a objective of
o clarifying thhe SLM Ti-6A
Al-4V micro-lattice failure issues that
hhave not been resolved in prrevious researrch work.
22. Material
SLM Ti-6A
Al-4V micro-struts were produced
p
from
m the same powder
p
materrial as the m
micro-lattice

sttructures in previous
p
research [7], whiich was Titannium Powderr Grade 5 AS
STM produceed by TLS
T
Technik GmbH
H & Co., Germ
many. As the micro-struts in
i the BCC arrchitecture of the micro-latttices was at
a 35° angle to the horizontal, the single sttruts were maanufactured at a 35° build angle,
a
with thee maximum
leength being limited to 433 mm by the manufacturring process. Fig 3 show
ws an image of the asm
manufactured SLM Ti-6All-4V micro-sttrut. For miccrostructure sttudies, metalllurgical sampples of the
m
micro-struts were
w
prepared by hot-mounnting in conduuctive resin using a 20 mm
m diameter m

mould. They
procedure, beefore being etcched and obseerved under
w
were then polished followinng a standard metallurgical
m
ooptical microsscopy and SE
EM, incorporaated with eleectron dispersive spectrosccopy (EDS) ffor element
aanalysis mappiing.

F
Fig. 3. A 43 mm micro-strut
m
of SLM
M Ti-6Al-4V maanufactured at the University of Liv
verpool (200W

X 1000—s)

33. Geometry, Microstructu
ure and Elem

ment Analysis of Micro-strrut
Fig 4(a) shoows the circuular cross-secttion of an as-m
manufactured SLM Ti-6All-4V micro-strrut build at
335° to the poowder bed, diiameter approoximately 3800 —m. The optical
o
microggraph Fig 4(bb) shows a
loongitudinal crross-section of
o a micro-struut, with micro-voids withiin the strut, annd a dendriticc structure,
similar to the welded
w
structuure of titanium
m alloy. Needdle like structu
ures are preseent within the grains due
too the solid staate transformaations that occur on coolinng. This structture is different to that norrmally used
w
with heat treateed Ti-6Al-4V
V [10].

Dendriticsttructures
microͲ

voids

100ђm

(b)

F
Fig. 4. (a) Opticall microscopic view of circular crooss-section; (b) Microstructure
M
off longitudinal secttioned; both figurres are the asm
manufactured SLM
M Ti-6Al-4V singgle strut built at 35°
3 angle laser beeam at 200 W laseer power and 10000 —s laser exposuure time

Rafidah Hasan et al. / Procedia Engineering 10 (2011) 536–541

Within the microstructurre were regionns that althouggh dendritic lo
ooked very diifferent from the bulk of
thhe material (F
Fig 5).

F
Fig. 5. A focused SEM image of dendritic
d
structuree with indicated points
p
for spectru
um 1 and 2 in eleemental analysis ((SLM Ti-6Al4V
V micro-strut witth 200W X 1000—
—s)

Elemental analysis
a
of thhese regions (Fig
(
6, spectrrum 1) showeed that compaared to the buulk (Fig 7,
sppectrum 2) thhey were richh in elements not normallyy found in Tiitanium Powdder Grade 5 A
ASTM [7].
T
These includedd relatively hiigh amounts of
o iron (Fe) annd chromium (Cr). It is likkely, thereforee, that these
sttructures are due to the crooss contaminaation of the powder
p
bed with
w 316L stainnless steel whhich is also
uused in this SL
LM machine. It is likely that
t
this contaamination occcurred during the manufactuuring stage
raather than poowder preparaation and maay have comee from the po
owder feed syystem as the argon gas
aatmosphere is carefully filteered [11]. Sinnce most of thhese dendriticc structures were
w
found at boundaries
aand within inteer-layer areas these phases probably form
m due to segreegation duringg freezing rathher than by
pprecipitation inn the solid staate and are likkely to form part
p of the lasst liquid to freeeze. It is alsoo suspected
thhat the dendriite formation affects the meechanical propperties as if th
hey are brittlee or soft they w
will form a
w
weak point in the micro-strrut initiating cracking
c
acrooss the strut in
n that region,, the strut onlly being as
sttrong as the weakest poinnt. It was foound that theese structures were signifiicantly harderr (419 HV
ccompared withh 373 HV) thaat the bulk of the
t structure. It has not yet been possiblee to determinee the phases
ppresent in these regions.

Fig. 6. The EDS plot
p of spectrum 1 for elemental annalysis of as-receiived micro-strut confirmed
c
with Ti-6Al-4V
T
elemennts
F

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Rafidah Hasan et al. / Procedia Engineering 10 (2011) 536–541

Fig. 7. The EDS plot of spectrum
m 2 for elementall analysis of as-rreceived micro-sttrut showed the existence
e
of Fe aand Cr peaks,
F
c
suuspected due to contamination

44. Heat Treatm
ment on SLM
M Ti-6Al-4V micro-strut
m
A simple heat
h
treatmentt process was used to studdy how the microstructure
m
affected the m
mechanical
pproperties of thhe struts. Fig 8(a) schematiically illustrattes the heat treeatment proceess while Fig 8(b) shows
thhe resultant microstructure
m
e, which is closer
c
to that normally useed and in whhich any segrregation of
eelements has been
b
redistributed. The heaat treatment was
w done in a normal furnace but with the sample
sealed within an
a evacuated quartz
q
tube. The
T heat treatm
ment led to a somewhat
s
coaarser microstruucture with
thhe formation of Į and ȕ reggions, the rem
moval of the chromium
c
and
d iron rich reggions and the removal of
thhe needle struucture. Resultts from EDS analysis shoowed that norrmal amounts of Fe were present, as
eexpected in Ti-6Al-4V,
T
w
while
no Cr was detected throughoutt the sampleed area, as tthe overall
ccontamination was below thhe detection limit
l
of the EDS
E
system. This
T
change in
i the microstructure on
hheat treatmentt has producedd a more uniform structure and is thereefore likely too produce struuts without
reegions of weaakness; howeever the heat treatment hass not induced
d significant grain
g
growth so that the
sttruts are one grain wide. The
T presence of
o a large graiin size within
n such small sttruts would siignificantly
aaffect their streength. Furtheer analysis to understand
u
thhe heat treatmeent effect on mechanical
m
prroperties of
S
SLM Ti-6Al-44V micro-latticce structure iss now being caarried out.
 Temp(°C)

Solutionheat
treatment

1000°C

Preecipitation
heattreatment

540°C
Water
W
q
quench
1hour

Air
cooled
4hours

Time(hour) (a)

Fig. 8. (a) Schemaatic of heat treatm
ment process on SLM
S
Ti-6Al-4V micro-strut;
m
(b) resulted microstruucture of heat treaated specimen
F
Al-4V
shhowed balance Į//ȕ phase of Ti-6A

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5. Conclusion and Ongoing Study
In this study the diameter of the SLM Ti-6Al-4V micro-struts was determined from the transverse
cross-section geometry. The characterization of the microstructure has led to the observation of dendritic
structures due to cross contamination with other powders, which had caused uneven mechanical
properties along the micro-struts. Elemental analysis identified the problem of contamination and it is
likely to be a significant problem with this technology if the equipment is used with more than one
powder. It has been shown that heat treatment can significantly change the microstructure dispersing the
contamination and creating a uniform microstructure, but without causing excessive grain growth that
would have a deleterious effect on the mechanical properties. Based on these results, the current ongoing
study focuses on effect of contaminant on mechanical properties of SLM Ti-6Al-4V micro-lattice
structure and looking forward in ways to improve the condition.

Acknowledgements
Financial support from grants sponsored by EPSRC/EP/C009525/1, EPSRC/EP/009398/1 and EU FP6 CELPACT is gratefully
acknowledged. Acknowledgements also extended to Dr S. Tsopanos and Dr C. Sutcliffe for specimen manufacturing; and Prof X.
Wu from University of Birmingham. R. Hasan acknowledges Malaysian Government and UTeM for her PhD study sponsorship.

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