* Corresponding author. Tel.: +44-0-151-794-4819; fax: +44-0-151-794-4819. E-mail address: r.mines@liv.ac.uk
ICM11
Characterization of selectively laser melted Ti-6Al-4V
micro-lattice struts
Rafidah Hasan
a,b, Robert Mines
a*, Peter Fox
aaDepartment of Engineering, University of Liverpool, The Quadrangle, Liverpool L69 3GH, UK
bFaculty 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
1877-7058 © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11 doi:10.1016/j.proeng.2011.04.090
im
mpact damage he impacted a howing almos oad bearing s 4V micro-lattic behavior of th failure of the m brittle fracture need to be dete
1.1. Micro-latt
The micro-p 1] from their attice block w truts protrudin hat fracture oc SLM Ti-6Al-4
trut ends can b
Fig. 1. (a) side vie nit cell with 8 str
Fig. 2. SEM image ode area [7]
Previous w possibility that hat a similar p
e, and this is p area of the T st flat fracture tructure perfo ce structures n he basic micro micro-struts i e failure obser ermined by th
tice strut
plastic and -e basic buildin with 2.5 mm u ng from a nod ccurred in the 4V micro-latti be clearly obs
ew of 20 mm X 2 ruts [2]
e of 200W X 100
work on the SL t the ‘cast’ mi problem may
preferred by t Ti-6Al-4V mic
e surfaces. Sin ormance, espe
needs to be fu o-strut build is thought to rved needs to e use of a reli
lastic properti ng block whic unit cell size, w
de at the centr e end part of c ice block fails served in the s
20 mm X 20 mm
00s SLM Ti-6Al
LM processin icrostructure m
occur with Ti
the aircraft ma cro-lattice str nce plasticity ecially in aero urther studied is an importa predict the be be found, and able micro-str
ies of SLM T ch is the micr
while Fig 1(b re. When subj certain struts, s on a 45° dia scanning elect
m BCC Ti 6Al 4V
l-4V micro-lattice
ng of stainles may contribut i-6Al-4V and
(a)
anufacturer [5 ructure core e is an importa ospace applica d and analyzed ant part of cu ehavior of the d in parallel, rut test.
Ti-6Al-4V mic ro-strut. Fig 1 b) graphically
jected to impa near the node agonal plane tron microscop
V micro-lattice str
e block which sho
ss steel micro e to the less d that this issu
5]. However, i experienced b ant criterion f ations [6], the d. Therefore, t urrent researc e micro-lattice
the elastic pro
cro-lattice stru 1(a) shows a shows a BCC act or compre e region. As r as shown in F py (SEM) ima
ructure block; (b)
ows failure at cer
-lattice struct ductile behavio ue could be cla
it has been ob brittle failure, for energy abs e failure of the the investigati ch, as the pro e. An explana operties of th
uctures can be BCC Ti-6Al-C unit cell wit ession load, it reported by Sh Fig 2. The fai age in the figu
) graphic illustrat
rtain point in the
tures [1] high or. It has been arified by exa
(b)
bserved that with struts sorbency in ese Ti-6Al-ion into the operties and ation of the e structures
e quantified -4V th 8
micro-strut, near
m phases presen ubsequent he tudied and rep have not been
2. Material
SLM Ti-6A tructures in p Technik GmbH a 35° angle to
ength being manufactured micro-struts w were then poli optical micros analysis mappi
Fig. 3. A 43 mm m
3. Geometry,
Fig 4(a) sho 35° to the po
ongitudinal cr imilar to the w o the solid sta with heat treate
Fig. 4. (a) Optical manufactured SLM
e present. The the strength o nt, while the eat treatment.
ported, with a resolved in pr
Al-4V micro-previous rese H & Co., Germ
the horizonta limited to 43
SLM Ti-6Al were prepared shed followin scopy and SE
ing.
micro-strut of SLM
Microstructu
ows the circu owder bed, di ross-section o welded structu ate transforma ed Ti-6Al-4V
l microscopic vie M Ti-6Al-4V sing
e effect of mi of SLM Ti-6A
microstructu In this pape an objective o revious resear
struts were p arch [7], whi many. As the l, the single st 3 mm by th l-4V micro-st
by hot-moun ng a standard m EM, incorpora
M Ti-6Al-4V ma
ure and Elem
ular cross-sect iameter appro of a micro-stru
ure of titanium ations that oc V [10].
w of circular cro gle strut built at 3
icrostructure o Al-4V produc ure varied w
r, the micros of clarifying th rch work.
produced from ich was Titan micro-struts i truts were ma e manufactur trut. For mic nting in condu
metallurgical ated with ele
anufactured at the
ment Analysis
tion of an as-m oximately 380
ut, with micr m alloy. Need cur on coolin
oss-section; (b) M 35° angle laser be
on behavior i cts varied wit ith the SLM structure of th he SLM Ti-6A
m the same p nium Powder in the BCC ar anufactured at ring process. crostructure st uctive resin u
procedure, be ectron dispers
University of Liv
of Micro-str
manufactured 0 m. The o o-voids withi dle like structu ng. This struct
Microstructure of eam at 200 W lase Dendriticst
s also implied th the micros M process par he SLM Ti-6 Al-4V
micro-powder mater r Grade 5 AS rchitecture of
a 35° build a Fig 3 show tudies, metall sing a 20 mm efore being etc ive spectrosc
verpool (200W
rut
SLM Ti-6Al optical microg in the strut, an
ures are prese ture is differe
f longitudinal sect er power and 100 100 tructures
microͲ
voids
d in other rese tructure form rameters used
Al-4V micro lattice failure
rial as the m STM produce the micro-latt angle, with the ws an image lurgical samp m diameter m
ched and obse copy (EDS) f
X 1000s)
l-4V micro-str graph Fig 4(b nd a dendritic ent within the nt to that nor
tioned; both figur 00 s laser exposu
ђm (b)
earch work med and the d and any -struts was
issues that
micro-lattice ed by TLS tices was at e maximum of the as-ples of the mould. They
erved under for element
rut build at b) shows a c structure, grains due rmally used
th
F 4V
sp T st u ra a a p th w st c p
F
Within the he material (F
Fig. 5. A focused V micro-strut wit
Elemental a pectrum 2) th These included tructures are used in this SL
ather than po atmosphere is and within inte precipitation in hat the dendri weak point in
trong as the compared with present in thes
Fig. 6. The EDS p
microstructur Fig 5).
SEM image of d th 200W X 1000
analysis of th hey were rich d relatively hi due to the cro LM machine. owder prepara carefully filte er-layer areas n the solid sta ite formation
the micro-str weakest poin h 373 HV) tha
e regions.
plot of spectrum 1
re were region
dendritic structure s)
hese regions ( h in elements
igh amounts o oss contamina It is likely t ation and ma ered [11]. Sin these phases ate and are lik
affects the me rut initiating c nt. It was fo at the bulk of t
for elemental an
ns that althoug
e with indicated p
(Fig 6, spectr not normally of iron (Fe) an ation of the p that this conta ay have come nce most of th
probably form kely to form p echanical prop cracking acro ound that the the structure.
nalysis of as-recei
gh dendritic lo
points for spectru
rum 1) showe y found in Ti nd chromium powder bed w amination occ e from the po
hese dendritic m due to segre part of the las perties as if th oss the strut in ese structures It has not yet
ived micro-strut c
ooked very di
um 1 and 2 in ele
ed that compa itanium Powd (Cr). It is lik with 316L stain
curred during owder feed sy c structures w
egation during st liquid to fre hey are brittle n that region, were signifi been possible
confirmed with T
ifferent from
emental analysis (
ared to the bu der Grade 5 A kely, therefore
nless steel wh the manufactu ystem as the were found at
g freezing rath eeze. It is also e or soft they w
, the strut onl icantly harder e to determine
Ti-6Al-4V elemen
the bulk of
(SLM
Ti-6Al-ulk (Fig 7, ASTM [7]. e, that these hich is also uring stage argon gas boundaries her than by o suspected will form a ly being as r (419 HV e the phases
F uspected due to c
4. Heat Treatm
A simple h properties of th
he resultant m elements has b
ealed within a he formation he needle stru expected in T contamination heat treatment
egions of wea truts are one affect their stre SLM Ti-6Al-4
Fig. 8. (a) Schema
plot of spectrum contamination
ment on SLM
heat treatment he struts. Fig microstructure been redistrib an evacuated q
of Į and ȕ reg ucture. Result Ti-6Al-4V, w was below th t has produced akness; howe grain wide. T ength. Furthe 4V micro-lattic
atic of heat treatm /ȕ phase of Ti-6A
m 2 for elemental
M Ti-6Al-4V m
t process was 8(a) schemati e, which is c uted. The hea quartz tube. T gions, the rem ts from EDS while no Cr he detection l d a more uni ever the heat The presence o
er analysis to u ce structure is
ment process on S Al-4V
4hours ecipitation
attreatment
l analysis of as-r
micro-strut
used to stud ically illustrat closer to that
at treatment w The heat treatm moval of the c
analysis sho was detecte limit of the E form structur
treatment has of a large grai understand th s now being ca
SLM Ti-6Al-4V m
Time(h Air cooled
received micro-st
dy how the m tes the heat tre
normally use was done in a ment led to a s chromium and owed that nor d throughout EDS system. T
e and is there s not induced in size within he heat treatme
arried out.
micro-strut; (b) r
our) (a)
trut showed the e
microstructure eatment proce ed and in wh a normal furn
somewhat coa d iron rich reg rmal amounts
t the sample This change i efore likely to d significant g n such small st ent effect on m
esulted microstru
existence of Fe a
affected the m ess while Fig hich any segr
ace but with arser microstru gions and the
of Fe were ed area, as t in the micros o produce stru grain growth
truts would si mechanical pr
ucture of heat trea
and Cr peaks,
mechanical 8(b) shows regation of
the sample ucture with removal of present, as the overall
tructure on uts without so that the ignificantly roperties of
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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