SAMPE Journal - July/August 2017 - 39
Article
AM process is a combination of
tens of thousands or millions of
multi-pass moving weld-tracks or
melt-pools joined together either
re-melting layer up-on-layer along
the build direction (in Z) and, or
overlap re-melting across the build
direction (in X-Y). In a typical single
pass fusion welding processes the
filler materials is fed into the focused
heat source either in the form of
wire or powder, whereas in AM raw
material is fed into the system either
as a powder bed or blown powder or
in the form of wire. Due to the nature
of the process, residual stresses are
present in all welded components
as a consequence of the localised
high cooling rate associated with the
moving liquid melt-pool surrounded
by a metallic mould/partially sintered
powder mould. The presence of
residual stresses could significantly
affect the crack growth rate and the
resulting mechanical performance
(of particular under dynamic
loading) of the EBM manufactured
components. Hence, it is critical to
understand and quantify the amount
of residual stresses present in the
EBM Ti6Al4V AM fabricated parts
as it is very similar to the multi-pass
welding processes, where a moving
localised rapid heating and cooling
cycle results in the introduction
of significant amount of residual
stresses in the structures.
A large number of investigations
have been carried out in the past to
understand and study the residual
stress and associated phenomenon
as this would affect the components
static and dynamic mechanical
performance and this may lead to
catastrophic and unexpected failures
during the service conditions.
However, very limited amount of
literature is available on the parts
that were fabricated through new
and emerging technologies such
as AM and, that too it is mostly
focussed on laser-based AM instead
of the electron beam based AM.
Although it was theoretically
expected (due to preheated bed to
SAMPE Journal, Volume 53, No. 4, July/August 2017
Figure 1. (a) Schematic representation of the EBM machine; (b) e-beam scanning strategy; and (c) Ti6Al4V samples built using powder bed EBM AM
(with axis of X=Rx, Y=Ry & Z=Nz).
above 740°C) that there shouldn't be
any presence of significant amount
of residual stresses, there is no
literature available with quantifiable
residual stresses developed during
AM Ti6Al4V materials using EBM
technology.
AM using Ti6Al4V was identified
as a first choice of material since it is
difficult to process and machining,
in addition to the high initial raw
materials cost. Thus, Ti6Al4V
alloy is a most interesting alloy for
early adaptation in both civil and
military aero-industries including
space applications. This paper aims
to address the quantity of residual
stresses that are present in the asfabricated Ti6Al4V parts that are
manufactured via a new pre-heated
Powder bed - Electron Beam Melting
(EBM) additive manufacturing2
technique using Portable X-ray
Diffraction methods.
Experimental Methods
EBM Samples
Figure 1a show a schematic
representation of Arcam EBAM
machine under vacuum2. In the
Arcam machine in each deposition
cycle, aa a first step a rake spreads
the powder on a stainless steel
platform to give a uniform layer
thickness of 50 to 90 mm. Rapid pre-
melt scanning is then performed
to sinter and preheat the powder
(to a temperature of 740°C), after
which the desired 2-D section is
consolidated with the electron beam
travelling at a appropriate speed to
ensure sufficient melting occurs. In
the consolidation stage, the beam first
draws the outline of each 2D-section
with a "contour" pass and then the
outlined area is filled-in by 'hatching',
with a raster pattern performed in
orthogonal directions (X or Y) that
alternate every deposited layer (see
Figure 1b). The contour stage uses a
lower current (in the range 5-8 mA)
and slower travel speed (~0.15-0.3
m s−1), while hatching takes place
at a higher traverse rate and beam
current (6-15 mA and 0.5-1ms−1)3.
Following consolidation of each
layer, the machine bed is incremented
downwards with a layer height
of 50-90 mm and this sequence is
repeated. Preheating is necessary for
EBM process due to the following
reasons: (i) To creates sinter-bond
between particles so that powder
clouds can be avoided during rapid
melting stage, (ii) residual stresses
in the build could be reduced,
and (iii) better heat flow through
semi-sintered powder-cakes. In the
control software, a speed function is
used to attempt to maintain a stable
39
Table of Contents for the Digital Edition of SAMPE Journal - July/August 2017
https://www.nxtbook.com/nxtbooks/sampe/journal_20240506
https://www.nxtbook.com/nxtbooks/sampe/journal_20240304
https://www.nxtbook.com/nxtbooks/sampe/journal_20240102
https://www.nxtbook.com/nxtbooks/sampe/journal_20231112
https://www.nxtbook.com/nxtbooks/sampe/journal_20230910
https://www.nxtbook.com/nxtbooks/sampe/journal_20230708
https://www.nxtbook.com/nxtbooks/sampe/journal_20230506
https://www.nxtbook.com/nxtbooks/sampe/journal_20230304
https://www.nxtbook.com/nxtbooks/sampe/journal_20230102
https://www.nxtbook.com/nxtbooks/sampe/journal_20221112
https://www.nxtbook.com/nxtbooks/sampe/journal_20220910
https://www.nxtbook.com/nxtbooks/sampe/journal_20220708
https://www.nxtbook.com/nxtbooks/sampe/journal_20220506
https://www.nxtbook.com/nxtbooks/sampe/journal_20220304
https://www.nxtbook.com/nxtbooks/sampe/journal_20220102
https://www.nxtbook.com/nxtbooks/sampe/journal_20211112
https://www.nxtbook.com/nxtbooks/sampe/journal_20210910
https://www.nxtbook.com/nxtbooks/sampe/journal_20210708
https://www.nxtbook.com/nxtbooks/sampe/journal_20210506
https://www.nxtbook.com/nxtbooks/sampe/journal_20210304
https://www.nxtbook.com/nxtbooks/sampe/journal_20210102
https://www.nxtbook.com/nxtbooks/sampe/journal_20201112
https://www.nxtbook.com/nxtbooks/sampe/journal_20200910
https://www.nxtbook.com/nxtbooks/sampe/journal_20200708
https://www.nxtbook.com/nxtbooks/sampe/journal_20200506
https://www.nxtbook.com/nxtbooks/sampe/journal_20200304
https://www.nxtbook.com/nxtbooks/sampe/journal_20200102
https://www.nxtbook.com/nxtbooks/sampe/journal_20191112
https://www.nxtbook.com/nxtbooks/sampe/journal_20190910
https://www.nxtbook.com/nxtbooks/sampe/journal_20190708
https://www.nxtbook.com/nxtbooks/sampe/journal_20190506
https://www.nxtbook.com/nxtbooks/sampe/journal_20190304
https://www.nxtbook.com/nxtbooks/sampe/journal_20190102
https://www.nxtbook.com/nxtbooks/sampe/journal_20181112
https://www.nxtbook.com/nxtbooks/sampe/journal_20180910
https://www.nxtbook.com/nxtbooks/sampe/journal_20180708
https://www.nxtbook.com/nxtbooks/sampe/journal_20180506
https://www.nxtbook.com/nxtbooks/sampe/journal_20180304
https://www.nxtbook.com/nxtbooks/sampe/journal_20180102
https://www.nxtbook.com/nxtbooks/sampe/journal_20171112
https://www.nxtbook.com/nxtbooks/sampe/journal_20170910
https://www.nxtbook.com/nxtbooks/sampe/journal_20170708
https://www.nxtbook.com/nxtbooks/sampe/journal_20170506
https://www.nxtbook.com/nxtbooks/sampe/journal_20170304
https://www.nxtbook.com/nxtbooks/sampe/journal_20170102
https://www.nxtbook.com/nxtbooks/sampe/journal_20161112
https://www.nxtbook.com/nxtbooks/sampe/journal_20160910
https://www.nxtbook.com/nxtbooks/sampe/journal_20160708
https://www.nxtbook.com/nxtbooks/sampe/journal_20160506
https://www.nxtbook.com/nxtbooks/sampe/journal_20160304
https://www.nxtbook.com/nxtbooks/sampe/journal_20160102
https://www.nxtbook.com/nxtbooks/sampe/journal_20151112
https://www.nxtbook.com/nxtbooks/sampe/journal_20150910
https://www.nxtbook.com/nxtbooks/sampe/journal_20150708
https://www.nxtbook.com/nxtbooks/sampe/journal_20150506
https://www.nxtbook.com/nxtbooks/sampe/journal_20150304
https://www.nxtbook.com/nxtbooks/sampe/journal_20150102
https://www.nxtbook.com/nxtbooks/sampe/journal_20141112
https://www.nxtbook.com/nxtbooks/sampe/journal_20140910
https://www.nxtbook.com/nxtbooks/sampe/journal_20140708
https://www.nxtbook.com/nxtbooks/sampe/journal_20140506
https://www.nxtbook.com/nxtbooks/sampe/journal_20140304
https://www.nxtbook.com/nxtbooks/sampe/journal_20140102
https://www.nxtbook.com/nxtbooks/sampe/journal_20131112
https://www.nxtbook.com/nxtbooks/sampe/journal_20130910
https://www.nxtbook.com/nxtbooks/sampe/journal_20130708
https://www.nxtbook.com/nxtbooks/sampe/journal_20130506
https://www.nxtbook.com/nxtbooks/sampe/journal_20130304
https://www.nxtbook.com/nxtbooks/sampe/journal_20130102
https://www.nxtbook.com/nxtbooks/sampe/journal_20121112
https://www.nxtbook.com/nxtbooks/sampe/journal_20120910
https://www.nxtbook.com/nxtbooks/sampe/journal_20120708
https://www.nxtbook.com/nxtbooks/sampe/journal_20120506
https://www.nxtbook.com/nxtbooks/sampe/journal_20120304
https://www.nxtbook.com/nxtbooks/sampe/journal_20120102
https://www.nxtbook.com/nxtbooks/sampe/journal_20111112
https://www.nxtbook.com/nxtbooks/sampe/journal_20110910
https://www.nxtbook.com/nxtbooks/sampe/journal_20110708
https://www.nxtbook.com/nxtbooks/sampe/journal_20110506
https://www.nxtbook.com/nxtbooks/sampe/journal_20110304
https://www.nxtbook.com/nxtbooks/sampe/journal_20110102
https://www.nxtbookmedia.com