SAMPE Journal - July/August 2017 - 43
Article
RS of -400 MPa at the top surface (for
both Phi=0°, and 90°) is approx. 50 %
of yield strength of Ti6Al4V.
High compressive residual stresses
were noticed on both the surfaces
(top XY and side XZ). However, the
magnitude of compressive residual
stresses noticed on the sides of XZ
surface were much higher than that
of top XY surfaces, around 2 and
2.5 times for Phi=0° and Phi=90°,
respectively. This can be explained
with the help of understanding the
temperature and stress field around a
melt-pool during welding processes.
At the leading edge of the melt-pool,
large areas of compressive residual
stresses will be generated ahead of
the moving melt-pool due to rapid
melting and thermal expansion
of the materials. Whereas at the
trailing, tensile residual stresses
would generate due to thermal
contraction
upon
solidification
(solidification induced residual
stresses). At any time when a weld
bead is deposited, the sum of the
stresses induced in any direction is
zero, i.e., the tensile residual stresses
in the build are compensated by
compressive stresses in the previous
layer or in the adjacent layer. The
magnitudes of the stresses can easily
be altered to a large extent since it is
dependent on the travelling speed
of the e-beam, depth of re-melt,
scanning strategy, and, plasticity of
the materials at given temperature.
In EBM AM (Figure 1b), during 'infill hatching' e-beam rapidly moves
(at a speed of 2 to 7 m/s depending
on speed functions7) from one end
to the other end and, then the speed
of the e-beam suddenly drops at the
edge (with turning function switched
on), so that the entire build will be
kept with a constant line energy per
unit length in order to promote a
homogeneous microstructure which
would result in uniform material
properties such as static and dynamic
mechanical properties. The higher
compressive stresses noticed close
to the edge or side surfaces, are the
accumulated local stresses ahead of
the rapidly moving melt pool during
SAMPE Journal, Volume 53, No. 4, July/August 2017
Figure 4. Residual stress profile measured using X-ray Diffractometer in the
powder EBM Ti6Al4V samples from the side of the build towards the interior
in XZ plane upon Electro-polishing at an approx. intervals of 100 mm.
rapid solidification. This is mainly
due to the sudden deceleration of
the e-beam combined with beam
reversal or turn around at the free
edges/surfaces of the components
(as the beam always raster in XY
plane and reverses with an offset
overlapping).
When e-beam turns-around or
reverses at the free surface or wall
corner, it leaves behind compressive
stresses developed locally at the
surface of the samples. The large and
complex orientation change noticed
near the herring-bone columnar
b-grains (being blue colour in Figure
2a-b could also be due to an influence
of localised heat flow and stress
relaxations interactions. After initial
transient, fine grains, a herring-bone
columnar grains <111>b direction is
aligned parallel to Z instead <001>
b(which is an easy growth direction
in cubic materials). <001> b is being
parallel to XY plane since maximum
grain growth is now rotated and
following beam movement along XY
plane. It was also speculated that the
higher surface roughness of Ra 70 mm
reported in the EBM builds by Rafi et
al11 on XZ surface (compared to Ra
30 mm in XY surface) could have also
contributed to the slight change in the
magnitude of compressive stresses
close to the surfaces. This can clearly
be seen from the error values shown
in Table 1 (all are showing the error
< ±25 MPa, whereas on XZ plane, one
of the result showed an error of up to
± 30 MPa).
Both analysis at the XY and XZ
plane has showed an insignificant
amount of residual stresses in their
bulk or core of the samples after 100
µm from the surface to up to a depth
of 600 mm (-17.5 MPa to +29.7 MPa
for Phi=0° and, from -17.16 MPa to
+26.8 MPa for Phi=90° in XY plane
and, of 42.5 MPa to +29.4 MPa for
Phi=0° and, from -37.7 MPa to +38.9
MPa for Phi=90° in XZ plane). The
RS after two layers just beneath the
final layer of 100 mm depth in XY
plane from top surface and/or from
100 mm depth from XZ plane surface
did not show any significant amount
residual stress, as the material
undergoes repetitive thermal cycles,
which intern acts as a stress relief
heat treatment (SRHT) cycle.
The overall insignificant amount of
low residual stresses noticed in the
bulk or core of the EBM components
can be summarised as ; (i) primarily
due to preheated powder bed (to
740°C), which reduces the thermal
mismatch between layers during
re-melting
thereby
reducing
43
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