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Abstract

The effect of two stop-off coatings on the structure and hardness of the surface of the titanium alloy VT6 has been investigated. Data for nondestructive diagnostics of the surface layer have been obtained by the method of acoustic emission. Results of this study can be of use for quality control of hollow parts fabricated from titanium alloys.
838
Russian Physics Journal, Vol. 58, No. 6, October, 2015 (Russian Original No. 6, June, 2015)
STUDY OF CRACK NUCLEATION USING THE ACOUSTIC
EMISSION METHOD
R. G. Khazgaliev,1 A. M. Dil’mukhametova,2 UDC 539.42+620.179
V. V. Astanin,3 and R. V. Safiullin1
The effect of two stop-off coatings on the structure and hardness of the surface of the titanium alloy VT6 has
been investigated. Data for nondestructive diagnostics of the surface layer have been obtained by the method of
acoustic emission. Results of this study can be of use for quality control of hollow parts fabricated from
titanium alloys.
Keywords: acoustic emission, stop-off coating, gas-saturated layer, titanium alloy.
INTRODUCTION
A promising method for the fabrication of key units and parts of aerospace equipment is superplastic forming
(SPF) combined with pressure welding (PW). This method can be used to fabricate complex-profile, lightweight
constructions with ribs made of titanium alloy sheet [1]. Special features of the technology envisage the use of stop-off
coatings, which are deposited on the welding surfaces of sheet-metal, in a definite order. The sheet-metal blanks are
joined together by pressure welding at those places where the coating is absent, and in the process of subsequent
superplastic forming hollow constructions are obtained [2, 3].
Pressure welding of titanium alloys is realized in the temperature interval 850–950°С over the course of
an extended time [4]. Under these conditions, it can be expected that the titanium alloy will interact with the
components of the coating, resulting in the formation of a brittle modified (-case) layer on the inner surfaces of the
cells. The formation of the modified layer at high temperature may be due to the influence of residual moisture,
decomposition of organics, and reduction by titanium of oxides and nitrides entering into the composition of the
coating. Similar layers were in fact detected in test constructions, which in turn served as a jumping-off point for the
present study. In [1–6] the most applicable stop-off coatings were presented; however, information on the effect of these
coatings on the properties of titanium alloy are absent in the literature. In PW/SPF technology two types of coatings are
most often used: the first is based on hexagonal boron nitride (BN) and the second is based on yttrium oxide (Y2O3).
Both are prepared in the form of suspensions with organic binding-agent additives.
In the present work we investigate the influence of the above two indicated coatings on the state of a titanium-
alloy surface layer and demonstrate the possibility of detecting a modified layer in finished parts by nondestructive
testing. For comparison, we fabricated and investigated specimens annealed in air (oxidation of the titanium alloy
surface) and in vacuum without a coating (clean surface without a modified layer).
1Institute for Metals Superplasticity Problems of the Russian Academy of Sciences, Ufa, Russia, e-mail:
sloth.usatu@gmail.com; dr_rvs@mail.ru; 2TruboprovodServis Engineering-Expert Center, Ufa, Russia, e-mail:
Lunasvetochek@mail.ru; 3Ufa State Aviation Technical University, Ufa, Russia, e-mail: vvastanin@yandex.ru.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 90–94, June, 2015. Original article
submitted March 6, 2015.
1064-8887/15/5806-0838 2015 Springer Science+Business Media New York
DOI 10.1007/s11182-015-0578-6
M
g
w
A
I
t
m
t
m
t
w
M
ATERIAL
S
For t
h
Commercial c
o
ground off an
d
a temperature
secon
d
speci
m
formed on the
Mech
a
electromecha
n
stress was det
e
w
here Р is the
The a
p
A
S-15A/2 co
attached to th
e
The
m
INSTRON-11
8
t
he acoustic si
g
The
p
m
icrohardnes
s
of 10 g for 10
t
o the original
The
d
m
icroscope a
n
t
he total widt
h
segment, whi
c
w
here ai is th
e
S
AND MET
H
h
ese investigat
o
atings contai
n
d
polished. T
h
of 915°C for
2
m
en was prepa
r
surface.
a
nical tests w
e
n
ical machine
w
e
rmined for th
e
load, l is the
d
p
pearance of
m
m
plex using t
h
e
bottom of th
e
m
inimum thre
85 test machi
n
g
nals and stre
s
p
resence and
s
was measure
d
s. To increase
surface of the
d
epth and widt
h
n
d a Tescan V
e
h
of all the cr
a
c
h could be cal
l
e
width of the i
n
H
ODS OF ST
U
ions, blanks
w
n
ing yttrium o
x
h
ey were then
2
4 h. The first
s
r
ed by anneal
i
e
re performed
w
ith a force o
e
outer fibers (
l
d
istance betwe
e
m
icrocracks w
a
h
e software p
a
e
bending tool
w
shold of sen
s
n
e. The signal
a
s
ses were dra
w
depth of the
d
on a Duram
i
the resolutio
n
specimen.
h
of the surfa
c
e
ga scanning e
l
a
cks intersecte
d
l
ed the relativ
e
n
dividual crac
k
Fig.
1
U
DY
w
ith di
m
ensio
n
x
ide (No. 1) a
n
dried in a de
s
s
pecimen was
i
ng in air at
9
according to
a
f 100 kN at r
o
l
ayers of the s
p
e
n supports, a
n
a
s inferred fro
m
a
ckage
A
EStu
d
w
ere use
d
.
s
itivity of 40
a
mplitude and
w
n on one grap
h
modified lay
e
i
n-2 microhar
d
n
of the metho
d
c
e cracks were
l
ectron micros
c
d
by a line se
g
e
crack width:

k
s and l is the
1
. Diagram of
a
n
s 2 × 10 × 7
0
n
d boron nitri
d
s
iccator at a t
e
processed acc
o
9
00°С for 2 h
,
a
three-
p
oint
b
o
om temperat
u
p
ecimen) acco
r
2
3
2
Pl
bh
,
n
d b is the wid
t
m
acoustic em
i
d
io. DR15 res
o
dB was dete
r
frequency of
e
h
for ease of v
i
e
r were deter
m
d
ness meter b
y
d
, the plane of
measured on
c
ope. From th
e
g
ment on the
s
/
i
al
,
length of the l
i
a
three-
p
oint b
0
mm were cu
t
d
e (No. 2) wer
e
e
mpe
r
ature of
2
o
rding to a si
m
,
as a result o
f
b
ending schem
e
u
re with a stra
r
ding to the fo
r
t
h an
d
h is the
ission (AE) si
g
o
nance-type s
e
r
mined on th
e
e
vents were a
n
i
sualization.
m
ined from
v
y
indenting usi
n
the micro-cut
s
elongated cut
s
e
measuremen
t
s
urface of the
i
ne on the sur
fa
ending test.
t
from a VT6
t
e
deposited on
2
00°С and an
n
m
ilar regime w
i
f
which a gas
e
on an INST
R
a
in rate of 1
m
r
mula
thickness of t
h
g
nals with the
e
nsors with fr
e
e
basis of th
e
n
alyzed. Sync
h
v
alues of the
n
g a diamond
s
was oriented
s
using an Oly
m
t
results we d
e
specimen to t
h
fa
ce of the spe
c
t
itanium alloy
blanks that ha
d
n
ealed in vac
u
i
thout a coatin
-saturated lay
e
R
ON-1185 un
i
m
m/min (Fig. 1
h
e specimen.
help of the M
a
e
quency of 15
0
e
noise level
h
ronized diagr
a
microhardnes
s
indentor with
at the angle α
m
pus GX-51
o
e
termined
t
he r
a
h
e length of t
h
c
imen.
839
sheet.
d
been
u
um at
g. The
e
r was
i
versal
). The
(1)
a
lakhit
0
kHz,
of the
a
ms of
s
. The
a load
= 2.5°
o
ptical
a
tio of
h
is line
(2)
R
i
v
r
t
h
w
g
v
m
840
R
ESULTS A
N
The
m
comparison w
i
The
h
annealed in ai
r
indicates the
p
at the surface
v
acuum witho
u
Figur
e
r
epresents the
at the very sta
r
t
he gas-satura
t
density of eve
n
h
igh amplitu
d
a coating, no
a
as a stress ar
o
specimens an
n
stresses that a
r
coating No. 1
(
To d
e
of the specim
e
w
ith depth u
p
signals with
h
earlier the cra
c
given case (T
a
v
acuum, crac
k
a study of wh
i
coating No. 2
appeared as a
r
m
uch less tha
n
F
(
a
N
D DISCUSS
m
easurement r
e
i
th the base m
a
h
ardness of th
e
r
the hardness
p
resence of a
g
is also obser
v
u
t a coating a
n
e
3 shows di
a
signal amplit
u
r
t of applicati
o
t
ed layer [7].
n
ts. As the tra
n
d
e falls, but t
h
a
coustic activi
t
o
und 95% of
t
n
ealed with c
o
r
e 40–55% of
(
Fig. 2d), no s
i
e
termine the s
o
e
ns after the b
e
p
to 20 μm an
d
h
igh amplitude
.
c
k arose, the
w
a
ble 1), which
k
s were not de
i
ch does not li
e
was deposite
d
r
esult of bend
i
n
in the first c
a
F
ig. 2. Values
(
curve 1), wit
h
a
nd with coati
n
ION
e
sults plotted
a
terial, which
a
e
alloy in its
b
increases wit
h
g
as-saturated l
a
v
ed for speci
m
n
d with coatin
g
a
grams of th
e
u
de and time o
o
n of the load
(
The signals a
r
n
sition is mad
e
h
e number of
t
y was observ
e
t
he yield poi
n
o
ating No. 2
(
the yield poi
n
i
gnals exceedi
n
o
urces of the a
e
nding tests (F
d
length up t
o
.
The relative
w
ider it opene
d
speaks of the
i
tected (Fig. 4
b
e
within the s
c
d
before ann
e
i
ng. Judging fr
a
se, but their
s
of the microh
h
out a coating
n
g No. 2 in va
c
in Fig. 1 sho
w
a
llows us to es
t
b
ulk at a sign
i
h
approach to
t
a
yer [7], whos
e
m
ens with coa
t
g
No. 1 does n
o
e
specimen lo
a
f the given ev
e
(
Fig. 3a), whi
c
r
e characteriz
e
e
from elastic
weaker sign
a
e
d in the elasti
n
t is reached.
T
(
Fig. 3c), aco
u
n
t and from th
e
n
g the sensitiv
i
coustic signal
s
ig. 4). In a sp
e
o
5 mm (Fig.
4
width of the
c
d
up with furth
i
r earlier appe
a
b
). Apparently
,
c
ope of the gi
v
e
aling, cracks
r
om the amplit
u
s
mall opening
ardness versu
s
in vacuum (c
u
c
uum (curve 4
)
w
that the gas
-
t
imate its dept
h
i
ficant distanc
t
he surface, re
a
e
thickness is
r
t
ing No. 2. Its
o
t lead to a not
a
ding (solid l
e
nt. In the spe
c
c
h is connecte
d
e
d by a large
deformation t
o
a
ls increases.
c region (Fig.
T
hey are rela
t
u
stic activity
w
e
re on gradual
l
i
ty threshold o
s
, we investig
a
e
cimen anneal
e
4
a). The appe
a
c
racks can ser
v
er deformatio
n
a
rance. In the
,
the acoustic
v
en work. In t
h
having a dep
t
u
de of the AE
width corresp
o
s
depth in the
s
u
rve 2), with
c
)
.
-
saturated lay
e
h
by measure
m
e from the su
r
aching 900 H
V
r
oughly 50 μm
depth is roug
h
iceable chang
e
ines) overlai
d
c
imens anneal
d
with the hig
h
amplitude, up
o
plastic defor
m
In the speci
m
3b). Signals
w
t
ively rare an
d
w
ith signal a
m
l
y weakens. I
n
f the device w
a
ted the micro
s
e
d in air, a nu
m
a
rance of suc
h
v
e as an indic
a
n
. The width o
structure of t
h
signals here a
r
h
e nea
r
-surfac
e
t
h of 11 μm (
F
signals, the fo
o
nds to their l
a
s
pecimens: wi
t
c
oating No. 1
i
e
r possesses e
n
m
ent of the mi
c
r
face is 350
Н
V
(Fig. 2, cur
v
. An increase
i
hly equal to
1
e
in the micro
h
d
by AE sign
a
ed in air, the
a
h
level of inter
n
to 80 dB an
d
m
ation, the nu
m
m
ens annealed
w
ith amplitude
d
possess a lo
m
plitude up to
n
runs on spec
i
ere detected (
F
s
tructure of th
e
m
ber of rectili
n
h
cracks was
a
a
tion of their
f
f the cracks w
h
e reference sp
r
e associated
w
e
layer of the
F
ig. 4c) and l
e
rmation energ
y
a
ter appearanc
t
hout a coatin
g
i
n vacuum (cu
r
n
hanced hard
n
c
rohardness (F
i
Н
V. In the sp
e
v
e 1). This reg
u
i
n the microh
a
1
5 μm. Annea
l
h
ardness.
a
ls (dots). Ea
c
a
coustic signal
n
al tensile stre
s
d
higher, and
a
m
ber of signa
l
in vacuum
w
above 45 dB
a
w amplitude.
54–56 dB ar
i
i
mens anneale
d
F
ig. 3d).
e
nea
r
-surface
n
ear cracks ap
p
a
ccompanied
b
f
ormation tim
e
as the greates
t
ecimens anne
a
w
ith other pro
c
specimens on
e
ngths of 0.5
y
of such crac
k
e. In the struc
t
g
in air
rve 3),
n
ess in
i
g. 2).
e
cimen
u
larity
a
rdness
l
ing in
c
h dot
s arise
s
ses in
a
high
l
s with
w
ithout
a
ppear
In the
i
ses at
d
with
layers
p
eared
b
y AE
e
– the
t
in the
a
led in
c
esses,
which
2 mm
k
s was
t
ure of
t
w
m
t
t
he specimens
w
as not possi
b
The
c
m
icrohardnes
s
t
he plots of th
Fig
.
wit
h
coa
t
a
Fig
.
wit
h
with coating
N
b
le to connect
t
c
orrelation of
s
data proves t
h
e acoustic em
i
.
3. Bending
s
h
out a coating
t
ing No. 1 (Y2
O
.
4. Microstru
c
h
out a coating
N
o. 1 individ
u
t
hem with the
A
the AE par
a
h
at the source
i
ssion signals
a
s
tress curves
a
in air, b) wit
h
O
3).
c
ture of speci
m
in vacuum, c)
u
al defects we
r
A
E signals du
e
a
meters with
of the signals
a
llowed us to
d
a
nd activity o
f
h
out a coating
b
m
ens in TEM
a
with coating
N
r
e observed w
h
e
to their smal
l
the presence
is namely the
d
etermine the
f
acoustic emi
s
in vacuum, c)
c
a
fter mechanic
N
o. 2 (BN), d)
w
h
ich were sim
i
l
size and the
v
and depth o
se cracks. Ov
e
stresses at w
h
s
sion signals
a
with coating
N
al tests: a) wi
t
w
ith coating
N
i
lar to microcr
a
v
iscous nature
o
f the cracks,
e
rlaying the lo
h
ich the first
m
a
s functions o
N
o. 2 (BN), a
n
d
t
hout a coatin
g
N
o. 1 (Y2O3).
a
cks (Fig. 4d)
,
o
f the opening
and also wi
t
ading diagra
m
m
icrocracks ap
p
f time: a)
n
d d) with
g
in air, b)
841
,
but it
.
t
h the
m
s with
p
ear in
842
the near-surface layer of the specimens. The cracks appear upon deformation in the elastic region long before the yield
point is reached, that is to say, before the appearance of residual deformations. This makes it possible to use the AE
method for nondestructive testing of hollow parts for the presence and thickness of a modified layer on inner coated
surfaces. As a result of these studies, we have determined that an yttrium-oxide (Y2O3) based coating has no discernible
effect on the titanium alloy surface and can therefore be successfully used in the technology of superplastic forming and
pressure welding.
CONCLUSIONS
To summarize, we have investigated the effect of stop-off coatings on the microstructure, hardness, and
acoustic emission of specimens of VT6 titanium alloy formed in temperature regimes corresponding to commercial
SPF/PW regimes. We found that a modified layer with increased hardness and thickness of about 10 μm is formed on
the surface of specimens coated with No. 2 (BN) coating. At stresses around 75% of the yield point cracks arise in this
layer having a depth up to 10 μm. In the specimen with coating No. 1 (Y2O3) a modified layer is not formed, and the
microstructure and hardness on the surface and in the bulk of the specimen are identical. The results obtained here can
be used to create a method of nondestructive acoustic testing, making it possible to determine the moment of crack
nucleation in the specimen under conditions of both plastic and elastic deformation.
REFERENCES
1. E. N. Petrov, V. V. Rodionov, É. N. Kuz’min, et al., Cellular Constructions [in Russian], Publishing House of
the Russian Federal Nuclear Center – All-Russian Scientific Research Institute of Experimental Physics,
Snezhinsk (2008).
2. R. V. Safiullin, Pis’ma Mater., 2, 32–35 (2012).
3. R. V. Safiullin, Pis’ma Mater., 2, 36–39 (2012).
4. G. Lütjering and J. C. Williams, Titanium, Springer Verlag, Berlin (2007).
5. D. G. Sanders and M. Ramulu, JMEPEG, 13, 744–752 (2004).
6. L. D. Hefti, JMEPEG, 17, No. 2, 178–182 (2008).
7. N. M. Pul’tsin, Interaction of Titanium with Gases [in Russian], Metallurgiya, Moscow (1969).
TABLE 1. Dimensions of the Cracks after Mechanical Tests
Specimens Depth, μm Relative width, % Length, mm
In air without a coating 18 7.4 3–5
In vacuum without a coating
In vacuum with coating No. 2 11 2.3 0.5–2
In vacuum with coating No. 1 (Y2O3) 1.5 0.07
... (ii) Field surveys include the electromagnetic radiation method, theological method, rebound method, and resistance method [13]. (iii) Applied mathematical methods [14] include the artificial neural network [15], ambiguity and mathematical evaluation [16], gray relational theory [17], cloud model [18], and catastrophe series method. For example, the optimal support vector machine is provided by Zhou et al. [9] to prove the higher accuracy of SVM on the prediction of rockburst intensity. ...
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The Superplastically Formed and Diffusion Bonded (SPF/DB) titanium structure in production today for Boeing products, not including engines, are all diffusion bonded using matched metal tooling and are all fabricated using the common 6Al-4V alloy. The matched metal tooling concept presents a challenge in obtaining high-quality bonds over large areas due to tolerance build-up in the tools and the titanium sheets. Boeing Commercial Airplanes (BCA) is currently advancing the state of the SPF/DB process in several ways. One of these advances is using stop-off between the sheets and diffusion bonding the pack first and then superplastically forming the stiffening features. This generates a component that is very well bonded in the required locations. However, this process also has its challenges. One of these involves how to apply the stop-off material in the proper location using the most cost effective process. Historically, silk screening has been used to define the required pattern for the stop-off material. This process requires several pieces of equipment including a wash booth since the screen needs to be cleaned after each part. A paper maskant and laser scribing process has been developed for defining the stop-off pattern. Also, because diffusion bonding is performed first, when the component is superplastically formed, there is a tendency to form creases on the surface of the part. Methods have been developed to eliminate these surface creases on the unformed surface. Another advance in the SPF/DB process is in the titanium alloys being used for products. A fine grain 6Al-4V material has been developed that bonds and forms at 775°C. The use of this material will minimize wear on the tools and presses as well as significantly reducing the amount of alpha case on the part surface.
Article
Superplastic forming (SPF) combined with diffusion bonding (DB) has been used successfully for the fabrication of titanium aerospace hardware. Many of these applications have been for military aircraft, whereby a complex built-up structure has been replaced with monolithic parts. Several methods for applying the two- and four-sheet titanium SPF/DB processes have been devised, including the welding of sheets prior to forming and the use of silk-screened stop-off (yttria) to prevent bonding where it is undesirable. Very little progress has been made in the past few years toward understanding and modeling the SPF/DB process using constitutive equations and data by laboratory testing. Concerns that engineers face in designing for fatigue life, acceptable design loads, and damage tolerance are currently being studied, but the database is very limited. This is a summary of past work found in the literature and forms the foundation for additional research.
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