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Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy
by Fine Particle Peening
Paper:
Formation of Hydroxyapatite Layer
on Ti–6Al–4V ELI Alloy by Fine Particle Peening
Shoichi Kikuchi∗1,†, Yuki Nakamura∗2, Koichiro Nambu∗3, and Toshikazu Akahori∗4
∗1Department of Mechanical Engineering, Graduate School of Engineering, Kobe University
1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
†Corresponding author, E-mail: kikuchi@mech.kobe-u.ac.jp
∗2Department of Mechanical Engineering, National Institute of Technology, Toyota College, Toyota, Japan
∗3Toyota Technological Institute, Nagoya, Japan
∗4Department of Materials Science and Engineering, Faculty of Science and Technology, Meijo University, Nagoya, Japan
[Received January 16, 2017; accepted May 11, 2017]
Fine particle peening (FPP) using hydroxyapatite
(HAp) shot particles can form a HAp layer on
room-temperature substrates by the transfer and mi-
crostructural modification of the shot particles. In
this study, FPP with HAp shot particles was applied
to form a HAp surface layer and improve the fatigue
properties of Ti–6Al–4V extra-low interstitial (ELI)
for use in bio-implants. The surface microstructures
of the FPP-treated specimens were characterized by
micro-Vickers hardness testing, scanning electron mi-
croscopy, energy-dispersive X-ray spectrometry, X-
ray diffraction, and X-ray photoelectron spectroscopy.
FPP with HAp shot particles successfully formed a
HAp layer on the surface of Ti–6Al–4V ELI in a rela-
tively short period by shot particle transfer at room
temperature; however, the thickness and elemental
composition of the HAp layer were independent of the
FPP treatment time. The original HAp crystal struc-
ture remained in the surface-modified layer formed
on Ti–6Al–4V ELI after FPP. Furthermore, FPP in-
creased the surface hardness and generated compres-
sive residual stresses at the treated surface of Ti–6Al–
4V ELI. Four-point bending fatigue tests were per-
formed at stress ratios of 0.1 and 0.5 to examine the
effect of FPP with HAp shot particles on the fatigue
properties of Ti–6Al–4V ELI. The fatigue life of the
FPP-treated specimen was longer than that of the un-
peened specimen because of the formation of a work-
hardened layer with compressive residual stress. How-
ever, no clear improvement in the fatigue limit of Ti–
6Al–4V ELI occurred after FPP with HAp shot parti-
cles because of subsurface failures from characteristic
facets.
Keywords: fine particle peening, titanium alloy, hydrox-
yapatite, fatigue, biomaterial
1. Introduction
The Ti–6Al–4V alloy is widely used in various engi-
neering fields because it possesses a high specific strength
and good heat resistance. In particular, Ti–6Al–4V extra-
low interstitial (ELI) is used as a substitute for hard bi-
ological tissues in biomaterials such as artificial joints,
dental implants, and fracture fixators, because it exhibits
excellent corrosion resistance, high tissue compatibility,
and a lower Young’s modulus than ferrous materials [1].
Osteoconductivity, or the characteristics permitting bone
growth on a material surface, is required to affix titanium-
based bio-implants [2] to human bones over a long period
of time.
Surface modification processes without the use of bone
cement have been introduced to improve the osteocon-
ductivity and bonding strength between human bones
and bio-implants [3–12] because the surfaces of bio-
implants are in contact with body tissues. For exam-
ple, Kokubo et al. [3, 4] investigated the effects of pre-
treatment with alkali hydroxide solutions on the forma-
tion of hydroxyapatite (HAp) on commercially pure (CP)
titanium in simulated body fluid to produce a bioactive
material surface. Plasma-sprayed HAp coatings are ef-
fective for improving the hard tissue compatibility of
titanium-based bio-implants because they form thick HAp
layers [5–11]. However, the thermal energy induced by
high-temperature and energetic processes, such as plasma
spraying, can change the crystalline structure of HAp [10]
or the titanium substrate [13], which negatively affects the
HAp-coated bio-implants.
To address this problem, various room-temperature
HAp coating methods have been developed for titanium-
based bio-implants [14–20]. Ishikawa et al. [17, 18] re-
ported the homogeneous surface coating of a titanium
plate with HAp using an ordinary sandblaster at room
temperature. In our previous study [20], fine particle
peening (FPP) with HAp shot particles was used to form
a HAp layer on a CP titanium plate at room temperature
through shot particle transfer [21–25] induced by the high
particle velocities used for FPP [26, 27]. Another impor-
tant aspect of HAp coatings formed on titanium-based
bio-implants is their behavior under cyclic loading, like
that applied during use in the human body. Kangasniemi
et al. [11] showed that fracture occurred at the interface
between the coating and titanium substrate by investigat-
Int. J. of Automation TechnologyVol.11 No.6, 2017 915
Kikuchi, S. et al.
5Pm
5 ȝm
Fig. 1. Image quality (IQ) map obtained from EBSD analy-
sis of Ti–6Al–4V ELI alloy.
ing the mechanical properties of bioactive coating mate-
rials using a developed testing system. In contrast, the
HAp layer remained on the FPP-treated CP titanium af-
ter fatigue testing without delamination on the fracture
surface [20]. Furthermore, FPP can improve the fatigue
properties of the titanium substrate by increasing the sur-
face hardness and forming fine grains [28–30].
In the present study, FPP using HAp shot particles was
used to form a HAp surface layer and improve the fatigue
properties of Ti–6Al–4V ELI, which is often applied in
bio-implants. The purpose of this study was to charac-
terize the HAp layer formed on Ti–6Al–4V ELI by FPP
using HAp shot particles at room temperature and to ex-
perimentally examine the fatigue properties of the resul-
tant coated alloy under four-point bending.
2. Experimental Procedures
2.1. Material and Specimen Preparation
The material used in this work was Ti–6Al–4V (ELI
grade) with the chemical composition of 6.31% alu-
minum, 4.13% vanadium, 0.12% iron, 0.002% hydrogen,
0.006% nitrogen, 0.11% oxygen, and 0.024% carbon by
mass, with titanium comprising the balance. Fig. 1 shows
an image quality (IQ) map obtained by electron backscat-
ter diffraction (EBSD) analysis for Ti–6Al–4V ELI con-
taining both the equiaxed
α
-phase and
β
-phase. This ma-
terial has the Vickers hardness of 340.2 ±4.7 HV, as mea-
sured for a polished surface with an indentation force of
0.098 N and a load holding time of 5 s (n=30). Ti–6Al–
4V ELI plates of 11 mm in thickness were machined into
1.5-mm-thick sheets and then cut into 3 ×20 mm spec-
imens using a wire electrical discharge machine. After
machining, the specimens were polished with emery pa-
per (#320 to #4000) to 1 mm in thickness and then to a
mirror finish using a SiO2suspension. The sides of the
specimen were also polished with emery paper (#500) to
remove the electro-discharge machined layer.
Table 1. Conditions for FPP.
Peening pressure 0.6 MPa
Peening time 1, 10, 20, 30 s
Nozzle distance 50 mm
Table 2. Residual stress measurement conditions.
Tube voltage 40 kV
Tube current 30 mA
Diffraction angle 2
θ
154.3 deg.
Diffraction plane (331)
Incident angle 10, 20, 30, 35, 40 deg.
Beam diameter 1 mm
Stress constant −170.77 MPa/deg.
FPP was performed on the polished specimens using a
direct pressure-type apparatus under the conditions given
in Tabl e 1 at room temperature in air. The shot particles
with diameters of 50
μ
m [20] were produced by pulveriz-
ing HAp (Ca10(PO4)6(OH)2) fabricated by ECCERA Co.,
Ltd. After performing FPP, the specimens were placed in
an ultrasonic bath of acetone for 600 s to remove free par-
ticles from the surfaces.
2.2. Characterization of the Surface-Modified
Layer
The surface microstructures of the specimens were
characterized using scanning electron microscopy (SEM)
with an accelerating voltage of 15 kV. The FPP-treated
surfaces were also analyzed using energy dispersive X-ray
spectrometry (EDX) in an area of 1.13 mm2observed at
100×magnification with an accelerating voltage of 20 kV,
as well as X-ray photoelectron spectroscopy (XPS) with
Mg K
α
radiation. The crystal structures of the speci-
mens were identified using X-ray diffraction (XRD) with
Cu K
α
radiation. The HAp layer was also analyzed us-
ing EDX at 5000×magnification in longitudinal cross-
sections.
The hardness distributions were measured along longi-
tudinal cross-sections of the FPP-treated specimens using
a micro-Vickers hardness tester with an indentation force
of 0.098 N and a load holding time of 10 s. The residual
stress was also measured at the top surface of a transverse
section of the specimen using XRD with Co K
α
radia-
tion and a position-sensitive proportional counter (PSPC)
system based on the sin2
ψ
method (n=2) [31, 32]. The
conditions for the residual stress measurement are shown
in Tabl e 2 .
2.3. Fatigue Tests Under Four-Point Bending
Fatigue tests were conducted in an electrodynamic fa-
tigue testing machine (loading capacity: 500 N) under
four-point bending using the mirror-finished and FPP-
treated specimens measuring 3×20×1 mm at the stress
916 Int. J. of Automation TechnologyVol.11 No.6, 2017
Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy
by Fine Particle Peening
(a) 0 s (Un-peened)
(b) 1 s (c) 10 s
(d) 20 s (e) 30 s
50 μm
50 μm50 μm
50 μm50 μm
Fig. 2. SEM micrographs of (a) un-peened specimen and
specimens treated with FPP for (b) 1 s, (c) 10 s, (d) 20 s, and
(e) 30 s.
ratios of 0.1 and 0.5, because fatigue crack propagation
in titanium alloys is influenced by the stress ratio [33–
37]. The frequency of stress cycling was 10 Hz based on
the JIS T 0309 standard and the tests were conducted in
air without temperature or moisture control. The fatigue
limit was defined based on the JSMS standard [38]. After
testing, the fracture surfaces of the failed specimens were
observed using SEM at an accelerating voltage of 10 kV.
3. Results and Discussion
3.1. Transfer of HAp Shot Particles by FPP to
Ti–6Al–4V ELI
Figure 2 shows backscattered electron (BSE) micro-
graphs of the un-peened and FPP-treated specimen sur-
faces, in which the contrast is related to the composi-
tion of the surface layer. The bright regions correspond
to the titanium substrate because the brightness increases
with increasing atomic number. A smooth surface with
high brightness is observed for the un-peened specimen,
as shown in Fig. 2(a), whereas the contrast in the BSE
images for the FPP-treated specimens is clearly different
(Figs. 2(b)–(e)). This is attributed to the presence of ir-
regularly shaped HAp shot particles transferred onto the
FPP-treated surface.
Figure 3 shows EDX maps of an un-peened specimen
surface and the surface of a specimen treated with FPP
for 30 s after ultrasonic cleaning. Only the substrate ele-
ments are detected in the un-peened specimen (Fig. 3(a)).
In contrast, calcium, phosphorus, and oxygen, present in
CaPO
(a) Un-peened
Ti
(b) FPP (30 s)
Not detected
Not detected
Not detected
200 μm
Fig. 3. EDX maps for (a) un-peened and (b) FPP-treated
specimen surfaces (peening time: 30 s).
FPP treatment time, s
0
100
Composition, mass%
20
40
60
80
0102030
P
Ca
OTi
Al
V
Fig. 4. Relationship between peening time and elemental
composition of FPP-treated surfaces determined from EDX
analysis.
the HAp shot particles, are also detected on the specimen
surfaces treated with FPP for 30 s (Fig. 3(b)). It is consid-
ered that the layer with these elements corresponds to the
transferred HAp layer. In addition, a trace amount of ti-
tanium is detected on the FPP-treated surface because the
elemental composition at depths of several micrometers
was also detected in the EDX analysis; however, the HAp
layer remained after performing ultrasonic cleaning.
To investigate the formation of the HAp layer dur-
ing FPP, specimens treated with FPP for various peening
times were analyzed using EDX. Fig. 4 shows the rela-
Int. J. of Automation TechnologyVol.11 No.6, 2017 917
Kikuchi, S. et al.
(Surface)
5 μm
CaPOTi
Not detected
Not detected
Not detected
(a) 0 s (Un-peened)
FPP treatment time
(b) 1 s (c) 10 s (d) 20 s (e) 30 s
Fig. 5. Longitudinal EDX maps for (a) un-peened and (b)–(e) FPP-treated specimens.
tionship between the elemental composition of the surface
layer analyzed by EDX and the FPP treatment time. The
data for an FPP treatment time of 0 s is similar to that
of the un-peened specimen shown in Fig. 3(a). Calcium,
phosphorus, and oxygen are detected in each FPP-treated
specimen, although there is no noticeable dependence of
the content of each element on the FPP treatment time.
This lack of influence of the FPP treatment time on the
content of each element in the HAp layer was also ob-
served in CP titanium [20]. Fig. 5 shows longitudinal
EDX maps of the un-peened and FPP-treated specimens.
For the un-peened specimen (Fig. 5(a)), only the substrate
elements are detected, which is consistent with the results
shown in Figs. 3(a) and 4. In contrast, calcium, phospho-
rus, and oxygen from the HAp shot particles are detected
near the surfaces treated with FPP, while titanium is not
detected (Figs. 5(b)–(e)).
Figure 5 also demonstrates that the HAp layer is non-
homogeneously formed at the FPP-treated surface. To ex-
amine the effect of the FPP treatment time on the thick-
ness of the HAp layer formed on Ti–6Al–4V quantita-
tively, the equivalent thickness of the HAp layer was cal-
culated from Eq. (1) if the HAp layer, consisting of the
HAp shot particles transferred onto the surface, was uni-
formly formed over the entire surface:
teq =A
b,............... (1)
where teq is the equivalent thickness of the HAp layer
[
μ
m], Ais the area of detection for the calcium element
[
μ
m2], and bis the width of the specimen in the analyzed
FPP treatment time, s
10
Thickness of the HAp layer, Pm
0102030
0
2
4
6
8
Un-peened
FPP
(0.04 Pm/s)
(6.90 Pm/s)
Fig. 6. Relationship between peening time and equivalent
thickness of the HAp layer, estimated by Eq. (1).
area (25.2
μ
m).
Figure 6 shows the relationship between the equivalent
thickness of the HAp layer estimated by Eq. (1) and the
FPP treatment time. The HAp layer thickness increases
significantly for FPP treatment times increasing to 1 s,
withtheincreaserateof6.90
μ
m/s. This result indicates
that FPP can form a HAp layer on Ti–6Al–4V ELI in a
short period at room temperature. In contrast, there is no
noticeable difference in the HAp layer thickness for FPP
treatment times greater than 1 s; the thickness increases
only slightly at the rate of 0.04
μ
m/s to the HAp layer
thickness of approximately 7
μ
m. Thus, the EDX analy-
ses suggest that FPP can form a HAp layer in a relatively
918 Int. J. of Automation TechnologyVol.11 No.6, 2017
Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy
by Fine Particle Peening
Un-peened
FPP (20 s)
Binding energy, eV
452467 462 457
Counts
Ti 2p
3/2
(TiO
2
)
O 1s (Phosphate)
Binding energy, eV
525540 535 530
Counts
FPP (20 s)
(a) Ti 2p (b) O 1s
Un-peened
O 1s (Ti-O)
O 1s (OH)
Mg KDMg KD
Ti 2p
1/2
(TiO
2
)
Fig. 7. XPS (a) Ti 2pand (b) O 1sspectra for un-peened
and FPP-treated specimens (peening time: 20 s).
short period (1 s) at room temperature, and that the thick-
ness and elemental composition of the HAp layer are in-
dependent of the FPP treatment time.
3.2. Characterization of HAp Layer Formed by
FPP on Ti–6Al–4V ELI
XPS and XRD analyses were conducted to examine the
microstructures of the surface HAp layers in more de-
tail. Fig. 7 shows the XPS Ti 2pand O 1sspectra for
the un-peened specimen and the specimen treated with
FPP for 20 s. No Ti 2 ppeaks are detected for the FPP-
treated specimen, whereas Ti 2ppeaks (TiO2) are clearly
evident for the un-peened specimen (Fig. 7(a)). Further-
more, an O 1speak (Ti–O) is detected from the un-peened
specimen, corresponding to the Ti 2 ppeak, as shown in
Fig. 7(b). In contrast, no titanium oxide peaks are de-
tected for the FPP-treated specimen, whereas O 1s(phos-
phate) peaks are clearly evident because of the formation
of the HAp layer at the top surface. These results indicate
that the HAp layer is formed over the entire surface of the
FPP-treated specimen.
Figure 8 shows the XRD patterns for an un-peened
specimen and a specimen treated with FPP for 20 s.
The un-peened specimen has diffraction peaks from the
substrate corresponding to the
α
-Ti and
β
-Ti phases
(Fig. 8(a)). However, XRD peaks associated with
HAp are also detected from the FPP-treated specimen
(Fig. 8(b)). The intensity of both the
α
-Ti and
β
-Ti peaks
observed from the FPP-treated specimen is decreased
slightly following FPP, and the Ti peaks are shifted to
lower angles. These changes are attributed to the plas-
tic deformation of the substrate during FPP, as discussed
in the next section.
The XPS and XRD analyses suggest that the origi-
nal HAp crystal structure remains in the surface-modified
layer formed on Ti–6Al–4V ELI after performing FPP.
3.3. Hardness and Residual Stress Measurements
of FPP-treated Ti–6Al–4V ELI
Figure 9 shows the distribution of Vickers hardness at
various depths in longitudinal sections of the FPP-treated
specimens. The specimen treated for 1 s exhibits al-
most the same hardness as the substrate (340.2 ±4.7 HV),
Cu KD
30 35 45
Intensity
Diffraction angle 2
T
, degree
40
E-Ti HAp
(a) Un-peened
(b) FPP (20 s)
D-Ti
(0002)
(1010)
(1011)
(110)
(202)
(300)
(211)
Fig. 8. XRD patterns for (a) un-peened and (b) FPP-treated
specimens (peening time: 20 s).
whereas the surface hardness values of the specimens
treated for longer than 10 s are higher than that of the sub-
strate. The surface hardness of the FPP-treated specimens
tends to increase with the FPP treatment time up to 20 s
and then remains constant. Furthermore, the thickness
of the surface hardened layer formed in the FPP-treated
specimens tends to increase with the FPP treatment time
up to 20 s.
Figure 10 shows the residual stress generated at the
surfaces treated with FPP for various peening times and
that for the un-peened specimen, as determined by XRD.
In the case of the un-peened specimen, tensile residual
stress is generated, whereas compressive residual stress
is generated at the surface of each FPP-treated speci-
men. This suggests that FPP using HAp shot particles can
generate compressive residual stresses at the surface of
Ti–6Al–4V ELI. In the FPP-treated specimens, the com-
pressive residual stress tends to decrease with increasing
FPP treatment time up to 20 s before increasing again.
The compressive residual stress generally increases as the
thickness of the surface hardened layer decreases; how-
ever, the compressive residual stress increases again for
the specimen treated by FPP for 30 s, although the thick-
ness of the surface hardened layer is almost the same, as
shown in Fig. 9. The same tendency was also observed for
a CP titanium specimen treated with FPP using HAp shot
particles [20] because the FPP treatment time affects both
the residual stress generated on the top surface and the
thickness of the compressive residual stress layer. Fur-
thermore, for excessive FPP treatment times, the degree
of plastic deformation in the substrate increases, and abra-
sion of the substrate and the transfer of HAp shot particles
simultaneously occur at the specimen surface during FPP;
therefore, compressive residual stress are not expected to
change linearly with the FPP treatment time.
Thus, FPP using HAp shot particles can form a work-
hardened layer with a compressive residual stress in Ti–
6Al–4V ELI at varied FPP treatment times.
Int. J. of Automation TechnologyVol.11 No.6, 2017 919
Kikuchi, S. et al.
300
400
320
Vickers hardness, HV (0.098 N)
0
Distance from surface, Pm
50 100 150 200
340
360
380
1 s
20 s
FPP treatment time
30 s
10 s
Un-peened specimen
(340.2㼼4.7 HV)
Fig. 9. Distributions of Vickers hardness at various
longitudinal-section depths.
FPP treatment time, s
-500
200
Residual stress, MPa
0 102030
-100
-200
-300
-400
Co KD
100
0
Un-peened
FPP
(Average 㼼Standard error)
Fig. 10. Residual stress measured on the treated surface as
a function of peening time.
3.4. Fatigue Properties of FPP-Treated Ti–6Al–4V
ELI Under Four-Point Bending
Four-point bending fatigue tests were conducted for the
specimen treated with FPP for 20 s, which had the lowest
compressive residual stress, as shown in Fig. 10.There-
sults of the fatigue tests under four-point bending are plot-
ted as S–Ndiagrams for the un-peened and FPP-treated
specimens tested at stress ratios of 0.1 and 0.5 (Fig. 11).
The plots with arrows indicate a run-out specimen with-
out failure. In every specimen series, a fatigue limit is
clearly observed, so that the S–Ncurve is determined by
using the bilinear S–Nmodel with fatigue limiting in the
JSMS standard regression models [38]. FPP using HAp
shot particles prolongs the fatigue life of Ti–6Al–4V ELI
compared to that of the un-peened specimen at a stress
ratio of 0.1, because FPP increases the surface hardness
of the Ti–6Al–4V ELI alloy and generates compressive
residual stress, as shown in Figs. 9 and 10.However,no
clear improvement occurs in the fatigue limit of Ti–6Al–
4V ELI by FPP using HAp shot particles. This may be
attributed to the FPP-induced surface roughness, because
the surface roughness affects the mechanical properties
of a specimen [39–41]. The FPP-treated specimen fails at
σ
a=314.3 MPa, whereas fatigue failure does not occur at
200
100
Number of cycles to failure N
f
, cycle
Stress amplitude
V
a
, MPa
10
4
10
5
10
6
10
7
10
8
300
400
: Run-out
Four-point bending
*: Subsurface failure
10 Hz
500
*
Un-peened
FPP (20 s)
R= 0.1 R= 0 .5
Fig. 11. Four-point bending fatigue test results for un-
peened and FPP-treated specimens in terms of stress ampli-
tude (peening time: 20 s).
700
600
Number of cycles to failure Nf, cycle
Maximum stress
V
max, MPa
104105106107108
1100
800
900
1000
Un-peened
FPP (20 s)
R= 0.1 R= 0.5
: Run-out
Four-point bending
*: Subsurface failure
10 Hz
*
Fig. 12. Four-point bending fatigue test results for un-
peened and FPP-treated specimens in terms of maximum
stress (peening time: 20 s).
σ
a=313.8 MPa in the un-peened specimen. When tested
at a stress ratio of 0.5, the FPP-treated specimens show
almost the same fatigue lifetimes and fatigue limits as the
un-peened specimen.
Figure 11 also reveals that the fatigue limit for the
specimens tested at R=0.1 is higher than that for the
specimens tested at R=0.5. This difference in the fa-
tigue limit is attributed to the value of the mean applied
stress. To investigate the effect of the stress ratio on the
fatigue properties of Ti–6Al–4V ELI, the fatigue test re-
sults are replotted as a function of the maximum stress
σ
max as shown in Fig. 12. For stress ratios of 0.1 and 0.5,
the plots of each specimen series are almost within the
same bands. This result implies that the effect of stress
ratio on the fatigue properties of Ti–6Al–4V ELI disap-
pears. A modified Goodman diagram and the Smith–
Watson–Topper (SWT) model [42] were used to exam-
ine the stress ratio effect; however, the fatigue properties
of Ti–6Al–4V ELI were determined using the maximum
stress applied to the specimen’s surface under four-point
bending. Therefore, fitting was performed using the data
for all the un-peened and FPP-treated specimens to statis-
tically investigate the effect of FPP with HAp shot parti-
920 Int. J. of Automation TechnologyVol.11 No.6, 2017
Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy
by Fine Particle Peening
(a) 35 mag.(b) 500 mag.(c) 3000 mag.
Fracture surface 1 Fracture surface 2
Facet Facet
5 μm
Facet
Facet
500 μm
(Tensile side)
Crack initiation site
Crack propagation region
Instantaneous fracture region
(Tensile side)
Crack initiation site
Instantaneous fracture region
Crack propagation region
50 μm
500 μm
50 μm
5 μm
Fig. 13. Typical features of both fracture surfaces of FPP-
treated specimen failing at Nf=4.35×106(stress amplitude:
314.3 MPa, stress ratio: 0.1, peening time: 20 s).
cles on the fatigue properties. Regression S–Ncurves for
the un-peened and FPP-treated specimens are respectively
expressed by the following formulae:
Un −peened :
σ
max =−108.7log(N)+1433.7,(2)
FPP −treated :
σ
max =−122.2log(N)+1543.0,(3)
where
σ
max is the maximum stress applied to the speci-
men surface [MPa], and Nis the number of cycles.
The fatigue life of the FPP-treated specimens was
longer than that of the un-peened specimens. The crit-
ical number of stress cycles giving the fatigue limit for
the FPP-treated specimens (Nw=8.82 ×106) was higher
than that for the un-peened specimens (Nw=5.80 ×106).
However, the fatigue limit of the FPP-treated specimen
(
σ
w,max =694.1 MPa) was almost equal to that of the un-
peened specimen (
σ
w,max =698.7 MPa). Thus, FPP with
HAp shot particles can prolong the fatigue life of Ti–6Al–
4V ELI, but does not improve the fatigue limit. In CP
titanium, FPP with HAp shot particles was reported to in-
crease the fatigue limit [20]; therefore, improvement of
the fatigue limit of titanium by FPP using HAp shot par-
ticles is different.
The fracture surfaces of the failed FPP-treated speci-
mens were observed using SEM to examine the fracture
mechanism. Fig. 13 shows the typical features of both
fracture surfaces observed at various magnifications for
the FPP-treated specimen that failed with a long lifetime
of Nf=4.35 ×106.InFig. 13, the tensile stress has been
applied to the lower surface. Microscopic observation re-
veals that only one fatigue crack is present near the spec-
imen surface; this propagates gradually across the cross-
10
Height, Pm
0
Distance from surface, Pm
51015
8
6
4
2
0
Analyzed line
(Surface)
2 μm
30.8o
Facet
Fig. 14. Three-dimensional analysis of the fracture surface
1 of FPP-treated specimen failed at Nf=4.35 ×106(stress
amplitude: 314.3 MPa, stress ratio: 0.1, peening time: 20 s).
section of the specimen (Fig. 13(a)). The fracture surface
is divided into two regions by a clear boundary, as indi-
cated by the dotted line. In addition, a characteristic facet
is clearly observed at the crack initiation site in the higher-
magnification SEM micrographs (Figs. 13(b) and 13(c)).
The FPP-treated specimen fails from the facet at a stress
ratio of 0.1; crack initiation can be observed below the
FPP-treated surface because the resistance to crack initia-
tion is higher at the high-hardness surface, which reduces
the fatigue limit of the Ti–6Al–4V ELI. In Figs. 11 and
12, the asterisk symbol “*” indicates that the specimen
failed in the subsurface fracture mode.
To examine the fatigue fracture mechanism of the
FPP-treated specimen in more detail, three-dimensional
fracture surfaces were produced by exclusive software
(Alicona’s MeX) for 3D-fracture surface reconstruction.
Fig. 14 shows the profile curve for the fracture surface 1
of the FPP-treated specimen shown in Fig. 13.There-
lationship between the height based on the lowest point
in the analyzed area and the distance from the specimen’s
surface is shown. The inclination angle of the facet ob-
served on the fracture surface is 30.8◦. The size of the
facet is almost equal to that of the
α
-grain shown in
Fig. 1; therefore, subsurface fracture occurs from a coarse
α
-grain with a weak microstructural orientation. In ac-
cordance with previous investigations on Ti–6Al–4V [43,
44], microstructural inhomogeneities in the phase distri-
bution are responsible for internal crack initiation, which
corresponds to the specific local conditions of plastic de-
formation in the
α
-phase and
β
-phase.
Int. J. of Automation TechnologyVol.11 No.6, 2017 921
Kikuchi, S. et al.
4. Conclusions
FPP was performed to form a HAp shot particle layer
on Ti–6Al–4V ELI at room temperature. The surface mi-
crostructure of the FPP-treated specimen was character-
ized, and the effect of FPP on the fatigue properties of
Ti–6Al–4V ELI was examined under four-point bending
at various stress ratios. The main conclusions of this study
are as follows:
1. FPP using HAp shot particles can form a surface-
modified layer with the original HAp crystal struc-
ture on Ti–6Al–4V ELI in a relatively short period at
room temperature.
2. The thickness and elemental composition of the HAp
layer are independent of the FPP treatment time.
3. FPP with HAp shot particles can increase the fatigue
life of Ti–6Al–4V ELI by forming a work-hardened
layer with compressiveresidual stress on Ti–6Al–4V
ELI.
4. The fatigue limit of Ti–6Al–4V ELI does not in-
crease by FPP using HAp shot particles, because the
FPP-treated specimen fails via subsurface fracture
from the characteristic facet, beginning at a coarse
α
-grain with a weak microstructural orientation. The
fatigue crack then propagates gradually across the
cross-section of Ti–6Al–4V ELI.
Acknowledgements
The authors would like to thank The Inamori Foundation and
the Advanced Machining Technology & Development Associa-
tion for their financial support. FPP using HAp shot particles was
based on Japanese Patent No.3314070, filed by FUJI KIHAN Co.,
Ltd and the Aichi Center for Industry and Science Technology.
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Name:
Shoichi Kikuchi
Affiliation:
Department of Mechanical Engineering, Gradu-
ate School of Engineering, Kobe University
Address:
1-1 Rokkodai-cho, Nada-ku, Kobe 6578501, Japan
Brief Biographical History:
2008- GCOE Researcher, Keio University
2010- Assistant Professor, Ritsumeikan University
2013- Visiting Researcher, University of Kaiserslautern
2014- Assistant Professor, Kobe University
Main Works:
•S. Kikuchi, T. Imai, H. Kubozono, Y. Nakai, M. Ota, A. Ueno, and K.
Ameyama, “Effect of harmonic structure design with bimodal grain size
distribution on near-threshold fatigue crack propagation in Ti-6Al-4V
Alloy,” Int. J. of Fatigue, Vol.92, pp. 616-622, March, 2016.
Membership in Academic Societies:
•Society of Materials Science, Japan (JSMS)
•Japan Society of Mechanical Engineers (JSME)
•Japan Institute of Metals and Materials (JIM)
•Japan Society for Abrasive Technology (JSAT)
Name:
Yuki Nakamura
Affiliation:
Department of Mechanical Engineering, Na-
tional Institute of Technology, Toyota College
Address:
2-1 Eisei-cho, Toyota 471-8525, Japan
Brief Biographical History:
2011- Assistant Professor, National Institute of Technology, Toyota
College
2014- Lecturer, National Institute of Technology, Toyota College
Main Works:
•Y. Nakamura, T. Sakai, H. Hirano, and K. S. Ravi Chandran, “Effect of
Alumite Surface Treatments on Long-life Fatigue Behavior of a Cast
Aluminum Alloy in Rotating Bending,” Int. J. of Fatigue, Vol.32,
pp. 621-626, March, 2010.
Membership in Academic Societies:
•Society of Materials Science, Japan (JSMS)
•Japan Society of Mechanical Engineers (JSME)
•Japan Society for Abrasive Technology (JSAT)
•Japan Society for Design Engineering (JSDE)
•Material Testing Research Association of Japan (MTRAJ)
Int. J. of Automation TechnologyVol.11 No.6, 2017 923
Kikuchi, S. et al.
Name:
Koichiro Nambu
Affiliation:
Toyota Technological Institute
Address:
2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan
Brief Biographical History:
2011- Assistant Professor, National Institute of Technology, Suzuka
College
2017- Assistant Professor, Toyota Technological Institute
Main Works:
•K. Nambu, K. Monda, K. Inagaki, Y. Maeyama, and S. Kikuchi, “Effect
of Hardness Ratio on the Behavior of Plastic Deformation in Various
Metallic Materials Treated with Fine Particle Peening,” The 30th Int. Conf.
on Surface Modification Technology (SMT30) proc., 2016.
Membership in Academic Societies:
•Society of Materials Science, Japan (JSMS)
•Japan Society of Mechanical Engineers (JSME)
•Japan Society for Abrasive Technology (JSAT)
•Japan Society for Heat Treatment (JSHT)
•Japanese Society of Tribologists (JAST)
Name:
Toshikazu Akahori
Affiliation:
Department of Materials Science and Engineer-
ing, Faculty of Science and Technology, Meijo
University
Address:
1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
Brief Biographical History:
2000- Assistant Professor, Toyohashi University of Technology
2006- Assistant Professor, Tohoku University
2008- Associate Professor, Tohoku University
2010- Associate Professor, Miejo University
Main Works:
•Y. Oguchi, T. Akahori, T. Hattori, H. Fukui, and M. Niinomi, “Change in
Mechanical Strength and Bone Contactability of Biomedical Titanium
Alloy with Low Young’s Modulus Subjected to Fine Particle Bombarding
Process,” Materials Transactions, Vol.56, pp. 218-233, November, 2015.
Membership in Academic Societies:
•Japan Institute of Metals and Materials (JIM)
•Society of Materials Science, Japan (JSMS)
•Japan Society of Mechanical Engineers (JSME)
924 Int. J. of Automation TechnologyVol.11 No.6, 2017
