ultrasonic.pdf

INTERFACIAL STABILITY IN MULTI-COMPONENT SYSTEMS
Ultrasonic Vibration-Strengthened Adhesive Bonding of CFRP/
Aluminum Alloy Joints with Anodizing Pretreatment
HUI WANG,1,3
CHENG GAO,1
YIZHE CHEN ,1,4
MIN WU,2
and LIN HUA1
1.—Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan
University of Technology, Wuhan 430070, China. 2.—Hubei Collaborative Innovation Center for
Automotive Components Technology, Wuhan 430070, China. 3.—Hubei Engineering Research
Center for Green and Precision Material Forming, Wuhan 430070, China.
4.—e-mail: yzchen@whut.edu.cn
Ultrasonic vibration has been applied to improve the penetration of adhesive
into the anodized layer, thereby enhancing the strength of carbon fiber-rein-
forced plastic/aluminum alloy bonding joints with anodizing pretreatment.
The ultrasonic vibration-assisted adhesive bonding process was designed by
orthogonal experiments, then verified experimentally. The strengthening
mechanism was studied by analyzing the morphology and elemental distri-
bution of the cross-section of the joint. The results show that the ultrasonic
vibration-assisted adhesive bonding process can further strengthen the
interfacial bonding. For the studied joints, the strength can reach 18.66 MPa,
being 55% higher than without ultrasonic strengthening. The ultrasonic
vibration creates shock waves in the adhesive layer, causing a high-speed
adhesive jet toward the adherend surface, which makes the interfacial bond-
ing tight and promotes penetration of the adhesive into the anodized layer.
The bonding strength is thereby remarkably improved by forming a larger
interfacial contact area and more mechanically interlocked structures.
INTRODUCTION
Carbon fiber-reinforced plastics (CFRPs) are
widely used in the aerospace and automotive indus-
tries due to their advantages of light weight, high
strength, high temperature resistance, and good
corrosion resistance. In practical applications, it is
impossible to avoid connections between CFRP and
conventional metal materials such as aluminum
alloy, high-strength steel, etc. How to improve the
strength of such connections between CFRP and
metal materials has become a focus of current
research.1
The connection methods that are com-
monly used at present include mechanical connec-
tion, adhesive bonding, etc. Mechanical connections
require perforation of the composite material, which
can cause damage to its structure and result in
stress concentrations.2
Adhesive bonding enables
the connection of parts into a nondetachable com-
ponent by means of a thermosetting resin.
Compared with mechanical connection, it offers
many unique advantages. Adhesive bonding does
not require mechanical fasteners (such as screws,
rivets, etc.), thus avoiding the breakage of fiber
continuity by drilling joint holes and making full
use of the material strength, while the problems of
severe anisotropy, poor toughness, and high notch
sensitivity of composite material can also be
avoided.3
In addition, adhesive bonding joints offer
the advantages of large bond area, strong load-
bearing capacity, uniform stress distribution, light-
weight structures, resistance to electrochemical
corrosion, and convenient connection of dissimilar
materials. This represents an important develop-
ment direction for connection of lightweight CFRP
materials and has become a research hotspot in
connection forming in recent years.4
Although
adhesive bonding offers many advantages, its dura-
bility is poor in harsh environments such as hot/
humid environments,5
freeze–thaw cycles,6
and
corrosive environments.7
Many studies have
described methods to improve the durability of
(Received December 28, 2019; accepted July 5, 2020)
JOM
https://doi.org/10.1007/s11837-020-04284-4
Ó 2020 The Minerals, Metals & Materials Society
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adhesive bonding. Similarly, insufficient joint
strength is also a disadvantage of adhesive bonding,
and a large number of scholars have conducted
research aimed at improving the bonding strength.
On the one hand, scholars have studied the effects
of modifying adhesive and physical parameters of
joints on the bonding strength. Zhou et al. 8
modified the epoxy with nanosilica and studied its
effect on the bonding strength. The failure mode
was analyzed by scanning electron microscopy
(SEM), and the enhancement mechanism was
revealed. The results showed that the bond strength
of the modified resin was increased by 20% com-
pared with neat epoxy. The failure mode of the
modified epoxy was cohesive failure. Konstantinos
et al. 9
studied the influence of the plate thickness,
plate length, adhesive layer thickness, and bonding
length on the quality of CFRP/steel bonding joints.
They found that the thickness of the joint had a
weak influence on the shear strength, while the
length of the lap significantly affected the quality of
the joint. Seong et al.10
found that the bonding
strength increased by 29.7% when the bonding
pressure was increased from 2 atm to 6 atm.
On the other hand, scholars have also treated the
surface of CFRPs by grinding, electron beam,
plasma, and ultraviolet radiation to enhance the
interfacial bonding force, thereby enhancing the
bonding strength of CFRP structural parts. Choi
et al.11
added a layer of aramid fiber with random
orientation to modify the surface of CFRP, finding
that the CFRP/CFRP bonding strength was greater
than achieved by surface grinding only. The
strength was increased by 37%. Zaldivar et al.12
found that using abrasive paper with a large
particle size to sand the surface of CFRP reduces
the quality of the adhesive bonding joint. When the
modulus of the fiber is low, the bonding performance
is more excellent. Okada et al.13
used a uniform low-
energy electron beam to treat the surface of CFRP/
aluminum alloy joints, improving the bonding
strength by 45%. Rhee et al.14
treated aluminum
sheets with direct-current plasma and CFRP with
Ar+
ionizing radiation (under oxygen environment).
This method has a significant effect on the CFRP/
aluminum sheet bonding performance. Reitz et al.4
found that infrared (IR) and ultraviolet (UV) laser
treatment of the CFRP surface could enhance the
shear strength of CFRP/aluminum alloy adhesive
bonding, but IR laser treatment resulted in greater
improvement of the bonding performance at the
adhesive/CFRP interface.
In addition, anodizing treatment of aluminum
alloy is an effective method to improve the bonding
strength. It can increase the contact area between
the adhesive and the aluminum alloy. Xu et al. 15
found that anodizing can improve the adhesion
strength of fiber–metal laminates. Their sample
with an apparent energy value of 84.62 mJ/m2
and
roughness of 0.720 lm exhibited a final interfacial
bonding strength of 52.45 MPa and the best
durability. He et al.16
found that the shear strength
of Ti6Al4V and epoxy resin increased by 51.6% after
anodizing. Wilson et al.17
found that tartaric acid
cross-contamination in the post-cascade reaction
after sulfuric acid anodization could increase the
bonding strength of the same material to 35 MPa.
Kim et al.18
modified the micro- and nanomorphol-
ogy of the aluminum alloy surface. After microscale
patterning, an anodic oxidation process was also
applied to bind the nanopores to the micropatterned
surface. It was found that this method had a certain
improvement effect on the bonding strength.
Aghamohammadi et al.19
found that Forest Prod-
ucts Laboratory (FPL) etching and anodizing treat-
ment could significantly improve the bonding
properties of fiber–metal laminates. Bland et al.20
compared grit-blasting and degreasing (GBD), phos-
phoric-acid anodizing (PAA), and PAA followed by
the application of an anticorrosion primer (PAAP).
It was found that the PAAP-treated samples showed
the best durability. Jang et al.21
anodized aluminum
and formed ZnO nanowires on the anodized layer to
increase the area and roughness of the adhered
surface. Drop-weight impact test results for the
alumina/aluminum laminate showed that the
nanostructure resulted in an increase in the bond-
ing strength between the laminates.
These surface treatment methods are based on
the properties of the material itself, using different
processes to change the surface energy and mor-
phology of the material. The wetting effect of the
interface and the contact area can be increased,
thereby improving the bonding strength. Based on
this review of previous literature, it is seen that
anodizing is an effective method to increase the
strength at metal–metal and metal–CFRP bonds.
However, adhesives can easily crosslink and solidify
before the interface is fully bonded, because of the
slow wetting process. It is difficult to penetrate
micropores and form a sufficient bond at the adhe-
sive/wall interface. Ultrasound has been used for
measurement and characterization,22–24
cleaning,25
welding,26
etc., because of its good directivity and
strong reflection ability, long travel distance, and
concentrated sound energy. In our previous study,27
the authors found that ultrasonic vibration has the
effect of enhancing the wettability and discharging
bubbles in the adhesive layer, thereby improving
the strength of CFRP/aluminum alloy bonds. This
method is suitable for most bonding situations,
because ultrasonic vibration only acts on the adhe-
sive layer regardless of material changes. Therefore,
a method is proposed herein to improve the strength
of CFRP/aluminum alloy bonding joints by using
ultrasonic vibration to increase the penetration of
the adhesive into the surface of the pretreated
anodized layer. Joints subjected to oxidation for
different times were bonded without ultrasonic
strengthening, and the influence of the anodizing
pretreatment on their strength was study. Based on
these results, the ultrasonic vibration-assisted
Wang, Gao, Chen, Wu, and Hua

adhesive bonding process was designed and opti-
mized, and experimental verification and analysis of
the underlying mechanism were carried out.
The remainder of this manuscript is organized as
follows: section ‘‘Experimental Procedures’’
describes the proposed ultrasonic strengthening
method and the experimental scheme. Section ‘‘Re-
sults and Discussion’’ presents the results and
discussion. Finally, section ‘‘Conclusion’’ summa-
rizes the conclusions of this work.
EXPERIMENTAL PROCEDURES
Materials and Anodizing Pretreatment
The size of the CFRP/aluminum joint samples
was designed according to international standard
ASTM D5868-01 Standard Test Method for Lap
Shear Adhesion for Fiber Reinforced Plastic (FRP)
Bonding. The dimensions of the CFRP (Toray T700)
sheet were 101.6 mm 9 25.4 mm 9 2.5 mm, the
dimensions of the aluminum alloy (7075) sheet were
101.6 mm 9 25.4 mm 9 1.5 mm, and those of the
adhesive layer were 25.4 mm 9 25.4 mm 9 0.76
mm, as shown in Fig. 1a. The thickness of the
adhesive was strictly controlled through the fixture,
as shown in Fig. 1b. The upper cavity where the
aluminum sheet was placed in the fixture was
3.26 mm higher than the lower one where the CFRP
sheet was placed. The thickness of the CFRP sheet
was 2.5 mm, thus the thickness of the adhesive
layer was ensured to be 0.76 mm. The overlapping
width was consistent with the width of the adher-
ends, which was 25.4 mm. In the fixture, the two
cavities overlapped by a distance of 25.4 mm. As
long as the two adherends are placed correctly in
the cavities, an overlap distance of 25.4 mm can be
ensured for the joint. During the application of
ultrasonic vibration, the Al sheet was pressed to
guarantee the layer thickness and prevent the sheet
from sliding. Therefore, the dimension of the adhe-
sive layer was controlled. The adhesive was 3M-DP
420 with excellent bonding performance. This is an
epoxy adhesive. The adhesive can be completely
cured at room temperature for over 24 h. The
surfaces of the CFRP and the aluminum alloy sheet
were sanded by using 40# sandpaper.
The power source for anodizing was a
MAISHENG direct-current (DC) power supply
(MP-3030D). The pretreatment process of anodizing
included sanding (sequentially with 280#, 400#,
600#, 800#, and 1000# sandpaper), anhydrous
ethanol degreasing (ultrasonic cleaning for 5 min),
washing with deionized water, etching (50 g/L
NaOH solution, 65°C, 2 min), washing with deion-
ized water, chemical polishing (30 vol.% HNO3,
30 s), washing with deionized water, and air drying.
The anodizing process was then conducted in an
electrolyte consisting of 184 g/L sulfuric acid solu-
tion at a temperature of 20 ± 2°C for 1 min, 2 min,
3 min, 4 min, 5 min, 10 min, 15 min, 20 min,
25 min, 30 min, 35 min, or 40 min. The current
density was set to 2 A/dm2
, and the electrolyte
temperature was controlled at 20 ± 2°C by using a
thermostatic water bath. After the anodizing treat-
ment, the aluminum alloy sheet was taken out,
washed with deionized water, and air-dried for use.
Fig. 1. Joint and ultrasonic equipment: (a) size of the single-lap joint, (b) fixture, (c) single-lap joint sample, (d) sample after strength test, and (e)
ultrasonic vibration platform.
Ultrasonic Vibration-Strengthened Adhesive Bonding of CFRP/Aluminum Alloy Joints with
Anodizing Pretreatment

Ultrasonic Vibration-Assisted Adhesive
Bonding
The aluminum sheet after the anodization
described in section ‘‘Materials and Anodizing Pre-
treatment’’ was then bonded to the CFRP. The
strength was tested, and thus the optimal anodizing
time was selected for the pretreatment of the
aluminum sheet. The samples before and after tests
are shown in Fig. 1c and d. The parameters of the
ultrasonic vibration-assisted adhesive bonding were
orthogonally optimized, the importance of each
parameter was analyzed, and the joint with the
highest strength was finally obtained.
Figure 1e shows the MAXWIDEÒ
ME-1800 ultra-
sonic platform used in this study. The high-fre-
quency signal is converted into ultrasonic vibration
by the ultrasonic transducer, and the amplitude is
amplified by the horn and transmitted to the
sonotrode. The pneumatic clamp presses the sono-
trode onto the CFRP sheet and thus transmits the
vibration to the adhesive layer through the sheet.
The ultrasonic vibration-strengthened bonding pro-
cess consisted of four steps: positioning, bonding,
exerting ultrasonic vibration, and curing. First, the
CFRP sheet was placed in the appropriate position
below the sonotrode. Secondly, the adhesive was
dispensed evenly on the bonding area using a 3MTM
EPX applicator with an EPX mixing nozzle. An
appropriate amount of adhesive was extruded to
ensure that there were no bubbles in the EPX
mixing nozzle and the adhesive was mixed evenly.
The adhesive was dispensed from one long edge to
the other across the width of the sheet. The
dispensing was conducted continuously in straight
lines without any gap or overlap until the entire
bonding area was evenly covered. The aluminum
sheet was slowly placed from one end to the other to
avoid bubbles. Then, the sonotrode was pressed
down on the CFRP sheet, and the ultrasonic device
was turned on to apply vibration on the joint.
Finally, the adhesive was cured at room tempera-
ture for 48 h. There are three key parameters in
this process: the vibration time, the vibration
position, and the vibration amplitude. The vibration
time is the period during which the sonotrode
applies ultrasonic vibration to the CFRP sheet.
The vibration position refers to the distance from
the sonotrode to the bonding area on the CFRP
sheet. The vibration amplitude is the maximum
vibration displacement of the sonotrode.
Characterization
Contact Angle
In the bonding process, the wettability of the
adhesive on the surface of the adherend has a great
influence on the quality of the joint. To determine
the effect of the anodizing on the wettability, the
contact angle was tested to characterize the wetta-
bility of the adhesive on the aluminum surface.
Water is commonly used as the liquid to character-
ize the surface wettability of materials. Many
studies14,15,21,28,29
have used water to characterize
the wettability of the adherend surface and obtained
clear insight. Therefore, water was selected as the
contact angle test liquid in this study. The contact
angle of the anodized aluminum alloy film was
measured by using a DSA100 contact angle mea-
suring instrument manufactured by KRUSS,
Germany.
Surface Morphology
To study the mechanism of the ultrasonic vibra-
tion-strengthened bonding, the surface morphology
of the anodized aluminum alloy film, and the cross-
sectional morphology and elemental distribution of
the single lap joint were analyzed. The sample was
cut by using a high-pressure water jet, and after
ultrasonic cleaning, gold sputtering was conducted
to observe the surface morphology. When observing
the cross-section, it was necessary to polish the
cross-section and sputter gold. The surface mor-
phology of the anodized aluminum alloy film and the
cross-section of the single lap joint were observed by
using an SU8010 ultrahigh-resolution field-emis-
sion scanning electron microscope manufactured by
Hitachi High-tech.
Single-Lap Specimen Shear Strength Test
To clearly indicate the strength of the bond, the
shear strength was used to evaluate the strength of
the CFRP/aluminum alloy joint. The strength for-
mula is
s ¼
P
B L
; ð1Þ
where P is the maximum load that the bonded joint
can withstand in units of N. B L is the bonding
area in units of mm2
. The overlap distance for all
samples was 25.4 mm, as ensured by the fixture.
The unit of shear strength s is MPa.
The MTS 810 ceramic experimental system was
used to measure the maximum load that each
bonded specimen could withstand in accordance
with ASTM D5868-01. Finally, the bonding strength
of the specimen was calculated using Eq. 1.
RESULTS AND DISCUSSION
Bonding Without Ultrasonic Strengthening
Twelve different anodizing times in the range
from 1 min to 40 min were used, and the charac-
teristics of the anodized surface determined. The
effect of anodizing on the surface characteristics of
the aluminum alloy was analyzed.
The bonding strength after anodizing for different
times was tested and is shown in Fig. 2. The
reference sample with time of 0 min in the fig-
ure refers to a sample without anodizing

pretreatment. It can be seen that, after the anodiz-
ing treatment of the aluminum alloy, the bonding
strength was remarkably improved. With increas-
ing anodizing time, the bonding strength showed
obvious fluctuation. When the anodizing treatment
time was 3 min, the bonding strength was the
highest, reaching 11.81 MPa. This can be attributed
to the characteristics of the anodized surface.
The surface morphology of the samples anodized
for different times was determined by SEM (Fig. 3),
revealing that the surface of the original aluminum
alloy sheet exhibited cracks generated by rolling.30
After anodizing treatment with sulfuric acid, a
porous oxide film was formed on the surface of the
aluminum alloy. As the anodizing time was
increased, the diameter of the pores became larger.
This porous structure allows the adhesive to pene-
trate into the nanopores to form a mechanically
interlocking structure, which is the main reason for
the improved bonding strength after the anodizing
treatment.
After anodizing pretreatment, the contact angle of
the anodized layer of the aluminum alloy changed
significantly (Table I). It can be seen that the
original sample exhibited a contact angle greater
than 90°, and it was difficult for the liquid to wet the
solid surface, consistent with previous study.31
The
anodized samples exhibited contact angles less than
90°. After treatment times of 2–10 min, the contact
angle generally decreased by about 32%, a signifi-
cant decrease. Such a reduction of the contact angle
can improve the wetting effect of the adhesive on
the surface of the aluminum alloy, thereby improv-
ing the bonding strength.
Interestingly, although the bonding strength was
highest when the anodizing time was 3 min, the
contact angle was not minimal. This is because the
bonding strength is not only determined by the
interface wetting but also by the strength of the
oxide film. As the anodization time was increased,
the pore diameter and the thickness of the oxide
film increased, and the oxide film became loose. This
caused a decrease in the strength between the oxide
film and the aluminum alloy substrate, which in
turn caused a decrease in the bonding strength.
Note that an anodized surface with high strength
does not exhibit good wetting performance as it
makes it slow and difficult to penetrate the
nanopores and form a sufficient bond at the adhe-
sive/wall interface. The adhesive can easily cross-
link and solidify before the interface is fully bonded,
because of the slow wetting process. In this situa-
tion, improving the wetting performance of the
adhesive on the anodizing surface can further
strengthen the bonding. The ultrasonic vibration
has the effect of enhancing the wetting of the
adhesive layer, thereby further increasing the pen-
etration of the adhesive into the surface of the
pretreated anodized surface to improve the bonding
strength.
Ultrasonic Strengthening
A method is proposed herein to improve the
strength of CFRP/aluminum alloy bonding joints
by using ultrasonic vibration to promote penetra-
tion of the adhesive into the anodized layer. There
are many parameters in the ultrasonic vibration
strengthening process, thus the orthogonal test
method was applied to optimize the process param-
eters and analyze their influence on the strength.
Orthogonal Experimental Design
The main parameters of the ultrasonic vibration-
strengthened bonding process include the vibration
time, vibration position, and vibration amplitude.
According to the results presented in section ‘‘Bond-
ing Without Ultrasonic Strengthening’’, anodization
for 3 min was chosen to pretreat the aluminum alloy
sheet, and the ultrasonic vibration parameters were
optimized on this basis. Based on previous trials,
the factors and levels of the orthogonal test were
determined as presented in Table II. The orthogonal
test was designed using Minitab software (V17.1),
resulting in the orthogonal L16 (43
) table presented
in Table III, according to which the experiments
were carried out. Here, to ensure the randomness of
the experiments, a random sequence was used. For
each test, four samples were prepared and mea-
sured. The results showed that the highest shear
strength obtained for the samples subjected to
ultrasonic vibration was 18.16 MPa, being 53.8%
higher than that of the sample that was anodized
only (11.81 MPa, section ‘‘Bonding Without Ultra-
sonic Strengthening’’).
Optimal Bonding Scheme
These results were then input into Minitab
software, and the orthogonal results were analyzed
visually using a main effects plot for the means
Fig. 2. Shear strength of CFRP/aluminum adhesive bonding joints
with different anodizing times.

(Fig. 4). The mean of each factor in Fig. 4 reflects
the extent to which it affects the joint bonding
strength.
Comparing Fig. 4 and Table III, it is found that
the ultrasonic process significantly improved the
bonding strength of the CFRP/aluminum alloy
joints. Analysis of Fig. 4 shows that the descending
order of importance of the process factors affecting
the bonding strength is: vibration time vibration
position vibration amplitude. Therefore, the
vibration time has the greatest effect on the bonding
strength, while the vibration amplitude has the least
significant effect. From the trend of each factor at
different levels shown in Fig. 4, the maximum mean
bonding strength of the sample will be achieved if
the three factors (vibration time, vibration position,
and vibration amplitude) are at level 4, 1, and 2,
respectively. Therefore, the vibration time, vibration
position, and vibration amplitude should take the
values 16 s, 10 mm, and 32 lm, respectively, which
are the optimal parameters for the ultrasonic vibra-
tion-strengthened bonding process.
Fig. 3. Oxide film surface morphology after anodizing for different times: (a) 0 min, (b) 3 min, (c) 10 min, (d) 25 min, (e) 35 min, and (f) 40 min.

Verification and Mechanism Analysis
Since the optimal process obtained by the orthog-
onal test is not included in the orthogonal test
scheme of Table III, it is necessary to test and verify
the obtained optimal process. Figure 5a shows the
bonding strength of the reference group and the
experimental group. No ultrasonic vibration was
applied in the reference group, while the optimal
process parameters were used in the experimental
group. Figure 5a indicates that, when applying the
optimal process parameters, the average bonding
strength of the CFRP/aluminum alloy single lap
joint was 18.66 MPa, which is 55% higher than that
of the samples without ultrasonic vibration in the
reference group, confirming that the ultrasonic
process had a significant effect on the bonding
strength of the CFRP/aluminum alloy joint. Fig-
ure 5b and c shows the failure interface after the
shear strength test. The main failure mode of the
samples without ultrasonic vibration was adhesive
failure. Residual adhesive was observed on both the
aluminum and CFRP sheets. The main failure mode
of the samples with ultrasonic vibration was fiber-
tear failure. Damaged fibers can be clearly observed
on the failure surface. This shows that ultrasonic
vibration improved the strength of the interface
between the aluminum and adhesive layer.
To study the mechanism of the strengthened
bonding, SEM was used to analyze the cross-section
morphology of the joint, and EDS was applied to
analyze the elemental distribution. Figure 5 shows
the morphology and elemental distribution of the
middle section. In Fig. 5d and e, the upper part is
the aluminum alloy while the lower part is the
adhesive layer. It can be seen that, at the same
magnification, the bond between the aluminum
alloy and adhesive layer in the experimental group
was better. The interface can hardly be distin-
guished. The aluminum alloy and adhesive layer
can be identified only by the difference in color
depth and morphology. However, there is a clear
interfacial line between the aluminum alloy and
adhesive layer in the reference group. This shows
that a tighter interface bond was achieved under
the action of the ultrasonic vibration.
Figure 5f and g shows elemental scans on the
cross-section. Three elements Al, C, and O are
marked. The interface between the aluminum alloy
and adhesive layer can be identified according to the
distribution of the O element. The content of the O
element at the interface is the largest, because the
anodizing treatment produces an oxide film on the
surface of the aluminum alloy. Furthermore,
besides the O element, the interface area also
contains the C element, which is the main element
of the adhesive 3M-DP 420. Therefore, it is believed
that the adhesive penetrated the anodized layer of
the aluminum alloy under capillary action, forming
a mechanically interlocked structure. As analyzed
by the ‘‘imread’’ function in MATLAB software
(V2016b), the green color representing the C ele-
ment covers about 7.2% of the area in the interface
Table I. Contact angle of anodized layer after various treatment times
Testing image
Time/min 5 10 15 20 25
Contact angle / ° 61.1 64.8 77.7 74.35 57.95
Testing image
Time/min 30 35 40
Contact angle / ° 84.35 85.9 54.3
Testing image
Time/min 0 1 2 3 4
Contact angle / ° 95.1 69.15 57.85 67.15 71.65

area in Fig. 5f, compared with 19.6% in Fig. 5g. It is
thus seen that the C element at the interface of the
experimental group was greater than that of the
reference group. It is thus confirmed that, under the
action of ultrasonic vibration, more adhesive pene-
trated into the tiny holes of the anodized layer,
which enhanced the strength by forming more
mechanically interlocked structures and greater
interfacial contact area. When the ultrasonic vibra-
tion is applied, the high-frequency vibration will be
transmitted to the adhesive to create shock waves in
the adhesive layer. In our previous study,32
the
temperature of the adhesive layer was measured via
an infrared thermometer (AicevoosÒ
AS-D400A).
Continuous ultrasonic vibration rapidly heated up
the adhesive layer, causing a decrease in its viscos-
ity.33
The penetration of the adhesive into the
anodized layer is thus promoted. Meanwhile, the
shock waves cause a high-speed adhesive jet onto
the adherend surface, which causes frequent colli-
sions between the adhesive and adherend.34
Since
Table II. Factors and levels of orthogonal experiments
Level
Factor
Vibration time (s) Vibration position (mm) Vibration amplitude (lm)
1 4 10 24
2 8 20 32
3 12 30 40
4 16 40 48
Table III. Orthogonal schemes and experimental results
Test
number
Vibration
time (s)
Vibration
position (mm)
Vibration
amplitude (lm)
Average maximum
load (N)
Bonding strength
(MPa)
1 4 10 24 7079þ1483
1310 10:97þ2:30
2:03
2 4 20 32 6838þ1060
737 10:60þ1:64
1:14
3 4 30 40 7717þ544
837 11:96þ0:84
1:30
4 4 40 48 8789þ577
583 13:62þ0:89
0:90
5 8 10 32 11682þ417
810 18:11þ0:65
1:26
6 8 20 24 8474þ939
724 13:13þ1:46
1:12
7 8 30 48 9781þ1176
875 15:16þ1:82
1:36
8 8 4 40 8538þ934
1024 13:23þ1:45
1:59
9 12 10 40 9399þ1026
1086 14:57þ1:59
1:68
10 12 20 48 7613þ829
684 11:80þ1:28
1:06
11 12 30 24 9025þ1018
932 13:99þ1:58
1:44
12 12 40 32 9598þ604
965 14:88þ0:94
1:50
13 16 10 48 11344þ1304
846 17:58þ2:02
1:31
14 16 20 40 8575þ1346
688 13:29þ2:09
1:07
15 16 30 32 10729þ607
221 16:63þ0:94
0:24
16 16 40 24 10715þ712
858 18:16þ1:10
1:33
Fig. 4. Main effect plot for means: factors 1, 2, and 3 represent the
vibration time, vibration position, and vibration amplitude,
respectively.

Fig. 5. Strength and cross-section characterization: (a) bonding strength of reference and experimental groups, failure surface of samples from
(b) reference and (c) experimental group, morphology of sample from (d) reference and (e) experimental group, and elemental distribution for (g)
reference and (f) experimental group.

the aluminum surface is porous, such collisions
make the bonding interface tight and also promote
penetration of the adhesive into the anodized layer.
Therefore, the bonding strength is remarkably
improved due to the formation of a greater interfa-
cial contact area and more mechanically interlocked
structures.
CONCLUSION
A method is proposed herein to improve the
penetration of the adhesive into the surface of the
anodized layer by using ultrasonic vibration,
thereby enhancing the strength of CFRP/aluminum
alloy joints. First, joints with different oxidation
times were bonded without ultrasonic strengthen-
ing. The influence of the anodizing pretreatment on
the strength was studied by analyzing the surface
morphology and contact angle. Accordingly, the
ultrasonic vibration-assisted adhesive bonding pro-
cess was designed and optimized by orthogonal
experiments. The effects of the process parameters
on the strength were analyzed, and the optimal
parameters for the joints were determined. Finally,
the ultrasonic strengthening process was experi-
mentally verified. The strengthening mechanism
was then studied by analyzing the morphology and
elemental distribution of the cross-section of the
joint by SEM and EDS. The specific conclusions
include:
1. An anodized surface with high strength does not
exhibit good wetting performance, making it
slow and difficult to penetrate the nanopores
and form a sufficient bond at the adhesive/wall
interface. Improving the penetration of the
adhesive into the anodizing surface can further
strengthen the bonding.
2. The ultrasonic vibration-assisted adhesive bond-
ing process can further improve the strength of
CFRP/aluminum alloy bonding joints after
anodizing pretreatment. For the studied bond-
ing joint, the ultrasonic strengthening process
was designed and optimized. The strength can
reach 18.66 MPa, 55% higher than when bond-
ing without ultrasonic vibration.
3. The application of ultrasonic vibration creates
shock waves in the adhesive layer, causing a
high-speed adhesive jet onto the adherend sur-
face. Meanwhile, continuous ultrasonic vibra-
tion rapidly heats up the adhesive layer, causing
a decrease in its viscosity. These effects make
the bonding interface tight and also promote
penetration of the adhesive into the anodized
layer. Therefore, the bonding strength is
remarkably improved by forming a greater
interfacial contact area and more mechanically
interlocked structures.
The ultrasonic vibration-assisted adhesive bonding
method further strengthens the adhesive bond of
anodized joints, and has practical value in the field
of high-performance bonding. This study is impor-
tant for guiding continuous optimization of adhesive
bonding.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the
financial support from the National Natural Science
Foundation Council of China (Grant Nos. 51775398,
51805392), the 111 Project (Grant No. B17034), the
Program for Innovative Research Team in Univer-
sity of Education Ministry (Grant No. IRT_17R83),
the Natural Science Foundation of Hubei Province
(Grant No. 2018CFB595), and the Fundamental
Research Funds for the Central Universities (Grant
Nos. WUT: 2019IVB022, 2018III074GX,
2018III067GX).
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