H. DURSUN et al.: EXPERIMENTAL INVESTIGATION OF THE CAVITATION EROSION OF A FLAT ALUMINUM PART ...
637–642
EXPERIMENTAL INVESTIGATION OF THE CAVITATION
EROSION OF A FLAT ALUMINUM PART USING A SONOTRODE
TEST DEVICE
EKSPERIMENTALNA RAZISKAVA KAVITACIJSKE EROZIJE
PLO[^ATIH DELOV IZ ALUMINIJA Z UPORABO SONOTRODA
Harun Dursun
1
, Gökhan Sevilgen
2*
, Mehmet Ýhsan Karamangil
2
1
Robert Bosch GmbH, Diesel Systems, TR-16140 Nilüfer, Bursa, Turkey
2
Department of Automotive Engineering, Faculty of Engineering, Bursa Uludað University, TR-16059 Bursa, Turkey
Prejem rokopisa – received: 2018-11-26; sprejem za objavo – accepted for publication: 2019-03-27
doi:10.17222/mit.2018.255
In this paper we aimed to determine the gap distance between a horn and an aluminum part that ensures the highest cavitation
erosion rate in a homogeneous delaminated area and provides the minimum energy consumption. For this purpose, a sonotrode
device, which is capable of generating cavitation bubbles in the laboratory and enabling the evaluation of the cavitation
resistance of parts with different parameters, was used in experiments. An optical microscope was used to visualize the surface
delamination area where the cavitation erosion occurs, and a high-precision scale was used for measuring the total mass loss due
to the cavitation erosion for each test. A high-speed camera was used to visualize the shape and behavior of the flow
characteristics below the sonotrode. From the experimental results it is clear that the delamination area decreased with the
increasing gap distance due to the impact region of bubble structures on the part surface. The gap distance clearly had a great
effect on the cavitation erosion rate that must be determined while considering the energy consumption, test time and maximum
homogeneous delaminated area in a specific cavitation test. The optimum distance was obtained as 3.5 mm, by considering the
maximum cavitation erosion rate and using minimum power.
Keywords: ultrasonic cavitation, sonotrode, cavitation erosion, flat horn
Avtorji opisujejo dolo~evanje velikosti re`e med oddajnikom ultrazvo~nih valov (sonotrodom) in delom iz aluminija, ki
omogo~a najvi{jo hitrost kavitacijske erozije v homogenem delaminiranem preseku in isto~asno zagotavlja najmanj{o porabo
energije. V ta namen so za preizkuse uporabili napravo (sonotrod), ki je sposobna generirati kavitacijske mehur~ke v laborato-
rijskih razmerah in omogo~a ovrednotenje kavitacijske upornosti Al izdelkov pri razli~nih procesnih parametrih. Za vizuali-
zacijo delaminirane povr{ine (odlu{~enih plasti) na mestih, kjer je nastopil proces kavitacije, so uporabili opti~ni mikroskop. Pri
vsakem preizkusu so uporabili natan~no merilno metodo za merjenje izgube celotne mase zaradi kavitacijske erozije. Uporabili
so visokohitrostno kamero za vizualizacijo oblike in karakteristik toka medija pod sonotrodom. Eksperimentalni rezultati so
jasno pokazali, da se delaminacijski presek s pove~evanjem re`e zmanj{uje zaradi strukture mehur~kov v udarnem podro~ju na
povr{ini Al dela. [irina re`e ima nedvoumno velik vpliv na hitrost kavitacijske erozije. Avtorji ugotavljajo, da mora biti le-ta
dolo~ena glede na porabo energije, ~as preizkusa in maksimalnega homogenega preseka delaminacije za to~no dolo~en (speci-
fi~en) preizkus kavitacije. Dolo~ili so optimalno velikost re`e. Ta je zna{ala 3,5 mm, upo{tevaje maksimalno hitrost kavitacijske
erozije pri uporabi najmanj{e mo~i.
Klju~ne besede: ultrazvo~na kavitacija, sonotrod, kavitacijska erozija, plo{~ato trobilo oz. rog (sonda za oddajanje ultrazvo~nih
valov)
1 INTRODUCTION
Cavitation is a phase-change process defined by the
formation of the vapor of a liquid as a result of a
pressure drop at constant ambient temperature.
1
Cavi-
tation effects are observed where the pressure and
velocity in a fluid changes dramatically, including in
many engineering components, such as pumps, turbines,
ships’ propellers and injectors.
2
On the other hand, the
stages of bubble formation in acoustic cavitation were
depicted in Figure 1.
3
The collapsing of these bubbles
near a solid wall produces the material erosion due to
cavitation. One of the test methods that can be used for
the formation of cavitation bubbles and observing their
effects on the metal parts is the acoustic cavitation test
method according to the ASTM G32-10 standard.
4
The
distance between the sample and the sonotrode in the
indirect method described in this standard influences the
intensity of the erosion and the test duration.
5
On the
other hand, the test results of the two types of samples
show that the different total test time was obtained for
these samples, although they had a similar cavitation
erosion resistance.
6
The total test time has to be opti-
mized for different materials considering the cavitation
erosion rate in the indirect method.
7
Cavitation erosion
had four stages, which were the incubation, acceleration,
maximum rate and deceleration.
8
Therefore, the total
time for getting the maximum cavitation erosion can be
determined by considering these different stages. On the
other hand, the cavitation erosion is strongly dependent
upon the flow velocity in the cavitation field and the
Materiali in tehnologije / Materials and technology 53 (2019) 5, 637–642 637
UDK 620.1:67.019:669.71:620.179.16 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)637(2019)
*Corresponding author's e-mail:
gsevilgen@uludag.edu.tr

material damage depends on the duration of the exposure
to cavitation.
9
Thus, the different flow structure occurred below the
sonotrode and the exposure time can cause different
material damage. A Conical Bubble Structure (CBS) was
observed for water and thick round layers were apparent
for glycerin at 100 % power.
10
These bubble structures
had different flow characteristics and also had different
effects in terms of material damage in the cavitation
field. Another important test parameter that effects the
cavitation erosion rate was amplitude, a typical CBS was
formed below the sonotrode tip in a water tank at a
50 μm amplitude, and the increase in amplitude led to a
higher number of small cavitation bubbles and selecting
a different gap distance between the sonotrode tip and
the opposite solid wall can cause a difference in the
number of detected micro bubbles.
11
Thus, the collapsing
bubbles density, which can cause material loss, varies by
selecting different gap distances. The acoustic power
level used in the acoustic cavitation tests also influenced
the cavitation erosion rate of the material. As the power
amplitude increased from 100 W to 400 W, the bubbles
reached a radius of 4.53 times greater than their initial
radius.
12
For this reason, an increase in the power
amplitude can cause higher material damage due to the
growing velocity and bubble radius in the acoustic
cavitation field. Consequently, the cavitation erosion rate
of different materials obtained by using a standard
vibratory device depends on the test parameters, such as
acoustic power level, amplitude of the sonotrode, liquid
type, test duration and the sonotrode type. However,
there are a small number of studies related to the gap
distance effect on the cavitation erosion in the available
literature.
13,14
In the present study, flat aluminum samples
were cavitated by using an indirect method and the
reason for this is to determine the minimum gap distance
that gives the highest delamination area in a particular
zone. This will reduce the total test time and also
energy-consumption costs for the industrial processes.
Our ultimate goal is to develop a new test methodology
in which coating quality and homogeneity of the valve
pieces exposed to the cavitation. In the magnet-type
injector, the valve piece that separates the high-pressure
and the low-pressure regions is one of the most critical
parts. This piece is coated with Cr-Ni coating material to
reduce or minimize the effect of cavitation. Therefore,
the results obtained by this work will then be used to
develop the coating quality test methodology of valve
pieces having angled surfaces.
2 EXPERIMENTAL PART
There are two types of application methods for vibra-
tion devices: (1) the direct method and (2) the indirect
method.
6–8
As a result of the oscillating of the horn, the
bubbles are generated and directed to the surface of the
sample to achieve cavitation erosion in a specified test
run. The experimental setup and the test parameters used
in this study are shown in Figure 2. The samples used in
this study were a cylindrical part made of Aluminum
6061-T6 material, having a diameter of 16 mm and a
thickness of 8 mm, with flat surface for the elimination
of geometrical effects. Eight different test cases were
H. DURSUN et al.: EXPERIMENTAL INVESTIGATION OF THE CAVITATION EROSION OF A FLAT ALUMINUM PART ...
638 Materiali in tehnologije / Materials and technology 53 (2019) 5, 637–642
Figure 2: a) The experimental setup and b) the test parameters of acoustic cavitation device
Figure 1: Stages of bubble formation in acoustic cavitation
3

selected to evaluate the effects of the distance between
the flat horn and the sample (Figure 2b) on the cavi-
tation erosion. The tests were repeated five times for
each case to confirm the experimental results under the
same conditions. According to our previous experiments,
if the gap distance is less than 2 mm or greater than
6 mm, the cavitation effect is dramatically reduced.
Therefore, the gap distance interval was varied from 2
mm to 6 mm and two different test durations of 45 min
and 90 min were applied to examine these effects,
whereas the other test parameters remained constant. An
optical microscope was used to determine the surface
delamination areas where the cavitation erosion occurs
and the high-precision scale with 0.01 mg readability
was used for measuring the total mass loss due to the
cavitation erosion during each test. A high-speed camera
with 1357 fps at VGA resolution was used to visualize
the shape and the behavior of the flow characteristics.
The test device used in this study allows recording
energy values with 100 ms steps via a direct connection
with a computer.
3 RESULTS
Before starting the cases, a high-speed camera was
used to determine the shape and the structure of the
bubble cloud that the flat horn created in the distilled
water shown in Figures 3a and 3b. As a result of these
images, we can easily say that the cone-shaped bubble
cloud was observed for all cases. The structure of the
bubble cloud derived from the image of the high-speed
camera for different gaps is shown in Figure 4. From
these results, the structure of cone-shaped bubble cloud
was maintained for all cases, but the dimensions of cone
shape changed depending on the gap distances. Thus, the
affected area on the material was changed by selecting
different gap distances, as shown in Figure 5.
The total mass-loss values depending on the cavi-
tation erosion are shown in Table 1. The cumulative
mass loss was ranged from 0.8 mg to 1.5 mg for Case-1,
whereas this value was measured between 2 mg and 2.80
mg for Case-2. From these results we can say that when
the gap distance increased from 2 mm to 3 mm, the
maximum total mass loss was nearly doubled. If the gap
distance continued to increase to a value of 3.5 mm
(Case-3), the cumulative mass loss was measured
between 4.80 mg and 6.50 mg at the end of the 90 min
testing period. From the comparison of the result of the
first three cases, the cumulative mass loss increased
nonlinearly with the gap distance. If the gap distance was
selected higher than 3.5 mm, then the cumulative mass
loss gradually decreased for Cases 4-6. This means that
the optimum gap distance considering the maximum
total mass loss was achieved for Case-3, which had a gap
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Figure 4: The locations of Al specimens in the bubble cloud with different gaps
Figure 3: The shape of the bubble cloud: a) the image taken from a high-speed camera, b) the structure of the bubble cloud derived from image of
a high-speed camera

distance selected of 3.5 mm. We can easily say that the
gap distance had a great effect on the cavitation erosion
of an aluminum sample.
Another result is that the test duration affects could
not be neglected and the total mass loss, which gradually
increased over time, and the maximum total mass loss
were measured for Case-3 in all experiments. This means
that there was no need for more testing time if the maxi-
mum cavitation erosion is required and less power is
spent. According to the optical microscope photographs
at 90 min, the surface area affected by cavitation erosion
was nearly the same for the first two cases and the
heterogeneous delamination area was observed for these
cases (Figure 5.1a). The mass losses are also low.
However, the delaminated surface area can be assumed
to be homogeneous for Case-3 compared to Case-1 and
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Table 1: The measured mass loss values for all cases
Sample
Total mass loss values obtained from experiments
Case-1 / d = 2.0 mm Case-2 / d = 3.0 mm Case-3 / d = 3.5 mm Case-4 / d = 4.0 mm
0-45 min 45-90 min Sum 0-45 min 45-90 min Sum 0-45 min 45-90 min Sum 0-45 min 45-90 min Sum
1 0.30 0.50 0.80 0.90 1.90 2.80 1.60 3.80 5.40 1.10 3.20 4.30
2 0.20 0.70 0.90 0.70 1.40 2.10 2.30 4.20 6.50 1.70 2.80 4.50
3 0.60 0.90 1.50 0.70 1.30 2.00 2.60 3.90 6.50 0.80 2.20 3.00
4 0.40 0.70 1.10 0.80 1.80 2.60 1.50 3.30 4.80 1.40 2.90 4.30
5 0.20 0.60 0.80 0.80 1.80 2.60 2.50 2.40 4.90 1.40 2.10 3.50
Avg 0.34 0.68 1.02 0.78 1.64 2.42 2.10 3.52 5.62 1.28 2.64 3.92
Case-5 / d = 4.5 mm Case-6 / d = 5.0 mm Case-7 / d = 5.5 mm Case-8 / d = 6.0 mm
0-45 min 45-90 min Sum 0-45 min 45-90 min Sum 0-45 min 45-90 min Sum 0-45 min 45-90 min Sum
1 1.30 1.60 2.90 0.90 1.90 2.80 0.40 1.10 1.50 0.20 0.80 1.00
2 0.90 1.30 2.20 1.30 0.80 2.10 0.20 0.80 1.00 0.20 0.50 0.70
3 0.30 2.20 2.50 0.40 2.10 2.50 0.50 0.80 1.30 0.40 0.80 1.20
4 0.70 2.00 2.70 0.60 1.70 2.30 0.50 0.70 1.20 0.20 1.00 1.20
5 1.10 1.90 3.00 0.20 2.90 3.10 0.40 1.40 1.80 0.10 1.30 1.40
Avg 0.86 1.80 2.66 0.68 1.88 2.56 0.40 0.96 1.36 0.22 0.88 1.10
Figure 5: Optical microscope photographs of the Al sample: (1) the distribution of delaminated area of the Al sample for Case 1-4, and Case-8 at
90 min of testing period, (2) optical microscope photographs of the Al samples for the same cases (a-e)

Case-2 (Figure 5.1b). According to the optical micro-
scope photographs in Figure 5, a certain gap distance
range is required to obtain a homogeneous distribution of
the delamination area. In addition, the mass losses are
also high. As the distance between the aluminum sample
and the sonotrode increases, the effects of cavitation
decrease, but the delaminated area concentrated at a
more central point (Figures 5.1c and 5.1d). It can be said
that the reason for this is the reduction of the affected
area of the cone-shaped bubble structure on the material.
The average mass loss and the calculated average
cavitation erosion rate obtained from all the experiments
for the two different test periods are shown in Figure 6.
From these results, the cavitation erosion rates were
calculated to be between 1.5 mg/h and 2 mg/h for case-2,
case-5 and case-6. This value was changed to be between
0.8 mg/h and 1.2 mg/h for case-7 and case-8. On the
other hand, the lowest and the highest values were ob-
tained for case-1 and case-3, respectively. From these
results, we can easily say that the calculated cavitation
erosion rates of case-2, case-5 and case-6 were close to
each other. The cavitation erosion rates of case-7 and
case-8 were similar.
The electric power of the horn for each case was
shown in Figure 7. From these results, the lowest energy
consumption was obtained for Case-7 and case-8, but
these cases had smaller delaminated areas and lower
cumulative mass loss. From the comparison of the
energy-consumption data for all cases, the homogenous
delaminated area that provides the minimum energy
consumption per unit mass loss was obtained for case-3
and the electric power was obtained about 203.82 W for
case-3.
4 DISCUSSION
The cumulative mass loss was changed nonlinearly
with different gap distances used in this experimental
study. There was a range of distances that the flat horn
works most effectively. The area affected by the cavita-
tion erosion decreased with the increasing gap distance
due to the geometry of the V-shape bubble cloud. How-
ever, where the gap distance was reduced too much, the
power of the cavitation decreases, even if the affected
area increased. The difficulty of the indirect method used
in this study was the complication of controlling the flow
characteristics between the flat horn and the sample. It
was confirmed that the flat horn generated a V-shaped
cavitation field (bubble cloud) during the oscillations at
high frequency in the water.
5 CONCLUSIONS
From the result of this experimental study, we can
easily say that the gap distance is an important parameter
that must be determined when considering the energy
consumption, test time and maximum homogeneous
delaminated area in a specific cavitation test. When the
gap distance was less than 3.5 mm, it was observed that
the distribution of the delaminated area was hetero-
geneous. It was determined that the 3.5-mm distance is
the optimum gap for the 22-mm-diameter flat horn and
the maximum total mass loss was measured for Case-3 in
all experiments. The homogenous delaminated area that
provides the minimum energy consumption per unit
mass loss was obtained for case-3 and the electric power
was obtained at about 203.82 W for case-3. The test
duration effects could not be neglected and this means
that there was no need for more testing time if the
maximum cavitation erosion was required and also less
power. In further studies we aim to develop the CFD
(Computational Fluid Dynamics) model for obtaining the
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Materiali in tehnologije / Materials and technology 53 (2019) 5, 637–642 641
Figure 7: The electric power (W) of horn for all cases
Figure 6: The average mass-loss measurements and calculated average cavitation erosion rate (mg/h) for each case

effects of other test parameters of this experiment, which
will be helpful to understand the control mechanisms of
the cone-like bubble structure below the sonotrode tip.
Acknowledgment
The authors would like to acknowledge the company
Bosch for supporting this research by providing the ex-
perimental test unit.
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H. DURSUN et al.: EXPERIMENTAL INVESTIGATION OF THE CAVITATION EROSION OF A FLAT ALUMINUM PART ...
642 Materiali in tehnologije / Materials and technology 53 (2019) 5, 637–642