Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
185–192
EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON
REFINEMENT OF Al-5Cu ALLOY INGOTS
VPLIV VISOKOINTENZIVNEGA ULTRAZVOKA NA
PRE^I[^EVANJE INGOTOV IZ ZLITINE Al-5Cu
Zheng Jia
*
, Zhi Zhuo Wang, Li Fu
College of Mechanical Engineering, Shenyang University, Shenyang 110044, China
Prejem rokopisa – received: 2023-11-05; sprejem za objavo – accepted for publication: 2024-02-07
doi:10.17222/mit.2023.1038
The effect of high-intensity ultrasonic treatment combined with bottom cooling treatment on the refinement of Al-5Cu alloy in-
gots was studied. The results show that ultrasonic treatment combined with forced cooling at the bottom of ingots has a good re-
fining effect, and the best refining effect can be obtained at 2000 W and after 120 s. The cross-section of an ingot exhibits al-
most 100 % refined equiaxed grains and no porosity. The method of combining ultrasonic treatment and forced cooling at the
bottom of ingots not only influences the ingot refinement, but also decreases the porosity of the ingots with an increase in the ul-
trasonic duration. The ultrasonic degassing effect is due to the release of hydrogen in biofilms, which expand and grow gradu-
ally; finally, they burst on the melt surface, achieving the effect of degassing.
Keywords: high-intensity ultrasonic treatment, Al-5Cu alloy ingots, grain refinement, porosities of ingot
V ~lanku je opisana {tudija vpliva visokointenzivne ultrazvo~ne obdelave v kombinaciji s specifi~nim hlajenjem taline na
rafinacijo ingotov iz zlitine Al-5Cu. Rezultati {tudije so pokazali, da ultrazvok v kombinaciji s prisilnim ohlajanjem dna ingotov
dobro vpliva na rafinacijo oziroma udrobljenje (zmanj{anje velikosti) kristalnih zrn ingotov. Najbolj{i u~inek obdelave ingotov
so avtorji dosegli z mo~jo 2000 W za 120 s. Ingoti so imeli v preseku skoraj 100 %-no enakoosna drobna kristalna zrna in so bili
prakti~no brez por. Izbrana metoda obdelave ingotov med ohlajanjem taline ni vplivala samo na udrobljenje kristalnih zrn
temve~ se je s podalj{evanjem ~asa trajanja ultrazvoka zmanj{ala tudi poroznost ingotov. Ugotovili so, da je razplinjevalni
u~inek ultrazvoka posledica spro{~anja vodika, ki potuje v obliki mehur~kov (biofilma) navzgor, dokler se le-ti ne razpo~ijo na
povr{ini raztaljene kovine.
Klju~ne besede: visokointenzivna ultrazvo~na obdelava, ingoti iz zlitine Al-5Cu, udrobljenje kristalnih zrn, poroznost ingotov
1 INTRODUCTION
Aluminum alloys are widely used in 3C (computer,
communication and consumer electronics), aerospace
and construction industries. Generally speaking, the
manufacture of aluminum alloys always involves smelt-
ing.
1–3
However, if ultrasonic treatment is applied during
smelting, the structure of an ingot can change from
coarse columnar crystals to fine equiaxed crystals, and
the porosity and segregation of the ingot are also re-
strained.
4,5
For aluminum alloys, refinement can be
achieved by adding refining agents such as titanium, bo-
ron, manganese and other elements. Refinement can also
be achieved through external field treatments such as ul-
trasonic field treatment, magnetic field treatment, pulse
electric field treatment, and so on. Compared with the
traditional chemical method, in grain refinement, the ul-
trasonic method is non-contaminative to the metal melt
and non-harmful to the body, nor does it pollute the sur-
roundings.
6–8
Ultrasonic treatment can refine a solidifica-
tion structure steadily and rapidly. Some researchers
found that the cavitation effect and sonic agitation
caused by the high-frequency ultrasonic vibration have
dual functions of refinement and degassing. The quality
and mechanical properties of alloy ingots were also fur-
ther improved, so high-intensity ultrasonic treatment has
been widely favored by scholars around the world.
9–14
According to previous investigations, the factors affect-
ing refinement comprise melt conditions, which include
the chemistry component, pouring temperature, melt
temperature at the beginning of ultrasonic vibration,
etc.
15–20
This research, based on the work above, took an
Al-5Cu alloy as an example, using forced cooling and ul-
trasonic treatment at the bottom of a clay-graphite cruci-
ble to achieve a combined refinement that was studied
and investigated. Our investigation provides a new
method for the industrial production of high-quality alu-
minum alloy ingots.
2 EXPERIMENTAL PART
2.1 Apparatus of the ultrasonic device
The experimental equipment includes an ultrasonic
generator (its maximum output power is 2000 W and fre-
quency is 20 kHz), a temperature control meter for the
temperature control of the melt, a resistance furnace, an
ultrasonic bracket (moving the ultrasonic probe up and
down) and a clay-graphite crucible. The diameter of the
Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192 185
UDK 661.862:544.57 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(2)185(2024)
*Corresponding author's e-mail:
jz140@163.com (Zheng Jia)

crucible is 30 mm and the height is 40 mm. The appara-
tus is shown in Figure 1.
2.2 Experimental procedure
The Al-5Cu alloy used in the experiments was made
of pure industrial aluminum (purity: 99.7 %) and electro-
lytic copper flakes in proportion to the melt. The speci-
mens were divided into six groups, with the mass of each
group being about 200 g, and put into the clay-graphite
crucible for remelting. During the remelting process,
when the temperature was stabilized at 800 °C, the
clay-graphite crucible containing the melt was placed on
a copper plate and ultrasonic treatment was performed
using a preheated 500 °C ultrasonic probe inserted into
the melt to a depth of 5 mm. When the melt temperature
dropped to 750 °C, the copper plate was internally fed
with circulating cooling tap water until the ingot solidi-
fied and cooled down to room temperature (about
25 °C). Experimental conditions were divided into six
types: no ultrasonic treatment applied, bottom water
cooling only, ultrasonic treatment only for 90 s without
water cooling, and ultrasonic treatment combined with
water-cooling treatment for (90, 120 and 150) s. Finally,
the ingots were cut into two symmetrical pieces from the
top downward for a microstructure observation. Sam-
pling positions are shown in Figure 2. The composition
and ratio of the macroscopic etching solution for the
Al-5Cu alloy were HCl : HF : H
2
O=15:10:90andthe
microstructure etching solution was a 5 % HF aqueous
solution. The microstructures were observed using polar-
ized light and the grain size was calculated. Image Pro
was used to calculate the grain size.
3 RESULTS AND DISCUSSION
3.1 Macrostructures
Figure 3 shows the macrostructure change of Al-5Cu
treated with ultrasonic treatment for different times and
forced water cooling at the bottom of the ingot. It can be
seen that the columnar crystals in the solidification struc-
ture of the naturally cooled ingot are relatively coarse,
dendritic dendrites are developed and large pores are vis-
ible, as shown in Figure 3a. When the copper plate is
placed in water without ultrasonic treatment, the coarse
grains are obviously refined without ultrasonic treatment,
the grain distribution is uniform and the grains of the
unit area of the solidified structure of the ingot increase,
as shown in Figure 3b. When ultrasonic treatment is
combined with forced water cooling treatment at the bot-
tom of an ingot for 90 s, the equiaxed crystal roundness
Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
186 Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192
Figure 1: Schematic layout of the ultrasonic field device:1–w ater in-
let;2–w ater outlet; 3 – copper plate; 4 – Al-5Cu alloy melt; 5 and 6 –
ultrasonic probe; 7 – ultrasonic transducer; 8 – ultrasonic generator; 9
– thermocouple
Figure 3: Macrostructures of Al-5Cu alloy ingots with or without ul-
trasonic treatment: a) normal cooling, b) forced water cooling, c) with-
out forced water cooling but with ultrasonic treat. for 90 s, d) forced
water cooling and ultrasonic treat. for 90 s, e) Forced water cooling
and ultrasonic treat. for 120 s, f) Forced water cooling and ultrasonic
treat for 150 s Figure 2: Sampling positions of Al-5Cu alloy ingots

of the solidification structure of the ingot increases, and
the coarser columnar crystals become shorter. Compared
with the ingot without the above treatment at the bottom
of the ingot but with ultrasonic treatment, it can be seen
from the solidification structure that the grain distribu-
tion is more uniform and the pores are reduced. When
ultrasonic treatment is combined with forced water cool-
ing treatment at the bottom of the ingot for 120 s, the ef-
fect of the combination is the best, and the grain refine-
ment is the most obvious, as shown in Figure 3d. When
ultrasonic treatment is combined with forced water cool-
ing treatment at the bottom of the ingot for 150 s, the
equiaxed grains begin to grow and the pores begin to be-
come larger. If the ultrasonic treatment time is too long
and the temperature of the melt is too low, cavitation
bubbles in the melt are not discharged in time. Therefore,
pores appear inside the ingot.
3.2 Microstructure
Figures 4 to 9 show a comparison of ingots’ micro-
structures affected by different melt treatment methods
as nine samples were taken from each ingot. When an in-
got is cooled down normally, treated with forced water
cooling or without forced water cooling but with ultra-
sonic treatment for 90 s, it can be found that the ingot
has a large number of coarse dendrites, as shown in Fig-
ures 4 to 6. However, the grain size is at the millimeter
level. When using forced water cooling and ultrasonic
treatment of the melt for 90–150 s, the dendrites of the
ingots are broken, and a large number of equiaxed crys-
tals appear. The grain-size statistics are shown in Figure
10.
3.3 Mechanism of ultrasonic refinement and degassing
by forced water cooling of the Al-5Cu alloy
clay-graphite crucible bottom
When a melt is treated with ultrasonic treatment, the
resulting cavitation bubbles grow and shrink in positive
and negative cycles, as shown in Figure 11. There are
two reasons for the grain refinement of the melt after
ultrasonication: first, the growth of the cavitation bubbles
can absorb the heat from the melt and then nucleate un-
der the influence of cooling at the bubble interface. The
subsequent rupture of the cavitation bubbles generates
enormous energy to crush the initial nuclei and dendrites
in the melt, increasing the number small grains. The gen-
erated acoustic flow is a liquid flow due to the acoustic
pressure gradient, which also facilitates grain refinement.
In addition, the macroscopic flow effect of the liquid
phase generated by ultrasonication is manifested as a
generalized reflux within the liquid, which can transport
a large number of nuclei caused by nonuniform nucle-
ation and dendrite fragmentation throughout the alumi-
num alloy melt. When forced cooling is applied to the
Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192 187
Figure 4: Microstructure of the Al-5Cu alloy ingot after normal cooling: a) position 1, b) position 2, c) position 3, d) position 4, e) position 5,
f) position 6, g) position 7, h) position 8, i) position 9

Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
188 Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192
Figure 6: Microstructure of the Al-5Cu alloy ingot without forced water cooling but with ultrasonic treat. for 90 s: position 1, b) position 2, c)
position 3, d) position 4, e) position 5, f) position 6, g) position 7, h) position 8, i) position 9
Figure 5: Microstructure of the Al-5Cu alloy ingot after forced water cooling: position 1, b) position 2, c) position 3, d) position 4, e) position 5,
f) position 6, g) position 7, h) position 8, i) position 9

Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192 189
Figure 8: Microstructure of the Al-5Cu alloy ingot after forced water cooling and ultrasonic treat. for 120 s: position 1, b) position 2, c) position
3, d) position 4, e) position 5, f) position 6, g) position 7, h) position 8, i) position 9
Figure 7: Microstructure of the Al-5Cu alloy ingot after forced water cooling and ultrasonic treat. for 90 s: position 1, b) position 2, c) position 3,
d) position 4, e) position 5, f) position 6, g) position 7, h) position 8, i) position 9

bottom of the clay-graphite crucible, the fine equiaxed
crystals produced by ultrasonication fall downward and
solidify into refined equiaxed grains due to the acoustic
flow. The pores below are gradually discharged, so a
dense structure starts to form from the bottommost part
of the melt. The best results are obtained when this stage
is achieved in 120 s. After further extending the ultra-
sonic treatment time to 150 s, many pores appear due to
a decrease in the aluminum liquid temperature, causing
the melt viscosity to be too large to hinder the upward
floating of the gas.
Figure 12 shows the ultrasonic treatment and forced
water cooling mechanism at the bottom of the
clay-graphite crucible for this experimental process.
When a copper plate is placed at the bottom of the
clay-graphite crucible for water cooling, the fine isomet-
ric crystals formed by ultrasonic vibration keep falling
and solidify quickly. The metal liquid above fills in the
gaps between them, thus forming a dense structure. In
contrast, in the case of non-forced cooling at the bottom
of the clay-graphite crucible, the melt solidifies every-
where although it also has a degassing effect under the
ultrasonic action. The cooling rates are comparable in all
directions, making most of the melt’s gas obstructed by
the dendrites formed by the melt before the solidifica-
tion; it remains in the melt to form pores before it can es-
cape completely.
Z. JIA et al.: EFFECT OF HIGH-INTENSITY ULTRASONIC TREATMENT ON REFINEMENT OF Al-5Cu ALLOY INGOTS
190 Materiali in tehnologije / Materials and technology 58 (2024) 2, 185–192
Figure 9: Microstructure of the Al-5Cu alloy ingot under forced water cooling and ultrasonic treat. for 150 s: position 1, b) position 2, c) position
3, d) position 4, e) position 5, f) position 6, g) position 7, h) position 8, i) position 9
Figure 10: Grain-size distribution of Al-5Cu alloy ingots after ultra-
sonic treatment Figure 11: Schematic of grain refinement

Recently, it has been reported that the porosities in
the ingots are due to the formation of bifilms in the liq-
uid metal. Campbell
21
indicates that there is a layer of
compact oxide films on the melt surface. When a metal
melt is poured stably, these oxide films have limited ef-
fects on the quality of ingots but they prevent the oxidi-
zation of the melt. However, there is turbulence when the
melt is poured. It makes the oxide films on the melt sur-
face fold collapse and finally enter into the interior of the
metal melt. During this process, air is entrained in the
melt, forming bubbles, which is a phenomenon of bubble
entrainment.
Firstly, bifilms are very thin, usually a few nano-
meters, thus, oxide films are broken by the ultrasonic
cavitation during the compression and expansion stages
of vibration. Hydrogen atoms in the melt enter biofilms;
being wrapped and continually diffused or separated in
bifilms, they cause an increase in the oxide film volume.
Otherwise, these bifilms continually expand and absorb
hydrogen from the melt by streaming, and when the
pressure in these bifilms reaches a certain level, the
bifilms burst and escape from the melt surface. Secondly,
these oxide films contain less gas; they continually affect
each other and bond with each other due to long-term
stirring of acoustic streaming, which also can remove the
gas from the melt. Therefore, the bond of oxide films
substantially reduces the hydrogen content and, finally,
the possibility of porosity formation. It is reported that
the partial/local temperature and pressure caused by the
cavitation effect are nearly 104 K and 103 MPa, respec-
tively. Therefore, the fragments of the oxide film are the
result of ultrasonic cavitation.
4 CONCLUSIONS
The method of combining ultrasonic treatment and
forced cooling at the bottom of ingots cannot influence
the ultrasonic ingot refinement. Conversely, with in-
creases in the ultrasonic duration, the equiaxed grain oc-
cupancy in ingots improves rapidly, even reaching 100
%. In this experiment, we obtained fine and uniform
equiaxed crystals in the ingot sections treated with 2000
W for 90–150 s. The USD degassing effect is due to a re-
lease of hydrogen in the bifilms, which expand and rise
gradually. Finally, they burst on the melt surface, result-
ing in the effect of degassing.
Acknowledgements
The authors acknowledge the financial support from
the Key Project of the Education Department of Liaoning
Province [LJKZ1171], China Postdoctoral Fund
[2020M680978], Liaoning Province Doctoral Start-Up
Fund Project [2020-BS-260], Natural Science Founda-
tion of Fujian Province [2021J05232] and Science and
Technology Planning Project of Longyan
[2021LYF9012].
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