TREATMENT OF BINARY ALUMINUM MELTS BY LOW-FREQUENCY ELASTIC
OSCILLATIONS IN THE CAVITATION MODE
Pastukhov E.A., Popova E.A., Bodrova L.E.
Institute of Metallurgy UD RAS, Ekaterinburg
The traditional ways for enhancement of metal quality are severely limited.
Technological methods involving changes in the melt state before or during cast by different
external effects hold much promise. Related to them the method of active applying of lowfrequency elastic oscillations in cavitation mode on melts was developed by us is very simple
and powerful.
Laboratory equipment for treatment of melts by elastic low-frequency oscillations in
cavitation mode is an acoustic resonator of a “mass-spring” type. With the help electromagnetic
vibration generator induced oscillations are fed by the plunger oscillator into melt. When
frequencies of both induced and natural low-frequency oscillations coincide, the system comes
into resonance and oscillation amplitude increases sharply. Under influence of intensive acoustic
wave a cavitation is formed and developed in liquid metal. Acoustic flows, always of turbulent
type, move to the bottom cavity bubbles and rough caverns (Fig.1). Changing the current power
we can move theirs in the metal volume and agitate the melt. Cavitation treatment of melts can
be carried out in wide temperature range: in two-phase liquid-solid region and upon various
overheating above liquidus.
A real melt contains a great number of nonsoluble impurities and suspended particles
with complicated microrelief, with a lot of cracks and hollows where a gas-vapor phase remains.
When the vapor-gas phase is removed from crack, its orifice is filled by liquid phase. In this
case, the magnitude of liquid metal adhesion to activated nonsoluble impurity is changed and the
nucleation takes place under less overcooling. Another factor affecting the crystallization
conditions after cavitation treatment of melt is dispersion of microclusters when treatment is
made above liquidus temperature, or breaking of primary segregated crystals when it is made in
a two-phase region. It also must be note, that cavitation treatment result in equalization of
temperature and composition gradients and origin of viscosity forces. These factors increase of
metal homogeneity and uniform distribution of the elements.
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Changes in the melt state lead to changes of crystallization conditions, causing in the cast
metal structure the following: a decrease of macrograin size and primary segregations size, an
increase of cast metal homogeneity, an uniform distribution of impurities, refining of inside grain
structure, a decrease or elimination of columnar crystal zone, and degassing of metal. All these
structural changes improve the cast metal properties.
Investigations of structural formation features for binary aluminum alloys (with Si, Cu,
Mg, Pb, Ti and Mn) after 5 minute long cavitation treatment of their melts at various
temperatures were carried out. Structural investigations were made by use of chemical,
metallographic, microspectrum X-ray and differential thermal analyses, and measurements of
phase component microhardness.
Al-Si alloys are the basis of silumins widely used in practice. One of the remarkable
features of their melts is an existence of microheterogeneities up to the high overheating
temperatures. Cavitation treatment of the melts lead to an increase in amount and a decrease in
size of the microheterogeneities playing the nucleus role. The melts of hypoeuthectic (2% Si),
euthectic (11%) and hypereuthectic (25%) compositions were cavity treated in a solid-liquid
state and upon 200K overheating above liquidus. The treatment gives a grinding effect on all
structural components of the cast metal. Depending on the alloy composition the primary phases
are grinded: α-solid solution dendrites and silicon crystals, euthectic is modified (Fig.2,3).
Composition equalization along the ingot cross-section promotes an improvement of their
properties. According to the DTA data, due to the cavitation treatment of the Al-Si melts the
magnitudes of phase transition hystereses, characterizing the difference between actual
temperatures of solid-liquid and liquid-solid phase transitions and that terminated in a state
diagram, are decreased to 1.5-5.5 K, i.e. crystal nucleation takes place upon less overcooling.
The Al-Cu melts are characterized by less extent of microheterogeneity. The melts of
hypoeuthectic (3-26% Cu) and euthectic (33%) composition were cavity treated. As a result of
melt treatment phase crystallization mode is changed from directional to chaotic one (Fig.4); and
euthectic segregation has more perfect differentiated nature (Fig.5). Treatment of the euthectic
alloy causes classical cellular structure to be grinding and improved (Fig.6). The influence of the
overheating on structural refining and uniform distribution of structural components over metal
is intensified, copper solubility in aluminum is increased, gas porouness is decreased, and the
macrograin size as well as liquation heterogeneity are reduced.
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Casting Al-Mg alloys have in their structure a brittle and difficult-to-dissolve β-phase, the
gross segregations of which form a continuous net and cause crack formation into cast metal.
Being segregated this phase, giving brittleness to the alloy, becomes almost harmless. Taking
into account that the secondary β-phase (Mg5Al8) crystallizes mainly in the interaxial space of
aluminum dendrites, for their grinding are used the modifying agents. Just in the effort to modify
the Al-16%Mg melt, it was cavity treated at 110K overheating above liquidus. The treatment
leads to a dispersion of gross β-phase segregations, a reduction of their size up to 5 times and a
disappearance of continuous segregation net (Fig.7). Besides that, it gives rise to a redistribution
processes of Mg between β- and α-phases usually take place during long (15 hours)
homogenizing annealing.
The Al-Pb system has a liquid immiscibility region in a wide temperature and
composition interval. Low-frequency elastic oscillation treatment of the Al-10%Pb melt in
cavitation mode were carried out above and below immiscibility cupola at 1340 and 1100K,
respectively. In both cases, the cavity treatment lead to an agitation liquid metals and dispersion
of spheroidal lead impurities in aluminum matrix (Fig.8). However the duration of stay in the
liquid state after treatment for the melt should not exceed 1 min. It defines coagulation and
precipitation of hard lead particles in the aluminum melt. Depending on residence of the melt in
liquid state, it is separated into two zones of different heights. In the upper zone a dredge of
small (up to 1μ) lead impurities remains and in the lower zone coagulated shperoidal impurities
up to 1mm are contained. After 15-minute exposure the ingot contains only small lead dredge
and the main part of the lead settle out at the bottom in a dense bed.
Changes in modifying effect of small (0.3 and 0.5%) titanium additions on aluminum
melt after its cavity treatment overheated up to 200K above liquidus were investigated. The
supersaturated by titanium aluminum solid solution is main structural unit of Al-Ti alloys under
non-equilibrium crystallization conditions. Untreated metal is characterized by heterogeneous
distribution of titanium and impurities within solid aluminum solution. Titanium enriched
dendrite formations are intensively etched and looked dark, whenever dendrite cells of α-phase,
almost free of titanium, are looked light. Cavitation treatment of melts results in a dispersion of
microstructural components of the alloy and a significant equalization of composition in titanium
and impurities, refining of internal structure of dendrite cells and a decrease of their size up to 4
times (Fig.9), that is an increasing of modifying effects of titanium. All of it has to improve
service properties of the alloy.
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Differential thermal analysis (DTA) of casting alloy samples with 0.3 and 0.5% Ti,
showed that dispersion and equalization of alloy composition caused by cavitation treatment was
preserved after melting and following crystallization of the samples during DTA. Equalization of
composition in titanium results in a decrease of differential signals, e.g. a decrease in thermal
effects of melting and crystallization processes (Fig.10).
Recently, great attention is paid to producing ligatures with transition metals, possessing
improved modifying and alloying properties. In modern ligature quality evaluation methods
homogeneity and chemical composition constancy, number of modifying particles (aluminides),
their dimensions (average and the highest) and uniformity of volume distribution are taken into
account. In order to enhance the quality of the Al-3,5% Ti and Al-10%Mn ligature, their melts
were cavity treated in different temperature conditions.
After cavitation treatment of the Al-3,5% Ti melt upon 50-70K overheating above
liquidus the gas porouness and number of irregular shaped gross plates of Al3Ti aluminide were
decreased and the average size of the plates became 10 times smaller (Fig.11a, b). More
effectively was the two-stage cavitation treatment of melt: above and below liquidus. The
ligature treated in such way is characterized with fine-grain structure, when essential amount of
titanium aluminides measure from 2 to 20 μ, with the highest size not greater than 30μ and with
uniform distribution, and dense ingot (Fig.11c).
The similar results were obtained for Al-10% Mn ligature. Cavitation treatment of the
melt upon 150K overheating above liquidus leads to grinding of MnAl4 needles up to one order
of magnitude on the average, and 4 times reduction of their highest length. The two-stage
treatment of this melt increases a dispersion of aluminides upon crystallization ten and hundred
times: thin growing needles are broke to equiaxed pieces about 20μ in diameter (Fig.12).
Thus cavitation treatment of aluminum-based melts promotes the melt state, characterized
by more homogeneous distribution of atomic groups through the microvolumes. As a
consequence, dispersity and homogeneity of the structure are increased, dendritic segregation is
decreased, and crystallization behaviour of metal phase components is changed. The aluminum
based alloys of the same structure possess advanced service properties and application of the
ligatures treated in such way allows one to intensify modifying and alloying effects, to cut down
time and heat expenses for aluminum solution and to enhance the quality of cast metal.
This work has been supported by the RFFI (Grant № 01-03-32708).
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Stages of cavity formation and development in liquid (KCl solution) when cylindric oscillator is
used
Fig. 1.
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Refining and uniform distribution of primary segregated dendrites of α-phase due to cavitation
treatment of the Al-11%Si melt upon 200K overheating above liquidus
a
b
115 мкм
115
мкм
Without treatment
After treatment
c
d
115

15
мкм
15
мкм
Without treatment
After treatment
a, b – by reflected light; с, d – by characteristic Kα Si radiation
Fig. 2
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Refinement of primary segregated silicon crystals
after cavitation treatment of the Al-25%Si melt
a
b
90 мкм
90 мкм
Without treatment
After treatment
Without treatment
After treatment
c
d
90 мкм
90 мкм
a, b – treatment in two-phase region, 70K below liquidus;
c, d – treatment upon 200K overheating above liquidus
Fig. 3
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Influence of cavitation treatment of the Al-19%Cu melt upon 200K
overheating above liquidus on change of crystallization mode
Without treatment
After treatment
b
a
50 мкм
50 мкм
a – phase crystallization mode directed to heat removal;
b – chaotic, uniform
Fig. 4
Change of euthectic segregation mode due to cavitation treatment of the Al-26%Cu melt upon
40K overheating above liquidus
Without treatment
After treatment
25
мкм
25
мкм
Imaging by secondary electrons. Cooling rate 0,01 m/s
Fig. 5
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Refinement and perfection of porous substructure after cavitation treatment
of the Al-34%Cu melt upon 40K overheating above liquidus
50 мкм
50 мкм
12,5
мкм
12,5
мкм
Without treatment
After treatment
Imaging by secondary electrons
Fig. 6
Destruction of β-phase (Mg5Al8) segregations due to cavitation
treatment of the Al-16%Mg melt upon 110K overheating above liquidus
Without treatment
After treatment
40
12,5
мкм
мкм
Fig. 7
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12,5
мкм
Coagulation of spheroidal lead impurities in alumina matrix after cavitation treatment
of the Al-10%Pb melt in homogeneous region at 1340K
1000 мкм
The general view of ingot throughout the height
Fig. 8
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Dispersion of microstructural components, equalization of composition
by titanium and distribution of the elements throughout A-A section
after cavitation treatment of the Al-0,5% Ti melt at 1200 K
А
А А
0
200
мкм
0
Imaging by secondary electrons and characteristic Kα Si radiation
Fig. 9
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А
200
мкм
DTA curves for the Al-0,3%Ti alloys
a
Differential-mode signal, μV
b
b
a
1 – when heating at the rate of 2 K/min in Ar atmosphere;
2 – under the same conditions during cooling
a – without treatment; b – after cavitation treatment of melt at 1200K
Sample weight – 69,7 mg
Fig. 10
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Refinement of Al3Ti intermetallic compound due to cavitation treatment
of the Al-3,5%Ti melt
a
80
мкм
Without treatment
b
c
80
мкм
After treatment
After treatment
80
мкм
a, b – upon 50K overheating above liquidus
c – in over-liquidus and two-phase regions (two-stage treatment)
Fig. 11
Dispersion of manganese aluminides after cavitation (two-stage)
treatment of the Al-10%Mn melt
Without treatment
After treatment
80
мкм
80
мкм
Imaging by reflected light, unetched samples
Fig. 12
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