Control of Non-Equilibrium Crystallization of Aluminum
V. Selivorstov1, Y. Dotsenko1, K. Borodianskiy2, M. Zinigrad2
1
Electrometallurgical Faculty, the National Metallurgical Academy of Ukraine,
Dnepropetrovsk, Ukraine
2
Advanced Materials Laboratory, Materials Research Center, Ariel University Center
of Samaria, Ariel, Israel
ABSTRACT
Lots of techniques are widely used in order to prevent and suppress defects in
ingot and to improve mechanical properties of metal alloys. In this article we showed
a theory of metal cast crystallization processes. Experimental work showed a
combined modification method by TiCN ultrafine particles followed by uniform gas
pressure casting process which caused to improve aluminum alloys mechanical
properties. It is found that tensile strength of Al-Si-Cu alloys increased by 5.5% when
the elongation increased by 25-35%.
INTRODUCTION
According to modern ingot crystallization concepts, higher quality of metal
production can be achieved when the liquid phase transforms to the solid phase. In
this context, use of various types of exposure at the crystallization stage to control
mass and heat exchange processes seems quite promising. In the meantime, a largescale industrial use of various exposures on ingots and blanks is largely restricted due
to a lack of complex recommendations and practicable theoretical models of mass and
heat exchange processes in liquid and solid-liquid phase during crystallization under a
forced exposure.
Lots of techniques are widely used in order to prevent and suppress defects in
ingot, blank, and cast production, which might be conveniently classified as static and
dynamic in terms of the type of exposure [1]. In this context, static techniques are
basically intended to optimize crystallization conditions by forming a geometric
shape, which would minimize the occurrence of defects in the finished product.
Dynamic techniques are based on a forced physical effect on the liquid phase while
crystallizing. In addition to a major effect on mass and heat exchange in the liquid
phase, these techniques also have a significant impact on the nature of processes in the
two-phase area. In specific cases, ingot and blank treatment techniques certainly
include elements of both static and dynamic exposures.
Techniques of static exposure include the following: riser heat mode management;
changing of ingot or section geometry; metal modification and microalloying with
special-purpose additives; spiking of various macrocoolers in the melt, etc. [2, 3].
Modification is eventually intended to improve mechanic, technological, and
performance properties of casts, ingots, and their derived semi-finished products by
fine crushing of a cast structure.
Dispersion of the cast structure is described by the first order distance between
axes or by size of a so called cast grain. The latter is a visible area on the polished
sections, which differs from the adjacent areas by its hue and clear boundaries. Cast
grains are formed under distinct thermal conditions, whose diversity attributes to a
different direction and, possibly, value of temperature gradient, and accordingly,
another solid phase growth direction; excessive volumes of segregated materials and
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crystalline lattice defects are accumulating along the interface of such areas, which
explains their high etching ability and accordingly, their visual identification.
Since the cast grain size depends on crystal nucleation (n) and growth (v) rates
relation [2], its modification is actually aimed at changing these parameters, as
required. The less is the first order distance between axes, the lower is the crystal
growth rate, and the higher is nucleation rate of crystallization centers. According to
crystallization theory, the rates of spontaneous nucleation and growth of crystals not
only depend on supercooling degree, but also on the surface tension along melt-crystal
interface, and on atom activation energy in the melt (U):
n = K1·exp[-E1/(R·T)]·ехр[-В·σ3/(T·ΔT2)],
v = К2·ехр[-E1/(R·T)] ехр [-E·σ2/(Т·ΔТ)],
(1)
(2)
where К1 - is proportionality factor approximately equaling the number of atoms in
the relevant volume of melt (К1 ~1023 per 1 mol);
К2 - is proportionality factor approximately equaling the number of atoms on the
surface of the relevant volume (К2 ~1016 per 1 mol);
E - is atom activation energy in the melt;
E1 - is activation energy that defines atom exchange rate between bi-dimensional
nucleus and the melt (E1= 0.25·E);
σ - is surface tension along melt-crystal interface;
σ1 - is melt surface tension at the periphery of bi-dimensional nucleus;
В - is matter constant = (2/k)·[4·M·T0/(ρ·q)]2;
М and ρ - is a molecular mass and density of crystal matter;
q - is one mol of matter melting heat;
k - is Boltzmann constant;
Е - is matter constant (E·σ2 ~ 10-3·В·σ3);
R - is gas constant;
Т - is temperature;
ΔТ- is supercooling degree.
As follows from these equations, increasing crystal nucleation and growth rates
can be only achieved by decreasing the activation energy and the surface tension.
The importance of surface tension along melt-crystal interface is more apparent
from the following equations for total work of nucleation (Аn) and nucleating seed
critical radius (rcr):
Аn = В·σ3/(Т·ΔT2),
(3)
rcr = 2·σ·Т/(L·ΔT·Т),
(4)
Equation for solid phase nucleus critical radius has been derived from the
following:
A new phase formation is always accompanied by emergence of a new liquidsolid surface. Therefore, in order to enable formation of a nucleus, energy reduction
of the mass of the matter used to form this nucleus shall be higher than the energy
consumed for interface surface formation. A new phase (cluster) can therefore only be
formed after it gains a certain critical radius. As long as the nucleus has not reached
its critical size, its growth will be accompanied by increase in energy. This is only
possible due to fluctuations.
Therefore, by denoting molar energy of liquid and solid phases as GL and GS, and
the newly-formed phase surface as S, we write down new phase formation conditions
as follows:
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ΔG = V·ρ/Mr·(GS - GL) + S·σL-S,
where V - is the volume of one mol of matter, m3/mol;
ρ - is matter density, kg/m3;
M - is molar mass, g/mol;
σL-S - is surface energy, J/m2.
Assuming that a nucleus is spherical:
ΔG = 4/3·π·r3· ρ/Mr·(GS - GL) + 4·π·r2·σL-S,
(5)
Cooling down of metal to a temperature below Tm results in below zero
subtraction (GS - GL). Therefore ΔG reaches its maximum in a liquid that has been
overcooled to a specific temperature at a certain critical value of r = rcr. Any further
increase in r results in lower ΔG.
Critical radius of nucleus can be determined provided that maximum ∂ΔG/∂r = 0.
Therefore, as follows from equation (5):
rcr = 2∙σL-S∙MrFe/[(GS - GL)∙ρFe]
(GS - GL) can be expressed through latent melting heat and Tm using a well-known
thermodynamic relationship:
ΔG = ΔH - T·ΔS = -L - T·ΔS,
At T = Tm, ΔG = 0, and therefore ΔS = -L/Tm.
At a relatively low degree of supercooling:
ΔGТmelt- ΔGТ = (ΔНТmelt - Тmelt·ΔSТmelt) - (ΔНТ - T·ΔSТ) = -ΔT·ΔS = ΔT·L/Тmelt
And therefore:
rc = 2∙σL-S∙MrFe∙Tm/(ρFe∙L∙ΔT),
Where rc - is cluster radius, m;
rFe - is iron atom radius, Å;
MrFe - is iron molecular mass, g-atom/mol;
ΡFe - is iron density, g/cm3; σL-S - is surface tension, J/cm2;
L - is melting heat, J/mol;
Tt - is melting temperature, K;
ΔT - is supercooling degree, K.
As follows from these equations, the lower is surface tension, the less is
nucleation work and critical size of a stable nucleus. Lower surface tension along the
melt-crystal interface facilitates nucleation of crystallization centers by increasing
center nucleation rate proportionate to:
у = ехр [-В·σ3/(Т·ΔT2)],
(6)
In view of the above, a higher degree of supercooling, which also promotes the
nucleation of new crystallization centers, works in the same direction. Based on
comparison between (1) and (2), it can be concluded that this is nucleation which is a
limiting factor rather than growth. It is attributed to supercooling degree = 2 in the
equation of the nucleation rate (1) (as opposed to growth rate equation, where
supercooling degree = 1). Therefore, a far higher supercooling degree is required for
nucleation of crystallization centers than for the growth. With this in mind,
modification studies are normally focused on increasing crystallization centers
nucleation rate by using modifying additives.
Methods of applying pressure to crystallization casting metal can be divided into
three main groups [4]. The first group includes uniform gas pressure casting (UGRC):
autoclave casting and the use of gasostatic extruders. The second group includes
casting techniques wherein gas or piston (plunger) pressure is transferred by melting
pot or chamber-contained melt compression to liquid metal inside the cast, and to
solidification front afterwards: low-pressure casting, counterpressure casting, vacuum
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pressure casting, etc. The third group includes piston pressure casting, with the
pressure transferred to liquid metal inside the cast by crushing crystallized outer
surface with the surface of pressure piston (plunger).
Uniform gas pressure casting is distinguished by changing thermal properties of
the gas [5] and casting mold material [6], which directly affect the intensity of castmold heat exchange, both at close contact and in the clearance [7]. Authors [4, 8, 9]
note that pressure is the major parameter that defines crystallizing and cooling
duration in uniform gas pressure casting: the higher is the pressure, the lower is
crystallization duration and the higher is its rate.
As opposed to the majority of known casting techniques, wherein metered
amounts of liquid metal are poured directly into working cavity of the casting mold or
into interim pouring device (like a pressurization chamber of gas casting machine), in
low pressure casting (LPC), a liquid metal contained in the melting pot, metal duct
and working cavity of the casting mold comprises a closed circuit over the entire
casting process. Low pressure casting has a significant influence on the structure,
physico-mechanical and performance properties of casts. So, according to [10], low
pressure modifies primary silicon when applied to Al-20Si-3Cu hypereutectic alloy
casts, so that primary silicon crystallites become ~ 1.5 times smaller under 0.05 MPa
and two times smaller under 0.075 MPa. The density of 10-50 mm thick cast metal
will be increased by 1.3%, with hardness increase by 8%, and tensile strength increase
over 20%.
Practice of casting production attests to a strong potential of modifiers used in
combination with physical exposures to enable production or super-fine and specialpurpose structural components.
RESULTS AND DISCUSSION
Experimental studies of the impact of combined gas-dynamic effect - TiCN
modification technology were conducted in aluminum alloy cast sections whose
chemical composition is shown in Table 1:
Table 1: Chemical composition of used aluminum alloys.
No. of alloy
Al
1
2
Base
Si
Fe
Mn
Ti
Mg
Cu
Zn
6.51
0.55
0.45
0.15
0.55
-
-
5.5
0.6
-
0.14
0.6
1.45
0.3
1.2 kg alloy No.1 cylindrical casts were poured into a heated painted cast iron
mold with 100 mm minimal wall thickness. Pouring temperature was 7200С. Casting
diagram is shown in Fig. 1.
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(a)
(b)
Fig. 1: Cylindrical casts produced using conventional technology (a), and produced
using combined effect (b).
1.1 kg alloy No.2 "Conveyor Rack Bearing Cap" cast was poured into a heated
painted cast iron mold with 40 mm minimal wall thickness. Pouring temperature was
6400С. "Conveyor Rack Bearing Cap" casting diagram is shown in Fig. 2.
Fig. 2: "Conveyor Rack Bearing Cap" Casting Diagram.
Technologically, gas-dynamic effect on the melt in the chill mold was
implemented at initial pressure 0.15 - 0.2 MPa with further increasing up to 2 - 3.5
MPa, in accordance with design dynamics of pressure growth in the cast - gas input
machine system. Fig. 3 shows the general view of "Conveyor Rack Bearing Cap" cast,
as produced using conventional technology (to the right) and using combined effect at
the crystallization process.
Fig. 3: General view of "Conveyor Rack Bearing Cap" cast, as produced using
conventional technology (to the right) and using combined effect at the crystallization
process.
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Fig. 4 and Fig. 5 show the microstructure of No.1 and No.2 alloys before and after
treatment.
(a)
(b)
Fig. 4: Microstructure of alloy No.1 before treatment (a), and after combined
treatment (b).
(a)
(a)
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(b)
Fig. 4: Microstructure of alloy No.2 before treatment (a), and after combined
treatment (b).
Tables 2 and 3 show experimental results of determining mechanical properties of
No.1 and No.2 alloys cast metal, as produced by means of gas-dynamic effect (GDE),
TiCN modification (М), and combined gas-dynamic effect and TiCN modification
(GDE+М) versus similar properties of cast metal, which is produced by conventional
metal mold casting technology.
Table 2: Mechanical properties of No.1 alloy.
No. of
alloy
before
treatment
after
treatment
σUTS (MPa)
HB (MPa)
δ (%)
503.3
1.93
164.6
GDE
М
GDE+М GDE
М
GDE+М GDE М GDE+М
184.5 189.8
194.4
506.6 507.6
510.3
2.26 2.28
2.3
Table 3: Mechanical properties of No.2 alloy.
σUTS (MPa)
No. of
alloy
before
treatment
after
treatment
162.0
GDE
М
181.2
185.1
GDE+
М
191.2
HB
δ (%)
68.7
0.93
GDE
М
GDE+М
GDE
М
GDE+М
71.7
72.7
73.7
1.25
1.27
1.29
Practical implementation of the proposed technology results in 28% reduction of
microporosity and blow holes in the finished casts. Appropriate instructions have been
issued and applied.
CONCLUSIONS
1. Technology of combined effect on aluminum alloy cast structure formation
has been theoretically substantiated and applied. This involves the impact on
structure formation process by ultrafine TiCN modification combined with
gas-dynamic effect.
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2. Mechanical properties of aluminum alloys produced by combined technology
(i.e., TiCN modification and gas-dynamic effect) versus cast metal produced
using conventional metal mold casting technology have been determined.
Tensile stress increased by 5-10%, with increasing of hardness (НВ) and
elongation by 3-7% and 25-35% respectively. The incidence of microporosity
and blow holes in "Conveyor Rack Bearing Cap" casts was reduced by 28%.
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