**1. Introduction**

538 Advances in Crystallization Processes

Szczepanik, Ryszard, 1963. Two- and multicomponent, solid-liquid systems formed by

Wakayama, N., Inokuchi, H., 1967. Heats of sublimation of polycyclic aromatic

hydrocarbons and their molecular packings. 40, 2267-2271.

Ser. A 7, 621-660.

aromatic hydrocarbons, anthraquinone, and coal-tar fractions. Chem. Stosowana

The phenomenon of crystallization following after pouring molten metal into the mould, determines the shape of the primary casting (ingot) structure, which significantly affects on its usable properties.

The crystallization of metal in the mould may result in three major structural zones (Fig.1) (Barrett, 1952; Chalmers, 1963; Fraś, 2003; Ohno, 1976):


Depending on the chemical composition, the intensity of convection of solidifying metal, the cooling rate i.e. geometry of casting, mould material and pouring temperature (Fig.2), in the casting may be three, two or only one structural zone.

Due to the small width of chilled crystals zone, the usable properties of casting depend mainly on the width and length of the columnar crystals, the size of equiaxed crystals and content of theirs zone on section of ingot, as well as on interdendritic or interphase distance in grains such as eutectic or monotectic. For example, you can refer here to a well-known the Hall-Peth law describing the influence of grain size on yield strength (Fig.3) (Adamczyk, 2004):

$$
\sigma\_y = \sigma\_0 + k \bullet d^{\frac{1}{\sqrt{2}}} \tag{1}
$$

where: σy – yield strength, MPa,

Fig. 3. The influence of grain size on yield strength of Fe, Zn and α brass (Adamczyk, 2004)

*kd k*

The primary structure of pure metals independently from the crystal lattice type creates practically only columnar crystals (Fig.4) (Fraś, 2003). According to presented data, this type of structure gives low mechanical properties of castings and mainly is unfavourable for the plastic forming of continuous and semi-continuous ingots, because causing forces extrusion rate reduction and during the ingot rolling delamination of external layers can occur

This structure can be eliminated by controlling the heat removal rate from the casting, realizing inoculation, which consists in the introduction of additives to liquid metal and/or influence of external factors for example infra- and ultrasonic vibrations or electromagnetic

*y* 01 2

 

σo – approximately yield strength of monocrystal, MPa

k1 and k2 – material constants, MNm-3/2,

– interphase distance in eutectic, mm.

(Szajnar & Wróbel, 2008a, 2008b).

where:

field.

σy – yield strength, MPa,

d – grain size, mm,


(2)

σo – approximate yield strength of monocrystal, for Al amount to 11,1 MPa, k – material constant characterizing the resistance of grain boundaries for the movement of dislocations in the initial stage of plastic deformation (strength of grain boundaries), for Al amount to 0,05 MNm-3/2,

Fig. 1. The primary structure of ingot: a – scheme, b – real macrostructure; 1 – chilled crystals zone, 2 – columnar crystals zone, 3 – equiaxed crystals zone

Fig. 2. The influence of pouring temperature on primary structure of ingot (Fraś, 2003)

Because the Hall-Peth law concerns only metals and alloys with the structure of solid solutions, therefore the solidification of alloys with eutectic transformation for example from Al-Si group in describing the influence of refinement of structure on the value of yield strength should take into account the value of interphase distance in eutectic (Paul et al., 1982; Tensi & Hörgerl, 1994; Treitler, 2005):

$$
\sigma\_y = \sigma\_0 + k\_1 \bullet d^1 \stackrel{\text{IV}}{\searrow} + k\_2 \bullet \stackrel{\text{IV}}{\searrow} \tag{2}
$$

where:

540 Advances in Crystallization Processes

k – material constant characterizing the resistance of grain boundaries for the movement of dislocations in the initial stage of plastic deformation (strength of grain boundaries), for Al

a) b)

Fig. 2. The influence of pouring temperature on primary structure of ingot (Fraś, 2003)

1982; Tensi & Hörgerl, 1994; Treitler, 2005):

Because the Hall-Peth law concerns only metals and alloys with the structure of solid solutions, therefore the solidification of alloys with eutectic transformation for example from Al-Si group in describing the influence of refinement of structure on the value of yield strength should take into account the value of interphase distance in eutectic (Paul et al.,

Fig. 1. The primary structure of ingot: a – scheme, b – real macrostructure; 1 – chilled crystals zone, 2 – columnar crystals zone, 3 – equiaxed crystals zone

σo – approximate yield strength of monocrystal, for Al amount to 11,1 MPa,

amount to 0,05 MNm-3/2, d – grain size, mm.

> σy – yield strength, MPa, σo – approximately yield strength of monocrystal, MPa k1 and k2 – material constants, MNm-3/2, d – grain size, mm, – interphase distance in eutectic, mm.

The primary structure of pure metals independently from the crystal lattice type creates practically only columnar crystals (Fig.4) (Fraś, 2003). According to presented data, this type of structure gives low mechanical properties of castings and mainly is unfavourable for the plastic forming of continuous and semi-continuous ingots, because causing forces extrusion rate reduction and during the ingot rolling delamination of external layers can occur (Szajnar & Wróbel, 2008a, 2008b).

This structure can be eliminated by controlling the heat removal rate from the casting, realizing inoculation, which consists in the introduction of additives to liquid metal and/or influence of external factors for example infra- and ultrasonic vibrations or electromagnetic field.

1999b; Fjellstedt et al., 2001; Fraś, 2003; Guzowski et al., 1987; Hu & H. Li, 1998; Jura, 1968; Kashyap & Chandrashekar, 2001; H. Li et al., 1997; P. Li et al., 2005; McCartney, 1988; Murty et al., 2002; Naglič et al., 2008; Pietrowski, 2001; Sritharan & H. Li, 1996; Szajnar & Wróbel,

Type of crystal

Parameters of crystal lattice [nm]

c = 0,321

c = 0,325

c =0,857

lattice

2008a, 2008b; Whitehead, 2000; Wróbel, 2010; Zamkotowicz et al., 2003).

Al 660 Cubical A1 a = 0,404 TiC 3200 Cubical B1 a = 0,431 TiN 3255 Cubical B1 a = 0,424 TiB 3000 Cubical B1 a = 0,421 TiB2 2900 Hexagonal C32 a = 0,302

AlB2 2700 Hexagonal C32 a = 0,300

Al3Ti 1400 Tetragonal D022 a = 0,383

Table 1. Characteristic of bases to heterogeneous nucleation of aluminum (Donnay & Ondik,

Moreover the effectiveness of inoculants influence can be assessed on the basis of the hypothesis presented in the paper (Jura, 1968). This hypothesis was developed at the assumption that the fundamental physical factors affecting on the crystallization process are the amount of give up heat in the crystallization process on the interphase boundary of liquid - solid and the rate of give up heat of crystallization. After analyzing the results of own researches, the author proposed to determine the index (α), which characterizes the

> *k s k p*

– characteristic frequency of atomic vibration calculated by the Lindemman formula, 1/s,

At α > 1 – additives which inhibit crystals growth by the deformation of the crystallization

At α < 1 – additives which accelerate crystals growth, favoring consolidation of the primary

In case of inoculation of Al the index α = 2.35 for inoculant in form of Ti and 1.76 for

*W*

(4)

*E*

*E* 

 

Ek – heat of crystallization of 1 mol of inoculant or inoculated metal, J/mol,

On the basis of equation (4) additives can be divided into three groups:

W – parameter dependent on the atomic mass of inoculant and inoculated metal.

s – symbol of inoculant, p – symbol of inoculated metal.

At α = 1 – additives do not affect on structure refinement.

front, thus are effective inoculants.

inoculant in form of B.

structure of the metal, thus are deinoculants.

(circa) [C]

Phase Melting point

1973)

type of inoculant.

where:

Fig. 4. Macrostructure of ingot of Al with a purity of 99,7%: a – transverse section, b – longitudinal section

#### **2. Endogenous inoculation of pure aluminum structure**

In aim to obtain an equiaxed and fine-grained structure, which gives high mechanical properties of castings, can use inoculation, which occurs in introducing into metal bath of specified substances, called inoculants (Fraś, 2003). Inoculants increase grains density as result of creation of new particles in consequence of braking of grains growth velocity, decrease of surface tension on interphase boundary of liquid – nucleus, decrease of angle of contact between the nucleus and the base and increase of density of bases to heterogeneous nucleation (Fraś, 2003; Jura, 1968). The effectiveness of this type of inoculation depends significantly on crystallographic match between the base and the nucleus of inoculated metal. This crystallographic match is described by type of crystal lattice or additionally by index (Fraś, 2003):

$$\mathcal{L} = \left( (\mathbf{1} \cdot \frac{\boldsymbol{\chi}\_b \cdot \boldsymbol{\chi}\_n}{\boldsymbol{\chi}\_n}) \right) \bullet 100\% \tag{3}$$

where:


xb, xn – parameter of crystal lattice in specified direction, suitable for base and nucleus.

When the value of index () is closer to 100%, it the more effective is the base to heterogeneous nucleation of inoculated metal.

Therefore active bases to heterogeneous nucleation for aluminum are particles which have high melting point i.e. TiC, TiN, TiB, TiB2, AlB2 i Al3Ti (Tab.1) (Easton & StJohn, 1999a, 1999b; Fjellstedt et al., 2001; Fraś, 2003; Guzowski et al., 1987; Hu & H. Li, 1998; Jura, 1968; Kashyap & Chandrashekar, 2001; H. Li et al., 1997; P. Li et al., 2005; McCartney, 1988; Murty et al., 2002; Naglič et al., 2008; Pietrowski, 2001; Sritharan & H. Li, 1996; Szajnar & Wróbel, 2008a, 2008b; Whitehead, 2000; Wróbel, 2010; Zamkotowicz et al., 2003).


Table 1. Characteristic of bases to heterogeneous nucleation of aluminum (Donnay & Ondik, 1973)

Moreover the effectiveness of inoculants influence can be assessed on the basis of the hypothesis presented in the paper (Jura, 1968). This hypothesis was developed at the assumption that the fundamental physical factors affecting on the crystallization process are the amount of give up heat in the crystallization process on the interphase boundary of liquid - solid and the rate of give up heat of crystallization. After analyzing the results of own researches, the author proposed to determine the index (α), which characterizes the type of inoculant.

$$\alpha = \frac{\left(\mathcal{A}\mathbb{E}\_k/\nu\right)\_s}{\left(\mathcal{A}\mathbb{E}\_k/\nu\right)\_p} \bullet \mathcal{W} \tag{4}$$

where:

(3)

542 Advances in Crystallization Processes

Fig. 4. Macrostructure of ingot of Al with a purity of 99,7%: a – transverse section,

**2. Endogenous inoculation of pure aluminum structure** 

heterogeneous nucleation of inoculated metal.

b – longitudinal section

index (Fraś, 2003):


where:

a) b)

In aim to obtain an equiaxed and fine-grained structure, which gives high mechanical properties of castings, can use inoculation, which occurs in introducing into metal bath of specified substances, called inoculants (Fraś, 2003). Inoculants increase grains density as result of creation of new particles in consequence of braking of grains growth velocity, decrease of surface tension on interphase boundary of liquid – nucleus, decrease of angle of contact between the nucleus and the base and increase of density of bases to heterogeneous nucleation (Fraś, 2003; Jura, 1968). The effectiveness of this type of inoculation depends significantly on crystallographic match between the base and the nucleus of inoculated metal. This crystallographic match is described by type of crystal lattice or additionally by

> - (1- ) 100% *b n n x x x*

 

xb, xn – parameter of crystal lattice in specified direction, suitable for base and nucleus.

When the value of index () is closer to 100%, it the more effective is the base to

Therefore active bases to heterogeneous nucleation for aluminum are particles which have high melting point i.e. TiC, TiN, TiB, TiB2, AlB2 i Al3Ti (Tab.1) (Easton & StJohn, 1999a, Ek – heat of crystallization of 1 mol of inoculant or inoculated metal, J/mol,

 – characteristic frequency of atomic vibration calculated by the Lindemman formula, 1/s, s – symbol of inoculant, p – symbol of inoculated metal.

W – parameter dependent on the atomic mass of inoculant and inoculated metal.

On the basis of equation (4) additives can be divided into three groups:

At α > 1 – additives which inhibit crystals growth by the deformation of the crystallization front, thus are effective inoculants.

At α = 1 – additives do not affect on structure refinement.

At α < 1 – additives which accelerate crystals growth, favoring consolidation of the primary structure of the metal, thus are deinoculants.

In case of inoculation of Al the index α = 2.35 for inoculant in form of Ti and 1.76 for inoculant in form of B.

a) b)

c) Fig. 6. Structure of thin foil from pure Al after inoculation with (Ti+B), a) TEM bright field mag. 30000x , b) diffraction pattern from the area as in Fig. a, c) analysis of the diffraction

Fig. 7. Result of X-ray diffraction of Al with a purity of 99,5% after inoculation with Ti

pattern from Fig. b

In case of aluminum casting inoculants are introduced in form of master alloy AlTi5B1. This inoculant has Ti:B ratio equals 5:1. This Ti:B atomic ratio, which corresponds to the mass content of about 0.125% Ti to about 0.005% B, assures the greatest degree of structure refinement (Fig.5). For this titanium and boron ratio bases of type TiB2 and Al3Ti are created (Fig.6) (Easton & StJohn, 1999a, 1999b; Fjellstedt et al., 2001; Guzowski et al., 1987; Hu & H. Li, 1998; Kashyap & Chandrashekar, 2001; H. Li et al., 1997; P. Li et al., 2005; Murty et al., 2002; Naglič et al., 2008; Pietrowski, 2001; Sritharan & H. Li, 1996; Szajnar & Wróbel, 2008a, 2008b; Whitehead, 2000; Wróbel, 2010). Type and amount of bases to heterogeneous nucleation of aluminum depend on Ti:B ratio. For example given in paper (Zamkotowicz et al., 2003) the possibility of application of master alloy AlTi1.7B1.4, which has Ti:B ratio equals 1.2:1 is presented. This ratio allows to increase in amount of fine phases TiB2 and AlB2 along with the Al3Ti phase decrease.

Moreover minimum quantities of carbon and nitrogen, which come from metallurgical process of aluminum, create with inoculant the bases in form of titanium carbide TiC and titanium nitride TiN (Fig.7) (Pietrowski, 2001; Szajnar & Wróbel, 2008a, 2008b).

Additionally, because there is a possibility of creation the bases to heterogeneous nucleation of aluminum in form of TiC phase without presence of bases in form of borides, in the practice of casting the inoculation with master alloy AlTi3C0.15 is used (Naglič et al., 2008; Whitehead, 2000). However, on the basis of results of own researches was affirmed that assuming of introducing to Al with a purity of 99,5% the same quantity of Ti i.e. 25ppm, the result of structure refinement caused by master alloy AlTi3C0.15 is weaker than caused by master alloy AlTi5B1 (Fig.8).

Fig. 5. Influence of Ti and B contents on the average size of Al ingots (H. Li et al., 1997)

In case of aluminum casting inoculants are introduced in form of master alloy AlTi5B1. This inoculant has Ti:B ratio equals 5:1. This Ti:B atomic ratio, which corresponds to the mass content of about 0.125% Ti to about 0.005% B, assures the greatest degree of structure refinement (Fig.5). For this titanium and boron ratio bases of type TiB2 and Al3Ti are created (Fig.6) (Easton & StJohn, 1999a, 1999b; Fjellstedt et al., 2001; Guzowski et al., 1987; Hu & H. Li, 1998; Kashyap & Chandrashekar, 2001; H. Li et al., 1997; P. Li et al., 2005; Murty et al., 2002; Naglič et al., 2008; Pietrowski, 2001; Sritharan & H. Li, 1996; Szajnar & Wróbel, 2008a, 2008b; Whitehead, 2000; Wróbel, 2010). Type and amount of bases to heterogeneous nucleation of aluminum depend on Ti:B ratio. For example given in paper (Zamkotowicz et al., 2003) the possibility of application of master alloy AlTi1.7B1.4, which has Ti:B ratio equals 1.2:1 is presented. This ratio allows to increase in amount of fine phases TiB2 and

Moreover minimum quantities of carbon and nitrogen, which come from metallurgical process of aluminum, create with inoculant the bases in form of titanium carbide TiC and

Additionally, because there is a possibility of creation the bases to heterogeneous nucleation of aluminum in form of TiC phase without presence of bases in form of borides, in the practice of casting the inoculation with master alloy AlTi3C0.15 is used (Naglič et al., 2008; Whitehead, 2000). However, on the basis of results of own researches was affirmed that assuming of introducing to Al with a purity of 99,5% the same quantity of Ti i.e. 25ppm, the result of structure refinement caused by master alloy AlTi3C0.15 is weaker than caused by

Fig. 5. Influence of Ti and B contents on the average size of Al ingots (H. Li et al., 1997)

titanium nitride TiN (Fig.7) (Pietrowski, 2001; Szajnar & Wróbel, 2008a, 2008b).

AlB2 along with the Al3Ti phase decrease.

master alloy AlTi5B1 (Fig.8).

a) b)

Fig. 6. Structure of thin foil from pure Al after inoculation with (Ti+B), a) TEM bright field mag. 30000x , b) diffraction pattern from the area as in Fig. a, c) analysis of the diffraction pattern from Fig. b

Fig. 7. Result of X-ray diffraction of Al with a purity of 99,5% after inoculation with Ti

Fig. 10. The influence of quantity of inoculants in form of Ti and B on electrical conductivity

Moreover the presence of the bases to heterogeneous nucleation in form of hard deformable phases for example titanium borides in structure in aluminum, generate possibility of point cracks formation (Fig. 11) and in result of this delamination of sheet (foil) during rolling

a) b) Fig. 11. Phase TiB2 in structure of pure Al (Fig. a) and produced in result from its present

Therefore important is the other method of inoculation, which consists of influence of electromagnetic field (Asai, 2000; Campanella et al., 2004; Doherty et al., 1984; Gillon, 2000; Griffiths & McCartney, 1997; Harada, 1998; Szajnar, 2004, 2009; Szajnar & Wróbel, 2008a, 2008b, 2009; Vives & Ricou, 1985; Wróbel, 2010) or mechanical vibrations (Abu-Dheir et al.,

crack in sheet (foil) during rolling (Fig. b) (Keles & Dundar, 2007)

2005; Szajnar, 2009) on liquid metal in time of its solidification in mould.

γ of Al with a purity of 99,5%

(Keles & Dundar, 2007).

Fig. 8. Macrostructure of ingot of Al with a purity of 99,5%: a – in as-cast condition, b – after inoculation with (Ti+B), c – after inoculation with (Ti+C)

However, this undoubtedly effective method of inoculation of primary structure of ingot is limited for pure metals, because inoculants decrease the degree of purity specified in European Standards, and Ti with B introduced as modifying additives are then classify as impurities. Moreover, inoculants, mainly Ti which segregates on grain boundary of Al (Fig.9) influence negatively on physical properties i.e. electrical conductivity of pure aluminum (Fig.10) (Wróbel, 2010).

Fig. 9. Segregation of Ti on grain boundaries of Al

a) b) c)

However, this undoubtedly effective method of inoculation of primary structure of ingot is limited for pure metals, because inoculants decrease the degree of purity specified in European Standards, and Ti with B introduced as modifying additives are then classify as impurities. Moreover, inoculants, mainly Ti which segregates on grain boundary of Al (Fig.9) influence negatively on physical properties i.e. electrical conductivity of pure

Fig. 8. Macrostructure of ingot of Al with a purity of 99,5%: a – in as-cast condition,

b – after inoculation with (Ti+B), c – after inoculation with (Ti+C)

aluminum (Fig.10) (Wróbel, 2010).

Fig. 9. Segregation of Ti on grain boundaries of Al

Fig. 10. The influence of quantity of inoculants in form of Ti and B on electrical conductivity γ of Al with a purity of 99,5%

Moreover the presence of the bases to heterogeneous nucleation in form of hard deformable phases for example titanium borides in structure in aluminum, generate possibility of point cracks formation (Fig. 11) and in result of this delamination of sheet (foil) during rolling (Keles & Dundar, 2007).

Fig. 11. Phase TiB2 in structure of pure Al (Fig. a) and produced in result from its present crack in sheet (foil) during rolling (Fig. b) (Keles & Dundar, 2007)

Therefore important is the other method of inoculation, which consists of influence of electromagnetic field (Asai, 2000; Campanella et al., 2004; Doherty et al., 1984; Gillon, 2000; Griffiths & McCartney, 1997; Harada, 1998; Szajnar, 2004, 2009; Szajnar & Wróbel, 2008a, 2008b, 2009; Vives & Ricou, 1985; Wróbel, 2010) or mechanical vibrations (Abu-Dheir et al., 2005; Szajnar, 2009) on liquid metal in time of its solidification in mould.

In addition, as presented in the paper (Szajnar, 2009) the rotating velocity of the liquid metal (V) is inversely proportional to the density of the metal (), because with some

> *<sup>V</sup> F B or*

Fig. 13. Dependence of peripheral velocity of liquid metal (V) in a cylindrical mould of inside diameter 45mm on magnetic induction (B) for example metals (Szajnar, 2009)

2004, 2009; Szajnar & Wróbel, 2008a, 2008b, 2009; Wróbel, 2010):


can convert in equiaxed crystals,

crystals.

Forced liquid metal movement influences by diversified way on changes in structure of casting i.e. by changes of thermal and concentration conditions on crystallization front, which decrease or completely stops the velocity of columnar crystals growth (Szajnar, 2004, 2009) and by (Campanella et al., 2004; Doherty et al., 1984; Fraś, 2003; Ohno, 1976; Szajnar,



One of the hypotheses regarding the mechanism of dendrites fragmentation caused by the energy of the movement of liquid metal is presented in work (Doherty et al., 1984). It is based on the assumption of high plasticity of growing dendrites in the liquid metal, which in an initial state are a single crystal with specified crystallographic orientation (Fig.14a). The result of liquid metal movement is deformation (bending) of plastic dendrite (Fig.14b), which caused creation of crystallographic misorientation angle Θ (Fig.14c). Created highangle grain boundary (Θ > 20) has the energy γGZ much greater than double interfacial energy of solid phase - liquid phase γS-L. In result of unbalancing and satisfying the dependence γGZ > 2 γS-L the grain boundary is replaced by a thin layer of liquid metal. This leads to dendrite shear by liquid metal along the former grain boundary (Fig.14d). Dendrite fragments of suitable size after moving into the metal bath can transform into equiaxed


(8)

approximation we can say that (Fig.13):
