**3. Exogenous inoculation of pure aluminum structure**

First research works on the application of stirring of liquid metal at the time of its solidification in order to improve the castings quality were carried out by Russ Electroofen in 1939 and concerned the casting of non-ferrous metals and their alloys (Wróbel, 2010). In order to obtain the movement of the liquid metal in the crystallizer in the researches carried out at this period of time and also in the future, a physical factor in the form of a electromagnetic field defined as a system of two fields i.e. an electric and magnetic field was introduced. The mutual relationship between these fields are described by the Maxwell equations (Sikora, 1998).

Generated by the induction coil powered by electric current intensity (I0) electromagnetic field affects the solidifying metal induces a local electromotive force (Em), whose value depends on the local velocity of the liquid metal (V) and magnetic induction (B) (Gillon, 2000).:

$$E\_m = \overline{V} \times \overline{B} \tag{5}$$

This is a consequence of the intersection of the magnetic field lines with the current guide in form of liquid metal. It also leads to inducing an eddy current of intensity (I) in liquid metal (Gillon, 2000; Vives & Ricou, 1985):

$$
\overline{I} = \sigma \left( \overline{V} \times \overline{B} \right) \tag{6}
$$

where:

σ - electrical conductivity proper to the liquid metal.

The influence of the induced current on the magnetic field results in establishing of the Lorenz (magnetohydrodynamic) force (F) (Gillon, 2000; Vives & Ricou, 1985):

$$
\overline{F} = \overline{I} \times \overline{B} \tag{7}
$$

that puts liquid metal in motion e.g. rotary motion in the direction consistent with the direction of rotation of the magnetic field. Strength (F) has a maximum value when the vector (V) and (B) are perpendicular (Fig.12).

Fig. 12. Scheme of electromagnetic field influence on the liquid metal

First research works on the application of stirring of liquid metal at the time of its solidification in order to improve the castings quality were carried out by Russ Electroofen in 1939 and concerned the casting of non-ferrous metals and their alloys (Wróbel, 2010). In order to obtain the movement of the liquid metal in the crystallizer in the researches carried out at this period of time and also in the future, a physical factor in the form of a electromagnetic field defined as a system of two fields i.e. an electric and magnetic field was introduced. The mutual relationship between these fields are described by the Maxwell

Generated by the induction coil powered by electric current intensity (I0) electromagnetic field affects the solidifying metal induces a local electromotive force (Em), whose value depends on the local velocity of the liquid metal (V) and magnetic induction (B) (Gillon,

This is a consequence of the intersection of the magnetic field lines with the current guide in form of liquid metal. It also leads to inducing an eddy current of intensity (I) in liquid metal

> *I VB*

The influence of the induced current on the magnetic field results in establishing of the

that puts liquid metal in motion e.g. rotary motion in the direction consistent with the direction of rotation of the magnetic field. Strength (F) has a maximum value when the

Lorenz (magnetohydrodynamic) force (F) (Gillon, 2000; Vives & Ricou, 1985):

Fig. 12. Scheme of electromagnetic field influence on the liquid metal

*E VB <sup>m</sup>* (5)

(6)

*F IB* (7)

**3. Exogenous inoculation of pure aluminum structure** 

equations (Sikora, 1998).

(Gillon, 2000; Vives & Ricou, 1985):

σ - electrical conductivity proper to the liquid metal.

vector (V) and (B) are perpendicular (Fig.12).

2000).:

where:

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 approximation we can say that (Fig.13):

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)

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


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 crystals.

(8)

In case of continuous ingots of square and circular transverse section, rotating electromagnetic field induction coils are used. Rotating electromagnetic field forces rotational movement of liquid metal in perpendicular planes to ingot axis (Fig.16a). Whereas, mainly for flat ingots, longitudinal electromagnetic field induction coils are used, which forced oscillatory movement of liquid metal in parallel planes to ingot axis (Fig.16b)

a) b)

Whereas the authors of paper (Szajnar & Wróbel, 2008a, 2008b) suggests the use of reversion in the direction of electromagnetic field rotation during permanent mould casting. The advantage of casting in rotating electromagnetic field with reversion compared to casting in rotating field, mainly based on the fact that the liquid metal located in the permanent mould and put in rotary-reversible motion practically does not create a concave meniscus, and thus is not poured out off the mould under the influence of centrifugal forces. Moreover, the influence of this type of field combines impact of high amplitude and low frequency vibration with action of rotating electromagnetic field. Also important is double-sided bending of growing crystals, causing the creation in the columnar crystals zone of ingot

Fig. 16. Scheme of an electromagnetic stirrer (induction coil) forced rotational (a) and

Fig. 17. Macrostructure of ingot of Al with a purity of 99,5% after cast with influence of

oscillatory movement of liquid metal (Adamczyk, 2004)

characteristic crystals so-called corrugated (Fig.17).

rotating electromagnetic field with reversion

(Adamczyk, 2004; Miyazawa, 2001).

Fig. 14. Schematic model of the grain boundary fragmentation mechanism: a – an undeformed dendrite, b – after bending, c – the reorganization of the lattice bending to give grain boundaries, d – for γGZ > 2 γS-L the grain boundaries have been "wetted" by the liquid phase (Doherty et al., 1984).

c) d)

The influence of electromagnetic field on liquid metal in aim of structure refinement (Fig.15), axial and zonal porosity elimination and obtaining larger homogeneity of structure, was applied in permanent mould casting (Griffiths & McCartney, 1997; Szajnar & Wróbel, 2008a, 2008b, 2009; Wróbel, 2010) and mainly in technologies of continuous (Adamczyk, 2004; Gillon, 2000; Harada, 1998; Miyazawa, 2001; Szajnar et al., 2010; Vives & Ricou, 1985) and semi-continuous casting (Guo et al., 2005).

Fig. 15. Macrostructure of ingot of Al with a purity of 99,5% after cast with influence of rotating electromagnetic field

a) b)

c) d)

undeformed dendrite, b – after bending, c – the reorganization of the lattice bending to give grain boundaries, d – for γGZ > 2 γS-L the grain boundaries have been "wetted" by the liquid

The influence of electromagnetic field on liquid metal in aim of structure refinement (Fig.15), axial and zonal porosity elimination and obtaining larger homogeneity of structure, was applied in permanent mould casting (Griffiths & McCartney, 1997; Szajnar & Wróbel, 2008a, 2008b, 2009; Wróbel, 2010) and mainly in technologies of continuous (Adamczyk, 2004; Gillon, 2000; Harada, 1998; Miyazawa, 2001; Szajnar et al., 2010; Vives & Ricou, 1985)

Fig. 15. Macrostructure of ingot of Al with a purity of 99,5% after cast with influence of

Fig. 14. Schematic model of the grain boundary fragmentation mechanism: a – an

phase (Doherty et al., 1984).

rotating electromagnetic field

and semi-continuous casting (Guo et al., 2005).

In case of continuous ingots of square and circular transverse section, rotating electromagnetic field induction coils are used. Rotating electromagnetic field forces rotational movement of liquid metal in perpendicular planes to ingot axis (Fig.16a). Whereas, mainly for flat ingots, longitudinal electromagnetic field induction coils are used, which forced oscillatory movement of liquid metal in parallel planes to ingot axis (Fig.16b) (Adamczyk, 2004; Miyazawa, 2001).

Fig. 16. Scheme of an electromagnetic stirrer (induction coil) forced rotational (a) and oscillatory movement of liquid metal (Adamczyk, 2004)

Whereas the authors of paper (Szajnar & Wróbel, 2008a, 2008b) suggests the use of reversion in the direction of electromagnetic field rotation during permanent mould casting. The advantage of casting in rotating electromagnetic field with reversion compared to casting in rotating field, mainly based on the fact that the liquid metal located in the permanent mould and put in rotary-reversible motion practically does not create a concave meniscus, and thus is not poured out off the mould under the influence of centrifugal forces. Moreover, the influence of this type of field combines impact of high amplitude and low frequency vibration with action of rotating electromagnetic field. Also important is double-sided bending of growing crystals, causing the creation in the columnar crystals zone of ingot characteristic crystals so-called corrugated (Fig.17).

Fig. 17. Macrostructure of ingot of Al with a purity of 99,5% after cast with influence of rotating electromagnetic field with reversion

Fig. 19. The influence of magnetic induction (B) and frequency (f) of the current supplied to

SKR = 0,0126f2

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 f [Hz]

Fig. 20. The influence of current frequency (f) supplied to the induction coil on equiaxed

crystals zone content (SKR) on transverse section of pure Al ingot

R2 = 0,9714


the induction coil on force value (F), which creates movement of liquid metal

0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00

SKR [%]

However in papers (Szajnar, 2004, 2009; Szajnar & Wróbel, 2008a, 2008b, 2009) was shown that influence of forced movement of liquid metal by use of electromagnetic field to changes in structure of pure metals, which solidify with flat crystallization front is insufficient. The effective influence of this forced convection requires a suitable, minimal concentration of additives i.e. alloy additions, inoculants or impurities in casting. Suitable increase of additives concentration causes at specified thermal conditions of solidification, occurs in change of morphology of crystallization front according to the scheme shown in Figure 18.

Fig. 18. Scheme of relationship between thermal and concentration conditions and type of crystallization; C0 – concentration of additives, GT – thermal gradient on crystallization front, V – velocity of crystallization (Fraś, 2003)

However it should be noted that, based on the latest results of author researches was affirmed that in some cases it is possible to obtain a sufficient refinement degree of pure aluminum structure in result of inoculation carried out only with the use of an electromagnetic field. Because it shows a possibility of increasing the force, which creates movement of liquid metal and in result of this the velocity of its rotation in mould, not only by increasing the value of magnetic induction according to the dependences (7) and (8), but also by increasing the frequency of the current supplied to the induction coil (Fig.19).

The effect of refinement of structure of Al with a purity of 99,5% caused by the rotating electromagnetic field produced by the induction coil supplied by current with frequency different from the network i.e. 50Hz is presented in Table 2. On the basis of macroscopic metallographic researches, which lead to the calculation of the equiaxed crystals zone content on transverse section of ingot (SKR) and average area of macro-grain in this zone (PKR) was affirmed, that application of frequency of supply current f 50Hz does not guarantee favourable transformation of pure aluminum structure (Fig.20 and 21). Whereas induction coil supplied with frequency of current larger than power network, mainly 100Hz generates rotating electromagnetic field, which guarantees favourable refinement of structure, also in comparison to obtained after inoculation with small, acceptable by European Standards amount of Ti and B i.e. 25 and 5ppm (Tab.2).

However in papers (Szajnar, 2004, 2009; Szajnar & Wróbel, 2008a, 2008b, 2009) was shown that influence of forced movement of liquid metal by use of electromagnetic field to changes in structure of pure metals, which solidify with flat crystallization front is insufficient. The effective influence of this forced convection requires a suitable, minimal concentration of additives i.e. alloy additions, inoculants or impurities in casting. Suitable increase of additives concentration causes at specified thermal conditions of solidification, occurs in change of morphology of crystallization front according to the scheme shown in

Fig. 18. Scheme of relationship between thermal and concentration conditions and type of crystallization; C0 – concentration of additives, GT – thermal gradient on crystallization

However it should be noted that, based on the latest results of author researches was affirmed that in some cases it is possible to obtain a sufficient refinement degree of pure aluminum structure in result of inoculation carried out only with the use of an electromagnetic field. Because it shows a possibility of increasing the force, which creates movement of liquid metal and in result of this the velocity of its rotation in mould, not only by increasing the value of magnetic induction according to the dependences (7) and (8), but also by increasing the frequency of the current supplied to the induction coil

The effect of refinement of structure of Al with a purity of 99,5% caused by the rotating electromagnetic field produced by the induction coil supplied by current with frequency different from the network i.e. 50Hz is presented in Table 2. On the basis of macroscopic metallographic researches, which lead to the calculation of the equiaxed crystals zone content on transverse section of ingot (SKR) and average area of macro-grain in this zone (PKR) was affirmed, that application of frequency of supply current f 50Hz does not guarantee favourable transformation of pure aluminum structure (Fig.20 and 21). Whereas induction coil supplied with frequency of current larger than power network, mainly 100Hz generates rotating electromagnetic field, which guarantees favourable refinement of structure, also in comparison to obtained after inoculation with small, acceptable by

European Standards amount of Ti and B i.e. 25 and 5ppm (Tab.2).

front, V – velocity of crystallization (Fraś, 2003)

Figure 18.

(Fig.19).

Fig. 19. The influence of magnetic induction (B) and frequency (f) of the current supplied to the induction coil on force value (F), which creates movement of liquid metal

Fig. 20. The influence of current frequency (f) supplied to the induction coil on equiaxed crystals zone content (SKR) on transverse section of pure Al ingot

B ingot

SKR [%]

21,01 0,44

parameters Macrostructure of

PKR [mm2]

Cast parameters Refinement


(Ti+B) [ppm]


4 10 21,36 0,35

5 15 20,66 0,33

6 60 20 - 22,63 0,37

No.

3

[mT]

60

f [Hz]

5

Fig. 21. The influence of current frequency (f) supplied to the induction coil on average area of equiaxed crystal (PKR) of pure Al ingot



of equiaxed crystal (PKR) of pure Al ingot

f [Hz]

[mT]

No.

PKR [mm2

]

PKR = 5E-05f2

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

f [Hz]

B ingot

SKR [%]

Fig. 21. The influence of current frequency (f) supplied to the induction coil on average area

Cast parameters Refinement


(Ti+B) [ppm]

1 - - - 19,94 0,64

2 - - 25+5 80,30 0,42

R2 = 0,9435


parameters Macrostructure of

PKR [mm2]


B ingot

SKR [%]

21,46 0,13

parameters Macrostructure of

PKR [mm2]

Cast parameters Refinement


(Ti+B) [ppm]


12 50 21,21 0,15

13 55 22,87 0,10

14 60 27,22 0,12

No.

11

[mT]

60

f [Hz]

45


B ingot

SKR [%]

parameters Macrostructure of

PKR [mm2]

Cast parameters Refinement


(Ti+B) [ppm]

7 25 18,90 0,33

8 30 21,42 0,33

9 35 21,44 0,19

10 40 21,68 0,21

No.

[mT]

f [Hz]


B ingot

SKR [%]

parameters Macrostructure of

PKR [mm2]

Cast parameters Refinement


(Ti+B) [ppm]

19 85 64,70 0,01

20 90 78,67 0,01


Table 2. The influence of rotating electromagnetic field on structure of Al with a

22 100 83,36 0,01

81,78 0,01

95

No.

21

purity of 99,5%

60

[mT]

f [Hz]


B ingot

SKR [%]

42,53 0,07

parameters Macrostructure of

PKR [mm2]

Cast parameters Refinement


(Ti+B) [ppm]


17 75 54,63 0,04

18 80 58,56 0,03

15 65 37,05 0,09

No.

16

[mT]

60

f [Hz]

70


Table 2. The influence of rotating electromagnetic field on structure of Al with a purity of 99,5%

a) b) Fig. 23. Macrostructure of ingot of Al with a purity of 99,5%: a – after common exogenous (electromagnetic field) and endogenous (Ti + B) inoculation, b – only after endogenous

Based on conducted calculations of number of macro-grains in equiaxed crystals zone

nex+en – number of macro-grains in equiaxed crystals zone of ingot which was cast with common influence of exogenous (electromagnetic field) and endogenous (Ti + B)

nex – number of macro-grains in equiaxed crystals zone of ingot which was cast only with

nen – number of macro-grains in equiaxed crystals zone of ingot which was cast only with

Summarize, was affirmed that application of common exogenous (electromagnetic field) and endogenous (Ti + B) inoculation strengthens effect of structure refinement in comparison with application of one type of inoculation, only if is used skinning of ingot

In conclusion can say, that even endogenous inoculation with small amount of (Ti + B) strongly increase on refinement in pure aluminum structure. It results from reactions, which proceed between inoculating elements and inoculated metal or charge impurities. These reactions lead to formation of active bases to heterogeneous nucleation of aluminum as high melting small particles of type TiB, TiB2, AlB2, Al3Ti and TiC or TiN, which have analogy in

However on the basis of conducted analysis of the literature and results of authors researches it was affirmed, that the rotating electromagnetic field generated by induction

*n nn ex en ex en* (9)

(Ti + B) inoculation

where:

inoculation,

**5. Conclusions** 

crystal lattice with Al.

following formula was formulated:

influence of exogenous (electromagnetic field) inoculation,

surface i.e. machining in aim of columnar crystals zone elimination.

influence of endogenous (Ti + B) inoculation.
