**2. Heat transfer mechanism in solidification**

To comprehend the heat transfer mechanism we need to know the behavior of solidification. The heat transfer from the liquid hot temperature cast to the mold is a very complex phenomenon and different modes of heat transfer can be observed while solidification in the cast. While heat transfer is predominant the resistance

**40**

*Casting Processes and Modelling of Metallic Materials*

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[12] Trivedi, R. and Kurz, W; 'Dendritic Growth', International Materials Reviews, 1994, Vol. 39 (2), p. 49-74

[13] Flemings, M.C., 'Solidification Processing', McGraw-Hill, New York,

[14] Church,N;Wieser,P;Wallace,J.K,

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[15] 'Foundry Technology'P.R.

[16] Mohapatra,S., Sarangi,H. and Mohanty,U.K, 'Effect Of Process Factors On The Characteristics Of Centrifugal Casting', Manufacturing Review, 2020,

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[2] Smith, William F.; Hashemi, Javad, *Foundations of Materials Science and Engineering* (4th ed.), McGraw-Hill, 2006, ISBN 978-0-07-295358-9

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[3] Degarmo,E.Paul, Black,J-T, Kohser,Ronald.A, Materials and Processing in Manufacturing', (9th edition) Willy Publication, ,2003, p. 277

[4] Campbell,J., Metal Casting

[5] Eskin,D.G. and Katgerman,L., Thermal Contraction During Solidification Of Alluminium Alloy, Materials Science Forum, 2006, Vols-

[6] Oxtobody,D.W., 'Homogenous Nucleation: Theory and Experiment, Journal of Physics: Condensed Matter, 1992, Volume 4, issue 38, p. 7627

[7] Mahata,Avik;Asle Zaeem Mohsen and Baskes,I.Michael, 'Understanding

[8] De Moor.P-Pe,Bleeten,T.A. and Van Sanken,R.A., In Situ Observation Of Nucleation And Crystal Growth In Zeolite Synthesis. A Small Angle X-Ray Scattering Investigation On Si-TPA-MFI, J.Phys.Chem B, 1999, Vol. 103(10),

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p. 326-327

**References**

edition

to the heat flow also has different dimensions to this solidification. This resistance mainly depends on liquid cast metal, latent heat release, interface, solidified cast, the type of mold and the ambient conditions. General solidification of an alloy is discussed in the **Figure 1** and specific cooling curve for Al6061 is shown in **Figure 2**.

Initially on pouring the liquid metal cast into the mold cavity the whole metal fluid flows and occupies the mold cavity, the liquid metal flowing with the velocity, mixes thoroughly and releases heat to the mold due to the very high temperature difference. Complete thermal contact is observed between the cast and the mold which causes the heat transfer to be purely conduction, where the resistance offered by this liquid metal is negligible since the entire fluid flow is the superheated cast metal. Once the cast metal reaches the liquidus point on cooling, the cast shrinks and releases latent heat and also a number of metal oxides are released which causes an air gap between the cast and the mold. Due to this air gap the heat transfer phenomenon now changes to a complex one where all modes of heat transfer can be observed simultaneously. This air gap is characterized with an Interfacial Heat transfer Coefficient (IHTC) "h" across the metal-mold interface. The rate of heat at the interface is found using the surface heat flux as q (W/m<sup>2</sup> ) and given by the Eq. 1.

$$q = h(Tc - Tm) \tag{1}$$

**43**

continue.

**Figure 2.**

*Heat Transfer Studies on Solidification of Casting Process*

the turbulence in the liquid metal. This rate of cooling is linear and a minimum amount of heat is transferred from the cast to the mold as it is having a complete

As the solidification progresses with time it reaches the liquidus point at the same time where the mold temperature increases significantly to a maximum temperature. Further the solid skin forms on the outer cast surface, the metal shrinks and an air gap starts forming between the metal and the mold. When the cast solidifies further the air gap separates the two surfaces. This is a common phenomenon in most of the alloys. The rate of heat transfer from the cast to the mold is very high as it releases larger quantity of latent heat to the mold and the cast temperature gradually reaches a solidus (TS) temperature of the alloy. The air gap plays a significant

Further solidification reduces the cast surface temperature, however the inner cast metal shrinks and it further releases the heat to the mold and there is rise in the mold temperature as shown in **Figure 2**. Thereafter further reduction in the cast temperature after the solidus point (Ts) was found as the third stage of solidification. The air gap size is further increased as the solidification time increase and its effects are felt till the end of solidification. However there is still a temperature difference between the cast and mold for the further heat transfer to

Once the complete air gap is formed between the cast and the mold, the gap will contain almost all kinds of gaseous except air that contradicts the air gap term. The sand mold which is used for the casting application, generates the mold gases which are often high in hydrogen, containing typically 50 percent which fills the air gap. The hydrogen gas thermal conductivity increases the heat transfer by 7 times more as the mold temperature rises to a high temperature of 500°C due to radiation. Therefore it is very essential to know or analyze the interface during the solidifica-

On comparing the green sand mold with dry sand mold the green sand mold expand homogeneously and release heat to the surrounding which leads to a lesser resistance for the heat flow whereas dry sand mold offers more resistance than the green sand mold. The high thermal conductivity die mold material has uniform

role in varying IHTC with various factors influencing solidification.

tion process as it is further discussed in the next section.

temperature variation and assumes homogeneous expansion.

*DOI: http://dx.doi.org/10.5772/intechopen.95371*

contact with the mold surface.

*Aluminum alloy (Al6061) solidification curve.*

Tc and Tm are the cast and mold surface temperatures at the interface in K or deg. C.

The dynamics of solidification of cast metal, mold temperature and the cast temperature can be clearly understood from the cooling curves shown in **Figure 2**. Once the molten metal fills the cavity the alloy cast reaches the maximum temperature. Generally the heat transfer analysis starts from this point onwards as the temperature drops from the liquid cast metal to the liquidus temperature (TL), the point at which the solidification begins and this freezing is called liquid cooling. The loss of superheat temperature of the cast metal after pouring is found due to

**Figure 1.** *Solidification curve for alloy.*

*Heat Transfer Studies on Solidification of Casting Process DOI: http://dx.doi.org/10.5772/intechopen.95371*

*Casting Processes and Modelling of Metallic Materials*

the interface is found using the surface heat flux as q (W/m<sup>2</sup>

to the heat flow also has different dimensions to this solidification. This resistance mainly depends on liquid cast metal, latent heat release, interface, solidified cast, the type of mold and the ambient conditions. General solidification of an alloy is discussed in the **Figure 1** and specific cooling curve for Al6061 is shown in **Figure 2**. Initially on pouring the liquid metal cast into the mold cavity the whole metal fluid flows and occupies the mold cavity, the liquid metal flowing with the velocity, mixes thoroughly and releases heat to the mold due to the very high temperature difference. Complete thermal contact is observed between the cast and the mold which causes the heat transfer to be purely conduction, where the resistance offered by this liquid metal is negligible since the entire fluid flow is the superheated cast metal. Once the cast metal reaches the liquidus point on cooling, the cast shrinks and releases latent heat and also a number of metal oxides are released which causes an air gap between the cast and the mold. Due to this air gap the heat transfer phenomenon now changes to a complex one where all modes of heat transfer can be observed simultaneously. This air gap is characterized with an Interfacial Heat transfer Coefficient (IHTC) "h" across the metal-mold interface. The rate of heat at

Tc and Tm are the cast and mold surface temperatures at the interface in K

The dynamics of solidification of cast metal, mold temperature and the cast temperature can be clearly understood from the cooling curves shown in **Figure 2**. Once the molten metal fills the cavity the alloy cast reaches the maximum temperature. Generally the heat transfer analysis starts from this point onwards as the temperature drops from the liquid cast metal to the liquidus temperature (TL), the point at which the solidification begins and this freezing is called liquid cooling. The loss of superheat temperature of the cast metal after pouring is found due to

) and given by the Eq. 1.

*q h Tc Tm* = − ( ) (1)

**42**

**Figure 1.**

*Solidification curve for alloy.*

or deg. C.

**Figure 2.** *Aluminum alloy (Al6061) solidification curve.*

the turbulence in the liquid metal. This rate of cooling is linear and a minimum amount of heat is transferred from the cast to the mold as it is having a complete contact with the mold surface.

As the solidification progresses with time it reaches the liquidus point at the same time where the mold temperature increases significantly to a maximum temperature. Further the solid skin forms on the outer cast surface, the metal shrinks and an air gap starts forming between the metal and the mold. When the cast solidifies further the air gap separates the two surfaces. This is a common phenomenon in most of the alloys. The rate of heat transfer from the cast to the mold is very high as it releases larger quantity of latent heat to the mold and the cast temperature gradually reaches a solidus (TS) temperature of the alloy. The air gap plays a significant role in varying IHTC with various factors influencing solidification.

Further solidification reduces the cast surface temperature, however the inner cast metal shrinks and it further releases the heat to the mold and there is rise in the mold temperature as shown in **Figure 2**. Thereafter further reduction in the cast temperature after the solidus point (Ts) was found as the third stage of solidification. The air gap size is further increased as the solidification time increase and its effects are felt till the end of solidification. However there is still a temperature difference between the cast and mold for the further heat transfer to continue.

Once the complete air gap is formed between the cast and the mold, the gap will contain almost all kinds of gaseous except air that contradicts the air gap term. The sand mold which is used for the casting application, generates the mold gases which are often high in hydrogen, containing typically 50 percent which fills the air gap. The hydrogen gas thermal conductivity increases the heat transfer by 7 times more as the mold temperature rises to a high temperature of 500°C due to radiation. Therefore it is very essential to know or analyze the interface during the solidification process as it is further discussed in the next section.

On comparing the green sand mold with dry sand mold the green sand mold expand homogeneously and release heat to the surrounding which leads to a lesser resistance for the heat flow whereas dry sand mold offers more resistance than the green sand mold. The high thermal conductivity die mold material has uniform temperature variation and assumes homogeneous expansion.
