**4. Interfacial heat transfer coefficient (IHTC)**

The heat transfer characteristics during casting are governed by IHTC. The molten metal is poured into the cavity it first enters the mold due to the fluidity of the metal, it occupies the cavity and ensures complete contact between the metal and the mold. In the early stage of solidification, the fluidity of the molten metal conformance and contact between the cast and mold surfaces is good. At this early stage of solidification due to the nucleation of the metal, higher initial surface heat flux is reached. Further the solid skin forms and then spreads to cover the entire casting surface. As the solidified

layer forms with sufficient strength, simultaneously air gap forms and as a consequence the contact between the casting and the mold are reduced. This leads to the sudden drop in the heat flux and the solid skin forms on the outer cast surface [3]. The cast liquid solid shrinks/contracts away from the mold surface. This further releases heat and it is absorbed by the mold surface and in turn increases the temperature of the mold as it expands. The mode of heat transfer is not only due to conduction at this stage because the heat from the metal to the mold takes place across the interface region but also due to other modes of heat transfer convection and radiation. The air gap varies for the different cast metals and depends on their factors of the release of metal oxides, hydrogen gases and material properties of the cast and mold, geometry etc.

Further the third stage of solidification is identified between the liquidus to solidus temperature of the cast as the fall in the casting surface temperature is suddenly halted, due to the release of latent heat. After the complete solid skin formation on the cast the heat transfer further diminishes and gap size increases and the mode for heat transfer is assumed to be conduction of heat through the gaseous phase in the interface using the air gap method. This air gap size is measured as x by assuming the expansion to be homogeneous, and the interfacial heat transfer coefficient is estimated as h = k/x: where k is thermal conductivity of the air (W/mK) as shown in **Figure4**. This concept of conduction as a mode of heat transfer in IHTC is reported by Kai- Ho and Robert D Pelhke, [4]. There are many factors that influence the IHTC and practically the IHTC becomes highly unpredictable if all the factors are not taken into account while designing. The various factors listed by the authors Lewis and Ransing, [5] and Guo Zhi-Peng et al. [6], that affect the interfacial heat during solidification is listed below.


**47**

*Heat Transfer Studies on Solidification of Casting Process*

solidification will have higher IHTC.

3.Geometry of Casting: The area of contact with the mold and the directional

4.Pouring temperature: Higher values of superheat will increase the initial value

5.Surface roughness: Higher initial value of IHTC for the better contact when the

6.Alloy composition: Higher initial value established for an alloy with a larger

7.Latent heat: Cast from superheat temperature to liquidus temperature ensures

8.Metallostatic pressure: During the pouring of molten metal into the cavity rises the metallostatic pressure, this is also responsible for higher IHTC at the initial stage.

9.Mold temperature: During initial stage higher IHTC due to the higher mold temperature and smaller temperature difference for higher peak heat flux.

10.Die Coating thickness: Increase of die coating thickness decreases the IHTC. While pouring the metal at the liquid stage the effect of die coating behaves as

As it is pointed out by many researchers the gap size mainly depends on the gas that is formed in the interface. The rate of solidification of castings made in a sand mold is generally controlled by the rate at which heat can be absorbed by the mold. In fact, compared to many other casting processes, the sand mold acts as an excellent insulator, keeping the casting warm. However, of course, ceramic investment and plaster molds are even more insulating, avoiding premature cooling of the metal, and aiding fluidity to give the excellent ability to fill thin sections for which these casting processes are renowned. It is regrettable that the extremely slow cool-

Extensive literature reviews have been made, in order to determine the interfacial heat transfer behavior during the solidification of casting at the metal-mold interfaces, since the 1970's. The boundary conditions as a surface heat flux and mold surface temperature established at the metal mold interface were used to determine the precise interfacial heat transfer coefficient value by using many mathematical methods described in the literature. The most common approaches can be distinguished here as follows for the determination of IHTC at the metal-mold interface

sharp slope in IHTC due to the evolution of latent heat.

a weaker influence at the interface as the air gap formed.

ing can contribute to rather poorer mechanical properties.

including surface heat flux and mold surface temperature:

1.Air gap measurement technique

2.Pure Analytical approach

3.Semi-analytical method

4.Numerical Methods

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

of IHTC.

surfaces are smooth.

freezing range.

11.Mold materials

12.Type of castings

**Figure 4.** *Schematic representation of IHTC during solidification of casting.*

*Casting Processes and Modelling of Metallic Materials*

gases and material properties of the cast and mold, geometry etc.

that affect the interfacial heat during solidification is listed below.

IHTC and the chills increases the IHTC.

*Schematic representation of IHTC during solidification of casting.*

gap formed.

layer forms with sufficient strength, simultaneously air gap forms and as a consequence the contact between the casting and the mold are reduced. This leads to the sudden drop in the heat flux and the solid skin forms on the outer cast surface [3]. The cast liquid solid shrinks/contracts away from the mold surface. This further releases heat and it is absorbed by the mold surface and in turn increases the temperature of the mold as it expands. The mode of heat transfer is not only due to conduction at this stage because the heat from the metal to the mold takes place across the interface region but also due to other modes of heat transfer convection and radiation. The air gap varies for the different cast metals and depends on their factors of the release of metal oxides, hydrogen

Further the third stage of solidification is identified between the liquidus to solidus temperature of the cast as the fall in the casting surface temperature is suddenly halted, due to the release of latent heat. After the complete solid skin formation on the cast the heat transfer further diminishes and gap size increases and the mode for heat transfer is assumed to be conduction of heat through the gaseous phase in the interface using the air gap method. This air gap size is measured as x by assuming the expansion to be homogeneous, and the interfacial heat transfer coefficient is estimated as h = k/x: where k is thermal conductivity of the air (W/mK) as shown in **Figure4**. This concept of conduction as a mode of heat transfer in IHTC is reported by Kai- Ho and Robert D Pelhke, [4]. There are many factors that influence the IHTC and practically the IHTC becomes highly unpredictable if all the factors are not taken into account while designing. The various factors listed by the authors Lewis and Ransing, [5] and Guo Zhi-Peng et al. [6],

1.Die coating thickness: The initial high peak value of IHTC is reduced with an increase of die coating thickness. While pouring the metal at the liquid stage the effect of die coating behaves as a weaker influence at the interface as the air

2.Insulating pads, chills, etc.: The IHTC has different behaviors with insulating pads and chills. It is obvious that always the insulating material reduces the

**46**

**Figure 4.**

