**3.2 Liquid- solid shrinkage**

Actually the liquid contraction leads to a solidification which is a complex problem in the casting industry. This requires a proper feeding mechanism to fill the cavity by maintaining high liquid cast temperature while pouring and if not then the partial liquid - solid contraction leads to shrinkage porosity. The specific volume of the solid metal is lesser than the liquid metal. All the solidifications are planned for the directional solidification which refers to the faster cooling rate at which solidification progresses from the cavity metal to the feeder mechanism. The faster cooling

**45**

*Heat Transfer Studies on Solidification of Casting Process*

rate and the movement of liquid in the solidification is due to the area of the surface which enables the liquid metal to drop its high temperature to solidus temperature. The runner, riser and the gating system is designed in the mold pattern enhances the directional solidification by transferring proper heat flow from the cast to the mold. The alloys of eutectic type allow lesser solidification shrinkage volume and also have a lower sensitivity to the solidification problems caused by sudden geometry changes. While they involve smaller risers, these can be omitted completely in certain cases by gates placed strategically and because the metal feed avenues stay open longer, it ensures a uniform solidifying process. While eutectic type of solidification is the most simplest, it requires the least reciprocity and can withstand a range of geometries. Directional solidification is more complex; however, when it has an ideally designed geometry, it is highly capable of extremely higher interior unity. Heat transfer is in fact the main process behind the bilaterally symmetrical and mutual state of connectedness in the process of solidification shrinkage and geometrical patterns. The heat transfer during solidification of castings involves three modes of heat transfer, namely radiation, conduction and convection, the rate of heat transfer is still dependent on the geometry of the casting as discussed later in the interfacial heat transfer coefficient section.

The final stages of shrinkage in the solid state which can cause a separate series of problems. As cooling progresses, and the casting attempts to reduce its size in consequence, it is rarely free to contract as it wishes. This stage of solidification is usually complex either by the types of mold, or by the other casting parts like the runner and riser that have already solidified and cooled as the air gap formed. The air gap formed is mainly due to the various factors like metal oxide formation, coefficient of thermal expansion, latent heat released, evaporation moisture in the case of sand mold, interfacial gap, mode of heat transfer etc. this type of solidification

These factors are the major causes for the heat to flow from the cast to the mold and it is found that it majorly affects the solidification and in turn affects the quality of the cast product. The amount of solid metal stretches like plastic casting, makes the solidification again into a complex problem. This shrinkage behavior leads to difficulty in predicting the size of the pattern since the degree to which the pattern is made oversize (the 'contraction allowance' or 'patternmaker's allowance') is not easy to quantify. This shrinkage also causes hot tearing or cracking of the casting

In general, liquids contract on freezing because of the rearrangement of atoms from a rather open 'random close-packed' arrangement to a regular crystalline array of significantly denser packing. The densest solids are those that have cubic close packed (face-centred-cubic, fcc, and hexagonal close-packed, hcp) symmetry. Thus

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

the greatest values for contraction on solidification are seen for these metals.

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

**3.3 Solid shrinkage**

shrinkage is also called as pattern shrinkage.

which lead to more localized problems.

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

**Figure 3.** *Specific volume changes against cast surface temperature.*

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

rate and the movement of liquid in the solidification is due to the area of the surface which enables the liquid metal to drop its high temperature to solidus temperature. The runner, riser and the gating system is designed in the mold pattern enhances the directional solidification by transferring proper heat flow from the cast to the mold.

The alloys of eutectic type allow lesser solidification shrinkage volume and also have a lower sensitivity to the solidification problems caused by sudden geometry changes. While they involve smaller risers, these can be omitted completely in certain cases by gates placed strategically and because the metal feed avenues stay open longer, it ensures a uniform solidifying process. While eutectic type of solidification is the most simplest, it requires the least reciprocity and can withstand a range of geometries. Directional solidification is more complex; however, when it has an ideally designed geometry, it is highly capable of extremely higher interior unity. Heat transfer is in fact the main process behind the bilaterally symmetrical and mutual state of connectedness in the process of solidification shrinkage and geometrical patterns. The heat transfer during solidification of castings involves three modes of heat transfer, namely radiation, conduction and convection, the rate of heat transfer is still dependent on the geometry of the casting as discussed later in the interfacial heat transfer coefficient section.

### **3.3 Solid shrinkage**

*Casting Processes and Modelling of Metallic Materials*

While melting the metal in the furnace has a higher specific volume hence it occupies more space by the metal and on pouring it results in the solidification in the mold which increases the complexity of the solidification [1]. After pouring the temperature of the cast reduces and the specific volume also reduces which causes shrinkage in the poured volume as shown in **Figure 3**. To understand the complex behavior of solidification we need to understand three different stages of shrinkage of metal during the solidification process; it includes liquid shrinkage, liquid- solid

The superheated metal which is poured in the liquid state has more specific volume than the liquid metal in the cavity [2]. This liquid metal occupies the mold cavity and is in superheated state and comes in complete contact with the mold surface. Here the mode of heat transfer is purely conduction shown in **Figure 3**. On solidification there is a liquid contraction due to reduction in specific volume, the metal cools further and reaches to a liquidus temperature. This contraction of liquid metal separates cast and mold surface and imitates the air gap formation which is

Actually the liquid contraction leads to a solidification which is a complex problem in the casting industry. This requires a proper feeding mechanism to fill the cavity by maintaining high liquid cast temperature while pouring and if not then the partial liquid - solid contraction leads to shrinkage porosity. The specific volume of the solid metal is lesser than the liquid metal. All the solidifications are planned for the directional solidification which refers to the faster cooling rate at which solidification progresses from the cavity metal to the feeder mechanism. The faster cooling

**3. Shrinkage behavior of casting**

shrinkage and solid shrinkage.

assigned as liquid shrinkage.

**3.2 Liquid- solid shrinkage**

**3.1 Liquid shrinkage**

**44**

**Figure 3.**

*Specific volume changes against cast surface temperature.*

The final stages of shrinkage in the solid state which can cause a separate series of problems. As cooling progresses, and the casting attempts to reduce its size in consequence, it is rarely free to contract as it wishes. This stage of solidification is usually complex either by the types of mold, or by the other casting parts like the runner and riser that have already solidified and cooled as the air gap formed. The air gap formed is mainly due to the various factors like metal oxide formation, coefficient of thermal expansion, latent heat released, evaporation moisture in the case of sand mold, interfacial gap, mode of heat transfer etc. this type of solidification shrinkage is also called as pattern shrinkage.

These factors are the major causes for the heat to flow from the cast to the mold and it is found that it majorly affects the solidification and in turn affects the quality of the cast product. The amount of solid metal stretches like plastic casting, makes the solidification again into a complex problem. This shrinkage behavior leads to difficulty in predicting the size of the pattern since the degree to which the pattern is made oversize (the 'contraction allowance' or 'patternmaker's allowance') is not easy to quantify. This shrinkage also causes hot tearing or cracking of the casting which lead to more localized problems.

In general, liquids contract on freezing because of the rearrangement of atoms from a rather open 'random close-packed' arrangement to a regular crystalline array of significantly denser packing. The densest solids are those that have cubic close packed (face-centred-cubic, fcc, and hexagonal close-packed, hcp) symmetry. Thus the greatest values for contraction on solidification are seen for these metals.
