Metal Shaping Processes. Vukota Boljanovic
mold casting differs from expendable processes in that the mold need not be destroyed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.
Steel cavities of molds are coated with some refractory wash layer like acetylene soot before processing to allow for easy removal of the workpiece and to promote longer tool life. The useful lifetime of permanent molds varies depending on maintenance: when the useful life is over, such molds require refinishing or replacement. Cast parts from a permanent mold generally show a 20% increase in tensile strength and a 30% increase in elongation as compared to the products of sand casting. Typically, permanent mold castings are used in forming iron, aluminum, magnesium, and copper-based alloys. The process is highly automated.
There are many different factors and variables that go into metal casting; each one can change the final product. That is why it is important that each one be considered in order to get a successful casting process.
Because different types of metal casting processes are available, it is one of the most used manufacturing processes. Casting technology is used to fabricate a large number of the metal components in designs we use every day. The reasons for this include the following advantages:
•Casting can produce very complex part geometries with internal and external shapes.
•It can be used to produce very small workpieces (a few hundred grams) to workpieces of very large size (over 100 tons).
•With some casting processes it is possible to manufacture final shapes that require no further manufacturing operations to achieve the required dimensions and tolerances of the parts.
•Casting is economical, with very little wastage. The extra metal in each casting is remelted and reused.
•Casting metal is isotropic: It has the same physical and mechanical properties in all directions.
•Some types of metal casting are very suitable for mass production.
There are also some disadvantages for different types of castings. These include the following:
•poor finish, wide tolerance (sand casting);
•limited workpiece size (shell molds and ceramic molds);
•patterns have low strength (expendable pattern casting);
•expensive, limited shapes (centrifugal casting);
•porosity (all types);
•environmental problems (all types).
To accomplish a casting process the worker must heat metal to the desired temperature for pouring. Heat is the energy that flows spontaneously from a higher temperature object to a lower temperature object through random interactions between their atoms. The heat energy required for heating metal to a pouring temperature is the sum of:
•the heat needed to raise the temperature to the melting point;
•the heat of fusion needed to convert it from a solid to a liquid;
•the heat needed to raise the molten metal to the desired temperature for pouring.
This energy can be expressed as a sum of phase energy by the following formula:
| Q | = | Qs + Qf + Ql | (1.1) | |
| Q | = | ρV[cs(Tm − T0) + Lf + c1(Tp − Tm)] | (1.1a) |
where
| Q | = | total heat energy, J (Btu) |
| Q s | = | heat energy for solid metal, J (Btu) |
| Q f | = | heat energy for fusion, J (Btu) |
| Q l | = | heat energy for liquid metal, J (Btu) |
| ρ | = | density, kg/m3 |
| V | = | volume of metal being heated, m3 (in3) |
| c s | = | specific heat for solid metal, J/kg°C (Btu/lbm°F) |
| c l | = | specific heat for liquid metal, J/kg°C (Btu/lbm°F) |
| T 0 | = | starting temperature, °C (°F) |
| T m | = | melting temperature of the metal, °C (°F) |
| T p | = | pouring temperature, °C (°F) |
| L f | = | heat of fusion of the metal, J/kg (Btu/lbm). |
The heat of fusion is the amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature.
Equation (1.1a) can be used for the approximate calculation of the total heat energy because values cs, and c1 vary with the temperature; in addition, significant heat losses to the environment during heating are not implied by this equation.
Figure 1.1 shows a phase-change diagram of the process of heating for metal casting.
Fig. 1.1 Phase change diagram of the process of heating metal to a molten temperature sufficient for casting.
After the metal is heated to pouring temperature Tp the metal is ready for pouring. As the introduction of the molten metal into the mold includes the fluid flow in casting, we will describe a basic gravity casting system as shown in Fig. 1.2.
Fig. 1.2 Cross-section of a typical two-part sand mold.
Figure 1.2 presents a cross-section of a typical two-part sand mold and incorporates many features of the casting process. These are:
Drag. The drag is the bottom half of any of these features.
Core. A core is a sand shape that is inserted into the