Fundamentals of Metal Casting- Part 2

By: doyouknow | Views: 2518 | Date: 04-Jan-2012

When manufacturing by metal casting, pouring refers to the process by which the molten metal is delivered into the mold. It involves its flow through the gating system and into the main cavity (casting itself).

Pouring, Fluidity, Risers, Shrinkage and other Defects

In the previous section we explained the fundamentals of metal casting, covering the two main branches of processes in manufacturing a part by metal casting. We discussed the setup and design of a system to perform a metal casting operation. Main topics were molds, patterns, cores, and the elements of a gating system. In this section we will explain the metal casting operation itself. We will begin assuming that there is a mold with a proper gating system in place and prepared for the metal casting operation.

Pouring of the Metal:

When manufacturing by metal casting, pouring refers to the process by which the molten metal is delivered into the mold. It involves its flow through the gating system and into the main cavity (casting itself).

Goal: Metal must flow into all regions of the mold, particularly the casting's main cavity, before solidifying.

Factors of Pouring:

Pouring Temperature:
Pouring temperature refers to the initial temperature of the molten metal used for the casting as it is poured into the mold. This temperature will obviously be higher than the solidification temperature of the metal. The difference between the solidification temperature and the pouring temperature of the metal is called the superheat.

Pouring Rate:
Volumetric rate in which the liquid metal is introduced into the mold. Pouring rate needs to be carefully controlled during the metal casting operation, since it has certain effects on the manufacture of the part. If the pouring rate it too fast then turbulence can result. If it is too slow the metal may begin to solidify before filling the mold.

Turbulences are inconsistent and irregular variations in the speed and direction of flow throughout the liquid metal as it travels though the casting. The random impacts caused by turbulence, amplified by the high density of liquid metal can cause mold erosion. An undesirable effect in the manufacturing process of casting, mold erosion is the wearing away of the internal surface of the mold. It is particularly detrimental if it occurs in the main cavity, since this will change the shape of the casting itself. Turbulence is also bad because it can increase the formation of metal oxides which may become entrapped creating porosity in the solid casting.

Since pouring is a key element in the manufacturing process of metal casting, and the main goal of pouring is to get metal to flow into all regions of the mold before solidifying. The properties of the melt in a casting process are very important. The ability of a particular casting melt to flow into a mold before freezing is crucial in the consideration of metal casting techniques. This ability is termed the liquid metals fluidity.

Test for Fluidity:
In manufacturing practice, the relative fluidity of a certain casting melt can be quantified by the use of a spiral mold. The geometry of the spiral mold acts to limit the flow of liquid metal through the length of its spiral cavity. The more fluidity possessed by the molten metal the farther into the spiral it will be able to flow before hardening. The maximum point the metal reaches upon the casting's solidification may be indexed as that melts relative fluidity.


Spiral Mold Test

How to Increase Fluidity in Metal Casting:
-Increase the superheat: If a melt is a higher temperature relative to its freezing point it will remain in the liquid state longer throughout the casting operation, and hence its fluidity will increase. However there are disadvantages to manufacturing a casting with an increased superheat. It will increase the melts likelihood to saturate gases, and the formation of oxides. It will also increase the molten metal’s ability to penetrate into the surface of the mold material.
-Choose a eutectic alloy, or pure metal: When selecting a manufacturing material consider that metals that freeze at a constant temperature have a higher fluidity. Since most alloys freeze over a temperature range they will develop solid portions that will interfere with the flow of the still liquid portions as the freezing of the casting occurs.
-Choose a metal with a higher heat of fusion: Heat of fusion is the amount of energy involved in the liquid-solid phase change. With a higher heat of fusion, the phase change, and hence the solidification of the metal casting will take longer and fluidity will be increased.

Most materials are less dense in their liquid state than in their solid state and denser at lower temperatures in general. Due to this nature a casting undergoing solidification will tend to decrease in volume, during the manufacture of a part by casting this decrease in volume is termed shrinkage. Shrinkage of the casting metal occurs in three stages:

1. Decreased volume of the liquid as it goes from the pouring temperature to the freezing temperature.
2. Decreased volume of the material due to solidification.
3. Decreased volume of the material as it goes from freezing temperature to room temperature.

When designing a setup for manufacturing a part by casting risers are almost always employed. As the metal casting begins to experience shrinkage the mold will need additional material to compensate for the decrease in volume. This can be accomplished by the employment of risers. Risers are an important component in the casting's gating system. Risers, (sometimes called feeders), serve to contain additional molten material. During the material's solidification process these reservoirs feed extra material into the casting as shrinkage is occurring, supplying it with an adequate amount of liquid metal. A successful riser will remain molten until after the casting solidifies. In order to reduce premature solidification of sections within the riser, in many manufacturing operations, the tops of open risers may be covered with an insulating compound, (such as a refractory ceramic), or an exothermic mixture, (such as iron oxide combined with powdered aluminum).

One of the biggest problems caused by shrinkage, in the manufacture of a cast part, is porosity. It happens in at different sites within the material, when liquid metal can't reach sections of the casting where solidification is occurring. As the isolated liquid metal shrinks a porous or vacant region develops.

Development of these regions can be prevented during the manufacturing operation, by strategically planning the flow of the liquid metal into the casting through good mold design, and by the employment of directional solidification. These techniques will be covered in detail in the Gating System and Mold Design section. Note that gases trapped within the molten metal can also be a cause of porosity. The effect of gases while manufacturing parts by casting will be discussed in the next section. Although proper casting methods can help mitigate the effects of shrinkage, remember some shrinkage, (like that which occurs in the cooling of the solid state to room temp.), can't be avoided. Therefore, the impression from which the casting is made is calculated oversized to the actual part, and the thermal expansion properties of the material used to manufacture the casting will be necessary to include in the calculation.

Other Defects:
The formation of vacancies due to shrinkage and improper solidification within the casting is a primary concern in the manufacturing metal casting process. There are numerous other defects that may occur, falling into various categories.

Metal Projections:
The category of metal projections includes all unwanted material projected from the surface of the casting, (i.e. fins, flashes, swells, etc.). The projections could be small, creating rough surfaces on the manufactured part, or be gross protrusions.

Any cavities in the casting material, angular or rounded, internal or exposed fit into this category. Cavities as a defect of casting shrinkage would be included here.

Cracks, tearing, and cold shuts in the casting qualify as discontinuities. Tearing occurs when the casting is unable to shrink naturally and a point of high tensile stress is formed. This could occur, for example, in a thin wall connecting two heavy sections. Cold shuts happen when two relatively cold streams of molten metal meet in the pouring of the casting. The surface at the location where they meet does not fuse together completely resulting in a cold shut.

Defective Surface:
Defects affecting the surface of the manufactured part. Blows, scabs, laps, folds, scars, blisters, ect.

Incomplete Casting:
Sections of the casting did not form. In a manufacturing process causes for an incomplete casting could be; insufficient amount of material poured, loss of metal from mold, insufficient fluidity in molten material, cross section within casting's mold cavity is too small, pouring was done too slowly, pouring temperature was too low.

Incorrect Dimensions or Shape:
The casting is geometrically incorrect. This could due to unpredicted contractions in the casting during solidification, a warped casting. Shrinkage of the casting may have been miscalculated. There may have been problems with the manufacture of the pattern.

Unwanted particles contained within the material act as stress raisers compromising the casting's strength. During the manufacturing process, interaction of the molten metal with the environment including the atmosphere, (chemical reactions with oxygen in particular), and the mold itself can cause inclusions within a casting. As with most casting defects good mold maintenance is important in their control.

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