the metal casting operation

8/12/2019 The Metal Casting Operation

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The Metal CastingOperation

Pouring, Fluidity, Risers, Shrinkage And Other Defects

In the previous section the fundamentals of the metal casting process, as the

basic starting point for metal fabrication and part manufacture, were covered.Setup and design of a system to perform a casting operation was explained. Maintopics were molds, patterns, cores, and the elements of a gating system. In thissection we will explain the operation itself. We will begin by assuming that there is

a mold with a proper gating system in place and prepared for the metal castingoperation.

Pouring of the Metal:

When manufacturing by metal casting, pouring refers to the process by which themolten 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 maincavity, before solidifying.

Factors Of Pouring:

Pouring Temperature:

Pouring temperature refers to the initial temperature of the molten metal used for thecasting as it is poured into the mold. This temperature will obviously be higher than thesolidification temperature of the metal. The difference between the solidification

temperature and the pouring temperature of the metal is called the superheat.

Figure:8

Pouring Rate:

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

Turbulence:

Turbulence is inconsistent and irregular variations in the speed and direction of flowthroughout the liquid metal as it travels though the casting. The random impactscaused by turbulence, amplified by the high density of liquid metal, can cause mold

erosion. An undesirable effect in the manufacturing process of metal casting, molderosion is the wearing away of the internal surface of the mold. It is particularlydetrimental if it occurs in the main cavity, since this will change the shape of thecasting itself. Turbulence is also bad because it can increase the formation of metaloxides which may become entrapped, creating porosity in the solid casting.

Fluidity:

Pouring is a key element in the manufacturing process of metal casting and the maingoal 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 particularcasting melt to flow into a mold before freezing is crucial in the consideration of metalcasting techniques. This ability is termed the liquid metals fluidity.

Test for Fluidity:

In manufacturing practice, the relative fluidity of a certain metal casting melt can bequantified by the use of a spiral mold. The geometry of the spiral mold acts to limit theflow of liquid metal through the length of its spiral cavity. The more fluidity possessedby 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 beindexed as that melts relative fluidity.

Figure:9

Spiral Mold Test




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How To Increase Fluidity In Metal Casting:

-Increase the superheat:If a melt is at a higher temperature relative to its freezingpoint, it will remain in the liquid state longer throughout the metal casting operation,and hence its fluidity will increase. However, there are disadvantages to manufacturinga metal casting with an increased superheat. It will increase the melts likelihood tosaturate gases, and the formation of oxides. It will also increase the molten metalsability to penetrate into the surface of the mold material.

-Choose an eutectic alloy, or pure metal:When selecting a manufacturing

material, consider that metals that freeze at a constant temperaturehave a higherfluidity. Since most alloys freeze over a temperature range, they will develop solidportions that will interfere with the flow of the still liquid portions, as the freezing of themetal casting occurs.

-Choose a metal with a higher heat of fusion:Heat of fusion is the amount ofenergy involved in the liquid-solid phase change. With a higher heat of fusion, thesolidification of the metal casting will take longer and fluidity will be increased.

Shrinkage:

Most materials are less dense in their liquid state than in their solid state, and moredense at lower temperatures in general. Due to this nature, a metal casting undergoingsolidification will tend to decrease in volume. During the manufacture of a part bycasting this decrease in volume is termed shrinkage. Shrinkage of the casting metaloccurs in three stages:

1. Decreased volume of the liquid as it goes from the pouring temperature tothe freezing temperature.

Figure:10

2. Decreased volume of the material due to solidification.

Figure:11




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Porosity:

One of the biggest problems caused by shrinkage, during the manufacture of a castpart, is porosity. It happens at different sites within the material, when liquid metal cannot reach sections of the metal casting where solidification is occurring. As theisolated liquid metal shrinks, a porous or vacant region develops.

Figure:14

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 becovered in detail in the gating system and mold designsection. Note that gasestrapped within the molten metal can also be a cause of porosity. The effects of gaseswhile manufacturing parts by metal casting will be discussed in the gases section.

Although proper metal casting methods can help mitigate the effects of shrinkage,some shrinkage, (like that which occurs in the cooling of the work metal from the top ofthe solid state to room temperature), can not be avoided. Therefore, the impressionfrom which the metal casting is made is calculated oversized to the actual part, and thethermal expansion properties of the material used to manufacture the part will benecessary to include in the calculation.


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Other Defects:

The formation of vacancies within the work material due to shrinkage is a primaryconcern in the metal casting process. There are numerous other defects that mayoccur, falling into various categories.

Metal Projections:

The category of metal projections includes all unwanted material projected from the

surface of the part, (ie. fins, flash, swells, ect.). The projections could be small, creatingrough surfaces on the manufactured part, or be gross protrusions.

Cavities:

Any cavities in the material, angular or rounded, internal or exposed, fit into thiscategory. Cavities as a defect of metal casting shrinkage or gases would be includedhere.

Discontinuities:

Cracks, tearing, and cold shuts in the part qualify as discontinuities. Tearing occurswhen the metal 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 thepouring of the casting. The surface at the location where they meet does not fusetogether completely resulting in a cold shut.

Defective Surface:

Defects effecting the surface of the manufactured part. Blows, scabs, laps, folds,scars, blisters, ect.

Incomplete Casting:

Sections of the metal casting did not form. In a manufacturing process causes for

incomplete metal castings could be; insufficient amount of material poured, loss ofmetal from mold, insufficient fluidity in molten material, cross section within casting'smold cavity is too small, pouring was done too slowly, or pouring temperature was toolow.

Incorrect Dimensions or Shape:

The metal casting is geometrically incorrect. This could be due to unpredictedcontractions in the part during solidification. A warped casting. Shrinkage of the metalcasting may have been miscalculated. There may have been problems with themanufacture of the pattern.

Inclusions:

Unwanted particles contained within the material act as stress raisers, compromisingthe casting's strength. During the manufacturing process, interaction of the moltenmetal with the environment, such as the mold surfaces and the outside atmosphere,(chemical reactions with oxygen in particular), can cause inclusions within a metalcasting. As with most casting defects, good mold maintenance and process design isimportant in their control.

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