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Casting defects are the undesired irregularities created in a metal casting process. Some defects can be accepted while others can be repaired, otherwise they must be eliminated. They are divided into five main categories: gas porosities, shrinkage defects, mold material defects, pouring metal defects and metallurgical defects.

Terminology
There are many types of defects which result from various reasons. Some solutions to certain defects can be the cause for other type of defect. The following defects can occur in sand castings. Most of these also occur in other casting processes.

Shrinkage defects  

Shrinkage defects can occur when standard feed metal is not available to compensate shrinkage when the thick metal solidifies. Shrinkage defects will have jagged or linear appearance. Shrinkage defects usually occur in either the cap or drag portion of the casting. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere, therefore as the shrinkage cavity forms, air compensates. There are two types of open air defects: pipes and caved surfaces.

Closed shrinkage defects, also known as shrinkage porosity, are defects that form inside the casting. Isolated liquid pool is formed inside solidified metal， which is called hot spot. The shrinkage defects are usually formed at the top of the hot spot. They require a nucleation point, so impurities and dissolved gas can result closed shrinkage defects. The defects are divided into macroporosity and microporosity (or microshrinkage), where macroporosity can be seen by the naked eye and microporosity cannot.

Gas porosities

Gas porosities are the bubbles formed within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength around. Nitrogen, oxygen and hydrogen are with the maximum void ratio. In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.

To prevent gas porosities, the material may be melted in a vacuum, in an environment of low-solubility gases, such as carbon dioxide, or with a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mold can introduce gases, so the molds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form dross that floats to the top.

Hydrogen is normally produced by the reaction of the metal with humidity or residual moisture in the mold. Drying the mold can eliminate this source of hydrogen formation.

Gas porosities sometimes are difficult to distinguish from microshrinkage, because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting does not have properly riser or if the solidification range of the cast is wide. If neither of these, then most likely it is the natural gas that generates the gas porosities.

Blowhole in iron casting

Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes or blisters. Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand, or other gasses from the melt or mold. (Vacuum holes caused by metal shrinkage (see above) may also be referred to as 'blowholes'). Proper foundry practices, including melt preparation and mold design, can reduce the occurrence of these defects. Because they are often surrounded by a layer of metal shell, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (i.e., industrial CT scanning) analysis.

Pouring metal defects 

Pouring metal defects include deviation, cold shuts, and inclusions. A deviation occurs when the liquid metal does not completely fill the mold cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities.

Misoperation and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity. These types of defects are serious because the area surrounding the defects is significantly weaker than intended. The castability and viscosity of the material can be important factors that cause these problems. Fluidity affects the minimum section thickness of the parts that can be cast, the maximum length of thin sections, fineness of casting thinner parts and the accuracy of filling mold extremities. There are various ways to measure the fluidity of a material, although it usually involves using a standard mould shape and measuring the flow distance of the material. Fluidity is affected by the composition of the material, freezing temperature or range, surface tension of oxide films, and, most importantly, the pouring temperature.

The point at which the material cannot flow is called the coherency point. The point is difficult to predict in mold design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid. Usually this value ranges from 0.4 to 0.8.

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