A casting defect is an undesired irregularity in a
metal casting
In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold (usually by a crucible) that contains a negative impression (i.e., a three-dimensional negative image) of the intended shape. The metal is ...
process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: ''gas porosity'', ''shrinkage defects'', ''mould material defects'', ''pouring metal defects'', and ''metallurgical defects''.
Terminology
The terms "defect" and "" refer to two specific and separate things in castings. Defects are defined as conditions in a casting that must be corrected or removed, or the casting must be rejected. Discontinuities, also known as "imperfections", are defined as "interruptions in the physical continuity of the casting". Therefore, if the casting is less than perfect, but still useful and in tolerance, the imperfections should be deemed "discontinuities".
Types
There are many types of defects which result from many different causes. Some of the solutions to certain defects can be the cause for another type of defect.
The following defects can occur in
sand casting
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced i ...
s. Most of these also occur in other casting processes.
Shrinkage defects
Shrinkage defects
can occur when standard feed metal is not available to compensate for
shrinkage as the thick metal
solidifies. Shrinkage defects will have jagged or linear appearance. Shrinkage defects usually occur in either the cope 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''. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting.
Closed shrinkage defects, also known as ''shrinkage porosity'', are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called ''hot spots''. The shrinkage defect usually forms at the top of the hot spots. They require a
nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into ''macroporosity'' and ''microporosity'' (or ''micro shrinkage''), where macroporosity can be seen by the naked eye and microporosity cannot.
Gas porosity
''Gas porosity'' is the formation of bubbles 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 in that vicinity.
Nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
and
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
are the most encountered gases in cases of gas porosity.
In aluminium castings, hydrogen is the only gas that dissolves in significant quantity, which can result in
hydrogen gas porosity
Hydrogen gas porosity is an aluminium casting defect in the form of a porosity or void in an aluminium casting caused by a high level of hydrogen gas (H2) dissolved in the aluminium at liquid phase. The solubility of hydrogen in solid aluminium is ...
. For casting that are a few kilograms in weight the pores are usually in size. In larger casting, they can be up to a millimetre (0.040 in) in diameter.
To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
or
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, or under 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 mould can introduce gases, so the moulds are often streamlined to minimize such turbulence. Other methods include
vacuum degassing,
gas flushing
In chemistry, sparging, also known as gas flushing in metallurgy, is a technique in which a gas is bubbled through a liquid in order to remove ''other'' dissolved gas(es) and/or dissolved volatile liquid(s) from that liquid. It is a method of de ...
, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance,
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
can be removed from
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
by adding
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
; aluminium or
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
can be added to steel to remove oxygen.
A third source consists of reactions of the molten metal with grease or other residues in the mould.
Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mould. Drying the mould can eliminate this source of hydrogen formation.
Gas porosity can sometimes be difficult to distinguish from micro shrinkage because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation.
![Foundry defect blowhole](https://upload.wikimedia.org/wikipedia/commons/2/20/Foundry_defect_blowhole.jpg)
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 mould. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as 'blowholes'). Proper foundry practices, including melt preparation and mould design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic,
ultrasonic
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
,
magnetic, or
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
(e.g.,
industrial CT scanning
Industrial computed tomography (CT) scanning is any computer-aided tomographic process, usually X-ray computed tomography, that uses irradiation to produce three-dimensional internal and external representations of a scanned object. Industrial CT ...
) analysis.
Pouring metal defects
Pouring metal defects include ''misruns'', ''cold shuts'', and ''inclusions''. A misrun occurs when the liquid metal does not completely fill the mould cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mould 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
Back pressure (or backpressure) is a resistance or force opposing the desired flow of fluid through pipes, leading to friction loss and pressure drop. The term ''back pressure'' is a misnomer, as pressure is a scalar quantity, so it has a magnit ...
from improperly vented mould cavities.
''Misruns'' and ''cold shuts'' are closely related and both involve the material freezing before it completely fills the mould cavity. These types of defects are serious because the area surrounding the defect is significantly weaker than intended. The
castability Castability is the ease of forming a quality casting. A very castable part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.Ravi, p. 2 Castability can refer to a part design or a material pro ...
and
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
of the material can be important factors with these problems. Fluidity affects the minimum section thickness that can be cast, the maximum length of thin sections, fineness of feasibly cast details, and the accuracy of filling mould extremities. There are various ways of measuring the fluidity of a material, although it usually involves using a standard mould shape and measuring the distance the material flows. 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 higher the pouring temperature, the greater the fluidity; however, excessive temperatures can be detrimental, leading to a reaction between the material and the mould; in casting processes that use a porous mould material the material may even penetrate the mould material.
The point at which the material cannot flow is called the ''coherency point''. The point is difficult to predict in mould design because it is dependent on the solid fraction, the structure of the solidified particles, and the local
shear strain
In physics, deformation is the continuum mechanics transformation of a body from a ''reference'' configuration to a ''current'' configuration. A configuration is a set containing the positions of all particles of the body.
A deformation can ...
rate of the fluid. Usually this value ranges from 0.4 to 0.8.
An inclusion is a metal contamination of
dross
Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low- melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. For ...
, if solid, or
slag, if liquid. These usually are impurities in the pour metal (generally
oxides, less frequently
nitride
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...
s,
carbide
In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.
Interstitial / Metallic carbides
The carbides of th ...
s, or
sulfides), material that is eroded from furnace or ladle linings, or contaminates from the mould. In the specific case of aluminium alloys, it is important to control the concentration of
inclusions by measuring them in the liquid aluminium and taking actions to keep them to the required level.
There are a number of ways to reduce the concentration of inclusions. In order to reduce oxide formation the metal can be melted with a
flux, in a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
, or in an
inert atmosphere. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mould. If this is not practical, then a special ladle that pours the metal from the bottom can be used. Another option is to install
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
filters into the gating system. Otherwise swirl gates can be formed which swirl the liquid metal as it is poured in, forcing the lighter inclusions to the center and keeping them out of the casting. If some of the dross or slag is folded into the molten metal then it becomes an
entrainment defect
An entrainment defect is a term used in metallurgy to describe a defect created in a casting by the folding-over of the oxidized surface layer of the molten metal into the bulk liquid.
Outside of special cases, the surface of a molten metal is cov ...
.
Metallurgical defects
There are two defects in this category: ''hot tears'' and ''hot spots''. Hot tears, also known as , are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. Proper mould design prevents this type of defect.
Hot spots are sections of casting which have cooled down more slowly than the surrounding material due to higher volume than its surrounding. This causes abnormal shrinkage in this region, which can lead to porosity and cracks. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.
Additional methods of minimising hot tears are not overheating the casting material and increasing the temperature of the mould.
Process specific defects
Die casting
In
die casting
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly ...
the most common defects are
misrun
A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: ''gas porosity'', ''shrinkage ...
s and
cold shut
A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: ''gas porosity'', ''shrinkage ...
s. These defects can be caused by cold dies, low metal temperature, dirty metal, lack of venting, or too much lubricant. Other possible defects are gas porosity, shrinkage porosity, hot tears, and flow marks. ''Flow marks'' are marks left on the surface of the casting due to poor gating, sharp corners, or excessive lubricant.
Continuous casting
A ''longitudinal facial crack'' is a specialized type of defect that only occurs in
continuous casting
Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the ...
processes. This defect is caused by uneven cooling, both
primary cooling and secondary cooling, and includes molten steel qualities, such as the chemical composition being out of specification, cleanliness of the material, and
homogeneity
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the Uniformity (chemistry), uniformity of a Chemical substance, substance or organism. A material or image that is homogeneous is uniform in compos ...
.
Sand casting
Sand casting has many defects that can occur due to the mould failing. The mould usually fails because of one of two reasons: the wrong material is used or it is improperly
rammed.
The first type is ''mould erosion'', which is the wearing away of the
mould as the liquid metal fills the mould. This type of defect usually only occurs in
sand casting
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced i ...
s because most other casting processes have more robust moulds. The castings produced have rough spots and excess material. The
moulding sand becomes incorporated into the casting metal and decreases the
ductility
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...
,
fatigue strength, and
fracture toughness of the casting. This can be caused by a sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates.
A related source of defects are ''drops'', in which part of the moulding sand from the
cope
The cope (known in Latin as ''pluviale'' 'rain coat' or ''cappa'' 'cape') is a liturgical vestment, more precisely a long mantle or cloak, open in front and fastened at the breast with a band or clasp. It may be of any liturgical colour.
A c ...
drops into the casting while it is still a liquid. This also occurs when the mould is not properly rammed.
The second type of defect is ''metal penetration'', which occurs when the liquid metal penetrates into the moulding sand. This causes a rough
surface finish. This is caused by sand particles which are too coarse, lack of mould wash, or pouring temperatures that are too high.
An alternative form of metal penetration into the mould known as
veining is caused by cracking of the sand.
If the pouring temperature is too high or a sand of low
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.
A ''run out'' occurs when the liquid metal leaks out of the mould because of a faulty mould or
flask
Flask may refer to:
Container
* Hip flask, a small container used to carry a small amount of liquid
* Laboratory flask, laboratory glassware for holding larger volumes than simple test tubes
** Erlenmeyer flask, a common laboratory flask wit ...
.
''s'' are a thin layer of metal that sits proud of the casting. They are easy to remove and always reveal a ' underneath, which is an indentation in the casting surface. ''s'' are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is ''s'', which are buckles that occur in the cope of sand castings. All of these defects are visual in nature and are no reason to scrap the workpiece. These defects are caused by overly high pouring temperatures or deficiencies of
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
aceous material.
A ''swell'' occurs when the mould wall gives way across a whole face, and is caused by an improperly rammed mould.
' occurs when metallic oxides interact with impurities in silica sands. The result is sand particles embedded in the surface of the finished casting. This defect can be avoided by reducing the temperature of the liquid metal, by using a mould wash, and by using various
additives
Additive may refer to:
Mathematics
* Additive function, a function in number theory
* Additive map, a function that preserves the addition operation
* Additive set-functionn see Sigma additivity
* Additive category, a preadditive category with fi ...
in the sand mixture.
See also
*
Hydrogen gas porosity
Hydrogen gas porosity is an aluminium casting defect in the form of a porosity or void in an aluminium casting caused by a high level of hydrogen gas (H2) dissolved in the aluminium at liquid phase. The solubility of hydrogen in solid aluminium is ...
*
Inclusions in aluminium alloys
*
Non-metallic inclusions for inclusions in steel
*
Porosity sealing
Porosity sealing is done through the process of vacuum impregnation. Vacuum impregnation is a preferred OEM process that seals porosity and leak paths in metal castings, sintered metal parts and electrical castings that form during the casting or ...
References
Bibliography
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{{DEFAULTSORT:Casting Defect
Casting (manufacturing)
Metallurgy