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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—known as ''casting sand''—as the mold material. The term "sand casting" can also refer to an object produced via the sand casting proces ...
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 Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the same temperature; however, certain substances possess dif ...
. 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 An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
, 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 In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...
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 a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
and
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
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. For casting that are a few kilograms in weight the pores are usually in size. In larger casting, they can be up to 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; it has 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 abu ...
or
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, 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 Degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous methods for removing gases from liquids. Gases are removed for various reasons. Chemists remove gas ...
,
gas flushing Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as o ...
, 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 a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
can be removed from
copper Copper is a chemical element; it has symbol Cu (from Latin ) 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 pinkish-orang ...
by adding
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
; aluminium or
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
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. 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 with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, includi ...
,
magnetic Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
, or
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
(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 C ...
) 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 the term for a resistance to the desired flow of fluid through pipes. Obstructions or tight bends create backpressure via friction loss and pressure drop. In distributed systems in particular event-driven archi ...
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 and
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
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 mechanics, strain is defined as relative deformation, compared to a position configuration. Different equivalent choices may be made for the expression of a strain field depending on whether it is defined with respect to the initial or the ...
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. Fo ...
, if solid, or
slag The general term slag may be a by-product or co-product of smelting (pyrometallurgical) ores and recycled metals depending on the type of material being produced. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be c ...
, if liquid. These usually are impurities in the pour metal (generally
oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
s, less frequently
nitride In chemistry, a nitride is a chemical compound of nitrogen. Nitrides can be inorganic or organic, ionic or covalent. The nitride anion, N3−, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitr ...
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
sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...
s), 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 Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...
, in a
vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
, 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, porcela ...
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.


Die casting

In
die casting Die casting is a casting (metalworking), 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 die (manufacturing), dies which have been ...
the most common defects are misruns and cold shuts. These defects can be caused by cold dies, low metal temperature, dirty metal, lack of venting, or excessive 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 melting, molten metal is solidified into a "semifinished" Billet (semi-finished product), billet, Bloom (casting)#Bloom, bloom, or Slab (casting)#Slab, slab for subsequent ro ...
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 relating to the Uniformity (chemistry), uniformity of a Chemical substance, substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, ...
.


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 A mold () or mould () is one of the structures that certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi ...
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—known as ''casting sand''—as the mold material. The term "sand casting" can also refer to an object produced via the sand casting proces ...
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 refers to the ability of a material to sustain significant plastic Deformation (engineering), deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic def ...
,
fatigue strength The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure. Some metals such as ferrous alloys and titanium alloys have a distinct limit, ...
, and
fracture toughness In materials science, fracture toughness is the critical stress intensity factor of a sharp Fracture, crack where propagation of the crack suddenly becomes rapid and unlimited. It is a material property that quantifies its ability to resist crac ...
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 A cope ( ("rain coat") or ("cape")) is a liturgical long mantle or cloak, open at the front and fastened at the breast with a band or clasp. It may be of any liturgical colour. A cope may be worn by any rank of the Catholic or Anglican clerg ...
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 Surface finish, also known as surface texture or surface topography, is the nature of a interface (matter), surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a ...
. 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 of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
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 liquid * Laboratory flask, laboratory glassware for holding larger volumes than simple test tubes ** Erlenmeyer flask, a common laboratory flask with a flat bottom, a c ...
. ''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; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
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-function see Sigma additivity * Additive category, a preadditive category with fin ...
in the sand mixture.


See also

* Hydrogen gas porosity * Inclusions in aluminium alloys *
Non-metallic inclusions Non-metallic inclusions are chemical compounds and nonmetals that are present in steel and other alloys. They are the product of chemical reactions, physical effects, and contamination that occurs during the melting and pouring process. These inc ...
for inclusions in steel * Porosity sealing


References


Bibliography

* . * . * . * . * . * . {{DEFAULTSORT:Casting Defect Casting (manufacturing) Metallurgy