Editing Metal casting (section)

==Theory==<!-- [[Metal casting theory]] redirects here -->
Casting is a [[solidification]] process, which means the solidification phenomenon controls most of the properties of the casting. Moreover, most of the casting defects occur during solidification, such as ''[[gas porosity (casting)|gas porosity]]'' and ''solidification shrinkage''.<ref>{{Harvnb|Degarmo|Black|Kohser|2003|pp=279–280}}</ref>

Solidification occurs in two steps: ''[[nucleation]]'' and ''[[crystal growth]]''. In the nucleation stage, solid particles form within the liquid. When these particles form, their [[internal energy]] is lower than the surrounded liquid, which creates an energy interface between the two. The formation of the surface at this interface requires energy, so as nucleation occurs, the material actually undercools (i.e. cools below its solidification temperature) because of the extra energy required to form the interface surfaces. It then recalescences, or heats back up to its solidification temperature, for the crystal growth stage. Nucleation occurs on a pre-existing solid surface because not as much energy is required for a partial interface surface as for a complete spherical interface surface. This can be advantageous because fine-grained castings possess better properties than coarse-grained castings. A fine grain structure can be induced by ''[[grain refinement]]'' or ''inoculation'', which is the process of adding impurities to induce nucleation.<ref name="degarmo280">{{Harvnb|Degarmo|Black|Kohser|2003|p=280}}</ref>

All of the nucleations represent a crystal, which grows as the [[heat of fusion]] is extracted from the liquid until there is no liquid left. The direction, rate, and type of growth can be controlled to maximize the properties of the casting. [[Directional solidification]] is when the material solidifies at one end and proceeds to solidify to the other end; this is the most ideal type of grain growth because it allows liquid material to compensate for shrinkage.<ref name="degarmo280"/>

===Cooling curves===
[[Image:Dendrite formation.gif|thumb|300px|Intermediate cooling rates from melt result in a dendritic microstructure. Primary and secondary dendrites can be seen in this image.]]
{{See also|Cooling curves}}

Cooling curves are important in controlling the quality of a casting. The most important part of the cooling curve is the ''cooling rate'' which affects the microstructure and properties. Generally speaking, an area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure. Below is an example cooling curve of a pure metal or [[eutectic]] alloy, with defining terminology.<ref>{{Harvnb|Degarmo|Black|Kohser|2003|pp=280–281}}</ref>

[[File:Cooling curve pure metal.svg|class=skin-invert-image|500px]]

Note that before the thermal arrest the material is a liquid and after it the material is a solid; during the thermal arrest the material is converting from a liquid to a solid. Also, note that the greater the superheat the more time there is for the liquid material to flow into intricate details.<ref>{{Harvnb|Degarmo|Black|Kohser|2003|p=281}}</ref>

The above cooling curve depicts a basic situation with a pure metal, however, most castings are of alloys, which have a cooling curve shaped as shown below.

[[File:Cooling curve alloy.svg|class=skin-invert-image|900px]]

Note that there is no longer a thermal arrest, instead there is a freezing range. The freezing range corresponds directly to the liquidus and solidus found on the [[phase diagram]] for the specific alloy.

===Chvorinov's rule===
{{Main article|Chvorinov's rule}}

The local solidification time can be calculated using Chvorinov's rule, which is:

:<math>t = B \left( \frac{V}{A} \right)^n</math>

Where ''t'' is the solidification time, ''V'' is the [[volume]] of the casting, ''A'' is the [[surface area]] of the casting that contacts the [[molding (process)|mold]], ''n'' is a constant, and ''B'' is the mold constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless.<ref name="degarmo282">{{Harvnb|Degarmo|Black|Kohser|2003|p=282}}</ref>

===The gating system===<!-- [[Runner (casting)]] and [[Gate (casting)]] redirect here -->
[[File:Casting gating system.svg|class=skin-invert-image|thumb|right|400px|A simple gating system for a horizontal parting mold.]]
{{See also|Sprue (manufacturing)}}

The gating system serves many purposes, the most important being conveying the liquid material to the mold, but also controlling shrinkage, the speed of the liquid, turbulence, and trapping [[dross]]. The gates are usually attached to the thickest part of the casting to assist in controlling shrinkage. In especially large castings multiple gates or runners may be required to introduce metal to more than one point in the mold cavity. The speed of the material is important because if the material is traveling too slowly it can cool before completely filling, leading to misruns and cold shuts. If the material is moving too fast then the liquid material can erode the mold and contaminate the final casting. The shape and length of the gating system can also control how quickly the material cools; short round or square channels minimize heat loss.<ref name="degarmo284">{{Harvnb|Degarmo|Black|Kohser|2003|p=284}}</ref>

The gating system may be designed to minimize turbulence, depending on the material being cast. For example, steel, cast iron, and most copper alloys are turbulent insensitive, but aluminium and magnesium alloys are turbulent sensitive. The turbulent insensitive materials usually have a short and open gating system to fill the mold as quickly as possible. However, for turbulent sensitive materials short sprues are used to minimize the distance the material must fall when entering the mold. Rectangular pouring cups and tapered sprues are used to prevent the formation of a vortex as the material flows into the mold; these vortices tend to suck gas and oxides into the mold. A large sprue well is used to dissipate the kinetic energy of the liquid material as it falls down the sprue, decreasing turbulence. The ''choke'', which is the smallest cross-sectional area in the gating system used to control flow, can be placed near the sprue well to slow down and smooth out the flow. Note that on some molds the choke is still placed on the gates to make separation of the part easier, but induces extreme turbulence.<ref name="degarmo285">{{Harvnb|Degarmo|Black|Kohser|2003|p=285}}</ref> The gates are usually attached to the bottom of the casting to minimize turbulence and splashing.<ref name="degarmo284"/>

The gating system may also be designed to trap dross. One method is to take advantage of the fact that some dross has a lower density than the base material so it floats to the top of the gating system. Therefore, long flat runners with gates that exit from the bottom of the runners can trap dross in the runners; note that long flat runners will cool the material more rapidly than round or square runners. For materials where the dross is a similar density to the base material, such as aluminium, ''runner extensions'' and ''runner wells'' can be advantageous. These take advantage of the fact that the dross is usually located at the beginning of the pour, therefore the runner is extended past the last gate(s) and the contaminates are contained in the wells. Screens or filters may also be used to trap contaminates.<ref name="degarmo285"/>

It is important to keep the size of the gating system small, because it all must be cut from the casting and remelted to be reused. The efficiency, or ''{{visible anchor|yield}}'', of a casting system can be calculated by dividing the weight of the casting by the weight of the metal poured. Therefore, the higher the number the more efficient the gating system/risers.<ref name="degarmo287"/>

===Shrinkage===
There are three types of shrinkage: ''shrinkage of the liquid'', ''solidification shrinkage'' and ''patternmaker's shrinkage''. The shrinkage of the liquid is rarely a problem because more material is flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmaker's shrinkage refers to the shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to [[thermal contraction]].<ref>{{Harvnb|Degarmo|Black|Kohser|2003|pp=285–286}}</ref>

====Solidification shrinkage====
{| class="wikitable" align="right"
|+ Solidification shrinkage of various metals<ref name="degarmo286"/><ref>{{harvnb|Stefanescu|2008|p=66}}.</ref>
! Metal !! Percentage
|-
| Aluminium || 6.6
|-
| Copper || 4.9
|-
| Magnesium || 4.0 or 4.2
|-
| Zinc || 3.7 or 6.5
|-
| Low carbon steel || 2.5–3.0
|-
| High carbon steel || 4.0
|-
| White cast iron || 4.0–5.5
|-
| Gray cast iron || −2.5–1.6
|-
| Ductile cast iron || −4.5–2.7
|}

Most materials shrink as they solidify, but, as the adjacent table shows, a few materials do not, such as [[gray cast iron]]. For the materials that do shrink upon solidification the type of shrinkage depends on how wide the freezing range is for the material. For materials with a narrow freezing range, less than {{convert|50|C|abbr=on}},<ref name="stefanescu67">{{harvnb|Stefanescu|2008|p=67}}.</ref> a cavity, known as a ''pipe'', forms in the center of the casting, because the outer shell freezes first and progressively solidifies to the center. Pure and eutectic metals usually have narrow solidification ranges. These materials tend to form a ''skin'' in open air molds, therefore they are known as ''skin forming alloys''.<ref name="stefanescu67"/> For materials with a wide freezing range, greater than {{convert|110|C|abbr=on}},<ref name="stefanescu67"/> much more of the casting occupies the ''mushy'' or ''slushy'' zone (the temperature range between the solidus and the liquidus), which leads to small pockets of liquid trapped throughout and ultimately porosity. These castings tend to have poor [[ductility]], [[toughness]], and [[fatigue (material)|fatigue]] resistance. Moreover, for these types of materials to be fluid-tight, a secondary operation is required to impregnate the casting with a lower melting point metal or resin.<ref name="degarmo286"/><ref>{{Citation | last1 = Porter | first1 = David A. | last2 = Easterling | first2 = K. E. | title = Phase transformations in metals and alloys | page = 236 | publisher = CRC Press | year = 2000 | edition = 2nd | url = https://books.google.com/books?id=eYR5Re5tZisC | isbn = 978-0-7487-5741-1}}.</ref>

For the materials that have narrow solidification ranges, pipes can be overcome by designing the casting to promote directional solidification, which means the casting freezes first at the point farthest from the gate, then progressively solidifies toward the gate. This allows a continuous feed of liquid material to be present at the point of solidification to compensate for the shrinkage. Note that there is still a shrinkage void where the final material solidifies, but if designed properly, this will be in the gating system or riser.<ref name="degarmo286">{{Harvnb|Degarmo|Black|Kohser|2003|p=286}}</ref>
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====Risers and riser aids====
[[File:Casting riser types.svg|class=skin-invert-image|thumb|right|400px|Different types of risers]]
{{Main article|Riser (casting)|chill (casting)}}

Risers, also known as ''feeders'', are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage. For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the ''yield'' of each casting; i.e. more metal is lost as scrap for each casting. Another way to promote directional solidification is by adding chills to the mold. A chill is any material which will conduct heat away from the casting more rapidly than the material used for molding.<ref>{{harvnb|Degarmo|Black|Kohser|2003|pp=286–288}}.</ref>

Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then it is called an ''open'' riser, otherwise it is known as a ''blind'' type. The second criterion is where the riser is located; if it is located on the casting then it is known as a ''top riser'' and if it is located next to the casting it is known as a ''side riser''. Finally, if the riser is located on the gating system so that it fills after the molding cavity, it is known as a ''live riser'' or ''hot riser'', but if the riser fills with materials that have already flowed through the molding cavity it is known as a ''dead riser'' or ''cold riser''.<ref name="degarmo287">{{Harvnb|Degarmo|Black|Kohser|2003|p=287}}</ref>

Riser aids are items used to assist risers in creating directional solidification or reducing the number of risers required. One of these items are ''chills'' which accelerate cooling in a certain part of the mold. There are two types: external and internal chills. External chills are masses of high-heat-capacity and high-thermal-conductivity material that are placed on an edge of the molding cavity. Internal chills are pieces of the same metal that is being poured, which are placed inside the mold cavity and become part of the casting. Insulating sleeves and toppings may also be installed around the riser cavity to slow the solidification of the riser. Heater coils may also be installed around or above the riser cavity to slow solidification.<ref name="degarmo288">{{Harvnb|Degarmo|Black|Kohser|2003|p=288}}</ref>
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====Patternmaker's shrink====
{| class="wikitable" align="right"
|+ Typical patternmaker's shrinkage of various metals<ref name="degarmo289"/>
! Metal !! Percentage !! in/ft
|-
| Aluminium || 1.0–1.3 || {{Frac|1|8}}–{{Frac|5|32}}
|-
| Brass || 1.5 || {{Frac|3|16}}
|-
| Magnesium || 1.0–1.3 || {{Frac|1|8}}–{{Frac|5|32}}
|-
| Cast iron || 0.8–1.0 || {{Frac|1|10}}–{{Frac|1|8}}
|-
| Steel || 1.5–2.0 || {{Frac|3|16}}–{{Frac|1|4}}
|}

Shrinkage after solidification can be dealt with by using an oversized pattern designed specifically for the alloy used. ''{{visible anchor|Contraction rule}}s'', or ''{{visible anchor|shrink rule}}s'', are used to make the patterns oversized to compensate for this type of shrinkage.<ref name="degarmo289">{{Harvnb|Degarmo|Black|Kohser|2003|p=289}}</ref> These rulers are up to 2.5% oversize, depending on the material being cast.<ref name="degarmo288"/>  These rulers are mainly referred to by their percentage change. A pattern made to match an existing part would be made as follows:  First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction rule, ensuring that the casting would contract to the correct size.

Note that patternmaker's shrinkage does not take phase change transformations into account. For example, eutectic reactions, [[martensitic]] reactions, and [[graphitization]] can cause expansions or contractions.<ref name="degarmo289"/>

===Mold cavity===
The mold cavity of a casting does not reflect the exact dimensions of the finished part due to a number of reasons. These modifications to the mold cavity are known as ''allowances'' and account for patternmaker's shrinkage, draft, machining, and distortion. In non-expendable processes, these allowances are imparted directly into the permanent mold, but in expendable mold processes they are imparted into the patterns, which later form the mold cavity.<ref name="degarmo289"/> Note that for non-expendable molds an allowance is required for the dimensional change of the mold due to heating to operating temperatures.<ref name="degarmo290"/>

For surfaces of the casting that are perpendicular to the parting line of the mold a draft must be included. This is so that the casting can be released in non-expendable processes or the pattern can be released from the mold without destroying the mold in expendable processes. The required draft angle depends on the size and shape of the feature, the depth of the mold cavity, how the part or pattern is being removed from the mold, the pattern or part material, the mold material, and the process type. Usually the draft is not less than 1%.<ref name="degarmo289"/>

The machining allowance varies drastically from one process to another. Sand castings generally have a rough surface finish, therefore need a greater machining allowance, whereas die casting has a very fine surface finish, which may not need any machining tolerance. Also, the draft may provide enough of a machining allowance to begin with.<ref name="degarmo290">{{Harvnb|Degarmo|Black|Kohser|2003|p=290}}</ref>

The distortion allowance is only necessary for certain geometries. For instance, U-shaped castings will tend to distort with the legs splaying outward, because the base of the shape can contract while the legs are constrained by the mold. This can be overcome by designing the mold cavity to slope the leg inward to begin with. Also, long horizontal sections tend to sag in the middle if ribs are not incorporated, so a distortion allowance may be required.<ref name="degarmo290"/>

Cores may be used in expendable mold processes to produce internal features. The core can be of metal but it is usually done in sand.<!-- This topic needs expansion -->

===Filling{{anchor|Filling}}===
[[File:Low pressure permanent mold casting schematic.svg|class=skin-invert-image|thumb|300px|upright|Schematic of the low-pressure permanent mold casting process]]
{{Expand section|date=February 2010}}

There are a few common methods for filling the mold cavity: ''gravity'', ''low-pressure'', ''high-pressure'', and ''vacuum''.<ref name="degarmo319"/>

Vacuum filling, also known as ''counter-gravity'' filling, is more metal efficient than gravity pouring because less material solidifies in the gating system. Gravity pouring only has a 15 to 50% metal yield as compared to 60 to 95% for vacuum pouring. There is also less turbulence, so the gating system can be simplified since it does not have to control turbulence. Plus, because the metal is drawn from below the top of the pool the metal is free from [[dross]] and slag, as these are lower density (lighter) and float to the top of the pool. The pressure differential helps the metal flow into every intricacy of the mold. Finally, lower temperatures can be used, which improves the grain structure.<ref name="degarmo319">{{harvnb|Degarmo|Black|Kohser|2003|pp=319–320}}.</ref> The first patented vacuum casting machine and process dates to 1879.<ref>{{Citation | last = [[Iron and Steel Institute]] | title = Journal of the Iron and Steel Institute | page = 547 | publisher = Iron and Steel Institute | year = 1912 | volume = 86 | url = https://books.google.com/books?id=Bz8ZAQAAIAAJ&pg=RA2-PA547 | postscript =.}}</ref>

Low-pressure filling uses 5 to 15&nbsp;psig (35 to 100&nbsp;kPag) of air pressure to force liquid metal up a feed tube into the mold cavity. This eliminates turbulence found in gravity casting and increases density, repeatability, tolerances, and grain uniformity. After the casting has solidified the pressure is released and any remaining liquid returns to the crucible, which increases yield.<ref>{{Citation | last = Lesko | first = Jim | title = Industrial design | page = 39 | publisher = John Wiley and Sons | year = 2007 | edition = 2nd | url = https://books.google.com/books?id=1_3snz7LgiMC&pg=PA39 | isbn = 978-0-470-05538-0 | postscript =.}}</ref>

====Tilt filling{{Anchor|Tilt casting}}====
''Tilt filling'', also known as ''tilt casting'', is an uncommon filling technique where the crucible is attached to the gating system and both are slowly rotated so that the metal enters the mold cavity with little turbulence. The goal is to reduce porosity and inclusions by limiting turbulence. For most uses tilt filling is not feasible because the following inherent problem: if the system is rotated slow enough to not induce turbulence, the front of the metal stream begins to solidify, which results in mis-runs. If the system is rotated faster it induces turbulence, which defeats the purpose. [[Durville (metallurgist)|Durville]] of France was the first to try tilt casting, in the 1800s. He tried to use it to reduce surface defects when casting coinage from [[aluminium bronze]].<ref>{{Citation | last = Campbell | first = John | title = Castings practice: the 10 rules of castings | pages = 69–71 | publisher = Butterworth-Heinemann | year = 2004 | url = https://books.google.com/books?id=MS-JFA04n0QC&pg=PA69 | isbn = 978-0-7506-4791-5 | postscript =.}}</ref>

===Macrostructure===
The grain macrostructure in ingots and most castings have three distinct regions or zones: the chill zone, columnar zone, and equiaxed zone. The image below depicts these zones.

[[File:Cast ingot macrostructure.svg|class=skin-invert-image|500px]]

The chill zone is named so because it occurs at the walls of the mold where the wall ''chills'' the material. Here is where the nucleation phase of the solidification process takes place. As more heat is removed the grains grow towards the center of the casting. These are thin, long ''columns'' that are perpendicular to the casting surface, which are undesirable because they have [[anisotropic]] properties. Finally, in the center the equiaxed zone contains spherical, randomly oriented crystals. These are desirable because they have [[isotropic]] properties. The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or [[wikt:inoculant|inoculants]].<ref name="degarmo282"/>

===Inspection===
Common inspection methods for steel castings are ''[[magnetic particle inspection|magnetic particle testing]]'' and ''[[liquid penetrant testing]]''.<ref>{{harvnb|Blair|Stevens|1995|p=4‐6<!-- This is not a range! -->}}.</ref> Common inspection methods for aluminum castings are ''[[industrial radiography|radiography]]'', ''[[ultrasonic testing]]'', and ''liquid penetrant testing''.<ref>{{harvnb|Kissell|Ferry|2002|p=73}}.</ref>

====Defects====
{{Main article|Casting defects}}

There are a number of problems that can be encountered during the casting process. The main types are: ''gas porosity'', ''shrinkage defects'', ''mold material defects'', ''pouring metal defects'', and ''metallurgical defects''.