Editing Metal casting (section)

====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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