Directional solidification
Directional solidification (DS) and progressive solidification are
types of solidification within castings. Directional solidification is
solidification that occurs from farthest end of the casting and works
its way towards the sprue. Progressive solidification, also known as
parallel solidification,[1] is solidification that starts at the walls of
[2]
the casting and progresses perpendicularly from that surface.
Directional solidification
Contents
Theory
References
Bibliography
Further reading
Theory
Most metals and alloys shrink as the material changes from a liquid
state to a solid state. Therefore, if liquid material is not available to
Progressive solidification
compensate for this shrinkage a shrinkage defect forms.[3] When
progressive solidification dominates over directional solidification a
shrinkage defect will form.[2]
The geometrical shape of the mold cavity has direct effect on
progressive and directional solidification. At the end of tunnel-type
geometries divergent heat flow occurs, which causes that area of the
casting to cool faster than surrounding areas; this is called an end
effect. Large cavities do not cool as quickly as surrounding areas
because there is less heat flow; this is called a riser effect. Also note
that corners can create divergent or convergent (also known as hot
spots) heat flow areas.[4]
In order to induce directional solidification chills, risers, insulating
sleeves, control of pouring rate, and pouring temperature can be
utilized.[5]
Directional solidification can be used as a purification process. Since most impurities will be more soluble in the liquid than in the
solid phase during solidification, impurities will be "pushed" by the solidification front, causing much of the finished casting to have
a lower concentration of impurities than the feedstock material, while the last solidified metal will be enriched with impurities. This
last part of the metal can be scrapped or recycled. The suitability of directional solidification in removing a specific impurity from a
certain metal depends on the partition coefficient of the impurity in the metal in question, as described by the Scheil equation.
Directional solidification (in zone melting) is frequently employed as a purification step in the production of multicrystalline silicon
for solar cells.
References
1. Stefanescu 2008, p. 67.
2. Chastain 2004, p. 104.
3. Kuznetsov, A.V.; Xiong, M. (2002). "Dependence of microporosity formation on the directio
n of solidification".
International Communications in Heat and Mass rTansfer. 29 (1): 25–34. doi:10.1016/S0735-1933(01)00321-9(http
s://doi.org/10.1016%2FS0735-1933%2801%2900321-9) .
4. Stefanescu 2008, p. 68.
5. Chastain 2004, pp. 104–105,..
Bibliography
Chastain, Stephen (2004),Metal casting: a sand casting manual for the small foundry
, Vol. II, 4, Stephen Chastain,
ISBN 978-0-9702203-3-2.
Stefanescu, Doru Michael (2008),Science and Engineering of Casting Solidification(2nd ed.), Springer, ISBN 978-0-
387-74609-8.
Further reading
Campbell, John, Castings (2nd ed.), Butterworth-Heinemann,ISBN 0-7506-4790-6.
Wlodawer, Robert (1966), Directional solidification of steel castings, Pergamon Press.
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