An entrainment defect is a term used in
metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the sc ...
to describe a defect created in a
casting
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
by the folding-over of the
oxidized
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
surface layer of the molten
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
into the bulk liquid.
Outside of special cases, the surface of a molten metal is covered with a thin, solid layer of oxide. If the surface of the liquid happens to fold in on itself, or if a separate droplet falls into it, there will be a solid-on-solid interface between the two films. This can happen as a consequence of any kind of turbulence during the processing of the molten metal. Once solidified, this interface become functionally indistinguishable from a crack.
The presence of such crack-like entrainment in solid metal articles constitutes a metalworking defect. The continuous casting process used in the initial forming of most wrought alloys is also susceptible to entrainment. Any entrainment defects thus formed are inherited by the wrought products.
When the entrainment is initially formed, it contains some amount of trapped air. This pocket of air, however, will be chemically consumed over time. The oxygen in this pocket of air will be used up by further thickening of the oxide layer. The nitrogen may then be converted into nitrides, although this process depends on which metal it happens in.
When liquid metal is subjected to high pressure, as during
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 t ...
, liquid metal may be forced through the oxide film. This phenomenon is able to partially bond the two facing oxide surfaces, thus reducing the severity of the defect.
See also
*
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 incl ...
References
* ''Castings'' by J. Campbell (Elsevier, 2003) Chapter 2.
Casting (manufacturing)
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