Casting Inclusion
Casting inclusion refers to non-metallic material that is entrapped or otherwise incorporated in a casting during the casting process. Common casting inclusions involve slag, sand, oxide films, and other materials. Casting inclusions can range from extremely harmful to almost negligible. It is important to understand the effects of casting inclusions on the mechanical properties of a casting.
Slag is largely composed of calcium oxide, silicon dioxide and magnesium oxide. It is the material which forms during the melting of a molten metal. It has a higher surface tension than the liquid metal and as a result it tends to remain in a liquid form even at low temperatures. Slag inclusions can cause porosity and inclusions in the casting and can lead to hot tears, stress concentrations, reduced fatigue strength and lower impact resistance.
Sand can be incorporated into castings during the pouring of the molten metal. Sand has a relatively high melting point and can form defects such as leathery pockets or gaps, blocking of channels, and increased porosity in the casting. Sand can also act as an entrapment site for gases, leading to trapped gas inclusions.
Oxide films form when the surface of the liquid metal comes into contact with oxygen or other substances in the atmosphere. Oxides can form a coating on the liquid metal, resulting in grain boundary oxidization, which reduces the grain boundaries and thus reduces the strength of the casting. Oxides can also form large inclusions in the castings leading to areas of reduced strength and porosity.
Other common casting inclusions include slurry, scales, flux, and residues from the flux. Slurry is a mixture of liquid and solid particles, which is formed during the casting process and can cause porosity, reduced impact strength, and inclusions. Scales are formed on the surface of the casting, leading to a porous surface, decreased fluidity, and reduced impact strength. Flux can form a hard and brittle coating on the casting, reducing its mechanical properties. Residues from the flux can also be entrapped in the casting and can form large areas of porosity.
In order to achieve the best mechanical properties in a casting, it is important to minimize the presence of casting inclusions. Proper monitoring of the chemical composition of the molten metal, control of the pouring temperature, and control of the atmosphere around the molten metal during the casting process are all important steps that must be taken to prevent the presence of casting inclusions. Additionally, post-casting processes such as trimming and descaling can be used to remove casting inclusions and are often a necessary part of the manufacturing process.
