Casting Defects and Their Remedies
Casting is a manufacturing process that involves the pouring of molten metal or other materials into a pre-designed mould or tool and then allowing the poured material to solidify. In spite of the long history of the practice, casting defects still arise during the fabrication of a product.
Essentially, any casting defect can be classified as a result of either the process of casting or the tooling of the mould used. Process-related defects arise due to the changing properties of the molten alloy material while tooling-related defects occur due to incorrect design or fabrication of the mould.
A wide variety of casting defects may occur during the process, some of the more common defects include: shrinkage defects, gas porosity, hot tearing, cold shut and misruns. Each of these issues has their own symptoms and remedies and it is important for any caster to understand their causes and effects in order to rectify the problem if a casting is rejected.
Shrinkage defects are commonly observed in castings and are due to the cooling and solidification of the alloy material, which tends to contract in size as it cools. Shrinkage defects are usually associated with the interior of the part, although they may be seen on the surface of the casting as well. The most common types of shrinkage defects seen in castings include shrinkage porosity, centreline shrinkage, and internal or external shrinkage cavities. In order to avoid shrinkage defects, it is important to make sure that the casting is completely filled out by the alloy when poured and that it is allowed to cool and solidify naturally.
Gas porosity is another frequently encountered casting defect and is most often caused by gases that become trapped in the molten alloy material. The gases expand as they reach the interior of the casting, thus creating voids in the material, also known as shrinkage voids. To correct this issue, it is important to ensure that the furnace and mould are properly vented to allow the escape of any entrapped gases.
The hot tearing and cold shut defects are usually a result of incorrect tooling or the incorrect pouring of the molten material. Hot tearing is a condition of tearing that occurs in the aggressive solidification of the casting and is often seen in castings with sharp internal corners that undergo gradual cooling from the inside to the outside. Cold shuts are also caused by cooling stresses in the material, but in this case, there is interfacial separations of the two metals in contact. To prevent these issues, it is important to ensure that the tooling is properly designed and that the molten material is poured into the mould evenly and without turbulence.
The last common issue, misruns, is the result of improper filling of the mould and is typically caused by the presence of ingates, vents and risers that are not properly sized. Misruns usually appear in the form of incomplete castings and are usually difficult to identify until after the part is removed from the mould. To prevent the occurrence of misruns, it is important to ensure the correct sizing and shape of all the solidification features of the mould.
In summary, over time, metal casting manufacturing has evolved and improved significantly, and todays casting operations are carried out in highly controlled and advanced environments. Despite this, casting defects are still a common issue that can occur during fabrication, and it is important to be aware of the causes and remedies associated with each type of defect. Understanding the different types of casting defects, their causes and remedies, can go a long way in ensuring improved product quality and greater customer satisfaction.
