Introduction
Castings are often found with subsurface defects attributed to shrinkage. These defects known as “shrinkage porosity”, “shrinkage cavity”, or “cold shut” are located in areas where shrinkage flow of the solidifying metal is impeded due to the geometry of the casting. Shrinkage porosity can cause a variety of problems, including mechanical failures, corrosion, and service wear. It is, therefore, important to understand the mechanism of the porosity and how to control it.
Types of Shrinkage Porosity
Shrinkage porosity refers to a type of subsurface defects caused by uneven solidification of the metal during casting. These defects are characterized by large cavities and pores in areas of the casting where the solidifying metal is prevented from flowing into the mold due to its geometry. Because the space in these areas cannot be filled with solid metal, the resulting voids form large cavities that look like wrinkles. These wrinkles are known as “shrinkage porosity” or “cold shut” defects.
Causes of Shrinkage Porosity
The most common cause of shrinkage porosity is the casting geometry. As the metal cools and contracts, it must fill the mold and fill any voids or spaces created by the geometry of the casting. In areas where the geometry of the casting restricts the flow of the metal, it will not be able to flow into the mold and the resulting “cold shut” or “shrinkage porosity” will form.
Other causes of shrinkage porosity include inadequate risers or risers that are too far from the casting, incorrect casting temperature, poor venting of the mold, and insufficiency mold wall thickness. All of these factors can prevent the metal from filling the mold and can contribute to the formation of shrinkage porosity.
Prevention and Control of Shrinkage Porosity
The first step in preventing shrinkage porosity is to properly design the casting and the riser system. This includes making sure that there are sufficient risers in the right locations and that they are properly vented. It also includes making sure that any areas that may be prone to shrinkage porosity are adequately sized and shaped and do not have geometries that may impede the flow of the metal.
It is also important to make sure that the casting is heated properly. This means controlling the temperature of the furnace and making sure that the furnace is in good condition and providing the correct amount of heat. It is also important to make sure that the mold is vented properly.
Conclusion
Shrinkage porosity is a common type of subsurface defect that can lead to a variety of mechanical and service wear problems. The best way to prevent shrinkage porosity is to properly design the riser system and the casting geometry and to make sure that the casting is heated properly and the mold is vented properly. By following these steps, it is possible to greatly reduce the incidence of shrinkage porosity.
