Residual shrinkage porosity is an important type of porosity found in castings, which has great influence on the integrity, mechanical and fatigue properties of the casting. In this paper, the formation and characteristic of residual shrinkage porosity is discussed in detail.
Definition of residual shrinkage porosity
Residual shrinkage porosity is a kind of open porosity which usually scattered in the casting products and occurs in the homogeneous primary solidification structure. Generally, the residual shrinkage porosity has solidifled form, which often appears in the form of spherical or dendritic cavities s. It has fine size, wall thickness and generally exceeds the normal wall thickness of chilled structure (R >2.0).
Features of residual shrinkage porosity
One feature is that the shape of residual shrinkage pores is spherical or dendritic, thus it looks like the copper salt crystal structure, and the size is at μm scale. Secondly, these residuals are distributed randomly in the casting or are concentrated in a certain area depending on the size of the casting. Thirdly, some of these residual shrinkage porosity may be connected with each other, forming a continuous chain due to the existence of channels.
Formation of residual shrinkage porosity
Residual shrinkage porosity is formed during the solidification and cooling of castings. It is caused by shrinkage of liquid metal during cooling and subsequent local solidification, which usually results from either lack of nucleation sites or lack of feeding which caused insufficient liquid metal to be filled in some areas.
On the one hand, in the liquid metal state, when the temperature is lower than the liquidus temperature, solidification will occur and the solidification structure with high viscosity will begin to form. On the other hand, when solidification structure becomes stable, shrinkage cavity is generated at local solidification area because of the uneven solidification speed or insufficient feeding, or even no feeding in certain areas.
Therefore, the quantity and distribution of residual shrinkage porosity in castings are related to the three attributes of solidification structure - cooling rate, filling rate and nucleation rate. Firstly, the cooling rate, referring to the speed of solidification, is too high, the solidified structure becomes very coarse, and the shrinkage rate is also high, resulting in more shrinkage porosity. Secondly, lower filling rate in casting mould will lead to local shrinkage due to insufficient liquid metal needed for solidification. Thirdly, insufficient number of nucleation sites, such as gas holes and micro slag particles, will result in worse occupation of solidification, thus more shrinkage porosity is generated.
Influence of residual shrinkage porosity
Residual shrinkage porosity has great influence on the service performance of castings, such as mechanical properties and fatigue properties. Firstly, residual shrinkage porosity is distributed randomly in the castings, which inevitably reduces the integrity and yields a significant drop of mechanical properties such as strength, plasticity, fatigue strength and so on. Secondly, residual shrinkage porosity will accelerate surface corrosion and fatigue cracking and then reduce material fatigue life.
In summary, residual shrinkage porosity is a kind of open porosity which usually scattered in the casting products and occurs in the homogeneous primary solidification structure. It has great influence on both integrity and fatigue performance of castings, thus it should be paid attention to in the manufacture of castings. The formation of residual shrinkage porosity is mainly due to the uneven cooling rate, insufficient filling rate and lack of nucleation sites. Understanding and controlling these factors will help to reduce the quantity of residual shrinkage porosity and thus improve the quality and performance of castings.
