Factors Affecting Shrinkage Cavities and Porosity
Shrinkage cavities and porosity in castings are formed during the period from the initial formation of a thin solid shell on the surface to the complete solidification of the center of the casting. Their volume is determined by the difference between the liquid and solid shrinkage (liquid contraction during cooling and solidification shrinkage) and the final solid-state shrinkage.
Main Influencing Factors:
1. Metal Properties
The higher the liquid shrinkage coefficient and the total shrinkage rate during liquid and solidification phases, the larger the volume of shrinkage cavities and porosity. However, the greater the shrinkage in the solid state, the smaller the volume of shrinkage defects, and the less likely they are to form.
2. Mold Conditions
The stronger the chilling capacity of the mold, the smaller the shrinkage cavity and porosity volume. A high chilling rate accelerates solidification, making the metal shrinkage more effectively compensated by subsequently poured molten metal, reducing the actual amount of liquid metal that undergoes shrinkage.
3. Pouring Conditions
Higher pouring temperatures increase liquid shrinkage and thus enlarge the volume of shrinkage cavities. However, under proper risering conditions, a high pouring temperature can improve feeding effectiveness and reduce shrinkage cavity volume. Slower pouring speeds and longer pouring times also help reduce shrinkage cavities.
4. Casting Dimensions
The thicker the section of the casting, the more prone it is to forming shrinkage cavities and porosity. After the surface solidifies, the inner molten metal in thick sections remains at a higher temperature, resulting in a larger liquid shrinkage volume and a higher chance of shrinkage defect formation.
5. Feeding Pressure
Applying feeding pressure during solidification can effectively reduce the tendency for shrinkage cavities and porosity to form.
Due to the influence of graphitization expansion and mold rigidity on the formation of shrinkage cavities and porosity in gray cast iron and ductile iron, the main factors affecting shrinkage defects in these materials also include the composition of the cast iron and the rigidity of the mold.
Prevention Measures for Shrinkage Cavities and Porosity
Preventing the formation of shrinkage cavities and porosity mainly depends on developing proper casting processes tailored to specific parts, ensuring adequate feeding conditions during solidification. Key measures include:
• Adopting appropriate solidification principles (directional or simultaneous solidification);
• Designing correct pouring processes;
• Introducing a well-structured gating system;
• Implementing effective feeding techniques such as risering or applying pressure during solidification.
Post time: Apr-11-2025