In the casting process, shrinkage feeding is the core link that determines the density and mechanical properties of castings. From engine blocks to aviation turbine blades, if the volume shrinkage of molten metal during solidification is not effectively compensated, it will directly lead to defects such as shrinkage cavities.
The essence of casting shrinkage feeding
The essence of shrinkage feeding is to compensate for the volume loss of liquid to solid state by directional control of molten metal flow and solidification timing. Its effect is directly affected by the solidification characteristics of the material:
- Iron castings (grey cast iron, ductile iron): The self-compensation ability generated by graphitization expansion can offset 60%-80% of the volume shrinkage. The process design focuses on using material characteristics to reduce riser dependence;
- Steel castings: There is a lack of self-compensation mechanism, and forced compensation is required through the riser system. The process focuses on the riser modulus design and solidification path control;
- Non-ferrous alloy parts (aluminum alloy, copper alloy): The shrinkage rate is high and the heat conduction is fast. A multi-stage riser and chill combination process is often used to balance the solidification gradient.
A certain automobile crankshaft casting case shows that by activating the graphite expansion effect of ductile iron, the process yield is improved while retaining the riser compensation amount.
The three rules of compensation process design
Hot node locking principle
The compensation path must cover the key hot nodes of the casting (the intersection of thick walls or the end of solidification). Taking the hub of a wind turbine as an example, an annular riser is usually set at the connection between the flange and the hub, and the static pressure of the metal liquid is used to penetrate the solidification front to eliminate the risk of shrinkage in the thick wall area.
Timing control technology
The solidification time of the shrinkage feeding system needs to be later than that of the casting body. Common technologies include:
- Insulating riser sleeve: ceramic fiber or expanded perlite is used to reduce heat loss and extend the shrinkage feeding window period;
- Heat-generating riser: releases additional heat energy through aluminothermic reaction, especially suitable for materials with long solidification intervals such as high-alloy steel.
In a marine propeller casting project, the shrinkage defect rate in the thick blade area decreased after the composite heat-generating and heat-insulating riser was used.
Channel efficiency optimization
The geometric design of the riser neck needs to take into account both flow efficiency and shrinkage termination control:
The tapered neck design reduces flow resistance and achieves a “self-cutting” effect through cross-sectional shrinkage;
For thin-walled parts, a flat neck structure can be used to accelerate the shrinkage liquid flow. A hydraulic valve body casting company has improved the utilization rate of shrinkage metal by optimizing the neck aspect ratio.
Innovation of feeding strategies for typical scenarios
Large structural parts (machine tool base, turbine runner): adopt the “chill iron + riser” combination process, use the chill iron to accelerate local solidification, and concentrate the feeding resources to solve the problem of thick and large hot joints;
Precision thin-walled parts (automobile steering knuckle, electronic housing): through the cross runner feeding and micro heating riser linkage, while reducing the weight of the riser to ensure density;
Complex art castings (bronze sculptures, decorative components): customized heating riser combined with graphite quenching to achieve complete molding of texture details.
Conclusion
Casting feeding is a deep fusion of scientific theory and engineering experience in casting technology. In the future, with the deepening application of artificial intelligence algorithms in solidification simulation, the feeding process will more accurately serve high-end manufacturing fields such as new energy equipment and aerospace, providing a solid technical foundation for “zero defect casting”.