How to Solve the Shrinkage Cavity and Porosity Problems in Die Castings and Other Castings

The existence of shrinkage cavities and porosity in die castings and other castings is a common phenomenon. Is there a way to completely solve this problem? The answer should be yes, but what exactly is it?

1. Causes of Shrinkage Cavities and Porosity in Die Castings

There is only one reason for the occurrence of shrinkage cavities and porosity in die castings: the inevitable phase change shrinkage that occurs when the molten metal transforms from a liquid phase to a solid phase after filling the mold cavity. Due to the solidification characteristic of die castings—cooling from the outside to the inside—when the wall thickness of the casting is relatively large, shrinkage cavities and porosity will inevitably form inside.

Therefore, for die castings, especially those with large and thick sections, the existence of shrinkage cavities and porosity is inevitable and cannot be completely eliminated through die casting process adjustments alone.

2. The Only Way to Resolve Shrinkage Cavity and Porosity Defects in Die Castings

The problem of shrinkage cavities and porosity in die castings cannot be completely solved by the die casting process itself. To thoroughly address this issue, it is necessary to go beyond the limitations of this process or seek solutions from outside the die casting system.

So, what is this solution?

In terms of process principles, solving shrinkage cavity and porosity defects in castings can only be achieved by following the technical concept of feeding (compensating for volume shrinkage). The phase change shrinkage during the solidification of castings is a natural physical phenomenon; we cannot go against the laws of this natural phenomenon, but only follow them to solve the problem.

3. Two Approaches to Feeding

There are two approaches to feeding castings: natural feeding and forced feeding.

To achieve natural feeding, the casting process system must include technical measures to enable "sequential solidification" (solidification progresses from the farthest part of the feeder to the feeder itself). Many people intuitively believe that using the low-pressure casting method can solve shrinkage cavity and porosity defects, but this is not the case. The application of low-pressure casting technology does not inherently eliminate such defects. If the low-pressure casting system lacks dedicated feeding measures, the blanks produced by this method may still have a 100% probability of shrinkage cavities and porosity.

Due to the inherent characteristics of the die casting process, it is relatively difficult and complex to implement natural "sequential solidification" measures. The most fundamental reason may be that "sequential solidification" always requires a relatively long solidification time for the casting, which conflicts with the core advantage of the die casting process (high production efficiency with short cycle times).

The most prominent feature of forced solidification and feeding is its short solidification time—usually only 1/4 of that required for "sequential solidification" or even shorter. Therefore, adding forced feeding measures to the die casting process system is compatible with the characteristics of the die casting process and can effectively solve the shrinkage cavity and porosity problems in die castings.

4. Two Levels of Forced Feeding: Extrusion Feeding and Forging Feeding

Forced feeding of castings can be achieved at two levels. One level is sufficient to basically eliminate shrinkage cavity and porosity defects, and the other level can make the internal structure of the blank reach a state of grain refinement or forged structure. To describe these two different levels with distinct terms, the former can be referred to as "extrusion feeding," and the latter as "forging feeding."

An important understanding and concept to clarify is that feeding is always a direct method and cannot be accomplished indirectly. In terms of process parameters, we can use a specific parameter to represent this direct action: "feeding pressure."

In terms of physical principles, the concept of pressure can occur in two scenarios: one in liquid systems (i.e., the context of "Archimedes' principle"), which we define as "liquid pressure" for clarity; the other in solid systems, which we define as "solid pressure." It is crucial to note the applicable conditions of these two pressure concepts under different states—confusing them will lead to major problems.

"Liquid pressure" is only applicable to liquid systems; its pressure direction can be transmitted and redirected, but it is completely inapplicable to solid systems.

The feeding of die castings occurs between the semi-solid and solid states. The pressure involved in this process has a specific direction and is a vector pressure, whose direction is the same as the direction of the applied feeding force.

Therefore, the idea that increasing the pressure of the die casting machine's injection cylinder or increasing the injection-specific pressure to solve shrinkage cavity and porosity in die castings—assuming that this injection-specific pressure can be transmitted throughout the entire solidification stage of the casting to achieve feeding—is completely incorrect.

5. Adopting the "First Die Casting Filling, Then Die Forging Feeding" Process: An Effective and Ultimate Solution to Shrinkage Cavity and Porosity Defects

The "first die casting filling, then die forging feeding" process can be simply referred to as the "die casting-forging process." Its essence is a continuous casting-forging process that combines the die casting process with the liquid die forging process, integrating the most effective functions of these two types of equipment to complete the entire production process.

This continuous casting-forging "die casting-forging machine" is very similar in appearance to ordinary vertical or horizontal die casting machines. In fact, it is a die casting machine equipped with an additional hydraulic forging head. The maximum forging feeding force that can be added can be equal to the maximum clamping force of the die casting machine.

It should be noted that the most important nominal parameter of this die casting-forging machine is not the clamping force, but the die forging feeding force—equivalent to the forging force of a four-column hydraulic press. This is a key point that must be fully considered when selecting equipment. Otherwise, if you purchase a die casting-forging machine with high clamping force but low die forging feeding force, its practical value will be greatly reduced.

The blanks produced by this die casting-forging machine have extremely high dimensional accuracy and surface finish, which can match the precision and surface roughness levels achievable by machining processes of grade 6 or higher. This process can be classified as an "ultimate forming" technology, representing a significant advancement beyond the "near-net shape forming" (no-cutting or minimal-machining allowance forming) process.