An Effective Solution to Shrinkage Cavity and Porosity of Die Castings

1. Reasons for shrinkage cavity and porosity of die castings

There is only one reason for the shrinkage cavity and porosity of die castings, that is, the inevitable phase change shrinkage when the metal melt transforms from liquid phase to solid phase after mold filling Because the solidification characteristic of die casting is cooling from outside to inside, when the wall thickness of the casting is large, shrinkage cavity and porosity will inevitably occur inside

Therefore, for die castings, especially for thick die castings, the problem of shrinkage cavity and porosity is inevitable and cannot be solved

2. The only way to solve the shrinkage defects of die castings

The problem of shrinkage cavity and porosity of die castings cannot be completely solved by the die casting process itself. To completely solve this problem, we can only go beyond this process, or seek solutions outside the system

What is this method?

From the technological principle, the solution to the shrinkage cavity and porosity defects of castings can only be carried out according to the technological idea of feeding The phase transformation shrinkage in the solidification process of castings is a natural physical phenomenon. We cannot reverse the law of this natural phenomenon, but can only follow its law to solve this problem

3. Two ways of replenishment

There are two ways to feed castings: natural feeding and forced feeding

In order to realize natural feeding, our casting process system should have technological measures that can realize "sequential solidification" Many people intuitively think that the low pressure casting method can solve the shrinkage cavity and porosity defects of castings, but this is not the case The application of low pressure casting process does not mean that the shrinkage cavity and porosity defects of castings can be solved. If the low pressure casting process system is not equipped with feeding process measures, then the blanks produced by this low pressure casting method may also have shrinkage cavity and porosity defects 100%

Due to the characteristics of the die casting process itself, it is difficult and complex to establish a natural "sequential solidification" process The most fundamental reason may also be that the process measure of "sequential solidification" always requires a relatively long solidification time for castings, which is somewhat contradictory to the die casting process itself

The biggest feature of forced solidification feeding is that the solidification time is short, generally only reaching a quarter or less of the "sequential solidification". Therefore, on the basis of the die casting process system, the addition of forced feeding process measures is compatible with the characteristics of the die casting process, which can well solve the problem of shrinkage cavity and porosity of die castings

4. Two levels of forced feeding: extrusion feeding and forging feeding

There are two ways to realize the forced feeding of castings One is basically the extent to eliminate shrinkage cavity and porosity of castings, and the other is the extent to make the inside of the blank reach the level of broken grain or forged structure If we want to use different words to express these two different degrees, we can use "extrusion feeding" to express the former, and "forging feeding" to express the latter

It should be noted that feeding is a direct means and cannot be completed indirectly. Technically, we can have a process parameter to express it, which is "feeding pressure"

In physical principle, the concept of pressure can appear in two ways. One is in the liquid situation, that is, the "Archimedes Law" situation. To make a clear distinction, we define it as the "liquid pressure", while the other is in the solid situation, that is, we define it as the "solid pressure". Note that the applicable conditions of the concept of pressure in these two different states. If we confuse, big problems will arise

"Liquid pressure" is only applicable to liquid system, its pressure direction can be transmitted and can turn, but it is not applicable to solid system at all

The feeding of die castings occurs between semi-solid and solid. Its pressure value is directional and a vector pressure. Its direction is the same as that of the feeding force applied

Therefore, it is totally wrong to think that the shrinkage cavity and porosity of the die casting can be solved by increasing the pressure of the injection cylinder of the die-casting machine and the injection filling specific pressure, and that the injection specific pressure can be transferred to the whole process of the casting solidification stage, so as to realize the casting feeding

5. The process of "die casting filling before die forging feeding" is an effective way to solve the shrinkage cavity and porosity defects of castings, and is also an ultimate means

”The process of die casting filling before die forging feeding can be referred to as "die casting die forging" Its essence is a continuous casting and forging process, which is to combine the die-casting process with the liquid die forging process, combine the most effective functions of these two devices, and complete the entire process.

The appearance of this continuous casting and forging "die casting die forging" equipment is very similar to that of ordinary vertical or horizontal die casting machines. In fact, the hydraulic forging head is added to the die casting machine. The maximum forging feeding force that can be added is equal to the maximum locking force of the die casting machine.

It should be noted that the most important nominal parameter of this die-casting die forging machine is not the clamping force, but the die forging feeding force, which is equivalent to the meaning of the forging pressure of the four column hydraulic press, which we must pay full attention to when selecting equipment Otherwise, if you buy a die casting die forging equipment with large clamping force but small die forging feeding force, its use value will be greatly reduced.

The blanks produced by this die casting die forging machine have high dimensional accuracy and high surface finish, which can be equivalent to the accuracy and surface roughness level that can be achieved by machining methods above level 6 It can already be attributed to the process means of "limit forming", which is more advanced than the process of "forming without cutting and less allowance".