The problem of shrinkage and porosity in die castings and other castings is a common phenomenon. Is there a complete solution to this problem? The answer should be yes, but what would it be?
1. Reasons for the existence of shrinkage and porosity in die-casting parts
There is only one reason for the phenomenon of shrinkage and porosity in die castings, which is the phase change shrinkage that inevitably occurs when the metal melt transforms from a liquid phase to a solid phase after filling. Due to the solidification characteristics of die castings, which are cooled from the outside to the inside, when the wall thickness of the casting is large, shrinkage and porosity problems will inevitably occur internally
So, for die-casting parts, especially for thick die-casting parts, the problem of shrinkage and porosity is inevitable and cannot be solved
2. The only way to solve the shrinkage and porosity defects in die-casting parts
The problem of shrinkage and porosity in die-casting parts cannot be completely solved from the die-casting process itself. To completely solve this problem, one can only go beyond the process or seek solutions outside the system
What is this method?
From the perspective of process principle, solving the shrinkage and porosity defects in castings can only be carried out according to the process concept of feeding. The phase change shrinkage during the solidification process of castings is a natural physical phenomenon, and we cannot reverse the laws of this natural phenomenon, but can only follow its laws to solve this problem
3. Two ways to supplement
There are two ways to feed castings, one is natural feeding, and the other is forced feeding
To achieve natural shrinkage, in our casting process system, there must be process measures that can achieve "sequential solidification". Many people intuitively believe that using low-pressure casting method can solve the shrinkage and porosity defects of castings, but the fact is not the case. Using low-pressure casting process does not mean that it can solve the shrinkage and porosity defects of castings. If the low-pressure casting process system does not have any shrinkage process measures, then, The blank produced by this low-pressure casting method may also have 100% shrinkage and porosity defects
Due to the inherent characteristics of the die-casting process, it is difficult and complex to establish a natural "sequential solidification" process. The fundamental reason may also be that the "sequential solidification" process requires a relatively long solidification time for the casting, which is somewhat contradictory to the die-casting process itself
The main characteristic of forced solidification and shrinkage is the short solidification time, which is generally only one fourth or shorter of the "sequential solidification". Therefore, adding forced shrinkage process measures on the basis of the die-casting process system is suitable for the characteristics of the die-casting process and can effectively solve the problem of shrinkage and porosity in die-casting
4. There are two levels of forced shrinkage: compression shrinkage and forging shrinkage
There are two levels to achieve forced shrinkage of castings. One is the basic degree to which shrinkage and porosity defects can be eliminated, and the other is the degree to which broken grains or forged structures can be achieved inside the blank. If different words are used to express these two different levels, then we can use "extrusion shrinkage" to express the former, and "forging shrinkage" to express the latter
A concept that needs to be fully noted is that feeding is a direct means and cannot be achieved indirectly. In terms of technology, we can have a process parameter to express it, which is "feeding pressure"
In physical principle, the concept of pressure can appear in two situations, one is in the liquid situation, that is, "Archimedes Law". To distinguish clearly, we define it as "liquid pressure", and the other is in the solid situation, that is, we define it as "solid pressure". It should be noted that the application conditions of the concept of pressure in these two different states will cause big problems if we confuse them
Liquid pressure "is only applicable to liquid systems, and its pressure direction can be transmitted and turned, but it is completely not applicable in solid phase systems
The feeding of die-casting parts occurs between semi solid and solid states, and its pressure value is directional, which is a vector pressure with the same direction as the applied feeding force
So, it is completely wrong to think that by increasing the pressure of the injection cylinder of the die-casting machine and increasing the injection filling specific pressure to solve the shrinkage and porosity of the die-casting, the idea that this injection specific pressure can be transmitted to the entire solidification stage of the casting and achieve the idea of casting shrinkage filling is completely wrong
5. The process of "first die-casting and filling, then die forging and shrinkage filling" is an effective way to solve the shrinkage defects of castings, and it is also an ultimate means
”The process of first die-casting and filling, followed by die forging and filling, can be abbreviated as the "die-casting and forging" process. Its essence is a continuous casting and forging process that combines the effective functions of the die-casting process with the liquid forging process, and completes the entire process
This type of continuous casting and forging "die casting die forging" equipment has a similar appearance to ordinary vertical or horizontal die casting machines. In fact, a hydraulic forging head is added to the die casting machine. The large forging and compression force that can be added can be equal to the large locking force of the die casting machine
It should be noted that the most important nominal parameter of this type of die-casting die forging machine is not the locking force, but the forging and shrinkage force, which is equivalent to the forging pressure of a four column hydraulic press. This is something we must pay full attention to when selecting equipment. Otherwise, buying a die-casting die forging equipment with a large locking force but a small forging and shrinkage force will greatly reduce its value in use
The blank produced by this die-casting die forging machine has high dimensional accuracy and extremely high surface finish, which can be equivalent to the precision and surface roughness level that can be achieved by machining methods above level 6. It can now be classified as a "limit forming" process, which is a significant step forward compared to the "no cutting less allowance forming" process