Die Casting Parts Solidification Characteristics
Shrinkage porosity of die castings is caused by only one, that's because the metal melts after filling, from the liquid phase into the solid phase of the inevitable phase change contraction. Die Casting Parts Because the solidification characteristic of the die casting is cooled from the outside, when the wall thickness of the casting is larger, the problem of shrinkage porosity must be produced internally.
Therefore, in terms of the die castings, especially the thick die castings, the existence of shrinkage porosity is inevitable, is not to be solved.
Die casting shrinkage porosity problem, can not be completely solved from the casting process itself, to solve this problem thoroughly, can only go beyond the process, or to seek solutions from outside the system. What is this approach?
According to the principle of technology, the solution to shrinkage shrinkage of castings can only be carried out in accordance with the technology of shrinkage. Die Casting Parts The phase change contraction of casting solidification process is a natural physical phenomenon, we can not counter the law of this kind of natural phenomenon, but only follow its law and solve this problem. There are two ways to make up the castings, one is natural shrinkage, the other is the forced shrinkage. In order to achieve natural shrinkage, we must have the process of "sequential solidification" in the casting process. Many people intuitively assume that the low pressure casting method can solve the shrinkage defects of the castings, but that is not the case. The use of low pressure casting process, Die Casting Parts does not mean that the shrinkage of castings can be resolved, if the low pressure casting process system does not have a shrinkage of the process, then, this low-pressure casting method produced by the blank, also may be 100% shrinkage porosity defects.
Due to the characteristics of the die casting process itself, it is difficult and complicated to set up a natural "sequential solidification" process. Die Casting Parts The most fundamental reason may also be that the "sequential solidification" of the process measures, the total requirements of the casting has a relatively long solidification time, this point, and the die-casting process itself a bit contradictory.
The most characteristic of the forced solidification shrinkage is the short solidification time, generally only and "sequential solidification" of one-fourth or less, so, on the basis of the die-casting process system, the addition of forced shrinkage process measures, and die-casting process characteristics, can be very good to solve the shrinkage of die castings.
There are two kinds of degree to achieve the forced shrinkage of castings. Die Casting Parts One is basically the degree to eliminate shrinkage defects of castings, one is the degree to which the internal of the broken grains or the forging state can be achieved. If you want to use different words to express these two different degrees, then, the former we can use "squeeze to make up", the latter, we can use the "forging and shrinkage" to express.
One of the concepts to be fully aware of is that the shrinkage is a direct means that it cannot be done indirectly. Process, we can have a process parameter to express, this is the "shrinkage pressure."
On the physical principle, the concept of pressure can appear in two cases, one is in the liquid situation, that is, "Archimedes ' law" occasions, Die Casting Parts in order to clarify we define it as "liquid pressure", while the other one appears in solid state, and we define it as "solid pressure". It is to be noted that the applicability of the concept of pressure in these two different states. If we are confused, there will be big problems.
"Liquid pressure", it is only applicable to the liquid system, Die Casting Parts its pressure direction can be transferred, can turn, but in the solid phase system is not applicable.
The shrinkage of the die castings is between semi-solid and solid state, its pressure value is directional, it is a kind of vector pressure, and its direction is the same as that of the applied shrinkage force.
Therefore, Die Casting Parts the kind of thought that by increasing the pressure of die-casting machine cylinder pressure, by increasing the pressure injection filling pressure to solve the die castings shrinkage porosity, that this pressure-injection pressure can be transferred to the whole process of casting solidification stage, the realization of the idea of casting shrinkage is completely wrong.