Answer: 1. Causes of shrinkage porosity in die castings
There is only one reason for the phenomenon of shrinkage porosity in die castings, which is due to the phase transition shrinkage that exists when the liquid phase is transformed into the solid phase after the metal melt is filled. Since the solidification characteristics of die castings are cooling from the outside to the inside, when the wall thickness of the castings is large, the problem of shrinkage cavities and porosity occurs inside.
Therefore, as far as die castings are concerned, especially for thick die castings, there is a problem of shrinkage cavities and porosity, which cannot be solved.
2. Ways to solve shrinkage porosity defects of die castings
The problem of shrinkage porosity and porosity in die castings cannot be completely solved from the die casting process itself. To solve this problem completely, only the process, or a solution from outside the system, can be found. What is this method?
In terms of process principle, to solve the shrinkage porosity defect of castings, it can only be carried out according to the process of feeding. The phase transformation shrinkage in the solidification process of castings is a natural physical phenomenon. We cannot go against the laws of this natural phenomenon, but can only follow its laws to solve this problem.
3. Two ways of feeding
There are two ways to feed castings, one is natural feeding, and the other is forced feeding. To achieve natural feeding, in the casting process system, there must be technological measures that can achieve "sequential solidification". Many people intuitively think that the shrinkage cavity and shrinkage defects of castings can be solved by using the low pressure casting method, but this is not the case. The use of low-pressure casting technology does not mean that the shrinkage and porosity defects of castings can be solved. If the low-pressure casting process system is not equipped with process measures for feeding, then the blanks produced by this low-pressure casting method may also be There are 100 shrinkage porosity defects.
Due to the characteristics of the die casting process itself, it is difficult and complicated to establish a natural "sequential solidification" process. The solidification time, at this point, is somewhat contradictory to the die casting process itself.
The major feature of forced solidification feeding is that the solidification time is short, generally only a quarter or less of "sequential solidification". If the process characteristics are suitable, the shrinkage porosity problem of die castings can be well solved.
4. Two levels of forced feeding
There are two levels of extrusion feeding and forging feeding to achieve forced feeding of castings. One is the degree to which the shrinkage porosity defect of the casting can be basically eliminated, and the other is the degree to which the inside of the blank can reach broken grains or forged structure. If we use different words to express these two different degrees, then, the former can be expressed by "extrusion and compression", and the latter can be expressed by "forging and compression".
An understanding that needs to be fully paid attention to and a clear concept is that feeding is a direct means, it cannot be done indirectly. In terms of technology, we can have a process parameter to express, which is "compression pressure".
In terms of physical principles, the concept of pressure can appear in two situations. One is in the case of liquids, that is, in the case of "Archimedes' law". For the sake of clarity, we define it as "liquid pressure", and the other appears. In the solid state, we define it as "solid state pressure". It should be noted that the applicable conditions of the concept of pressure appearing in these two different states. If we confuse, there will be big problems.
"Liquid pressure", it is only applicable to liquid systems, its pressure direction can be transmitted and can be turned, but it is completely inapplicable in solid phase systems.
The feeding of die castings occurs between semi-solid and solid states. Its pressure value is directional, and its direction is the same as the direction of the applied feeding force.
Therefore, it is thought that by increasing the pressure of the injection cylinder of the die-casting machine, the shrinkage porosity of the die-casting part can be solved by increasing the injection-filling specific pressure, and that the injection-specific pressure can be transmitted to the whole process of the solidification stage of the casting to realize The idea of casting feeding is completely wrong.
5. The process of "filling by die casting and then feeding by die forging" is an effective way to solve the shrinkage porosity defect of castings, and it is also a means.
The process of "filling by die-casting and then feeding by die forging" can be referred to as "die-casting and die forging" for short. Its essence is a continuous casting and forging process, which is to combine the die casting process with the liquid die forging process, and combine the more effective functions of these two equipment to complete the entire process.
This continuous casting and forging "die-casting die forging" equipment is very similar in appearance to ordinary vertical or horizontal die-casting machines. In fact, hydraulic forging heads are added to the die-casting machine. The larger forging feeding force that can be added can be equal to the larger clamping 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 feed force, which is equivalent to the forging force of the four-column hydraulic press. This is what we must pay full attention to when selecting equipment. of. Otherwise, if you buy a die-casting die-forging equipment with strong clamping force but small die-forging feeding force, its use value will be greatly reduced.
The blank produced by this die-casting die forging machine has high dimensional accuracy and high surface finish, which can be equivalent to the level of precision and surface roughness that can be achieved by machining methods above grade 6. It can already be attributed to the "extreme forming"-the process method, which is a step further than the "no cutting and small allowance forming" process.
Research on the rapid selection of die-casting process parameters of die-casting molds under given conditions. Before the new mold is commissioned and produced, its die-casting process parameters are preselected and calculated. In the actual commissioning and production, based on this, there are fewer detours in the process parameter setting, and the mold commissioning can be quickly completed to produce qualified products.
The cost of die-casting molds accounts for a large proportion of the cost of die-casting parts, and the cost of die-casting molds is allocated to the cost of each die-casting part, which requires us to minimize the number of unnecessary mold productions to improve the overall die-casting mold cost. Life, as much as possible to reduce the cost of die-casting molds in the cost of each die-casting part allocation, to create greater benefits.
For how to improve the life of the mold, we often think of the mold temperature control system, the regular stress relief treatment and surface strengthening of the mold forming part, the reasonable pouring and overflow system, and the use of lower pressure while meeting the product requirements. , speed and temperature and other process parameters. However, the debugging and production process of the new mold is often ignored. If the process is not controlled, it is even possible that the production times of the mold has reached the first stress relief mode, but the debugging has not been completed, and the product that meets the customer's requirements has not been produced. This virtually increases the cost of a single die casting. In order to avoid this situation as much as possible and lay a good foundation for future production, this paper studies the pre-quick selection of the die-casting process parameters of the new die-casting mold under the given conditions.
Selection of die casting machine
Before the mold is manufactured, the designer of the mold should work with the die-casting technologist of the mold to determine the die-casting machine to be used and the diameter of the pressure chamber.
Fast pre-determined die casting process parameters
Take the cold chamber die casting machine for aluminum alloy die casting as an example. According to the three-dimensional model of the mold, the metal weight of each mold G0 (kg), the net weight of the product G1 (kg), the total weight of the slag collection tank G2 (kg), the total projected area of the parting surface S (m2), together with the The rated clamping force T(N) of the first good die-casting machine and the diameter of the pressure chamber D1 are determined by the following process parameters as the basic data.
1. Determination of Injection Specific Pressure Po
The ultimate specific pressure during injection: P limit = T/S In the formula, T - rated clamping force of die casting machine; S - total projected area of parting surface;
Po is less than the P limit to avoid mold expansion in production, and according to product structure, appearance and internal quality requirements. At the same time, refer to Table 2 to determine a relatively low value to reduce the maintenance frequency of the mold and improve the life of the mold.
2. Determination of pressure P1 after pressurization of injection cylinder of die casting machine
After the injection process is completed, the same forces act on the punch and the piston of the injection cylinder, namely:
Therefore, a die-casting machine with real-time control can directly set P1 in its control computer; an ordinary die-casting machine basically adjusts the opening process of the booster valve manually, and adjusts the nitrogen filling pressure of the booster accumulator to complete the setting.
3. Determination of injection speed
(1) The first stage low-speed injection V1. It is generally composed of two parts. First, the punch is from static to just past the pouring. At this time, it needs to be slow, mainly to prevent the alloy liquid from overflowing from the pouring port, which is conducive to the discharge of gas; secondly, the metal liquid continues to be filled to the inner pouring. Before the passage (the speed at this time is higher than the previous part), the main purpose is to avoid the entrainment of the alloy liquid, and at the same time, try to avoid the alloy liquid entering the cavity in advance.
Reference data: Generally, it can be set to 0.1-0.5m/s; thin-walled parts and exterior decorative parts are 0.25-0.35m/s; high compressive strength parts are 0.15-0.25m/s.
(2) The second stage high-speed injection V2. When the alloy liquid reaches the inner runner, it can be switched at high speed, so that the alloy liquid can be filled under high pressure and high speed. Experience data: high-speed injection speed: above 2~4.5m/s, high-speed injection acceleration time t1 is 0.01s, and boost time t2 is 0.01s.
(3) Decelerate before the end of the third stage of molten metal filling. Adding a deceleration action before the end of the filling can reduce the impact of the alloy liquid at the end of the filling, protect the die-casting mold, and reduce the generation of flash; but it should be noted that the deceleration point should not be set too early, otherwise it will affect the filling effect.
4. Selection of important injection speed switching positions
(1) Usually the position of the high-speed injection starting point is II (normal speed switching position), that is, when the alloy liquid reaches the inner runner.
(2) For die castings with high surface quality requirements, the switching position can be advanced between I and II.
(3) If you want to reduce the local pores of the die casting, you can delay the switching position to the important part of the die casting, that is, at III, so as to reduce the pores of the important part and increase the density. However, great attention should be paid to preventing cold defects of die castings caused by too slow filling speed. This method should not be used when the important part of the die casting is at the end.
(4) For large die castings and large die casting machines, the switching position can be set at about 30% of the alloy liquid entering the cavity to reduce the generation of pores.
(5) When the switching position is below I, the amount of entrained air is large, which is not recommended.
The following data calculations were performed for the study subject based on the normal speed switching position.
L0 is the low-speed injection stroke, that is, the punch stroke when the alloy liquid reaches the high-speed injection switching position. L1 is the high-speed injection stroke, that is, the sum of the net weight of the product G1 and the total weight of the overflow system G2 occupies the proportion of the alloy liquid in the pressure chamber. length, so L1 can be calculated by:
The density ρ of the alloy liquid in the above formula, the aluminum alloy liquid can be calculated as 2.65XlO3kg/m3. L2 is the thickness of the material handle (experience data is 30 ~ 50mm).
L=L. +L1+L2: It can be obtained by measuring after pouring and pressing room.
According to the measured L, the calculated value, and the self-determined value that can be obtained, that is, the switching position of the high-speed injection is determined.
5. Relevant settings of boost pressure
In cold-chamber die-casting, the pressure build-up time represents the response speed of the boost pressure, which is achieved by adjusting the boost speed and adjusting the handwheel for ordinary die-casting machines. Advanced die casting machines can set the curve of boost pressure and time directly on the control panel. The start of the boosting process can be triggered by position, pressure and speed.
Generally speaking, the pressurization is triggered by setting the position, which is easy to set and easy to adjust. The empirical data for this position setting is: 10-30mm before the end of the die-casting stroke of the punch.
6. Setting of pouring temperature and die-casting mold temperature
(1) The pouring temperature can be determined according to the alloy grade and the quality requirements of the die casting.
(2) The temperature of the die-casting mold can be controlled at about 1/3 of the pouring temperature, and the die-casting parts with thin walls and complex structures can be appropriately increased, but it should be noted that the mold should be preheated before starting production, and the preheating temperature should be controlled at 150～180℃.
7. Setting of pressure holding time and mold retention time
Aluminum alloy die casting recommendations based on wall thickness holding time and mold retention time. If the quality requirements of the product are not met after the above process parameters are set and the die-casting parts are adjusted, the pouring and overflow system on the mold needs to be modified and adjusted. (Source/Internet, Editor/Jing Liwen)
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