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Answer: 1. Causes of shrinkage porosity in die castings

There is only one reason for the shrinkage porosity of die castings, that is, the phase change shrinkage that exists when the liquid phase changes to the solid phase after the metal melt is filled. Since the solidification of die castings is characterized by cooling from the outside to the inside, when the casting wall thickness is large, the problem of shrinkage porosity occurs inside.

Therefore, as far as die castings are concerned, especially for thick die castings, there is a problem of shrinkage porosity, which cannot be solved.

2. Ways to Solve the Shrinkage Porous Defect of Die Casting

Die casting shrinkage shrinkage problem, can not be completely solved from the die casting process itself, to completely solve this problem, only the process, or from outside the system to find a solution. What is this way?

From the process principle, to solve the casting shrinkage defects, can only be carried out in accordance with the process of feeding. Casting solidification process of phase transformation shrinkage, is a natural physical phenomenon, we can not disobey 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 make up castings, one is natural feeding and the other is forced feeding. To achieve natural feeding, in the casting process system, it is necessary to achieve "sequential solidification" process measures. Many people intuitively think that the use of low-pressure casting method can solve the casting shrinkage porosity defects, but the fact is not the case. The use of low-pressure casting process does not mean that the casting shrinkage defects can be solved. If the low-pressure casting process system does not have feeding process measures, then the blank produced by this low-pressure casting method may also 100% exist shrinkage defects.

Due to the characteristics of the die-casting process itself, it is difficult and complicated to set up a natural "sequential solidification" process. The more fundamental reason may also be that the "sequential solidification" process measures always require a relatively long solidification time for castings, which is somewhat contradictory to the die casting process itself.

Forced solidification feeding is characterized by a short solidification time, generally only and "sequential solidification" 1/4 or less, so, on the basis of the die casting process system, the addition of forced feeding process measures, is adapted to the characteristics of the die casting process, can be a good solution to the problem of shrinkage cavity shrinkage of die castings.

4. Two degrees of forced replenishment

There are two levels of forced feeding of castings by extrusion and forging. One is the basic degree that can eliminate the shrinkage porosity defect of casting, and the other is the degree that can make the internal of the blank reach the degree of broken grain or forged organization. If you want to use different words to express these two different degrees, then the former can be expressed by "squeezing and replenishing", and the latter can be expressed by "forging and replenishing.

One of the concepts that should be fully paid attention to and distinguished is that supplementation is a direct means and it cannot be accomplished indirectly. Technically, we can have a process parameter to express, which is "feeding pressure".

In terms of physical principle, the concept of pressure can appear in two situations. One is in liquid situations, that is, in the case of "Archimedes Law". In order to distinguish clearly, we define it as "liquid pressure", while the other is in solid situations, we define it as "solid pressure". It should be noted that the applicable conditions of the concept of pressure in these two different states. If we get confused, there will be big problems.

"Liquid pressure", it only applies to liquid systems, its pressure direction can be transferred, can turn, but in the solid phase system is not applicable.

The feeding of die castings occurs between semi-solid and solid. Its pressure value has a direction, and its direction is the same as the direction of the applied feeding force.

Therefore, it is totally wrong to think that the shrinkage and shrinkage of die castings can be solved by increasing the pressure of the injection cylinder of the die casting machine and the specific pressure of injection and filling, and that the specific pressure of injection can be transmitted to the whole process of the solidification stage of the casting to realize the idea of feeding the casting.

5, the use of "first die-casting filling, after die forging feeding" process, is an effective way to solve the casting shrinkage shrinkage defects, but also a means.

"First die-casting filling, after die-forging feeding" process, we can be referred to as "die-casting die-forging" process. 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 devices to complete the entire process.

This continuous casting and forging "die-casting die forging" equipment, the appearance and ordinary vertical or horizontal die-casting machine is very similar, in fact, in the die-casting machine, an increase of hydraulic forging head. The larger forging feeding force that can be added can be equal to the larger locking force of the die casting machine.

It should be noted that the more important nominal parameters of this die-casting die forging machine are not the clamping force, but the die-forging feeding force, which is equivalent to the forging force significance of the four-column oil press. This is what we must pay full attention to when choosing equipment. Otherwise, buy a clamping force is very large, but the die forging feeding force is very small die-casting die forging equipment, its use value is greatly reduced.

The use of this die-casting forging machine production of blank, high dimensional accuracy, surface finish is also high, can be equivalent to more than 6 levels of machining means to achieve the accuracy and surface roughness level. It has been able to belong to the "limit forming"-the process means, more than the "no cutting less margin forming" process.

The rapid selection of die-casting process parameters for die-casting molds under established conditions is studied. Before the commissioning and production of the new mold, the pre-selection and calculation of its die-casting process parameters, the actual commissioning and production on this basis, in the process parameter setting less detours, quickly complete the mold commissioning, to produce qualified products.

The cost of die-casting mold accounts for a large proportion of the cost of die-casting parts, and the cost of die-casting mold is allocated to the cost of each die-casting part. This requires us to minimize unnecessary mold production times to improve the overall life of die-casting molds.

For how to improve the life of the mold, we often think of the possibility of using the mold temperature control system, mold forming part of the regular stress relief treatment and surface strengthening, a reasonable pouring overflow system, as well as to meet the requirements of the product at the same time using lower pressure, 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 have reached the first stress elimination mode, but the debugging has not been completed, and no products that meet the customer's requirements have been produced. This virtually increases the cost of a single die casting. In order to avoid such a situation as far as possible and lay a good foundation for future production, this paper studies the pre-rapid selection of die-casting process parameters for new die-casting molds under established conditions.

Selection of Die Casting Machine

Before the mold is made, the mold designer should work with the die casting technologist to determine the die casting machine to be used and determine 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 G0(kg) per mold, the net weight G1(kg) of the product, the total weight G2(kg) of the slag collecting tank, and the total projected area S(m2) of the parting surface of the product can be obtained, together with the rated clamping force T(N) of the die casting machine and the diameter D1 of the pressure chamber, which have been determined as basic data by the following process parameters.

1. Determination of injection specific pressure Po

Limit specific pressure during injection: P limit = T/S type T-rated clamping force of die casting machine; S-total projected area of parting surface;

Po is less than 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 frequency of mold maintenance 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 force acting on the punch and the injection cylinder piston is the same, namely:

Therefore, a die casting machine with real-time control can directly set P1 in its control computer. Ordinary die casting machines basically manually adjust the opening and starting of the booster valve and cooperate with the adjustment of the nitrogen filling pressure of the booster accumulator to complete the setting.

3. Determination of injection speed

(1) the first stage of low-speed injection V1. Generally, it is composed of two parts. First, the punch is from stationary to just after pouring. At this time, it needs to be slow, mainly to prevent alloy liquid from overflowing from the gate and facilitate gas discharge. Secondly, before the metal liquid continues to be filled into the inner runner (the speed at this time is greater than the previous part), the main purpose is to prevent the alloy liquid from entering the cavity in advance.

Reference data: generally, it can be set to 0.1~0.5 m/s; Thin-walled parts and exterior decoration parts are 0.25~0.35 m/s; High compressive strength parts are 0.15-0.25 m/s.

(2) the second stage of high-speed injection V2. When the alloy liquid reaches the in-gate, high-speed switching can be performed, so that the alloy liquid is filled at high pressure and high speed. Empirical data: high-speed injection speed: above 2~4.5 m/s, high-speed injection acceleration time t1 is 0.01s, and pressurization time t2 is 0.Ols.

(3) Slow down before the end of the third stage metal filling. Adding deceleration action before the end of filling can reduce the impact of alloy liquid at the end of filling, protect the die-casting mold and reduce the generation of flash; but pay attention to the setting of deceleration point should not be too early, otherwise it will affect the filling effect.

4. Selection of important shot speed switching position

(1) Usually, the starting point of high-speed injection is located at II (normal speed switching position), that is, when the alloy liquid reaches the inner runner.

(2) If the surface quality requirements of high die casting, you can switch the position in advance between I, II.

(3) If you want to reduce the local porosity of the die casting, you can lag the switching position to the important part of the die casting, that is, III, to reduce the porosity of the important part and increase the density. However, great attention should be paid to prevent the 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 volume of volume is large and does not recommend.

The following data calculations were performed for the study subject according to the normal speed switching position.

L0 is the low-speed injection stroke, I .e. the punch stroke L1 at which the alloy liquid reaches the high-speed injection switching position is the high-speed injection stroke, I .e. the length of the alloy liquid occupied in the pressure chamber by the sum of the net weight of the product G1 and the total weight of the overflow system G2, so L1 can be calculated:

The density ρ of the alloy liquid in the above formula can be calculated as 2.65 XlO3kg/m3. L2 is the thickness of the material handle (empirical data is 30~50mm).

L = L. L1 L2: It can be measured after passing through the pouring material ironing chamber.

According to the measured L, calculated and self-determined, the value that can be obtained, that is, the switching position of the high-speed injection is determined.

5. Relevant setting of boost pressure

In the cold chamber die-casting, the pressure-building time indicates the response speed of the boost pressure, and the ordinary die-casting machine realizes by adjusting the boost speed and adjusting the hand wheel. The advanced die casting machine can directly set the curve of boost pressure and time on the control panel. The starting point of the pressurization 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 of the 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 die castings.

(2) the die casting mold temperature can be controlled at about 1/3 of the pouring temperature, thin-walled, complex structure of the die casting can be appropriately improved, but it should be noted that the mold should be preheated before the start of production, and the preheating temperature should be controlled at 150~180 ℃.

7. Setting of pressure holding time and mold retention time

recommend values of holding time and mold retention time based on wall thickness for aluminum alloy die castings. If the quality requirements of the product are not met after the above process parameters are set and adjusted according to the die casting, the pouring and overflowing system on the mold needs to be modified and adjusted.