About the aluminum alloy die-casting mold

    Die-casting mold defects are extremely common in actual production, which directly affect the appearance of the product. In severe cases, the surface of the casting will be peeled, lack of meat, and pulled and cracked. Especially for castings with sealing requirements, the mold will cause partial leakage of the casting. Gas, causing castings to be scrapped directly. The essence of the mold-die defect is that the die-cast alloy and the steel mold are combined, and the cast material is adhered to the surface of the mold. During the die casting process, the molten metal enters the mold cavity, which will have a strong physical impact on the surface of the mold, and also cause chemical corrosion. The physicochemical action of the molten metal on the mold causes fine pits on the surface of the mold. Each shot will cause a change in the surface of the mold, and the small pits on the surface of the mold will gradually become larger. After the time is accumulated, the pit will reach a certain level, which will cause the aluminum liquid to enter and combine with the steel mold. The surface of the mold originally has a dense oxide layer, which begins to aluminate after the oxide layer breaks, and the intermetallic compound phase Al Fe Si begins to form. These phases grow in an expanded manner into the steel, and this diffusion is controlled by time and temperature. At this time, the sticky aluminum defect appeared. The following analyzes the influencing factors and solutions of sticky aluminum from four aspects.

1.mold and surface coating

    Adhesive aluminum is a complex mechanical wear and chemical reaction process for the mold, which has great damage to the surface of the mold. In severe cases, the mold will fail. The mold needs to use a good material and a reasonable surface treatment, which can effectively reduce the aluminum adhesion.

1.1 Mold material

    For aluminum alloy die-casting mold steel, it should have: 1) excellent high temperature strength and toughness; 2) excellent high temperature wear resistance and thermal fatigue resistance; 3) good heat treatment and machinability. Mold manufacturers should select appropriate mold materials according to actual production needs.

1.2 mold surface hardness

    If the surface hardness of the mold is insufficient, the wear resistance will be worse, which will cause thermal fatigue failure of the mold, cracking and pitting, and then sticking aluminum; if the surface hardness of the mold is too high, the mold will be brittle. Therefore, it is necessary to select a reasonable mold surface hardness. For example, H13 steel generally has an optimum hardness of 44-48HRC after quenching, and then refines the selection for different structural components. Large-sized cavities can improve the toughness to avoid early cracking, and the hardness can be appropriately reduced. The core is mainly caused by bending deformation failure, and the occurrence of crack failure is small, so that the toughness can be reduced and the hardness can be improved. For large-sized aluminum alloy castings or molds with complicated shape and structure, the heat treatment process is very difficult to reduce the hardness properly; on the contrary, the small and medium-sized aluminum alloy die-casting mold can appropriately increase the hardness.

1.3 mold surface roughness

    The mold surface should have a reasonable roughness. If the roughness is too large, the damage to the mold is large, but the smaller the roughness, the better, and the surface of the mold should be prevented from being excessively polished. The surface of the mold can be sandblasted, oxidized, and sanded by a fine sandpaper, which is advantageous for uniform distribution of the release agent, and also prevents the die-casting alloy from being bonded to the surface of the mold caused by the mirror surface, thereby causing pinholes on the surface of the mold. 

1.4 Surface treatment

    The surface coating is extremely important for the protection of the mold. Commonly used treatment methods include CVD coating, PVD coating, oxidation, nitriding, and salt bath treatment under various conditions. Nitriding may be the most common treatment method in mold processing. This method is also extremely advantageous against erosion performance, but the heat cracking resistance may be impaired if the treatment method is not appropriate. Oxidation treatment is also a common treatment method. The mold is slightly oxidized when the mold is first used. It is usually heated to 450-550 ° C in air or pure oxygen atmosphere for 1-2 h to produce an oxide layer of 1-10 μm on the surface of the mold. The oxide layer is mainly composed of oxides of C, Si and Fe. It is proved that the oxide layer has a great protective effect on the mold and can greatly resist the erosion wear. From the results of the preliminary research, the generation and control of the oxide layer is likely to be the most important direction of the surface treatment of the mold. However, considering the multiple causes of mold failure, it is difficult to use a single surface treatment to resist all failure modes during mold strengthening. Many studies have begun to consider designing a composite coating system.

    After the mold is glued to aluminum, the traditional method is to use a grinding wheel and a whetstone for polishing, which is easy to cause damage to the mold. It is also possible to use a sodium hydroxide solution for cleaning, which is relatively small in damage to the mold, but it is not easy to handle completely and the price is slightly higher.

2. alloy liquid

2.1 Iron content in alloy liquid

    A large number of experiments have proved that the Fe content in the alloy liquid has a definite influence on the aluminum alloy die-casting aluminum defect.

    Fe is the main impurity in Al-Si alloys and is mainly derived from charge and smelting tools. If the Fe content is too much, the fluidity of the alloy will be greatly reduced, and the filling performance of the alloy will be deteriorated. If the Fe content is too low, Fe in the alloy liquid will react with Al, Si, etc. to form an intermetallic compound phase Al Fe Si. Produces sticky aluminum. Therefore, the Fe content in the alloy solution is an important check factor and should be kept within a certain range and cannot be high or low. For example, the Fe content of ADC12 should be kept between 0.6-1.3%.

2.2 Impurities

    If there are more impurities in the alloy liquid, it will directly wash the surface of the mold, increasing the roughness of the mold surface, making the aluminum sticking phenomenon more serious. Therefore, the alloy liquid needs to be refined to reduce the generation of impurities.

3. spraying

    Spraying is an indispensable process in the die casting process. Spraying can form a separation layer on the surface of the mold to prevent the alloy liquid from directly adhering to the mold, which can effectively reduce the direct scouring effect of the molten metal on the mold, and can also improve the surface quality of the casting and smooth the surface of the casting. In the spraying process, the following requirements are imposed on the release agent:

1. The volatilization point is low. At 100-150 ° C, the diluent can be quickly evaporated, without increasing the gas in the cavity;

2. good coating, no accumulation at the low temperature of the mold, but in the high temperature area of the mold can produce a separation layer to ensure its release properties, and easy to clean;

3. No corrosion to the mold and castings;

4. The environmental pollution is as small as possible, that is, it is odorless, does not precipitate or decompose harmful gases;

5. the performance is stable, not easy to volatilize in the air, no precipitation, no decomposition, etc. during the storage period.

    When spraying, it is necessary to ensure that the release agent can perform well. For example, after the mold temperature is higher than 300 ° C, the Leidenfrost Phenomenon effect is generated, so that the release agent is completely separated from the mold surface, and the separation layer cannot be produced, and the spraying effect is lost. When spraying, it is necessary to face the surface of the mold at 90 degrees and keep a certain distance for a sufficient time. At the sticking point, it is very likely that the spraying is not in place, so that the molten metal is directly washed with the mold, so it is necessary to adjust the inspection spraying process in time.

4.die casting process

  4.1 Filling speed

    The effect of filling speed on the adhesion of aluminum is also very straightforward. Filling speed is too fast, it will bring a series of problems, and it will cause great damage to the mold. When the filling speed is too fast, the molten metal will enter the cavity in the form of particles or mist, which has a strong scouring effect on the mold cavity, or directly destroys the surface layer of the release agent, adheres to the cavity wall, and produces a sticky aluminum. defect. Therefore, verifying the rationality of the filling speed is of great help to the sticky aluminum defect. 

4.2 mold temperature

    The effect of mold temperature on the adhesion of aluminum is also evident. In the production process, the mold temperature is not reasonable, not only will produce a variety of product quality problems, such as cold insulation, bubbles, shrinkage, aluminum, etc., but also affect the life of the mold. From the point of view of the adhesive aluminum defect, if the mold temperature is too high, the mold release agent will be invalid, the filling speed will be too fast, and the affinity between the mold and the alloy liquid will be increased, and the chemical reaction between the mold and the alloy liquid will be accelerated. Therefore, in places where the mold temperature is high, it is necessary to ensure that the temperature is maintained within a reasonable range. Generally, the aluminum alloy castings are measured within 1-3 s after mold opening, and the surface temperature of the mold cavity is not more than 300 ° C, preferably 240 ± 40 ° C. The surface temperature of the mold cavity was measured within 1-3 s before mold clamping after spraying, and should not be lower than 160 °C. The most common cooling methods are water cooling, heat pipe heat conduction, and mold temperature control.

    For mold internal cooling, the most common is water cooling, including local cooling and overall cooling. Careful analysis of the heating of the mold is required during the mold design phase, which can be analyzed by mold flow software and then added to the water at the appropriate location. Point cooling is a common and effective way of water cooling, which is to cool the mold core, the deep cavity of the mold, or the location where the individual local temperatures are high. For the surface cooling of the mold, it is generally done by the spraying process. The cooling capacity of the spray is actually the cooling effect of the water in the release agent. If the concentration of the release agent is too high, the surface cooling ability of the mold will be greatly reduced, and carbon deposition on the surface of the mold will be easily caused, and the concentration of the release agent is too low, and a protective layer cannot be formed, which does not protect the mold. Therefore, the concentration of the release agent needs to be appropriately adjusted according to the product to ensure that it can both protect the mold and have a good cooling effect on the mold surface.

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