Abstract: A series of analyzes were conducted on causes of aluminum alloy die-casting mold sticking, and some corresponding countermeasures were proposed.
Die-casting mold sticking defects are extremely common in actual production and directly affect appearance of product. In severe cases, surface of casting will peel, lack flesh, be strained and cracked. Especially for castings with sealing requirements, severe mold sticking will cause local air leakage in casting, leading to direct scrapping of casting. Essence of sticking defect is that die-casting alloy and steel mold are combined, and casting material sticks to the surface of mold. During die-casting process, molten metal entering mold cavity will have a strong physical impact on mold surface and will also cause chemical corrosion. Physical and chemical effects of molten metal on mold will cause small pits on mold surface. Each injection will cause mold surface to change, and small pits on mold surface will slowly become larger. After time accumulates, pits will reach a certain level, allowing aluminum liquid to enter and combine with steel mold. There will originally be a dense oxide layer on mold surface. After oxide layer breaks, aluminization begins. At this time, intermetallic compound phase Al Fe Si begins to form. These phases grow into steel in an expanding manner, this diffusion is controlled by time and temperature. At this time, aluminum sticking defects appear. Following analyzes influencing factors and solutions to aluminum sticking from four aspects.

 

Aluminum sticking is a complex process of mechanical wear and chemical reaction for mold, which will cause great damage to the surface of mold. In severe cases, it will cause mold to fail. Mold needs to use good materials and carry out reasonable surface treatment, which can effectively reduce aluminum sticking.

Mold steel for aluminum alloy die-casting should have: 1) excellent high-temperature strength and toughness; 2) excellent high-temperature wear resistance and thermal fatigue resistance; 3) good heat treatability and cutting processability. As shown in Table 1-1, commonly used mold steels include domestic 4Cr5Mo Si V steel, Swedish ASSAB's 8407, Japanese SKD61, American H13, etc. Mold manufacturers should select appropriate mold materials based on actual production needs.
Commonly used die-casting mold steels for aluminum alloys

If surface hardness of mold is insufficient, wear resistance will be poorer, which will cause thermal fatigue failure of mold, cracks and pitting corrosion, then aluminum sticking; if surface hardness of mold is too high, brittle cracking will occur in mold. Therefore, a reasonable mold surface hardness needs to be selected. For example, H13 steel generally has an optimal hardness of 44-48HRC after quenching, then refines selection for different structural components. In order to improve toughness and avoid early cracking of large mold cavities, hardness can be appropriately reduced; if core mainly fails due to bending deformation and crack failure occurs rarely, its toughness can be reduced and hardness can be increased. For large-sized aluminum alloy castings or molds with complex shapes and structures, hardness can be appropriately reduced if heat treatment process is very difficult; conversely, hardness can be appropriately increased for small and medium-sized aluminum alloy die-casting molds.

Mold surface should have reasonable roughness. Excessive roughness will cause greater damage to mold, but smaller roughness is not always better. Over-polishing of mold surface should be avoided. Mold surface can be sandblasted, oxidized, and polished with fine sandpaper. This will help distribute release agent evenly and prevent die-cast alloy caused by mirror surface from adhering to mold surface, causing pinholes on mold surface. Recommended mold surface roughness is shown in Table 2.
Commonly used mold surface roughness

Surface coating is extremely important to protect mold. Commonly used treatment methods include CVD coating, PVD coating, oxidation, nitriding, and salt bath treatment under various conditions. Nitriding treatment may be the most commonly used treatment method in mold treatment. This method is also extremely beneficial to corrosion resistance. However, if treatment method is inappropriate, thermal crack resistance may be damaged. Oxidation treatment is also a commonly used treatment method. When mold is first used, mold is slightly oxidized. It is usually heated to 450-550℃ in air or pure oxygen environment and kept for 1-2 hours to produce an oxide layer of 1-10 μm on the surface of mold. This oxide layer is mainly composed of oxides of C, Si and Fe. Studies have proven that this oxide layer has a great protective effect on mold, can resist erosion and wear to a great extent. Judging from results of previous research, generation and control of this oxide layer is likely to be the most important direction in future mold surface treatment. However, considering 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 design of combined coating systems.
After aluminum is adhered to mold, traditional method is to use grinding wheels and oil stones for polishing. This method can easily cause damage to mold. You can also use sodium hydroxide solution for cleaning, which will cause less damage to mold, but it is not easy to clean thoroughly and price is slightly higher.

A large number of experiments have proven that Fe content in alloy liquid has a definite impact on aluminum sticking defects in aluminum alloy die casting.
Fe is main impurity in Al-Si alloy, which mainly comes from charge and smelting tools. If Fe content is too much, fluidity of alloy will be greatly reduced and filling performance of alloy will be deteriorated; if Fe content is too low, Fe in alloy liquid will react with Al, Si, etc. to generate intermetallic compound phase Al Fe Si, resulting in aluminum sticking. Therefore, Fe content in alloy liquid is an important inspection factor, should be maintained within a certain range and cannot be too high or too low. For example, Fe content of ADC12 should be maintained between 0.6-1.3%.

If there are too many impurities in alloy liquid, they will directly wash away mold surface, increase roughness of mold surface, and make aluminum sticking phenomenon more serious. Therefore, alloy liquid needs to be refined to reduce generation of impurities.

 

Spraying is an essential process in die-casting production process. Spraying can form an isolation layer on the surface of mold to prevent alloy liquid from directly adhering to mold. It can effectively reduce direct erosion effect of molten metal on mold. It can also improve surface quality of casting and make surface of casting smooth. In spraying process, there are following requirements for release agent:
1. Low evaporation point. At 100-150℃, diluent can evaporate quickly without increasing gas in cavity;
2. It has good coating properties and does not produce accumulation in low-temperature areas of mold. It can produce an isolation layer in high-temperature areas of mold to ensure its release performance and is easy to clean;
3. No corrosive effect on molds and castings;
4. It pollutes environment as little as possible, that is, it is odorless and does not precipitate or decompose harmful gases;
5. Stable performance, not easy to volatilize in air, and will not precipitate or decompose during storage period. Commonly used release agent brands include Henkel, Chem-Trend, Quaker and EPS.
When spraying, it is necessary to ensure that release agent can achieve good results. For example, when mold temperature is higher than 300℃, Leidenfros effect will occur, causing release agent to completely separate from mold surface, unable to form an isolation layer, and thus losing spraying effect. When spraying, you need to face mold surface at 90 degrees, keep a certain distance, and spray for sufficient time. At mold sticking point, it is very likely that spraying is not in place, causing molten metal to directly wash away from mold. Therefore, spraying process must be adjusted and checked in time.

Impact of filling speed on aluminum adhesion is also very intuitive. If filling speed is too fast, it will cause a series of problems and cause great damage to mold. When filling speed is too fast, molten metal will enter mold cavity in the form of granules or mist, which will have a strong flushing effect on mold cavity, or directly damage surface layer of release agent and adhere to cavity wall, resulting in aluminum sticking. defect. Therefore, verifying rationality of filling speed is of great help to aluminum sticking defects. Recommended gate speed is shown in Table 3.
General gate speed

Influence of mold temperature on aluminum adhesion is also obvious. During production process, unreasonable mold temperature will not only cause various product quality problems, such as cold insulation, bubbles, shrinkage cavities, aluminum sticking, etc., but will also affect life of mold. From perspective of aluminum sticking defects alone, if mold temperature is too high, release agent will fail, filling speed will be too fast, affinity between mold and alloy liquid will be increased, chemical reaction between mold and alloy liquid will be accelerated. Therefore, in places with high mold temperatures, it is important to ensure that temperature is maintained within a reasonable range. For general aluminum alloy castings, surface temperature of mold cavity within 1-3 seconds after mold opening cannot be greater than 300℃, preferably 240±40℃. surface temperature of mold cavity within 1-3 seconds after spraying and before closing mold should not be lower than 160℃. The most commonly used cooling methods include water cooling, heat pipe heat conduction, mold temperature machine control, etc.
For internal cooling of mold, water cooling is most commonly used, including local cooling and overall cooling. In mold design stage, heating condition of mold needs to be carefully analyzed, which can be analyzed through mold flow software, then water cooling can be added at appropriate location. Point cooling is a common and effective way of water cooling. It targets mold core, deep cavity of mold or individual local locations with higher temperatures. As for cooling of mold surface, it is usually completed by spraying process. Cooling ability of spraying is actually cooling effect of water in release agent. If concentration of release agent is too high, it will greatly reduce surface cooling ability of mold and easily cause carbon deposition on the surface of mold. However, if concentration of release agent is too low, it will not be able to form a protective layer and will not protect mold. Therefore, concentration of release agent also needs to be adjusted appropriately according to product to ensure that it can not only protect mold, but also have a good cooling effect on mold surface.

 

There are many factors that cause aluminum sticking defects. When solving them, we must observe them carefully and analyze and consider them from many aspects. This article analyzes causes of aluminum sticking from four aspects and proposes some corresponding countermeasures, which has certain reference value for die-casting practitioners.