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2021.10.27
Die casting is a precision metal forming method with high efficiency and less cutting. The material of the die casting mold has a direct influence on the mold design, heat treatment and casting process. What are the best mold materials for aluminum and zinc die casting? Let’s figure out the requirements for die casting dies used in metal casting.
The service life of die casting die is closely related to the mold material. Die casting mold components are mainly divided into parts in contact with liquid metal, sliding mating parts, and mold base structure parts. Die casting mold cavity, sprue and other components are in direct contact with high temperature, high pressure and high-speed liquid metal in the process of metal pressure casting production. On the one hand, it is subject to direct scouring, wear, high-temperature oxidation and various corrosion of liquid metal; on the other hand, due to the high efficiency of production, the increase and decrease of die temperature are very violent, and periodic changes are formed. Therefore, die casting die steel requires high thermal fatigue resistance, thermal conductivity, good wear resistance, and corrosion resistance mechanical properties.
The material selection of die casting mold is mainly determined by the temperature of casting metal and the type of casting metal. The higher the temperature, the more serious the damage and wear of die casting die. Since each part of the die-casting die works under different conditions and is subject to different impact and wear of the cast metal, the hardness requirements of the die-casting die should be different according to the purpose of the parts and the cast metal.
The melting point of zinc alloy is 400 ~ 430 ℃. The surface temperature of the zinc alloy die casting mould shall not exceed 400 ℃. Due to the low working temperature, in addition to common die steel, alloy structural steel can also be used to make dies, and even low-carbon steel can be used. After medium temperature solid carbonitriding, quenching and low-temperature tempering, the application effect is also very good. The materials commonly used in manufacturing zinc alloy die casting molds are alloy structural steel, such as 40Cr, 30CrMnSi, 40CrMo, etc; die steel 5CrMnMo, 4Cr5MoSiV, 4Cr5M0SiV1, etc. Life of zinc alloy die casting mold: alloy structural steel can reach 200,000 ~ 300,000 cycles, and die steel can reach 1 million cycles.
At present, the common aluminum alloy die casting mold steels include H13, H11, new steel Y10, and HM3. The tempering resistance and thermal stability of 3Cr2W8V are the best and have high enough heat resistance. However, due to the high tungsten content, the thermal conductivity decreases and the thermal expansion coefficient of steel is also large. Therefore, the cold and hot fatigue resistance is poor, and the cold and hot fatigue cracks are easy to appear on the surface of the die cavity. The tempering resistance and thermal stability of 4Cr5MoSiV1 (H13) and 4Cr5MoSiV (H11) steel are slightly worse than that of 3Cr2W8V steel, but the cold and hot fatigue resistance is much higher than that of 3Cr2W8V steel. The life of aluminum alloy die casting die made of H13 steel is much longer than that made of 3Cr2W8V steel.
According to the service conditions and common failure forms of aluminum alloy die casting mold, the main performance requirements of steel for mold are as follows:
1) High tempering resistance and cold and hot fatigue resistance.
Aluminum alloy die-casting molds produced continuously shall maintain their high hardness under a certain temperature for a long time, and should not stick to the die and produce oxide scale. Therefore, the die should have good oxidation resistance and tempering stability. The surface of the aluminum alloy die casting die is repeatedly heated and cooled by high temperature, which expands and shrinks continuously, resulting in alternating thermal stress. When the stress exceeds the elastic limit of the die material, repeated plastic deformation occurs, resulting in thermal fatigue. Colleagues, when the die surface is corroded and oxidized by molten metal for a long time, it will gradually produce micro-cracks. In most cases, thermal fatigue is the most important factor to determine the life of die casting mold.
2) Sufficient strength, hardness, plasticity and heat resistance
Aluminum alloy die casting die is easy to deform and even crack under the action of high temperature, high pressure, and thermal stress during the molten metal injection. Therefore, die steel should have sufficient high-temperature strength and toughness, high hardness, and heat resistance at working temperature.
3) Good thermal conductivity.
Under the high temperature of 600 ~ 700 ℃, in order to ensure other properties. Must have good thermal conductivity.
4) Good resistance to molten metal damage.
With the large-scale die-casting machine, the die-casting pressure is also increasing, from 20 ~ 30MPa of low pressure to 150 ~ 500MPa of high pressure. High temperature and high-pressure casting can produce obvious melting damage, and the mold should have great resistance to this. Therefore, the die material must have high-temperature strength. With a small affinity for molten metal, the mold shows that the roughness is small, and is attached with appropriate protective layers such as oxidation mold and nitriding layer, but there is no decarburization layer.
5) Good hardenability and small heat treatment deformation
The general manufacturing method of the die-casting die is to carve the annealed die material into the cavity, and then heat treat to obtain the required hardness, or heat treat the die material to obtain the required hardness, and then carve the cavity. The manufacturing method of engraving the cavity first and then heat treatment has high hardness and strength, which is not easy to produce melting loss and thermal fatigue. No matter which method is used for heat treatment, it is necessary to obtain uniform hardness, so it is required to have good hardenability, especially carving the cavity before heat treatment. Materials with small heat treatment deformation should be used, which is particularly important for large molds.