The service condition of aluminum alloy die-casting mold is harsh, the temperature of aluminum alloy melt is usually around 650-700℃, pressed into the mold cavity with the speed of 40-180m/s, the pressure is about 20-120Mpa, the holding time is 5-20s, the interval time of each press injection is about 20-70s. The surface of die-casting mold cavity is repeatedly impacted by high temperature and high speed aluminum liquid, which produces large pressure.
At present, most of the failure forms of Chinese die-casting mold is due to thermal fatigue cracking. With the improvement of die life. Die-casting die by the liquid aluminum alloy dissolution loss viscous corrosion effect will become the main form of mold failure. Therefore, the life of aluminum alloy die casting mold depends on two factors, namely, whether the occurrence of sticky mold and cavity surface cracking.
According to the service conditions of aluminum alloy die casting mold and common forms of failure, the main performance requirements for aluminum alloy die casting mold steel are.
1.High tempering resistance and hot and cold fatigue resistance.
A large number of continuous production of aluminum alloy die-casting mold, a long time at a certain temperature, should continue to maintain its high hardness, and should not stick to the mold and do not produce oxide skin. Therefore, the mold should have good oxidation resistance and tempering stability. Aluminum die-casting mold surface is repeatedly heated and cooled by high temperature, constantly expanding and contracting, resulting in alternating thermal stress. When this stress exceeds the elastic limit of the mold material, repeated plastic deformation occurs, causing thermal fatigue.
At the same time, the mold surface for a long time by the corrosion and oxidation of molten metal, will also gradually produce micro cracks, in most cases, thermal fatigue is the most important factor to determine the life of the die casting mold.
2. Strength, hardness, plasticity and heat resistance.
Aluminum die-casting mold by the molten metal injection of high temperature, high pressure and thermal stress, easy to deformation, and even cracking. Therefore, the mold steel in the working temperature should have sufficient high-temperature strength and toughness, as well as high hardness and heat resistance performance.
3. Good thermal conductivity.
Aluminum die-casting mold for a long time in 600 ~ 700 ~ C high temperature effect, in order to ensure other performance. Must have good thermal conductivity.
4.Resistance to damage of molten metal.
With the massification of die-casting machine, die-casting pressure is also increasing, has been from low pressure 20 ~ 30MPa, raise to high pressure 150 ~ 500MPa. high temperature and high pressure casting can produce obvious melt damage, die-casting mold should have a greater resistance to this.
For this reason, the mold material must have a large high-temperature strength. Smaller affinity for molten metal, mold surface roughness to be small, and with the appropriate oxide film, nitride layer and other protective layer, and there is no decarburization layer.
5. Good hardenability, heat treatment deformation is small.
General die-casting mold manufacturing method is to anneal the state of the mold material carved cavity, and then heat treatment, to get the required hardness, or the mold material first heat treatment, to get the required hardness, and then carved cavity. The manufacturing method of engraving cavity first and then heat treatment has high hardness and strength, and is not easy to produce melt loss and thermal fatigue.
No matter which method is used for heat treatment, it is necessary to get uniform hardness, so it is required to have good hardenability, especially if the cavity is carved first and then heat treated, it is necessary to use the material with small heat treatment deformation, which is especially important for the large size of the mold.