In the production of aluminum alloy die-casting, the most common way of mold damage is cracks and cracks. Stress is the primary cause of mold damage.
In the process of die casting production
1. Mold temperature
The mold should be preheated to a certain temperature before production. Otherwise, chilling occurs when the high-temperature molten metal is filled, which will cause the temperature gradient between the surface and the inner layer of the mold to increase, forming thermal stress, and causing the mold to crack or even crack.
In the production process, the mold temperature rises from time to time. When the mold temperature is overheated, the mold sticks easily, and the movable parts fall off and cause damage to the mold surface.
A cooling temperature control system should be installed to keep the mold task temperature within certain limits.
2. Alloy filling
The molten metal is filled with high pressure and high speed, which will inevitably cause severe impact and erosion on the mold, so mechanical and thermal stress will occur. During the impact process, the molten metal, impurities, and gas will have complex chemical effects on the surface of the mold and accelerate the occurrence of corrosion and cracks. When molten metal is encased in gas, it will expand first in the low-pressure area in the cavity. When the gas pressure increases, inward blasting occurs, pulling out the metal particles on the surface of the cavity to form damage, and cracks due to cavitation.
3. Mold opening
In the process of core pulling and mold opening, mechanical stress will also occur when certain components are deformed.
4. Production process
In the production process of each aluminum alloy die casting, due to the heat exchange between the mold and the molten metal, periodic temperature changes occur on the surface of the mold, causing periodic thermal expansion and contraction, and periodic thermal stress. For example, the surface of the mold is subjected to compressive stress due to heating during pouring, and after the mold is opened to eject the casting, the surface of the mold is subjected to tensile stress due to cooling. When this alternating stress repeats cycles, the stress accumulated inside the mold becomes larger and larger. When the stress exceeds the fatigue limit of the material, cracks occur on the surface of the mold.
The problem of rough forging
1. Some molds show crack patterns when only a few hundred pieces are produced, and the cracks develop quickly. It may be that the outer dimensions are only guaranteed during forging, and the loose defects such as dendrites, doped carbides, shrinkage cavities, and bubbles in the steel are elongated and elongated along the processing method to form a streamline. This streamline will affect the future *The final quenching deformation, cracking, embrittlement in the application process, and the tendency to lose effectiveness have a great influence.
2. The cutting stress that occurs during final processing such as turning, milling, and planing can be eliminated by center annealing.
3. Grinding stress occurs during the grinding of hardened steel, friction heat occurs during grinding, softening layer and decarburization layer occur, which reduces the thermal stress and easily causes hot cracks and early cracks. After fine grinding, H13 steel can be heated to 510-570℃, and the thickness is kept at 25mm for one hour for stress relief annealing.
4. Stress occurs in EDM. A bright white layer enriched with electrode elements and dielectric elements is formed on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. High frequency should be used in EDM to reduce the white layer to the smallest. It must be polished to remove and tempered. The tempering is performed at the third tempering temperature.
Mold disposal process
Inadequate heat treatment will lead to die cracking and premature scrapping, especially if only quenching and tempering is used, and the surface nitriding process is carried out without quenching, surface cracks and cracks will appear after several thousand die castings.
The stress that occurs when steel is quenched is the result of the superposition of the thermal stress during the cooling process and the structural stress during the phase transformation. The quenching stress is the cause of deformation and cracking, and it is necessary to temper to eliminate the stress.
In the mold processing and manufacturing process
1. Quality problems of rough forging
Some molds show cracks after only a few hundred pieces are produced, and the cracks develop quickly. It may be that the outer dimensions are only guaranteed during forging, and the loose defects such as dendrites, doped carbides, shrinkage cavities, and bubbles in the steel are elongated and elongated along the processing method to form a streamline. This streamline will affect the future *The final quenching deformation, cracking, embrittlement in the application process, and the tendency to lose effectiveness have a great influence.
2. The cutting stress that occurs during final processing such as turning, milling, and planing can be eliminated by center annealing.
3. Grinding stress occurs during the grinding of hardened steel, friction heat occurs during grinding, softening layer and decarburization layer occur, which reduces the thermal stress and easily causes hot cracks and early cracks. After fine grinding, H13 steel can be heated to 510-570℃, and the thickness is kept at 25mm for one hour for stress relief annealing.
4. Stress occurs in EDM. A bright white layer enriched with electrode elements and dielectric elements is formed on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. High frequency should be used in EDM to reduce the white layer to the smallest. It must be polished to remove and tempered. The tempering is performed at the third tempering temperature.