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03
2020
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08
The three main reasons for cracking in die casting molds.
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1. During the mold processing and manufacturing process
1. Quality issues with the forged blank
Some molds develop cracks after only producing a few hundred pieces, and the cracks develop rapidly. This may be due to ensuring only the external dimensions during forging, while the loose defects such as dendritic crystals, inclusions of carbides, shrinkage cavities, and bubbles in the steel are elongated along the processing method, forming flow lines. These flow lines greatly affect the final quenching deformation, cracking, brittle fracture during use, and failure tendency.
2. Cutting stress generated during finishing processes such as turning, milling, and planing can be eliminated through intermediate annealing.
3. Grinding stress generated during the grinding of quenched steel produces friction heat, resulting in softening layers and decarburization layers, which reduce thermal fatigue strength and easily lead to thermal cracking and early cracks. For H13 steel after precision grinding, stress relief annealing can be performed by heating to 510-570°C and holding for one hour for every 25mm thickness.
4. Electrical discharge machining generates stress. A bright layer rich in electrode elements and dielectric elements forms on the surface of the mold, which is hard and brittle; this layer itself may have cracks and stress. High frequency should be used during electrical discharge machining to minimize the bright layer, which must be removed through polishing methods and subjected to tempering treatment at a three-stage tempering temperature.
2. During the mold treatment process
Improper heat treatment can lead to mold cracking and premature scrapping, especially when only tempering is used without quenching followed by surface nitriding processes. After several thousand die-casting cycles, surface crazing and cracking may occur. The stress generated during steel quenching is a result of the superposition of thermal stress during cooling and organizational stress during phase transformation; quenching stress is a cause of deformation and cracking, so tempering must be performed to eliminate stress.
3. During the die-casting production process
1. Mold temperature
The mold should be preheated to a certain temperature before production; otherwise, when high-temperature molten metal fills the mold, it causes rapid cooling, leading to an increased temperature gradient between the inner and outer layers of the mold, forming thermal stress that causes surface crazing or even cracking. During production, the mold temperature continuously rises; when it overheats, it can easily lead to sticking molds and malfunctioning moving parts that damage the mold surface. A cooling temperature control system should be set up to maintain the working temperature of the mold within a certain range.
2. Filling
The molten metal fills at high pressure and speed, inevitably causing intense impact and scouring on the mold, thus generating mechanical stress and thermal stress. During impact, molten metal, impurities, and gases can also interact chemically with the mold surface, accelerating corrosion and crack formation. When molten metal is wrapped with gas, it will first expand in low-pressure areas within the cavity; as gas pressure increases, it causes inward explosions that pull metal particles from the cavity surface causing damage and cavitation-induced cracks.
3. Production process
In every die-casting production process, due to thermal exchange between the mold and molten metal, periodic temperature changes occur on the mold surface, causing periodic thermal expansion and contraction that generate periodic thermal stress. For example, during pouring when the mold surface heats up it experiences compressive stress; after ejecting the casting from the mold, it experiences tensile stress due to cooling. When this alternating stress cycles repeatedly, it accumulates internal stress in the mold; when this stress exceeds the material's fatigue limit, cracks will form on the mold surface.
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