1, Thermal fatigue cracking
Die casting production, die repeatedly by the role of cold and heat, forming surface and its internal deformation, mutual involvement and repeated cycles of thermal stress, resulting in structural damage and loss of toughness, triggering the emergence of micro-cracking, and continue to expand, once the crack expands, there is molten metal liquid extrusion, coupled with repeated mechanical forces are to accelerate the expansion of the crack. Therefore, in actual production, most of the mold failure is thermal fatigue cracking failure.
2, Brittle cracking
Under the action of pressure injection force, the mold will sprout cracks at the weakest point, especially the mold forming surface scribing traces or electric machining traces are not polished, or forming at the angle of inclination will be the first to appear fine cracks, when the grain boundary exists brittle phase or grain coarse, that is easy to fracture. When the brittle fracture is present in the grain boundary or the grain is coarse, it will be easy to fracture. The expansion of the crack is very fast. The brittle failure of the mold is a very dangerous factor, the mold material's plastic toughness is the most important mechanical property corresponding to this phenomenon.
3, Dissolution corrosion
Commonly used die casting alloys are zinc alloys, aluminum alloys, magnesium alloys, and copper alloys, but also pure aluminum die casting, Zn, Al, Mg is more active metal elements, they have a good affinity with the mold material, which is due to the combined effect of mechanical and chemical corrosion. Especially Al is easy to bite the mold, molten aluminum alloy shot into the cavity at high speed, resulting in mechanical corrosion of the cavity surface, at the same time, the metal aluminum and mold material to generate brittle iron-aluminum compounds, become a new source of hot crack sprouting, in addition, aluminum filling to the crack and crack wall mechanical action, and superposition with the thermal stress, intensify the tensile stress at the tip of the crack, thus accelerating the expansion of the crack. When the mold hardness is higher, the corrosion resistance is better, and if there are soft spots on the forming surface, it is not good for corrosion resistance. However, in actual production, the dissolution corrosion is only the local place of the mold, generally, the part where the inner gate is directly flushed (core, cavity) is prone to dissolution corrosion, as well as the hardness is soft where the aluminum alloy is prone to sticky mold. Improve the high-temperature strength and chemical stability of the material is conducive to enhance the corrosion resistance of the material.