Abstract:

The chemical composition, surface defect morphology and microstructure of failure parts of the aluminum alloy die-casting mold made of 8418 steel were analyzed by means of direct reading spectrometer, digital Rockwell hardness tester, optical microscope, scanning electron microscope and energy spectrometer. The results show that, the grain size of matrix is coarse. The average diameter of coarse grains is 1647 μm, and the degree of material superheat is extremely serious. The original coarse grains lead to an increase of the quenching stress after the final heat treatment, and thus the fracture morphology of cleavage characteristic and intergranular dimple are observed. Carbides were precipitated along grain boundaries during the subsequent slow cooling, as a result, the intergranular strength is decreased remarkably. Under the influence of the quenching microstructure stress, internal crack along the grain boundary is formed. The reason for the early cracking of the aluminum alloy die-casting mold is the overheated coarse structure and internal crack along the grain boundary, which greatly reduces the strength and toughness of the material. During the use of the mold, it is difficult to withstand the working stress of the die-casting process, and finally the crack is generated at the minimum position of the surface of the mold cavity, that is, the position where the stress concentrated maximumly.

Key words: superheated structure, cleavage fracture, intergranular dimple, intragranular crack