Internal defects of die-cast parts: Characteristics and causes of metal hard spots

(1) Characteristics:
Similar to the non-metallic hard points in 2.6, during mechanical processing or metallographic examination, fine particles or blocks with hardness higher than the metal matrix are found on the castings, resulting in severe tool wear and showing different brightness on the machined surfaces.
(2) Causes:
① Silicon elements that were not melted were mixed in the metal hard points;
② Primary crystalline silicon: The temperature of the aluminum liquid was low and the storage time was long.
③ The segregation of iron and manganese elements led to the formation of intermetallic compounds.