1. Casting crack
Due to the complex shape and uneven thickness of the casting, the cooling rate of each part is inconsistent during solidification, resulting in large internal stress. When the stress exceeds the tensile strength of the alloy material, the casting is pulled apart.
Cracks can be divided into hot cracks and cold cracks. The thermal crack is intergranular crack, with oxidation black, irregular shape and sawtooth shape; The cold crack is intergranular cracking, the fracture is not oxidized, and it is refractive silver and shiny.
Process measures to eliminate cracks: the chemical composition of the alloy shall be accurate and the impurity content shall not exceed the standard; Avoid Melt Overheating and stay in the furnace for a long time; Formulate scientific and reasonable casting temperature and casting speed; Liquid metal flow and cooling shall be uniform; Prevent foreign inclusions from falling into the melt, etc.
2. Lace tissue
The boundary grains are wavy, sawtooth and broken line lace, which are similar to cast twins, often in feather shape. The microstructure is parallel to each other and in thin flakes. It is caused by improper adjustment of chemical composition, overheating of melt and too long residence time, too fine diameter of filter tube hole, too high casting temperature, too low crystallizer and loss of function of metamorphic refiner.
Measures to eliminate lace like structure: strictly control the chemical composition and impurity content of the alloy, and shall not exceed the standard; Scientifically and reasonably design the filtration system and crystallization device; Accurately control the casting melting temperature and time; Add qualified metamorphic refiner, etc.
3. Bright grain
When the poor solid solution with low alloy element content appears in the casting, the grain is coarse, the dendritic network is thin and bright, and the hardness is lower than that of the normal structure. Due to the low funnel temperature in the casting process, the primary crystal structure of low-component solid solution is formed at the bottom, which grows continuously according to the original composition. When the weight reaches a certain degree, bright grains will be formed and the strength and toughness of the alloy will be reduced.
Preventive measures: the funnel material has good thermal conductivity, the surface shall be bright, the height from the funnel to the bottom shall be moderate, and the funnel coating shall be uniform; Preheating funnel and sinking into molten metal shall not be too deep before pouring; Strict control of casting temperature and uniform and stable supply of liquid aluminum can effectively eliminate the bright grain defects of castings.