1. Defect category identification
According to the shape and location of the stoma, the defect was initially identified as subcutaneous stoma. The subcutaneous porosity of nodular cast iron may be caused by H2, because the following reactions will occur in the molten iron after spheroidizing treatment [1]:
(Fe, Mg) C + H2O (in mold) → (Fe, Mg) O + H2 ↑ + C (graphite film)
Mg + H2O (in mold) → mg O + H2 ↑ (2)
2. Brake caliper structure comparison
Figure 1 shows the jaw structure of 693 brake caliper, and Figure 2 shows the jaw structure of 812 brake caliper. Through comparison, it is found that the jaw structure of 693 brake caliper is different from that of 812 brake caliper: (1) the jaw height of 693 brake caliper is higher, 3.4 mm higher than that of 812 brake caliper; (2) the jaw thickness of 693 brake caliper is thinner, 2 mm thinner than that of 812 brake caliper. The higher and thinner jaw structure will make the temperature of the molten iron on the top of the casting drop faster, the solidification and the formation of oxide film are faster, and the gas inside the molten iron is more difficult to be discharged. This is probably the reason why 693 brake caliper is more likely to produce subcutaneous porosity defects than 812 brake caliper.
3. Simulation comparison of casting filling process
In order to accurately compare the differences between the two kinds of brake caliper, the simulation comparison of the casting filling process was carried out. The results show that the casting filling process is stable without turbulence and splashing of molten iron; when the casting filling is completed, the temperature at the casting jaw of 693 brake caliper is about 19 ℃ lower than that at the same part of 812 brake caliper. The analysis shows that the temperature difference caused by the structural difference may be more conducive to the generation of subcutaneous pores. On the premise of not changing the casting structure, the “gas source” of subcutaneous pores needs to be controlled.