The figure shows the graphite morphology of gray cast iron prepared under different vibration frequencies. Among them, figures (a) – (d) show the graphite morphology of gray cast iron prepared at the vibration frequencies of 0, 35, 50 and 100Hz respectively.
It can be seen from the figure that in the gray cast iron without vibration, the length of type a flake graphite is relatively large. In addition to type a flake graphite, there are many types of graphite, as shown in figure (a). When vibration is applied to the lost foam casting process of gray cast iron, the length, thickness and quantity of A-type flake graphite in gray cast iron become shorter and smaller.
The composition of gray cast iron usually belongs to hypoeutectic composition. According to the iron carbon alloy phase diagram, the alloy with this composition precipitates austenite first during solidification. When the temperature is reduced to the eutectic temperature, the alloy will undergo eutectic reaction to generate eutectic austenite and eutectic graphite, and the graphite morphology generated during the eutectic reaction depends on the cooling rate, undercooling degree of the metal body during the eutectic reaction and the nucleation ability of the graphite.
In EPC of gray cast iron, the cooling rate of liquid metal is faster and primary austenite is formed rapidly in the negative pressure stage. When the temperature of the metal solution drops to the eutectic temperature, the thermal conductivity of the molding sand decreases significantly because the temperature of the molding sand has increased, which slows down the cooling rate of the alloy, reduces the undercooling during Eutectic Transformation, and is easy to form A-type graphite.
Vibration applied during solidification of gray cast iron can make the composition in the alloy become uniform, reduce the concentration gradient of carbon atoms in the metal melt, and reduce the driving force of graphite growth, resulting in shorter length and smaller thickness of type a graphite.
| Vibration frequency /Hz | 0 | 35 | 50 | 100 |
| Actual length of graphite / mm | 0.36 | 0.35 | 0.27 | 0.33 |
The table shows the results of statistical analysis on the graphite length in the figure. It can be seen from the table that under the condition that the alloy composition of cast iron is the same, with the increase of vibration frequency, the length of flake graphite in gray cast iron first decreases and then increases. When the vibration frequency is 35Hz, the length of flake graphite in gray cast iron is the shortest.