In order to research the change of austenite dendrite structure of gray cast iron from the perspective of thermodynamics, two conditions without vibration and vibration frequency of 35Hz were selected to measure the solidification temperature field of gray cast iron, and the effect of vibration on primary austenite was studied by hot alkali corrosion candle method.
Figure 1 shows the austenite morphology of gray cast iron prepared under the above two conditions. Among them, figure 1 (a) and (b) are the austenitic morphology of gray cast iron prepared at vibration frequencies of 0Hz and 35Hz respectively. It can be seen from Figure 1 that in the lost foam casting gray cast iron without vibration, the content of primary austenite is more and the grain size is larger, while in the gray cast iron prepared at the vibration frequency of 35Hz, the content of primary austenite is less and the grain size is smaller, as shown in the yellow texture in Figure 1. Comparing the eutectic structure of the gray cast iron prepared by the two processes, it can be found that the content of eutectic austenite is less and the size of eutectic cluster is larger in the lost foam casting gray cast iron without vibration; However, the content of eutectic austenite is higher and the size of eutectic cluster is smaller in the gray cast iron with vibration frequency of 35Hz. The primary austenite grain size of the vibration lost foam casting gray cast iron is small, which is mainly because the vibration increases the bending stress on the root of the primary austenite dendrite arm, makes it broken, increases the number of austenite nucleation, and refines the size of the primary austenite. In addition, the proper vibration can reduce the local crack point of primary austenite in lost foam casting gray cast iron and cause the austenite dendrite to dissolve, which is also a reason for the reduction of austenite grain size.
Fig. 2 shows the solidification cooling curve of gray cast iron under the above two process conditions. Comparing the cooling curves of columnar crystal zone of vibration lost foam casting gray cast iron with that of non vibration lost foam casting gray cast iron (Fig. 2), it can be seen that the eutectic point temperature of vibration lost foam casting gray cast iron is about 1110 ° C, which is nearly 44 ° C lower than the theoretical eutectic point (1154 ° C). However, the eutectic point temperature of lost foam casting gray cast iron without vibration is about 1134 ° C, which is 20 ° C lower than the theoretical eutectic point (1154 ° C). Comparing the eutectic point temperature of grey cast iron prepared by the two processes, it can be seen that the eutectic temperature of vibration lost foam casting grey cast iron is 24 ° C lower than that of lost foam casting grey cast iron without vibration.
The critical radius formula of heterogeneous nucleation during metal solidification can be expressed as:
According to the formula, the critical nucleation radius of grain is inversely proportional to the undercooling degree in the solidification process of metal. The smaller the undercooling degree is, the larger the critical nucleation radius of crystal is, which is more unfavorable to the nucleation of metal.
It can be seen from the test results that the undercooling degree of gray cast iron prepared at the vibration frequency of 35Hz is large, so the critical nucleation size is small, which is conducive to the nucleation of primary phase. In addition, the eutectic temperature of vibration lost foam casting gray cast iron is low, which reduces the diffusion of atoms in the metal body, inhibits the growth of eutectic austenite and eutectic graphite, and generates primary phases and eutectic clusters with small grain size. However, the solidification undercooling of lost foam casting gray cast iron without vibration is small, which increases the critical nucleation size of primary phase and is not conducive to the nucleation of metal solution. Therefore, the number of primary phase in gray cast iron is small. In addition, it can be seen from Fig. 2 that the eutectic temperature of lost foam casting gray cast iron without vibration is high (1134 ° C), and the diffusion speed of atoms is fast, which makes the growth speed of primary phase and eutectic phase faster. Finally, the size of primary phase and eutectic cluster generated in gray cast iron is large.