In order to better compare the solidification structures of different sampling positions under different pressures, the solidification structures of top, middle and bottom samples were compared. The characteristic parameters of solidification structure (eutectic spacing, eutectic proportion, primary phase size, eutectic length diameter ratio, etc.) were statistically analyzed
Firstly, the eutectic spacing of samples at different positions under different pressures is counted, and the results are shown in Figure 1. The smaller the solidification rate is, the greater the effect of pressure on eutectic spacing is. When the pressure is less than 130 MPa, the eutectic spacing decreases with the increase of solidification rate. When the temperature exceeds 130 MPa, the eutectic spacing increases with the increase of solidification rate. At the bottom of the sample, although the eutectic spacing gradually decreases with the increase of pressure, the effect is not as obvious as that of small solidification rate.
Secondly, the volume proportion of eutectic structure of the sample at different positions under different pressures is counted, and the results are shown in Figure 2. When the solidification rate is low in the middle sample, the content of eutectic structure is directly related to the value of pressure. With the increase of pressure, the content of eutectic structure gradually decreases. With the increase of solidification rate, the effect of pressure on the content of eutectic structure begins to fluctuate, which is less affected by pressure.
In addition, the primary phase sizes of samples at different positions under different pressures were counted, and the results are shown in Figure 3. When the solidification pressure is low, the polygonal primary phase in the solidification structure is fully grown in the middle of the lower solidification rate, the size ratio is large, and the refining effect is not obvious. When the pressure is less than 130 MPa, the primary phase can be refined with the increase of solidification rate. However, when the pressure is large enough, the pressure promotes the growth of primary phase obviously. It is worth noting that the distribution of primary phase is more uniform.
According to the statistics of the aspect ratio of eutectic structure at different positions under different pressures, the results are shown in Figure 4. With the increase of solidification rate, the aspect ratio of eutectic carbide decreases gradually, and the eutectic carbide transforms from long lath to short rod. After the pressure is applied, the equiaxed eutectic structure in the middle of the specimen is the most obvious, and the aspect ratio decreases the fastest. However, with the increase of solidification rate, although the aspect ratio of eutectic carbide is affected by pressure, it decreases to a certain extent. However, under the condition of high solidification rate of the bottom sample, the aspect ratio increases to a certain extent. This may be due to the influence of heat flow direction on the specimen near the die wall, even if it is refined under pressure, it still presents fine strip shape rather than equiaxed shape.
In conclusion, the analysis of the morphology parameters of solidification structure such as eutectic spacing, eutectic volume and primary phase size shows that increasing the solidification pressure can not change the thermal field and heat flow in the pressurized metal mold. Higher mold temperature, rather than solidification pressure, has a significant effect on heat transfer coefficient and heat flux. By comparing the solidification structures under different solidification rates and pressures, it can be seen that when the solidification rate of the alloy is large, the effect of pressure is not very obvious in the range of this study, regardless of whether it is dominated by thermal supercooling or not. When the solidification rate is small, the effect of pressure on microstructure is obvious. Therefore, in the process of pressure casting, it is very important to ensure the proper undercooling. For example, the morphology of solidification structure near the edge of the mold can be improved by increasing the preheating temperature of the mold while increasing the undercooling.