The volume of 7075 aluminum alloy shrinks when it crystallizes. For squeeze casting, solidification under pressure will increase the undercooling of the alloy, which greatly improves the solidification speed of the parts and helps to refine the grains. From the analysis of the metallographic diagram, as shown in Fig. 1, it is the metallographic structure diagram of the same part a (see Fig. 2) under different specific pressures. The grain size of the metallographic diagram was measured by IPP software (using the marking method).
From the analysis of calculation results, it can be seen that with the increase of specific pressure, the grain size of metallographic structure in the same part of the workpiece decreases gradually. Therefore, increasing the pressure is conducive to grain refinement, but according to the number of nuclei in unit volume, zv is related to the number of nuclei N and the linear velocity g of the crystal, which can be expressed as follows:
It can be seen that the higher the nucleation rate and the smaller the linear growth velocity, the more the number of nuclei per unit volume and the smaller the grain size. However, the increase of pressure will increase the growth rate of crystal and the nucleation rate at the same time. Therefore, the effect of crystallization under pressure on the grain size should be analyzed by integrating various process factors, which has certain complexity.
The metallographic structure near part B (see Fig. 2) in the same position of successfully manufactured parts under different specific pressures was analyzed, as shown in Fig. 3. The influence of different specific pressures on liquid phase segregation was discussed. The solid fraction was calculated by IPP software.
Generally, the macro process of liquid-phase flow in the liquid-solid two-phase region can be calculated by Darcy law D’Arcy. If gravity is omitted, the semi-solid metal melt is assumed to be porous medium, and its expression is: