Liquid segregation of 7075 alloy in rheo squeeze casting

By analyzing the metallographic structure of the parts successfully manufactured by rheo squeeze casting, it is found that liquid phase segregation occurs on the upper surface of the parts, especially at the outer corner of the upper surface of position 1 and position 3 (see Fig. 1). From the analysis of the cross section of the part, the solid rate near the center (positions 4 and 5) is high, and the number of solid particles is more, while the number of solid particles near the edge of the upper surface of the part is less, and the proportion of liquid phase increases significantly (see Fig. 1, the proportion of liquid phase at position 1 increases, resulting in obvious liquid phase segregation). In the experiment, the position Y as shown in Fig. 2 is selected for metallographic analysis, and the microstructure is shown in Fig. 3.

It can be seen from Fig. 3 that the number of solid particles in u site is more, the number of solid particles in V site is significantly reduced, and the number of liquid phase is increased. The results show that the position V and the position 1 (see Fig. 1) near the upper surface have higher degree of liquid segregation.

The primary solid phase with near spherical shape is the main characteristic of semi-solid alloy, and its forming process has its own characteristics. Liquid phase segregation is also one of the basic characteristics of semi-solid forming, and special analysis will be carried out later. The main viewpoints of deformation mechanism of semi-solid alloy are as follows: there are constantly changing contact morphology between liquid phase and solid phase; the viscosity of liquid phase and deformation resistance of solid phase are affected by melting, diffusion and solidification of crystal interface; the liquid phase is evenly distributed among solid phase particles, so that solid phase particles float in liquid phase, and there is almost no knot between solid phase particles The results show that the resultant force has little resistance to deformation and flow; when the solid ratio is very low, the semi-solid alloy will be fluid, and flow and deformation can occur under very small external pressure; when the semi-solid metal is subjected to external force, the solid and liquid phases tend to flow separately, and generally flow first due to the fluid characteristics of the liquid phase.

According to the indirect squeeze casting process, solid particles and liquid phase flow and deform separately in the process of rheo squeeze casting. As a result, the liquid phase flow from the upper part of the die cavity to the lower part of the die is constrained by the hydrostatic pressure.

① Because the upward movement of injection hammer is the power source of semi-solid slurry flow and deformation. During injection, the semi-solid slurry is in direct contact with the upper surface of the mold cavity, and the rate of metal solidification and shrinkage is relatively high. The volume of the part near the upper surface is solidified and contracted first, which will produce a certain capillary adsorption on the liquid phase, and the liquid phase will be replenished up and around the mold cavity in time. Therefore, the solid phase in the center of the cross section of the part is continuously extending, the central part of the casting is mainly composed of the solid phase before forming, and the upper surface of the part is mainly composed of the liquid phase before solidification.

② The main space that semi-solid slurry can fill under pressure is the cavity composed of upper mold, lower mold and injection hammer head. There are certain overflow grooves around the upper surface of the workpiece, and the outer part of the top of the workpiece is the surface with the least constraint. Therefore, during semi-solid slurry filling, part of the liquid phase preferentially flows into this part, forming liquid phase segregation.

③ Fig. 4.6 position Y, because the corner is close to 90 ° angle, when the semi-solid slurry flows through it under pressure, there is a large pressure gradient, and the liquid phase is extruded from the gap between solid particles by pressure, resulting in the separation of solid-liquid two phases, resulting in liquid phase segregation, and the proportion of liquid phase region near part V is significantly higher than that near part U. Therefore, in the design of semi-solid squeeze casting parts, it is necessary to avoid changing the right angle as far as possible, and adopting large angle design is conducive to inhibiting the liquid segregation defects.