The results show that the segregation zone with high impurity element content is located at position 1 and the segregation zone with low impurity element content is located at position 4. On the surface, there is no relationship between them. In fact, both of them belong to “zero rheological segregation”. Position 1 is in the center below the punch (indenter). When the punch contacts the alloy liquid, a solidified shell is formed on its lower surface. With the extrusion of the punch, the metal liquid under it flows along the radial direction. At position 1 in the center below the punch, the metal liquid has only vertical compression movement, but no horizontal movement, That is, there is a “dead zone” similar to plastic deformation. The thickness of the “deformation dead zone” increases with the increase of solidification time, and the range of the “deformation dead zone” also expands with the increase of solidification time. Therefore, “zero flow segregation” is a segregation phenomenon in the region without obvious flow. The segregation law is the same as gravity casting, which is the final solidification position. The solute content is higher than the average value, which belongs to positive segregation.
The reason for the low impurity content of No.4 site is also “zero rheological segregation”. It can be seen from Fig. 2 that position 4 is located at the outer rim. The wall thickness of the workpiece at this position is only 4mm, and it has basically solidified before the start of pressurization. Therefore, this position has almost no rheology during the subsequent reverse extrusion filling and feeding process, which also belongs to the “zero rheology” position. Because of the small wall thickness, it is the first solidification position, and the impurity element content is relatively low, that is, negative segregation occurs. Different from position 1, this is the first solidification position, and the discharged solute is “rheologically transferred” to the rim position, so it belongs to negative segregation.
This kind of “zero flow segregation” occurs in the parts where no obvious flow occurs in squeeze casting, “deformation dead zone” and the parts of thin wall, rib and rib which have basically solidified before extrusion are the typical parts of “zero flow segregation”, and the segregation law is the same as that of gravity casting. In the last solidification position, there will be positive segregation, while in the first solidification position, there will be negative segregation.