Due to the effect of flow and high pressure in squeeze casting, it is difficult to avoid the component segregation in the formed parts. The segregation of alloy elements has been concerned as early as the 1990s, and it is found that there is a significant difference between squeeze casting and gravity casting. After comparing the microstructure and properties of Al Cu Mg alloy samples with different thickness by gravity casting and squeeze casting, gany et al. Found that Cu in gravity casting is reverse segregation, but Cu in squeeze casting is positive segregation, and the degree of segregation in squeeze casting is less than that in gravity casting. Many other studies on the segregation of direct squeezehave also proved that the segregation of alloy elements in squeeze casting is the heavier positive segregation. Further study shows that the position and degree of segregation are also controlled by the cooling rate.
The segregation of impurity elements in alloy materials also has an important influence on the microstructure and properties of the materials. However, the segregation law of impurity elements in extrusion castings is still unclear. Babichevari et al. Studied the effect of grain boundary segregation of impurity elements such as Fe, Co, Cu, Ti, Mg and Pb on the grain boundary strength of nanocrystalline aluminum alloy, and found that the grain boundary segregation of Fe and CO can strengthen the grain boundary, while the grain boundary segregation of Cu and Pb can weaken the grain boundary. Yang Jian systematically studied the partition coefficients and segregation mechanism of common elements in aluminum by using molecular dynamics calculation method, and proved that the equilibrium partition coefficients of all impurity elements change periodically with the increase of atomic number, which provides a basis for the segregation prediction of impurity elements. However, the macrosegregation of impurity elements in squeeze casting is closely related to the shape, size and pressure mode of the workpiece. In this paper, the macro segregation behavior of impurity elements P, CA, Zn, Pb, Cu, Fe, Zr, Mn in A356 aluminum alloy wheel hub was analyzed, in order to provide reference for the control of impurity segregation in aluminum alloy casting.
(1) The results show that there are channel segregation of impurity elements along feeding channel, zero flow segregation at no flow position and density segregation of Ca and Cu in squeeze casting wheel hub. Both channel segregation and zero flow segregation are positive segregation, while density segregation is reverse segregation.
(2) Under the condition of backward squeeze casting, the segregation degree of each impurity element is ranked as follows: Zn, Pb and P are serious segregation elements, and the segregation level is 4; CA belongs to the element with heavy segregation, and the segregation level is 3; The segregation degree of Fe, Cu, Mn, Zr and other impurity elements is not different, they are all light segregation elements, and the segregation level is below grade 2.
(3) The segregation position of impurities is controlled by solidification sequence, rheological state and spatial position. The center position (zero flow zone) under the punch (indenter) and the local hot spot position of reverse filling are the impurity element concentration area of K0 < 1.