Many people think that adding silicon to aluminum alloy or magnesium alloy is mainly to increase the fluidity of the alloy, but this is not completely correct and misleading.
Adding silicon to the alloy has two main functions:
The first is to increase liquidity. But this is mainly for gravity casting and other very low pressure mold filling. Both inspection and practice show that there is little difference between silicon-free alloy and silicon-containing alloy when the filling pressure exceeds 65438±0 MPa. In the present die casting machine and die casting process, the filling pressure can exceed 100MPa, even for the alloy with the worst fluidity (wrought aluminum alloy, wrought magnesium alloy, etc.). ) There is no difficulty in underfilling.
The second is to reduce the phase change volume shrinkage of "liquid-solid" phase-this is the most important and key. Studies have shown that the phase transition volume of aluminum alloy (such as A390) with silicon content of about 20% can be basically unchanged. Therefore, aluminum pistons used at high temperature always have alloys with high silicon content. Due to the essential characteristics of die casting process, it belongs to unidirectional high-pressure forced filling casting, and has no reverse feeding function, which is completely different from low-pressure casting and gravity casting. It is precisely for this reason that aluminum alloy grades with relatively low phase transformation shrinkage and as high silicon content as possible are specially prepared in industry, especially for the purpose that reverse feeding cannot be carried out during die casting.