In-situ X-ray investigation of growth behavior of primary silicon particles and aluminum dendrites during solidification of a hypereutectic Al-Si-Cu alloy

The solidification behavior of hypereutectic Al-18 wt%Si-10 wt%Cu alloy was investigated using in-situ X-ray radiography. Sheet-like samples were solidified both under isothermal cooling conditions and in a moderate temperature gradient with different process parameters. The real-time observation method allows for the investigation of the growth of faceted primary Si particles and non-faceted Al dendrites. Cooling down below the liquidus temperature, firstly primary silicon particles nucleate and grow. At low cooling rates just a few Si particles nucleate and grow continuously to large sizes, whereas at higher cooling rates a larger number of smaller Si particles appear. As cooling continues below the nucleation temperature of aluminium, simultaneous growth of both columnar and equiaxed Al dendrites in the melt surrounding the primary Si particles was observed. In-situ observation as well as post-mortem analysis of the solidification morphology demonstrated that nucleation and growth of Al dendrites in the melt was nearly independent of existing Si particles and no indication was found that the Al phase starts growing from the surface of Si particles. The growth of the equiaxed Al dendrites can be explained by the thermal undercooling controlled by the application of a constant and continuous cooling, but the reason for the fast growing columnar Al dendrites in an isothermal melt is still not clear.