Influence of variations in alloy composition on castability and process stability. Part 2: Semi-solid casting processes

Abstract

For stabilising semi-solid casting processes it is important to keep the variations in solid fraction low. The solid fraction and the shape of the solid phase strongly influence fluidity during die filling and homogeneity of the resulting casting. With the use of a meander die on a horizontal die casting machine, and semi-solid slugs prepared by the application of the new rheocasting recipe to variations of the aluminium alloy A356, the influence of fluctuations in alloy composition and the influence of minor alloying elements on the flow behaviour of semi-solid slurries was investigated. It becomes clear that temperature, time or viscosity measurements alone are not sufficient for proper process control in semi-solid casting. A new process parameter, the sensitivity to compositional fluctuations KC is introduced, which shows that it is essential to keep the alloy composition stable. The application of laser induced plasma spectroscopy (LIPS) is proposed as a possible solution. The findings are applicable to practically all currently available semi-solid casting processes.