Macro-Segregation and Microstructural Characteristics in Rheo-Diecasting of a High Strength Al–4.8 mass%Si–0.7 mass%Mg Alloy

Abstract

Rheo-diecasting with electro-magnetic stirring has been carried out to investigate macro-segregation and microstructural characteristics of a high strength Al–Si–Mg alloy under various process conditions. The amount of initial heterogeneous nucleation and the degree of temperature uniformity in the slurry are dependent on the superheat of the melt at pouring into the slurry making vessel. The formation of macro-segregation in rheo-diecasting was closely related to the microstructural characteristics of semi-solid slurries. Slurries which have fine and globular α-Al particles prevent the formation of macro-segregation throughout the rheo-diecast specimens. However, coarse and non-globular α-Al particles result in the formation of macro-segregation, leading to a non-uniform hardness distribution, dependent on the thickness of the product. Various casting process parameters were examined to prevent the formation of macro-segregation, and the optimal ranges of the process conditions for rheo-diecasting were determined as follows: the injection velocity of 0.3–1.0 m·s⁻¹ and the mold temperature above 150°C.