Formation of Spheroidal Microstructure in Semi-solid State of Al-4.8Cu-0.5Mg AluminiumAlloy

This paper presents the outcomes of the work on the formation of spheroidal microstructure in the semisolid state of Al-4.8Si-2.8Cu-0.5Mg aluminium alloy. The starting material of the aluminium alloy was produced employing a conventional casting process. The microstructure of the starting material consists of fine dendritic and interdendritic structures. The fine nondendritic microstructures of the alloy were generated from the cooling slope casting process. The starting material was initially melted using a resistance furnace before pouring (at 650 degrees C) onto the cooling slope plate (with 60 degrees tilt angle and cooling slope length of 400 mm) and finally collected in a 150 degrees C preheated cylindrical stainless steel mould. The formation of the spheroidal structure in a liquid matrix was obtained when the nondendritic alloy billet was rapidly reheated into its semisolid temperature at 575 degrees C using a high-frequency induction heating system (30-80 kHz, 35 kW). The semisolid alloy billet was successfully thixoformed into a preheated stainless steel die using a hydraulic cylinder press that provided a load of 20 kN with a maximum compression velocity of 85 mm/s. The thixoformed sample showed a uniform distribution of nearly globular alpha-Al grains without micropores throughout the sample. The results demonstrate that the grain distribution is more homogenous after the T6 heat treatment which involves the solid solution treatment at 480 degrees C for 8 hrs, quenched in warm water at 60 degrees C and followed by ageing at 155 degrees C for 4 hrs.