Pembentukan Mikrostruktur Berbentuk Sfera dalam Keadaan Separa Pepejal Aloi Aluminium Al-4.8Si- 2.8Cu-0.5Mg

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°C) onto the cooling slope plate (with 60° tilt angle and cooling slope length of 400 mm) and finally collected in a 150°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°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 α-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°C for 8 hrs, quenched in warm water at 60°C and followed by ageing at 155°C for 4 hrs.