Effects of Heat Treatment on the Microstructure and Hardness of A356 (AlSi\({}_{7}\)Mg\({}_{0.3}\)) Manufactured by Vertical Centrifugal Casting  
[PERSON]  
1 Department of Ocean Advanced Materials Convergence Engineering, Korea Maritime and Ocean University, Busan 49112, Korea; [EMAIL_ADDRESS]  
2 Advanced Forming Process R&D Group, Korea Institute of Industrial Technology (KITECH), Ulsan 4476, Korea  
3H/W Development, S-FAC Solution Factory, Daejeon 34025, Korea  
Correspondence: [EMAIL_ADDRESS] (K.J.); [EMAIL_ADDRESS] (C.J.); [EMAIL_ADDRESS] (E.L.); Tel.: +82-(051)-410-4353 (E.L.)  
[PERSON]  
3H/W Development, S-FAC Solution Factory, Daejeon 34025, Korea  
3  
[PERSON]  
2 Advanced Forming Process R&D Group, Korea Institute of Industrial Technology (KITECH), Ulsan 4476, Korea
than the gray cast iron and of higher thermal conductivity, were used to reduce the weight and improve the heat dissipation of the disk.  
Unlike gravity casting, centrifugal casting is a method in which molten metal is injected during the rotation of the mold. It adheres to the mold wall by centrifugal force and then solidifies [14]. This casting process has been applied to the development of structural steel pipes, composite rolls, bearings, and cylinder liners. It has also been used broadly to produce functionally graded materials for special purposes [15; 16; 17; 18]. The content of each solute atom depends on the particle densities and the rotational speed of the motor. For example, solute atoms that are heavier than the matrix alloy atoms move outward as the rotation speed increases. It is easy to segregate and to remove intermetallic compounds (IMCs) that act as impurities [18; 19]. The materials fabricated via centrifugal casting possess a finer microstructure than those obtained via gravity casting, resulting in improved mechanical properties, such as the tensile strength, hardness, and fatigue limit [20].  
The A356 alloy is widely applied in many automotive industries owing to its excellent castability and good mechanical properties [21; 22]. The A356 alloy contains the eutectic Si, Mg\({}_{2}\)Si, \(\beta\)-Al\({}_{5}\)FeSi, and \(\pi\)-Al\({}_{8}\)Mg\({}_{3}\)FeSi\({}_{6}\). The Mg\({}_{2}\)Si phases exhibit a transformation mechanism that changes from a supersaturated solution to a Mg and Si nanocluster formation, and then transforms into the Guinier-Preston (GP) zone, \(\beta^{\prime}\), \(\beta^{\prime}+\) Si, and finally to \(\beta\). These phases could improve the mechanical properties of the material through artificial aging [23]. The \(\beta\)-Al\({}_{5}\)FeSi phase generated at 841 K is needle-shaped, and its hardness and brittleness degrade its tensile strength and elongation [24]. The \(\pi\)-Al\({}_{8}\)Mg\({}_{3}\)FeSi\({}_{6}\) phase generated at 823 K has a Chinese script-like shape, which inhibits the formation of \(\beta\)-Al\({}_{5}\)FeSi and improves the tensile strength [25; 26]. During the heat treatment, the eutectic Si undergoes three morphological transformations, which change from fragmentation to spheroidization and finally to coarsening. Some researchers have reported that the more circular the eutectic Si, the greater the tensile strength and elongation [27].  
In brake disks manufactured via the centrifugal casting process, the presence of the non-uniform IMC fraction in some parts, owing to the centrifugal force, adversely affects the safety and reliability of the brake disks. To solve this problem, heat treatment is required to improve the mechanical strength of the A356 alloy and to alleviate segregation during solidification. However, there are limited studies on the effects of heat treatment on centrifugally cast A356 aluminum alloys, and related experimental data are insufficient. Therefore, the effects of heat treatment, such as solid solution and artificial aging, on the microstructure and mechanical properties of A356 alloys produced by centrifugal casting were investigated in this study.