## 4 Results and Discussion
### Effects of Heat Treatment on the A356 Alloys  
Figure 4 shows the microstructures of the A365 alloys after heat treatment. Compared to the non-heated A356 alloys (Figure 3), the Fe-rich IMCs adjacent to the dendrite decreased in ST-WQ. When ST-E was used, the contents of the Fe-rich IMCs were similar to those in the non-heated A356 alloys. Figure 5a shows the SDAS measured using the linear intercept method (crossline in Figure 3). Depending on the heat treatment, the SDAS in sections A and B did not change significantly. However, they varied according to the cooling rates after ST and grew slightly, even with AG. Figure 5b shows the area fractions of the intermetallic compounds. The area fractions of the IMCs decreased in the samples with WQ after ST. In contrast, the area fractions of the IMCs in the ST-E A356 alloys was similar to that in the non-heated A356 alloys. In the ST-WQ and ST-WQ-AG A356 alloys, the difference of area fraction between sections A and B was decreased from 0.42% to 0.2% (compared to that in the non-heated A356 alloys). The reductions of the area fraction and the difference of area fraction occurred because the Mg constituting the \(\uppi\)-Al\({}_{8}\)Mg\({}_{3}\)FeSi\({}_{6}\) phases was re-dissolved into the Al matrix during ST [29]. The aspect ratio about the horizontal axis/vertical axis of eutectic Si was shown in Figure 5c; if the value were closer to 1, it would adopt a more spherical shape. As a result of the ST, the aspect ratio of eutectic Si decreased by approximately 0.3 compared to that of the non-heated A356 alloys. This  
Figure 3: Optical microscopy images of non-heated A356 alloy; crosslines in the images indicate measurements of secondary dendrite arm spacing. (**a**) section A and (**b**) section B. Section A is closer to the center of rotation than section B.  
was because inter-diffusion occurred to reduce the surface energy of the Si solute atoms owing to the driving force during the heat treatment [27]. The spheroidization of eutectic Si in heat-treated A356 alloys yielded a similar aspect ratio.  
Figure 5d shows the Vickers hardness values of both sections after heat treatment. In both sections the HV values were similar because the values existed within deviation ranges. The differences are observed between the specific heat treatment processes. The average hardness was calculated to this Equation ((section A + section B)/2). Compared to the non-heated A356 alloys, the average hardness of the ST-E A356 alloys decreased  
Figure 4: Optical microscopy images of heat-treated A356 alloy: (**a**–**c**) section A; (**d**–**f**) section B; (**a**,**d**) ST-E, (**b**,**e**) ST-WQ, and (**c**,**f**) ST-WQ-AG (section A is closer to the center of rotation than section B).  
Figure 5: Quantitative microstructural analysis using I-solution DT image software: (**a**) SDAS, (**b**) area fraction of IMCs, (**c**) aspect ratio of eutectic Si, and (**d**) Vickers hardness.  
by 13 HV because of grain boundary segregation caused by the formation of eutectic-Mg\({}_{2}\)Si adjacent to eutectic Si and \(\pi\)-AlMgFeSi [30]. In contrast, the average hardness of the ST-WQ A356 alloys were 64 HV, which were higher than the non-heated A356 alloys. These changes corresponded to the reduction in the area fractions of IMCs that occurred as Mg solute atoms re-dissolved into the Al matrix during ST. This led to the solid-solution strengthening effect owing to the re-dissolution of Mg in the Al matrix.