## 5 Conclusions  
This research was aimed at increasing the quality of centrifugally cast A356 alloys by improving the mechanical properties and reducing segregation caused by the Fe-rich IMCs through heat treatment.  
1. Concerning the non-heated A356 alloys, the SDAS of section A was 1.48 times coarser than section B, and the area fraction of Fe-rich IMCs in section A (0.95%) was 1.72 times higher than that in section B. This difference in microstructure of sections A and B can be reduced through heat treatment.
2. The SDAS in the ST-E heat-treated A356 alloys was longer than that in the non-heated A356 alloys by 3.92 \(\upmu\)m, and the eutectic-Mg\({}_{2}\)Si phases were precipitated at the grain boundaries. This phase caused grain boundary segregation and decreased the average hardness by 13 HV. The differences in the area fractions of Fe-rich IMCs between section A and section B were similar to those from before heat treatment.
3. In the ST-WQ (T4) heat-treated A356 alloys, the SDAS was decreased by 3.1 \(\upmu\)m, compared to the non-heated alloys. In addition, the area fraction of the Fe-rich IMCs decreased by 0.25. This decrease occurred because the Mg solute atoms included in the \(\pi\)-AlMgFeSi phases were re-dissolved into an aluminum matrix. This caused the average hardness to increase by 11 HV. Moreover, the difference in area fraction of Fe-rich IMCs between section A and section B was reduced from 0.42% to 0.2%.
4. The segregation of Si and Mg solute atoms was reduced by ST-WQ and ST-WQ AG. The average hardness of the ST-WQ-AG (T6) heat-treated A356 alloys was the highest value owing to GP zone formation during artificial aging.  
This study presented the potential for the heat treatment of aluminum alloys manufactured by vertical centrifugal casting processes. The consequence would guarantee the quality of castings by reducing segregation and improving mechanical properties, and would be a reference for optimizing heat treatment of aluminum alloys manufactured by the vertical centrifugal casting.  
Figure 8: DSC curve of A356 alloy (water quenching and annealing after ST at 813 K for 10 h).  
**Author Contributions:** Conceptualization, W.K. and E.L.; methodology, W.K and C.J.; visualization, W.K.; validation, C.J., K.J. and E.L.; formal analysis, W.K. and E.L.; investigation, W.K. and E.L.; resources, K.J.; data curation, W.K.; writing--original draft preparation, W.K.; writing--review and editing, K.J., E.L. and C.J.; project administration, E.L. and C.J.; funding acquisition, E.L. and C.J. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the National Research Foundation of Korea (NRF) granted by the Korean Government (NRF-2019R11A3A01062863). And this study has been financially supported by the Ministry of Economy and Finance and conducted with the support of the Korea Institute of Industrial Technology as \"Development of Smart Manufacturing Technology for Low Temperature Fuel Tank for LNG Ships project KITECH JA210008\".  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.