Mold–Slug Interfacial Heat Transfer Characteristics of Different Coating Thicknesses: Effects on Slug Temperature and Microstructure in Swirled Enthalpy Equilibration Device Process
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Interfacial Heat Transfer Coefficient and Heat Flux
3.2. Temperature Distribution and Evolution of Semi-Solid Slug
- The duration of the unstable stage became shorter as the coating thickness reduced (see Figure 8b). These results are coincident with those of heat flux shown in Figure 6a. Figure 6a shows that, for a slug with a thinner coating, less time is required to reduce the heat flux to a nearly constant low value.
- Figure 5 shows that the highest temperature gradient decreased with an increase in coating thickness. To show this clearly, the largest temperature difference between points 1 and 3 in Figure 5 was used to represent the largest temperature gradient and is plotted in Figure 8b. Figure 8b shows that this temperature difference decreased from 21 °C to 9 °C as the coating thickness increased. As shown in Figure 6b, the IHTC reduced from ~2700 W·m−2·K−1 to ~500 W·m−2·K−1 when the coating increased from 0.0 mm to 1.0 mm, which caused the highest heat flux to decrease from 800,000 W·m−2 to 180,000 W·m−2, respectively (see Figure 6a). This is the reason for the variation of this temperature gradient.
- The final temperature (Figure 5, location 1 at 180 s) decreased from 586 °C to 581 °C with the decrease in coating thickness. This observation is supported by the temperature data of the mold (see Figure 4a): a mold with a thick coating exhibited a lower temperature than a mold with a thin coating. Less heat was absorbed by the mold with a thick coating and the rate of heat loss to the air also reduced.
- As the coating thickness reduced, the temperature difference between the slug center and surface during the quasi-stable stage decreased (see Figure 5 and Figure 7). Figure 8a shows that the temperature difference between slug center and middle was almost same for various coating thicknesses. The increasement in Figure 7 could result from measurement error: the thermocouple at the slug surface touched the mold during the experiments, which may have resulted in the experimental values being slightly lower than the true values. The influence of this effect would have increased as the mold temperature reduced (Figure 4a shows that the mold temperature reduced as the coating thickened). It was concluded that the coating thickness had no obvious effect on the radial temperature distribution of the slug during the quasi-stable stage. This conclusion is supported by the data of Figure 6a, which shows that the interfacial heat flux of different coating thicknesses was almost same during the quasi-stable stage.
3.3. Microstructures of Different Coating Thicknesses
4. Conclusions
- The IHTC of the mold–slug interface in the SEED process was estimated. The IHTC fluctuated within a small range and decreased with an increase in coating thickness. The IHTC reduced from ~2700 W·m−2·K−1 to ~1100 W·m−2·K−1, 800 W·m−2·K−1, and 500 W·m−2·K−1 when the coating increased in thickness from 0.0 mm to 0.1 mm, 0.5 mm, and 1.0 mm, respectively. The IHTC was sensitive to the BN coating thickness when this was less than 0.1 mm.
- The heat flux of the mold–slug interface decreased sharply from its highest value to a nearly constant low value during the SEED process. The reduction rate decreased as the BN coating thickness and swirling time increased. The highest value decreased from ~800,000 W·m−2 to ~180,000 W·m−2 when the coating increased from 0.0 mm to 1.0 mm.
- The temperature difference in the slug dropped off to a constant low value as the swirling time rose. According to this characteristic, the temperature evolution was divided into two stages: unstable and quasi-stable stages. As the coating thickened, the duration of the unstable stage increased and the highest temperature gradient in the slug decreased. The coating thickness had no obvious effect on the quasi-stable stage.
- The microstructures strongly depended on the BN coating thickness. When the coating thickened, the grain size enlarged and the grain morphology transformed from a globular shape to a dendritic structure. The grain size enlarged from 78 µm to 106 µm at the edge, from 80 µm to 98 µm at the middle, and from 83 µm to 100 µm at the center when the BN coating thickness increased from 0.0 mm to 1.0 mm.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
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Experiment Number | 1 | 2 | 3 | 4 |
---|---|---|---|---|
Spray time (s) | 2 | 15 | 60 | 120 |
Coating thickness (mm) | ~0.0 | ~0.1 | ~0.5 | ~1.0 |
Parameter | ||||||
---|---|---|---|---|---|---|
Unit | kg·m−3 | J·kg−1·°C−1 | °C | W·m−2·K−1 | m | m |
Value | 7850 | 600 | 30 | 60 a | 0.0413 | 0.0390 |
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Luo, M.; Li, D.; Qu, W.; Hu, X.; Zhu, Q.; Fan, J. Mold–Slug Interfacial Heat Transfer Characteristics of Different Coating Thicknesses: Effects on Slug Temperature and Microstructure in Swirled Enthalpy Equilibration Device Process. Materials 2019, 12, 1836. https://doi.org/10.3390/ma12111836
Luo M, Li D, Qu W, Hu X, Zhu Q, Fan J. Mold–Slug Interfacial Heat Transfer Characteristics of Different Coating Thicknesses: Effects on Slug Temperature and Microstructure in Swirled Enthalpy Equilibration Device Process. Materials. 2019; 12(11):1836. https://doi.org/10.3390/ma12111836
Chicago/Turabian StyleLuo, Min, Daquan Li, Wenying Qu, Xiaogang Hu, Qiang Zhu, and Jianzhong Fan. 2019. "Mold–Slug Interfacial Heat Transfer Characteristics of Different Coating Thicknesses: Effects on Slug Temperature and Microstructure in Swirled Enthalpy Equilibration Device Process" Materials 12, no. 11: 1836. https://doi.org/10.3390/ma12111836