# Improvement of Radiant Heat Efficiency of the
## 3 Experiment  
### Material Used to Print the Honeycomb Pattern  
The INCONEL 718 alloy powder (CarTech(r) 718, UNS N07718), with its chemical composition given in Table 2 (supplied by Carpenter Technology Corporation), was used to print a set of honeycomb patterns on the RT made of INCONEL 601. Ideally, the pattern's metal should be the same as the RT metal to achieve high bonding strength between the pattern and the RT's surface and prevent the separation of patterns from the RT when the pattern-printed RTs are heated and cooled repeatedly. However, the selected pattern material was the INCONEL 718 as an alternative because INCONEL 601 powder for 3D printing is not commercially available.  
Table 3 shows that INCONEL 718 has higher tensile and yield strength than INCONEL 625 at elevated temperatures. A higher strength pattern can support the RT and prevent collapse at high temperatures. Table 4 shows that the difference between the thermal expansion coefficients of INCONEL 601 and INCONEL 718 is smaller than that between INCONEL 601 and INCONEL 625. Therefore, INCONEL 718 is more effective in preventing the separation of the patterns from the RT when the pattern-printed RTs is heated and cooled repeatedly.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{3}{c}{**Thermal Expansion Coefficient (1/\({}^{\circ}\)C)**} \\ \cline{2-5}  & **INCONEL 601** & **INCONEL 625** & **INCONEL 718** \\ \hline
600 & 15.62 & - & - \\
649 & - & 14.8 & 15.1 \\
700 & 16.11 & - & - \\
760 & - & 15.3 & 16 \\
800 & 16.67 & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: Thermal expansion coefficient of INCONEL alloy candidates.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fe**} & **Ni + Co** & **Cr** & **Nb + Ta** & **Mo** & **Ti** & **Co** & **Al** & **Si** & **Mn** & **Cu** & **C** & **S** & **P** & **B** \\ \hline Bal. & 50.00-55.00 & 17.00-21.00 & 4.75-5.50 & 2.80-3.30 & 0.65-1.15 & \(\leq\)1.00 & 0.35-0.80 & \(\leq\)0.35 & \(\leq\)0.35 & \(\leq\)0.15 & \(\leq\)0.08 & \(\leq\)0.015 & \(\leq\)0.015 & 0.001-0.006 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Chemical composition (% mass) of INCONEL 718 alloy powder provided by Carpenter Technology Corporation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{3}{c}{**Tensile Strength (MPa)**} & **Yield Strength (0.2\% Offset) (MPa)** \\ \cline{2-5}  & **INCONEL 625** & **INCONEL 718** & **INCONEL 625** & **INCONEL 718** \\ \hline
649 & 1010.1 & - & 734.3 & - \\
704 & - & 1006.6 & - & 930.8 \\
760 & 584.7 & 851.5 & 544.7 & 799.8 \\
816 & - & 723.9 & - & 689.5 \\
871 & 284.1 & - & 275.8 & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: High temperature strength of INCONEL alloy candidates.  
The particle size distribution of INCONEL 718 powder was performed using a Malvern Mastersizer 3000, and the particle morphology was observed via scanning electron microscopy (SEM). Figure 7a shows the particle size distributions for D10, D50, and D90 are 57.1 \(\upmu\)m, 95.5 \(\upmu\)m, and 155 \(\upmu\)m, respectively. Unlike in PBF method, powders with a wide range of particle sizes can be used in the DED process, which is an advantage contributing to the reduction of DED's manufacturing cost. The powder particle sizes for DED method are similar to those used in powder metallurgy processes, 50-150 \(\upmu\)m, and the powder used has a comparable size.  
Figure 7b shows SEM images of the INCONEL 718 powders. The overall shape is nearly spherical (Figure 7c), and individual powder particles have a dendritic microstructure, typical results of rapid solidification while some satellites are sticking on the surface of larger particles (Figure 7c).