# Parameter Optimisation in Selective Laser Melting on C300 Steel
## 4 Result and Discussion  
To carry out this experimental plan, a round tensile specimen (4 mm of diameter) was printed for each of the matrix point; these conditions were subsequently tested in accordance with the ASTM ESM standard [21]. The printer software allows all the specimens to be manufactured together, assigning their corresponding parameters (\(P\), \(S\) and \(H\)) to each one of them. Figure 1 shows the newly manufactured tensile specimens, as well as their final appearance (SLM condition without finishing treatment) before testing. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Experiment** & \multicolumn{4}{c}{**Experiment Matrix**} & \multicolumn{4}{c}{**Experimentation Plan**} \\ \hline
**NUM** & **Power** & **Speed** & **Hatching** & **Power (W)** & **Speed (mm/s)** & **Hatching (mm)** \\ \hline
1 & \(-1\) & \(-1\) & \(-1\) & 200 & 500 & 0.08 \\ \hline
2 & \(1\) & \(-1\) & \(-1\) & 250 & 500 & 0.08 \\ \hline
3 & \(-1\) & \(1\) & \(-1\) & 200 & 800 & 0.08 \\ \hline
4 & \(1\) & \(1\) & \(-1\) & 250 & 800 & 0.08 \\ \hline
5 & \(-1\) & \(-1\) & \(1\) & 200 & 500 & 0.14 \\ \hline
6 & \(1\) & \(-1\) & \(1\) & 250 & 500 & 0.14 \\ \hline
7 & \(-1\) & \(1\) & \(1\) & 200 & 800 & 0.14 \\ \hline
8 & \(1\) & \(1\) & \(1\) & 250 & 800 & 0.14 \\ \hline
9 & \(-1\) & \(0\) & \(0\) & 200 & 650 & 0.11 \\ \hline
10 & \(1\) & \(0\) & \(0\) & 250 & 650 & 0.11 \\ \hline
11 & \(0\) & \(-1\) & \(0\) & 225 & 500 & 0.11 \\ \hline
12 & \(0\) & \(1\) & \(0\) & 225 & 800 & 0.11 \\ \hline
13 & \(0\) & \(0\) & \(-1\) & 225 & 650 & 0.08 \\ \hline
14 & \(0\) & \(0\) & \(1\) & 225 & 650 & 0.14 \\ \hline
15 & \(0\) & \(0\) & \(0\) & 225 & 650 & 0.11 \\ \hline
16 & \(0\) & \(0\) & \(0\) & 225 & 650 & 0.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Proposed design of experiments. After the test of each specimen, force-displacement data are processed in order to obtain the mechanical behaviour of each parameter combination. In this sense, Figure 2 shows some of the typical stress-strain curves obtained. Two types of behaviour can be clearly differentiated: one more brittle (specimens 7 and 8) and the other more ductile (specimens 1 and 15) where necking occurs in the specimen. In order to better compare this different behaviour, Figures 3 and 4 show the optical micrographs of all the specimens tested. The degree of ductility in each of them is directly related to the reduction in area that occurs in the failure zone. SEM micrographs have also been carried out. Figures 5 and 6 show those of specimens 1 and 15 with a ductile behaviour, dominated by the growth and coalescence of microvoids. In contrast, Figures 7 and 8 show the brittle behaviour of specimens 7 and 8. In the latter, it can be seen that the laser beam has not had enough energy to completely melt the net area of the specimen, which causes brittle failure. Figure 3: Optical micrographs of specimens 1 to 8. Figure 4: Optical micrographs of specimens 9 to 16. Figure 5: SEM micrographs (\(25\times\), \(50\times\) and \(200\times\)) of specimen 1. Figure 6: SEM micrographs (\(25\times\), \(50\times\) and \(200\times\)) of specimen 15. Figure 7: SEM micrographs (\(25\times\), \(50\times\) and \(200\times\)) of specimen 7. Figure 8: SEM micrographs (\(25\times\), \(50\times\) and \(200\times\)) of specimen 8. For each of the stress-strain curves, the values of yield strength, ultimate stress, elongation and area under the curve have been obtained and can be found in Table 4. By introducing these values in the Design Expert software, it is possible to calculate the coefficients of the function \(f(P^{*},S^{*},H^{*})\) that defines the response surfaces of \(\sigma_{0.2}\), \(\sigma_{tts}\), \(\varepsilon_{r}\) and \(A\), shown in expressions 3 to 6, respectively. The coefficients of determination (\(R^{2}\)) have been included for the corresponding fitted response surface.