# Process Parameter Optimisation in Laser Powder Bed Fusion of Duplex Stainless Steel 2205
## 2 Materials  
DSS-2205, along with the rest of the category of duplex steels, belongs to a variety of stainless steels distinguished by their exceptional corrosion resistance and characterized by a very strong resistance to fracture, while also being a very ductile type of steel. However, the defining element of this category is its characteristic microstructure, composed of approximately 50% ferritic and 50% austenitic phases [25]. The characteristics of the base material (DSS-BM), obtained from previous tests, are depicted in Table 1 and have been used as references to compare the properties of additive manufactured steel (DSS-SLM). DSS-BM was manufactured by hot rolling. Then, it was heat-treated by quenching at 1050 \({}^{\circ}\)C and water quenching. The whole process was performed at atmospheric pressure. SLM technology uses spherical powder as a material, with a certain variety of types, depending on the size and the kind of material. It is therefore necessary to properly define the type of powder used in this study. DSS-SLM specimens are to be manufactured from duplex spherical purchased powder obtained by an atomization process. The size of the powder particles is in the range of 15 \(\upmu\)m to 45 \(\upmu\)m, below the layer height (Th) set at 50 \(\upmu\)m. The grains of powder cannot exceed this height, as this would impair the properties of the final product. According to the manufacturer's certificate, the mean value is 32.4 microns in diameter, and the standard deviation is 8.1 microns. In addition, an excessive powder size results in a worsening of the surface finish and a higher porosity of the finished product [34]. Figure 1 depicts the DSS-SLM powder used. \begin{table}
\begin{tabular}{c c c c c} \hline Elastic Modulus (GPa) & \(\sigma_{0.2}\) (MPa) & \(\sigma_{\text{uts}}\) (MPa) & \(\varepsilon_{\text{r}}\) (\%) & A (\%) \\ \hline
195 & 541 & 773 & 52.44 & 83.53 \\ \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of DSS-BM. Figure 1: SEM microstructure of DSS-SLM powder. Another fundamental factor is the chemical composition of the steel used. The high chromium and molybdenum contents allow the use of DSSs under conditions where pitting, crevices, and stress corrosion cracking would be critical for other types of steel. A lower amount of nickel than austenitic steel reduces manufacturing costs. The ferrite grains are enriched in P, Mo, Si, and Cr, whereas austenite grains are enriched in N, Ni, Cu, and Mn [35; 36]. Table 2 shows the chemical composition of both the powder and the base In the response surface method, these three parameters are related to each other with the expression S = \(f\) (P*, S*, H*) of Equation (2). In this quadratic function, the variables P*, S*, and H* represent the coded equivalences of the real parameters P, S, and H within the interval defined for the experiment design of [1, 1]. Parameter coding is performed using Equation (3). For a real value Xi, its encoded value will be xi when XiNNup and XiNNif are the maximum and minimum real values and \(\tilde{X}_{i}\) is the average of real values that Xi can take [14]. Aiming to calculate the material properties from these coefficients, the Design Expert software tool v13 was used. Some characteristics can be highlighted:  
* The use of the three parameters P*, S*, and H*. * Choosing a cubic domain. * Coding the factors with the specified interval, using Equation (3). * In order to measure test variability, the cubic matrix has three levels for each input parameter, with two replicates at the midpoint of the domain. * The application of Equation (2) to adapt the five response surfaces to the five material properties analyzed. \[