# Evaluating the Reliability of Powder Bed Fusion for Biomedical Materials: An Experimental Approach
## 2 Materials and Methods
### Working Conditions for PBF Process  
Many studies were conducted at the Technical University of Cluj-Napoca (TUCN, Cluj-Napoca, Romania) to improve the Selective Laser Melting process, utilizing the Realizer II SLM 250 equipment (manufactured by MCP Hek Tooling GMBH, Lubeck, Germany), which is available within the Manufacturing Engineering Department. Most of this research was carried out by PhD students [34; 35; 36]. This paper builds upon the extensive knowledge of the SLM process developed at TUCN through previous studies that optimized SLM process parameters for specific powder types [37; 38; 39]. The powder used was stored under controlled humidity conditions, and the working platform was not preheated. The key parameters of the SLM equipment include the Nd: YAG laser power (up to 200 W), layer thickness (0.03-0.1 mm), deposition volume (5-30 cm\({}^{3}\)/h), and spot diameter (0.08-0.23 mm). In this study, the authors leveraged the most relevant findings from previous research on the SLM process to configure the optimal process parameters required for sample production. These crucial parameters are detailed in Table 4. Throughout the manufacturing processes, a constant layer thickness of 50 \(\upmu\)m was maintained to ensure good productivity. Additionally, the average grain size of the powder was carefully considered to achieve optimal results. Table 4 also presents the methodology used in this research. Each specimen was produced with specific parameter values listed in each column: laser power, scanning speed, layer thickness, and hatch distance. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Chemical Element** & **Co** & **Cr** & **Mo** & **Mn** & **Si** & **Ni** & **Fe** & **C** \\ \hline Maximum & 58.9–69.5 & 27–30 & 5–7 & max. 1 & max. 1 & max. 1 & max. 0.75 & max. 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of the CoCr powder. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Chemical Element** & **Co** & **Cr** & **W** & **Mo** & **Si** & **Other Elements (C, Fe, Mn, N)** \\ \hline Maximum & 59 & 25 & 9.5 & 3.5 & 1 & \textless{}1 \\ weight percentage [\%] & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of the CoCrWMo powder. For each combination of parameters shown in Table 4, at least seven specimens were produced to ensure reliable results. The laser power range was established based on prior knowledge of the SLM process at TUCN. The lower limit was set to guarantee the compactness of the metallic structure, preventing material separation. The laser scanning strategy followed an \"X/Y\" pattern, where the current layer (\(n\)) was scanned along the X direction, and the next layer (\(n+1\)) was scanned along the Y direction. In this study, only the laser power was varied to investigate its influence on PBF reliability. According to previously published studies [8; 11; 22; 40; 41; 42], reducing the scanning speed decreases the risk of internal cracks and other defects in the parts. Based on this recommendation, the scanning speed was set between 350 and 500 mm/s (see Table 4). In the PBF process, the hatch distance parameter refers to the distance between successive parallel scanning lines created by the laser beam on the metal powder layer. Essentially, it is the distance between two consecutive paths that the laser follows when melting the material layer by layer. Hatch distance influences the quality of the part, as a distance that is too large can lead to gaps between the melted lines, resulting in higher porosity and lower mechanical strength. On the other hand, if the distance is too small, the scanning/melting lines overlap, leading to higher density but also a longer production time. For these studies, the hatch distance was varied to up to half the width of the wall tracks (solidified melt traces). Previous measurements indicated that the track width ranged from 0.18 to 0.25 mm. Consequently, the hatch distance was set between 0.10 and 0.12 mm (see Table 4).