# SHS Produced TiB\({}_{2}\)-Si Powders for Selective Laser Melting of Ceramic-Based Composite
## 3 Results and Discussion
### Selective Laser Melting  
The structures designed by CAD were adjusted to optimize the SLM procedure in terms of a laser current (I, mA). The X-ray diffraction pattern of the SLM-ed sample reveals no new or spurious phase formation during the SLM process (Figure 3). The XRD patterns of the SHS powder indicate the presence of 44.1 %Si, while after the SLM process, the amount of Si is calculated to be 43.5%. The difference might be attributed to the accuracy of the Rietveld refinement method; moreover, some evaporation of silicon during the SLM process cannot be avoided at 10-20 mbar pressure. Comparative analysis of the XRD patterns of the powders and printed parts revealed no changes of peak shapes. The geometric density of the TiB\({}_{2}\)-Si compacts printed at the laser current ranging from 1400 to 2200 mA is specified in Table 4. Taking into consideration the theoretical density of the 56 wt.%TiB\({}_{2}\)-44 wt.%Si composite (3.19 g\(\cdot\)cm\({}^{-3}\)), the density of the bulks varies from 2.77 to 3.05 g\(\cdot\)cm\({}^{-3}\) corresponding to a relative density range from \(\sim\)87 to \(\sim\)97%. Therefore, a laser current of a high intensity (at least I = 2200 mA) is needed for the production of a high-density material. Cylinder 3, printed at a high energy input, exhibits the highest density due to larger/deeper melt pools, which provides sufficient stitching, melt track overlapping, and improved interlayer connection, resulting in the elimination of the defects caused by lack of fusion. Rapid infiltration of liquid Si into the pores implies readiness of Si to wet TiB\({}_{2}\), assuming a low contact angle resulting in the restricted removal of TiB\({}_{2}\) solid inclusions from silicon [17]. With an increase in a laser current, the viscosity of liquid silicon decreases, and the flattened silicon inclusions homogeneously occupy the distance between the titanium diboride particles being promoted by the \"pomegranate-like\" morphology of the feedstock. \begin{table}
\begin{tabular}{c c c} \hline
**Powder Size, \(\upmu\)m** & **PSD**, \(\upmu\)m & **Hall Flow Rate (FR\({}_{\text{H}}\)), \(\mathbf{s}\)/50 g** \\ \hline \(\upmu<20\) & - & - \\  & Dv10 \(-\) 5.48 & \\
20 \(<\upmu<45\) & Dv50 \(-\) 21.2 & 29.75 \(\pm\) 0.31 \\  & Dv90 \(-\) 41.4 & \\  & Dv10 \(-\) 12.3 & \\
45 \(<\upmu<90\) & Dv50 \(-\) 57.56 & 24.68 \(\pm\) 0.34 \\  & Dv90 \(-\) 85.4 & \\ \hline \end{tabular}
\end{table} 
Table 3: Particle size distribution (PSD) and Hall flow rate of TiB\({}_{2}\)-Si powders. Due to the \"pomegranate-like\" structure of the initial powder particles, during the process of cooling, ultra-fine grained TiB\({}_{2}\) particles serve as nucleation centers making the solidification of molten silicon more homogeneous. This prevents the coarsening of silicon grains and decreases the chance of cracking during shrinkage. The microstructures of the composites processed at different laser currents are shown in Figures 4 and 5, which demonstrate some reduction in porosity level with increase in the laser current. Process induced non-spherical (irregular shaped) porosity was substantiated by an insufficient energy density (or unexpected disturbances during the printing process) that is needed for complete melting of the feedstock and for promoting consolidation (Figure 4a). If insufficient power is supplied, a lack of fusion regions is identifiable by un-melted powder particles visible near the pores. During the powder bed fusion process, the powder layer and some amount of the underlying substrate undergoes melting; however, an insufficient energy input can result in a decrease in molten pool dimensions and, therefore, in the deficient melting of powder particles and/or re-melting of previously solidified layers. With the application of a high-power laser, the powder absorbs higher energy as compared to a solidified layer; hence, the temperature of the molten pool increases above the boiling point leading to the vaporization of silicon and possible formation of a plasma state during the SLM process.