## 3 Results  
### Characterization of the Melt Pool  
The melt pool during electron beam melting is characterized by the melt pool width, also called lateral extension, the melt pool depth, and the peak melt pool temperature. The focus of this study is on the melt pool width and depth. It is to note that it was not possible to determine either the width or the depth of the melt pool for 60 W beam power due to the small size of the molten area. The results of the light microscopic measurement of the melt pool characteristics are summarized in Figure 4. The average value of four samples (grey dots) are analyzed per beam power and a linear fit of these four measurements is presented (black dotted line). Figure 3: Schematic cut section of one of the stripes as depicted in Figure 1. The power-adapted beam size is displayed by the orange hemicycles (here for 900 W). The scan lines are separated by a line offset of 75 \(\upmu\)m and follow a so-called “cross-snake” hatch pattern, indicated by the “\(\times\)” and “\(\bullet\)”. Hence, the electron beam moves in and out of the presented plane. On the bottom, a cross-section of the sample is shown. The first scan line is determined by approximately half the width of the electron beam. Following the first scan line, the melt pool depth (fully lamellar microstructure, FL) is measured every 225 \(\upmu\)m corresponding to every third scan line. Additionally, a small nearly lamellar (NL) area is found and measured at the bottom of the melt pool for the first few scan lines. \begin{table}
\begin{tabular}{c c} \hline
**Property** & **Value** \\ \hline Thermal diffusivity 2 [m\({}^{2}\)/s] & 6 \(\times\) 10\({}^{-6}\) \\ Density 2 [kg/m\({}^{3}\)] & 3533 \\ Specific heat 2 [J/(kg\cdot\)K] & 1160 \\ Absorption coefficient & 0.85 \\ Preheat temperature [K] & 1023 \\ Liquidus temperature 2 [K] & 1832 \\ Beam diameter (4\(\sigma\)) [\(\upmu\)m] & 340 \(\upmu\)m \\  & or Equation (5) \\ \hline \end{tabular}
\end{table} 
Table 3: Thermophysical properties for semi-analytic heat conduction model with TiAl. The lateral extension steadily increases until 900 W. From there, the rise of the melt pool width is less pronounced up to the maximum tested beam power of 1500 W. The increased melt pool width is caused by the widening of the electron beam, which is discussed in detail in Section 4.2. The lateral extension of the melt pool is up to two times larger than the 4\(\sigma\) beam width. For example, a beam diameter of 402 \(\upmu\)m leads to a width of 802 \(\upmu\)m at 540 W beam power. Therefore, the melt pool can be characterized as persistent. The lateral extension scatters due to the stochastic powder bed, irregular melt pool borders (see Figure 1b), and manual measurement via light microscopy. The numerical calculation with a constant beam diameter (4\(\sigma\)) of 340 \(\upmu\)m displays a constant melt pool width. Since the numerical calculations are based on a heat conduction model, geometry-induced changes in the heat conduction should be revealed in the simulations with constant beam diameter. In contrast, the numerical calculation with beam widening shows an increasing lateral extension with increasing beam power. However, the simulated lateral extension is smaller than the experimental values. Reasons for this difference include but are not limited to deviations of the beam size from the formulation by Equation (5), material properties taken from Ti-43.5 Al-4 Nb-1 Mo alloy, and lack of latent heat in the semi-analytical heat conduction model. Nevertheless, the simulation reveals that an increasing electron beam diameter can lead to a wider melt pool, while the melt pool width is unaffected if the beam diameter is constant.