## 2 Materials and Methods  
Experiments are performed with two powder batches of the same \(\gamma\)-TiAl alloy, which only differ slightly in aluminum content (Table 1). The primary rods are produced by GFE Metalle und Materialien GmbH (Nuremberg, Germany). The rods are gas atomized into spherical powder with a particle size distribution from 50 \(\upmu\)m to 120 \(\upmu\)m by TLS Technik GmbH & Co. Spezialpulver KG (Bitterfeld, Germany). The powder is processed on an Arcam A2X (Arcam AB, Molndal, Sweden) with the software version EBM Control 5.2. The x-deflection of the electron beam is parallel to the direction of rake movement, while the y-deflection is perpendicular to that. Thirteen defined stripes, each consisting of 35 scan lines with a line offset \(l_{o}\) of 75 \(\upmu\)m, are molten in a single, 100 \(\upmu\)m thick layer, see Figure 1a. The single layer with the stripes is molten on top of a 5 mm thick plate with an edge length of 45 mm to get as close to the melting conditions of real parts as possible. To investigate the melt pool during hatching, 35 scan lines are chosen to get into a steady melting state in contrast to measuring single scan lines. Each stripe has beam power between 60-1500 W with a step size of 120 W. The scan lengths inside each stripe are set in a way to provide integer beam power. The beam velocity is adjusted to the beam power ensuring constant line energy, area energy, return time, and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(\gamma\)-TiAl BMBF3 1  & **Ti** & **Al** & **Nb** & **W** \\  & **at.\%** & **at.\%** & **at.\%** & **at.\%** & **at.\%** \\ \hline batch 1 & nom. & bal. & 47.5 & 5.5 & 0.5 \\ batch 1 & measured & bal. & 47.5 & 5.6 & 0.4 \\ \hline batch 2 & nom. & bal. & 47.5 & 5.7 & 0.5 \\ batch 2 & measured & bal. & 46.8 & 5.7 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the used powder batches While the nominal (nom.) composition is equal, the measured (meas.) composition differs slightly. Figure 2: Beam diameter over beam power is calculated by Equation (5) according to [12]. lateral velocity (see Table 2). The process parameter set used as a basis was evaluated in preliminary experiments for scan lengths from roughly 3-7 mm and produced dense samples without misconnections, a homogeneous microstructure, and isotropic mechanical properties [14]. The melt pool width is determined in the middle of the as-built samples with a light microscope Zeiss Axioskop 2 MAT (Carl Zeiss AG, Jena, Germany). The melt pool width and the measurement method are defined as shown in Figure 1b. Afterward, the samples are cut along the dotted lines in Figure 1a to determine the melt pool depth. To measure the melt pool depth, one-half of the sample is heat-treated at 1320 \({}^{\circ}\)C for 1 h in a vacuum furnace LHTM 250 (Carbolite Gero GmbH & Co. KG, Neuhausen, Germany). The heat treatment (HT) temperature was set based on preliminary experiments. The stripes are heat-treated in the \(\alpha\)-phase field, while the base plate with slightly less energy input and hence more aluminum content is heat-treated in the \(\alpha+\gamma\)-phase field. Consequently, the microstructure of the stripes is transformed into a fully lamellar (FL) microstructure, while the base plate remains nearly lamellar (NL), see Figure 3. More information on the HT of \(\gamma\)-TiAl parts built via PBF-EB can be found in [17]. Finally, the contrast of microstructures is used to determine the melt pool depth (Figure 3). To make the microstructures visible, the samples are ground and polished with a suspension consisting of 50 mL OPS, 50 mL distilled water, 10 mg KOH, and 10 mL 30% H\({}_{2}\)O\({}_{2}\). The melt pool depth is measured at 12 positions 225 \(\upmu\)m apart, which equates to three scan lines. The geometry for evaluating the defects and aluminum evaporation Is shown in Figure 1e.