# Dimensional Errors Due to Overhanging Features in Laser Powder Bed Fusion Parts Made of Ti-6 Al-4V
## 3 Results and Discussion
### Testing of Prediction Slice Plot  
The empirical prediction slice plot that was created based on the regression equation was then tested by the printing of new samples using various process parameters. For these test prints, a strip scanning strategy was chosen and the new samples were printed with a layer thickness of 60 \(\upmu\)m. The results can be seen below in Table 3.  
These results indicate a high level of prediction accuracy in samples which have a lower dimensional error and as the inherent dimensional error of the sample increases, the predicted error percentage also increases, as seen in Table 1. In sample 1, the predicted error was 4.45% and the measured error % was 4.08%, while in sample 5, the predicted error % was 26.04% and the measured error % was 29.74%. The reason for this needs to be further explored, and full-scale validation will be conducted in the next step of the research. However, the prediction accuracy of this regression equation at this stage is considered to be good, as the accuracy is quite high at lower dimensional deviations. This is required since the prediction accuracy only decreases as the dimensional deviations increase and in our tests that is when the  
Figure 6: Prediction slice plots created using the developed regression equation. dross formation leads to a dimensional error % of around 20%, which realistically, would never be an acceptable printing window anyway. Figure 7 depicts the average measured error % of the samples with a 60 \(\upmu\)m layer thickness and various scanning patterns. The measured points depict an average of five measurements and it can be seen that for most of the trials, even the average measurements display significant overlap or are close to each other. For trial 15, which includes the repeated center point samples, it can be seen that there is significant overlap among the different scanning patterns and the variation of the other trial points also established the stability of the process and thus the analysis process. This can be explained as the effective area where the parameters are varied is relatively small, since the down-facing area only extends for approximately 200 \(\upmu\)m into the part and the three different scanning strategies that were tested were not different enough to have a significant effect. The reason for this is that the cells in the rectangular and hexagonal cell scanning strategies are normally much larger than the entire down-facing area and the effective scanning strategy that was employed was thus similar to the stripe scanning strategy, regardless of the chosen strategy. Therefore, in further studies, the scanning strategy will not be considered as a parameter for optimization within the down-facing area. It is important to stress that the scanning strategy does play a significant role when varied within the bulk of the part and this has been the focus of other studies. Figure 7: Average measurements of dimensional error % amongst three scan strategies for a 60 \(\upmu\)m layer thickness. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Laser Power**} & \multirow{2}{*}{**Scan Speed**} & \multirow{2}{*}{\begin{tabular}{c} **Scan** \\ **(mm/s)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Scan** \\ **Spacing** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Measured** \\ **Error \%** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Prediction Made** \\ **by the Regression** \\ **Equation (\%)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Difference in** \\ **Equation (\%)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Error of the** \\ **Equation (\%)** \\ \end{tabular} } \\
150 & 1500 & 75 & 4.08 & 4.45 & 0.37 & 8.47 \\
140 & 1000 & 75 & 6.43 & 6.53 & 0.09 & 1.49 \\
200 & 500 & 90 & 20.93 & 18.03 & 2.90 & 16.10 \\
175 & 390 & 60 & 17.44 & 18.05 & 0.60 & 3.32 \\
250 & 370 & 90 & 29.74 & 26.04 & 3.70 & 14.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test results.