## 2 Materials and Methods  
### Materials and Samples Preparation  
Cylindrical Ti6 Al4V(ELI) test specimens with a diameter of 6 mm and height of 80 mm were manufactured in an EOSINT M280 DMLS machine using gas-atomised spherical powder of Ti6 Al4V(ELI) obtained from TLS Technik GmbH. Different levels of interstitial elements are usually present in different grades of Ti6 Al4V alloy and generally affect its mechanical properties. The Ti6 Al4V(ELI) (grade 23) used in this study is very similar to Ti6 Al4V (grade 5), in addition to that, it contains a reduced level of interstitial elements, which enhances the ductility and toughness of the alloy [27]. As shown in Table 1, the chemical composition of Ti6 Al4V(ELI) as received from the supplier (TLS Technik GmbH) complied with the ASTM F3001-14 standard, a specification that establishes the chemical composition of Ti6 Al4V(ELI) components manufactured using a full-melt powder bed fusion process [27]. The samples were manufactured in an EOSINT M280 DMLS machine with the set of process parameters as follows: scanning speed 1400 mm/s, laser power 175 W, layer thickness 30 \(\upmu\)m, laser beam diameter 80 \(\upmu\)m and hatch spacing 100 \(\upmu\)m. The back-and-forth raster scanning strategy was employed in the manufacturing of samples for use in this study. The strategy generally uses stripe patterns in each layer that shift at an angle of 67\({}^{\circ}\) in the next layer. This scanning strategy is illustrated schematically in Figure 1. During the manufacturing process, the layers were at first offset by 100 \(\upmu\)m from the external edges to form an internal region. This internal region was first scanned with laser energy using the strategy described in Figure 1. The region between the outer and the offset borders was then scanned. This manufacturing strategy ensures reduction in defects near the surface of the samples, as well as improving the surface finish of parts produced. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Element** & **Al** & **V** & **Fe** & **O** & **C** & **N** & **H** & **Ti** \\ \hline Composition (wt.\%) & 6.34 & 3.944 & 0.25 & 0.082 & 0.006 & 0.006 & 0.001 & Bal. \\ ASTM F3001-14 & 5.50–6.50 & 3.50–4.50 & \textless{}0.25 & \textless{}0.13 & \textless{}0.08 & \textless{}0.05 & \textless{}0.012 & Bal. \\ \hline \end{tabular}
\end{table} 
Table 1: The chemical composition of the Ti6 Al4V(ELI) as received from the supplier and that specified in the ASTM F3001-14 standard [27]. The cylindrical rods shown in Figure 2, which were manufactured for use in this study, were subdivided into three major groups for experimental testing, designated in this study as samples C, D and E.  
The designations A and B were reserved for as-built and stress-relieved samples, respectively, which were taken to be reference specimens and were not used for experimental analysis in this study. The microstructure of these reference specimens was discussed in detail in the authors' previous research published in [PERSON] et al. [28; 29]. Therefore, the microstructure analysis of these reference materials was omitted from this study. At first, the three main groups of specimens were stress relieved in a vacuum furnace while still on the steel platform with support structures. Thereafter, an electrical discharge machine (EBM--wire cutting) was used to cut these specimens from the DMLS machine platform, while at the same time removing the support structures. Stress relieving to alleviate process-related macro-residual stresses was conducted in a vacuum chamber at a temperature of 650 \({}^{\circ}\)C, with a soaking period of 3 h, followed by  
Figure 1: Schematic diagram illustrating the EOSINT M280 machine scanning strategy employed in production of samples for use in this study. The arrows show the scan vectors in every stripe (shown by the dashed lines). The stripes in the subsequent layer are rotated at 67\({}^{\circ}\) from the previous layer. Figure 2: A photograph showing produced cylindrical rods on the EOSINT M280 machine table.