## 2 Materials and Methods  
### Production of Test Specimens  
Cylindrical rods with a diameter 6 mm and a height of 80 mm were fabricated from gas atomised spherical Ti6 Al4V(ELI) alloy powder, using the EOSINT M280 DMLS machine (EOS GmbH, Munich, Germany). The chemical composition of this powder [26] is shown in Table 1. The optimised set of process parameters shown in Table 2 was used based on research work carried on this equipment to obtain the best mechanical and physical properties of Ti6 Al4V(ELI) [27].  
The SHPB test specimens were fabricated along the build direction. A photographic image of these specimens on the EOS M280 machine platform is shown in Figure 1. The specimens were first stress relieved at a heat treatment temperature of 650 \({}^{\circ}\)C for 3 h followed by furnace cooling, while attached to the base plate. The cylindrical rods were then cut-off from the steel base plate using an electrical discharge machine (wire cutting),  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \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. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of Ti6 Al4V(ELI) alloy powder as determined in [26].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Processing Variable** & **Value** \\ \hline Laser power setting & 175 W \\ Laser diameter & 80 \(\upmu\)m \\ Hatch spacing & 100 \(\upmu\)m \\ Layer thickness & 30 \(\upmu\)m \\ Scanning speed & 1400 mm/s \\ \hline \hline \end{tabular}
\end{table}
Table 2: EOSINT M280 DMLS processing parameters.  
ready for further heat treatment using a SuperSeriesTM vacuum furnace system, Model SS12-24/13 MX, with a horizontal vacuum chamber. These cylindrical rods were further subdivided into three groups designated as samples C, D and E for further heat treatment. These subsequent heat treatment cycles are summarized in Figure 2. The designations samples A and B were reserved for the reference microstructures of the as-built and stress relieved material, which were presented and discussed in the authors previous work in Reference [28]. The further details on criteria for selection of these heat treatment cycles for DMLS T6A14V(ELI) shown in this figure can also be found in [28].  
Figure 1: A photographic image showing the manufactured cylindrical rods of T6A14V(ELI) on the EOS M280 machine platform. The insert schematic drawing shows the length of the rods in millimetres.  
Figure 2: Heat treatment cycles for high temperature annealed DMLS T6A14V(ELI) for (**a**) samples C, (**b**) samples D and (**c**) samples E, all after the initial stress relieving heat treatment.  
Specimens for SHPB testing, with a diameter and height of 6 mm were cut from the heat-treated cylindrical rods. These specifications of their dimensions were adopted to ensure an aspect ratio of 1. The 0.5 to 1.5 aspect ratio for SHPB test specimens was shown to represent an optimal balance between reducing the longitudinal inertia effect by using a shorter specimen and reducing radial inertia by using a thinner specimen [29]. The ends of these cut specimens were faced off on a lathe machine to ensure that they were flat and parallel, to ensure the best possible contact between specimen and pressure bars during impact.