## 1 Introduction
### Mechanical Properties of Ti-6 Al-4V Alloys  
Ti-6 Al-4V alloys fabricated by laser or electron beam melting display different mechanical properties than their conventionally-fabricated counterparts. Both methods produce alloys with higher hardness than cast or wrought alloys [55]. For example, the microhardness of SLM-fabricated solid samples can be increased from 2.3 GPa to around 4.0 GPa by reducing the laser scan rate from 0.8 m/s down to 0.4 m/s [56]. The microhardness of solid samples obtained by the EBM method is less dependent on the scan rate and varies from 4.4 to 4.9 GPa for electron scan rates of 0.1 and 1.0 m/s, respectively [57]. The laser scan speed has a much higher influence on microhardness than the electron scan speed, because it has critical influence on the sample density. The hardness of the EBM fabricated elements depends on their size and shape. Thin sections or hollow cell structures with a wall thickness below 2 mm yield values of micro-indentation hardness of up to 40% higher than the thicker solid structures [58]. This is due to rapid material solidification in thin samples facilitated by their small dimensions. Large thermal gradients during the scaffold fabrication processes for bone tissue engineering produce structures with small grain size and, thus, high micro-indentation hardness. The Ultimate Tensile Strength (UTS) of laser fabricated items is about 1200 MPa [59], which is matched by EBM fabrication at values of 1150-1200 MPa [60]. Both UTS values are significantly larger than a value of 1000 MPa obtained for wrought materials. The SLM and EBM processes give similar yield strength (\(\sigma_{\mathrm{y}}\)) values of 1000-1100 MPa [59; 61; 60]. The reported elongation range for laser fabricated Ti-6 Al-4V is 6% [59] to 9% [61]. This value exceeds elongations obtained for cast materials, but is lower than elongations of wrought materials. Reported elongation values for EBM materials are  
\begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Realizer SLM50** & **Arcam EBM S12** \\ \hline Environment & argon & Vacuum 10\({}^{-4}\)–10\({}^{-5}\) (mbar) \\ Preheating (\({}^{\circ}\)C) & 200 (building table & 700 (powder bed heating by \\  & resistive heating) & defocused electron beam) \\ Maximum beam power (W) & 120 & 3500 \\ Laser/electron beam spot (\(\upmu\)m) & 30–250 & 200–1000 \\ Average powder layer thickness (\(\upmu\)m) & 20–100 & 50–200 \\ Beam scan speed (m/s) & 0.3–1.0 & \(>\)1000 \\ \hline \end{tabular}
\end{table}
Table 1: A comparison between the.Realizer SLM50 (Selective Laser Melting) and the Arcam EBM S12 machines.  
very inconsistent: some authors report elongation values between 16% and 25% [60], while others quote a value of 2% [55; 62] when oxygen content in the powder exceeded 0.34%. Both SLM and EBM fabrication methods can produce implants meeting the ASTM F136-13 [63] strength standards for wrought surgical implants. These requirements are: a minimum yield strength of 795 MPa and a minimum tensile strength of 860 MPa. Only the EBM method meets the minimum ASTM F136-13 standard for 10% elongation. However, post-production heat treatment of laser-processed Ti-6 Al-4V alloys might produce surgical implants meeting the ASTM F136-13 elongation requirement, as well [24].