## 3 Results and Discussion  
### Microstructures  
Figure 3 shows different microstructures of LPBF Ti6 Al4V (ELI) obtained after exposing the stress-relieved samples of the alloy to the heat treatment cycles presented in Table 1. The heat-treated microstructures of LPBF Ti6 Al4V (ELI) shown in this figure consist of equilibrium mixtures of \(\alpha\) and \(\beta\) phases. The \(\alpha\)-phase appears as the darker phase while the \(\beta\)-phase is brighter. The heat treatment at 800 \({}^{\circ}\)C resulted into the formation of a finer lamellar microstructure where the \(\alpha\)-phase appeared in needle-like morphology, as seen in Figure 3a. At higher heat treatment temperatures just below the \(\alpha\)\(\rightarrow\)\(\beta\) transformation temperature, the growth of the \(\alpha\)-phase is evident in Figure 3b, from which the thickness of the laths is seen to be coarse. Samples E that were heat-treated at a temperature above the \(\alpha\)\(\rightarrow\)\(\beta\) transformation temperature was characterized by a typical Widmanstatten microstructure, where colonies consisting of several parallel \(\alpha\)-laths were present, as shown in Figure 4c. While the \(\beta\)-grains appeared in small patches and in particle-like morphology in samples C, the \(\beta\)-lamellar grains were clearly visible along the \(\alpha\)-laths boundaries in samples C and D. The \(\alpha\)-laths in samples C had an average thickness of about 2.5 \(\upmu\)m, with lengths extending to over 100 \(\upmu\)m. In samples D, the \(\alpha\)-laths had an average thickness of 6 \(\upmu\)m and lengths of about 150 \(\upmu\)m. In Figure 3c the thickness of the \(\alpha\)-laths within the colonies were in the range of 7-15 \(\upmu\)m, while the average thickness was determined as 9 \(\upmu\)m.  
Figure 3: The microstructure of samples (**a**) C, (**b**) D, and (**c**) E after exposing stress-relieved LPBF Ti6 Al4V (ELI) parts to the heat treatment cycles presented in Table 1.  
The lengths of the \(\alpha\)-laths in each colony were limited by the adjacent colonies, as was observed in references [14,15,19,20]. There was also evidence of a globalization of \(\alpha\)-grains in some regions, believed to have been initially grain-boundary \(\alpha\)-grains in samples E, as shown in Figure 3c.  
A statistical analysis of the volume fraction of \(\beta\)-phases present in these three different forms of the alloy was carried out on the surface of the polished samples using the X-ray diffraction method. The obtained results are presented in Figure 4. Different peaks belonging to the \(\alpha\)-phase (hcp-titanium) and \(\beta\)-phase (bcc-titanium) were indexed on the three different samples of LPBF Ti6 Al4V(ELI), which confirmed the observations made here about the microstructures shown in Figure 3. While the peak reflections in the three different microstructures of the alloy were largely for the \(\alpha\)-phase, a peak reflection of the plane at an angle of \(2\theta^{\alpha}\) of the XRD profile was detected in each category of samples. The volume fraction of the \(\beta\)-phase was determined from the relative peak intensities as 3.6%, 6.4%, and 6.6% for samples C, D, and E, respectively.