## 4 Fatigue Analysis and Discussion
### DMLs Ti6 Al4V  
Ti-6 Al-4V is perhaps one of the most studied alloys in AM [4; 5; 6; 14; 20; 21; 22; 23; 24]. Since titanium alloys are generally lightweight, bio-compatible and resistant to corrosion, it has a potential of wide applications in various industries with AM. The conventional wrought Ti-6 Al-4V processes a dual-phase microstructure with equiaxed \(\alpha+\beta\) grains or lamellar \(\alpha\) + \(\beta\) colonies interspersed between nearly equiaxed \(\alpha\) grains. The as-built SLM Ti-6 Al-4V has a martensitic acicular \(\alpha^{\prime}\) microstructure. Therefore, it has high static strength but poor ductility compared with cast or wrought Ti-6 Al-4V [4].  
Due to the limited scope, the current analysis cannot cover all previous studies but can only take data from references [14; 21]. The wrought material fatigue data are taken from reference [21], which is considered as the baseline, and the behaviour is described by Equations  
Figure 4: The TMW model-calculated fatigue life in comparison with the experimental data (arrow indicates runout) for 17-4 PH stainless steel.  
(3) and (4) with \(R_{s}=1\) and M = 1, i.e., the baseline Equations (1) and (2), as given in Table 7. The DMLS Ti6 Al4V fatigue data are digitized from reference [14]. The fatigue behaviours of AM Ti6 Al4V are described by Equations (3) and (4), while \(R_{s}\) and M are evaluated in a similar way by Equations (5) and (6), and the values are given in Table 8. The calculated curves are shown in Figure 5, which are in good agreement with the experimental data.  
The HIPed DMLS Ti6 Al4V has a microstructure factor of 1.5, which is comparable to AM 17-4 PH, reasonably, with pores. However, the as-fabricated DMLS Ti6 Al4V has a microstructural factor of 316, which is much larger than that raised by round pores. Therefore, it is suspected that this must be caused by LOF defects, which are usually larger and sharper than pores. The post-mortem fractographic examination in the original work revealed that fatigue crack initiation did start from surface defects, which propagated, linked up and prompted the final failure. Although mechanical polishing might remove some of the obvious defects on the surface, some remained due to the emergence of occasional voids from within the bulk during material removal. Treatment of DMLS specimens with either mechanical polishing or electropolishing produced negligible improvement in  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \(\sigma_{a}\) (MPa) & \(\varepsilon_{a}\) (\%) & \(\Delta\varepsilon_{p}\)/2 &
\begin{tabular}{c} **2N\({}_{\rm f}\)** \\ **(Reversals)** \\ \end{tabular} & **TMW (2N\({}_{\rm c}\))** \\ \hline Strain-controlled & 749 & 1.65 & 0.957 & 620 & 475 \\  & 722 & 0.999 & 0.331 & 3294 & 3969 \\ condition & 724 & 0.669 & 0 & 46,790 & 73,702 \\  & 725 & 0.669 & - & 56,704 & 72,528 \\ Load-controlled & 676 & 0.625 & - & 84,012 & 196,200 \\ condition & 649 & 0.6 & - & 137,428 & 471,991 \\  & 622 & 0.575 & - & 3,057,102 & 2,341,425 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Fatigue test results of wrought Ti-6 Al-4V with the machined surface under axial loading (test information and data taken from [21]).  
Figure 5: Fatigue behaviours of wrought and AM Ti6 Al4V.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Orientation** & **Treatment** &
\begin{tabular}{c} **Surface** \\ **Condition** \\ \end{tabular} & \(R_{s}\) (\(\mu\)m) & \(R_{s}\) & M \\ \hline Wrought Ti6 Al4V & - & - & Machined & 0.010 & 1 & 1 \\ DMLS Ti6 Al4V & Horizontal & HIPed & Polished & 10 & 0.57 & 1.5 \\ DMLS Ti6 Al4V & Horizontal & - & As fabricated & 11–13 & 1 & 316 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Surface finish and microstructural factors for Ti6 Al4V (\(\sigma_{0}=600\) MPa).  
fatigue life. The surface finish factor \(R_{s}\) has values ranging from 0.57 to 1, but it does not seem to correlate with the arithmetic average surface roughness, \(R_{a}\), for Ti6 Al4V.