## 4 Discussion
### Influence of Scanning Strategy on Hardness and Microstructure  
The average hardness values for each of the three specimens are given in Table 3 and are higher than the values found in the literature [11; 28]; around 440 HV was stated for similar energy densities [28]. It is noted that in the present work a low load was used, which can explain part of the discrepancy. The relatively large spread in hardness values as reflected by the error bar and the hardness ranges in Figure 9 is common for AM parts and was observed earlier for SLM parts [11]. In general, the large variation is related to variations in microstructure, most notably the banding, the lath size, and the decomposition of \(\alpha^{\prime}\). Figure 10: Volume averaged size distribution of pores detected in Avizo on reconstructed CT using LFOV and 4\(\times\) objectives in three different Ti-6 Al-4V SLM specimens. The average hardness values for the island scan specimens (i.e., 492 and 496 HV\({}_{0.01}\) for stripes and modeled, respectively) are slightly, but not significantly, higher than 474 HV\({}_{0.01}\) for the line scan specimen. This is largely determined by the hardness decline in the bottom part of the line scan specimen. All three specimens show a slight increase in hardness in the top 0.5 mm, showing that something is changing; most likely caused by the reheating of underlying metal upon scanning of the next layers. It has been reported that the growth conditions for SLM stabilize after a build height of 10-15 mm is reached [8]. This means that for the present specimens no stable growth conditions were reached yet. Nevertheless, investigation of the unstable region is very relevant, as many SLM parts are thin walled. Decomposition of martensite into lamellar \(\alpha/\beta\) in forged Ti-6 Al-4V is normally found to enhance hardness [26], rather than reduce the hardness as observed in the present work. It is noted that such a hardness increase is observed for a martensitic structure with a hardness of about 330 HV\({}_{0.01}\). Clearly, regularly quenched martensite in bulk samples as achieved by heat treatment is different from the martensite formed in the present SLM specimens. Generally, SLM leads to microstructure with a hardness above that normally achieved by quenching and tempering, attributed to the development of fine multiply-twinned martensite. Upon tempering SLM Ti-6 Al-4V a hardness reduction can result by the formation of the coarser dual phase \(\alpha/\beta\) microstructure. Another effect that could contribute to a relatively high hardness of the SLM specimens as compared to conventional TiAl6V4 is the uptake of oxygen during the build process. Titanium has a strong affinity for oxygen and even a very low partial pressure of oxygen in the protective atmosphere will react with the alloy at high temperatures. The \(\alpha\)-Ti phase has a high solubility for oxygen and is stabilized to higher temperatures by the dissolution of oxygen. A solid solution of oxygen in the \(\alpha\)-Ti phase is associated with effective solid solution strengthening [29]. Moreover, surface oxidation of the melt can occur, forming Ti- and Al-based oxides, which could lead to dispersion hardening. In addition, an augmented oxygen content (as compared to the nominal powder composition) will enhance the propensity for Ti\({}_{3}\)Al formation in the low temperature regime, as oxygen is dissolved in the precipitate phase [30]. The promoted formation of Ti\({}_{3}\)Al by increased oxygen content leads to significant precipitation hardening, accompanied by increased brittleness. Promoted Ti\({}_{3}\)Al formation could mean that the material is more sensitive to thermal inputs, possibly adding increased oxygen levels as a reason for the high hardness variations. The decomposition of martensite was only observed in the line scan specimen, while the martensite appeared unchanged in the island scan specimens.