*3.2. Density Analysis*

Figure 5 shows the effect of the added milled fine powder on the relative density of different green and sintered samples. Based on the density measurements, the milled powder has a notable effect on the properties of the samples. Due to the applied high compacting force (1600 MPa), the relative density of the green sample, which does not contain any milled titanium, was determined to be around ~97.5%. Due to the powder production, the ~2.5% porosity was typically closed porosity. The high green density is beneficial to obtaining high sintered density and high mechanical properties. Robertson et al. applied the same load (1600 MPa) during the cold pressing of Ti6Al4V powder, and the measured green density was ~95% [36]. Raynova et al. has investigated the effect of the green density (65.6% to 96.5%) on the sintered density in the case of HDH Cp-Ti powder. After sintering, the density of the sample with the highest green density remained the highest (~97%) [24]. In our research, the results showed the same tendency. The density of the green sample, which does not contain any milled titanium, practically does not change during the sintering temperatures. As it can be seen in Figure 5, the green density values were decreased by increasing the fraction of the milled powder. This is due to the fact that porosities take place between the hard particles of the milled fraction (Figure 3b) which add to the micro porosities inside the powder particles. Moreover, the increase in sintering temperature resulted in an exponential increase in density. This is due to the fact

that the specific interface increases with fine grains, which increases the driving force of sintering [37].

**Figure 5.** Relative density of the samples containing different amount of milled Ti.

Despite the applied low sintering temperature and short sintering time, the sintered density of the produced samples was found to be similar or better, compared with those observed in the literature (Table 3). As the values show, the applied processing parameters have a significant effect on the density.

