*3.4. Compression Test Results*

The yield strength is probably the most important mechanical property of the material. This value determines the maximal mechanical loads that the material can withstand without deformation. This value is also influenced by the direction of the load. There are some studies which applied both the tensile and compression test on the sintered Ti samples. Schulze et al. determined the YS of SLM-produced Ti-42Nb alloy on tensile and compression specimen. The YS was 674 MPa upon tensile test and 831 MPa upon compression test, respectively [26]. Raghavendra et al. compared the tensile and compressive properties of additively manufactured (SLM) Ti6Al4V porous samples. The ultimate tensile strength was lower than compressive strength by an average value of 100 MPa [44]. We have to account for the fact that the PM samples have a significant number of defects, e.g., porosities. Generally, the sensitivity of the tensile test properties on the presence of defects is much higher than in the case of a compression test. Figure 7 shows the measured engineering stress–engineering strain curves of samples sintered at 800 ◦C, determined by compression test.

**Figure 7.** Stress–strain curves of the samples sintered at 850 ◦C with different milled Ti content.

Strength values (YS, compressive strength) determined on the measured curves show the same tendency as the hardness results. The samples without milled Ti content had an average YS (500–640 MPa). On the other hand, for the samples with an increased milled Ti content, the YS increased proportionally (Figure 8). The highest strength values in most cases belong to the samples sintered at 850 ◦C. Regarding the strain values of the compression tests, it can be concluded that increasing the milled fraction decreases the deformability of the material.

The samples with 8 wt.% milled Ti presented a reduced formability around 5%, meaning that the material is too brittle for engineering applications. Comparing the measured YS values with data from the literature, it can be stated that the sample that has 6 wt.% milled powder shows a higher YS than the wrought Ti6Al4V (Table 4).

**Table 4.** Mechanical properties of titanium alloys fabricated by various processing technologies. (compressive YS; ultimate compressive strength, plastic strain to maximum compressive stress).

