PSI - Issue 46

David Liović et al. / Procedia Structural Integrity 46 (2023) 42 – 48 D. Liovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3. Average surface roughness comparison for upper surfaces of cubic specimens.

3.3. Porosity estimation By estimating the porosity on the cubic specimens based on surface images, it was found that by using the SLM process parameters previously listed in Tab. 2, components with low porosity fractions can be manufactured. Moreover, by using a laser power of 250 W and a scanning speed of 1250 mm/s it is possible to achieve almost full density of the material (Fig. 4). The combination of process parameters used and the stability of the SLM process itself significantly influence the porosity fraction inside processed material. Low porosity fractions of SLM-ed components can be achieved within another process window as well, as stated by Pal et al. (2020) where a porosity fraction of 0.55% was achieved using a laser power of 75 W and scanning speed of 600 mm/s. In addition, Vrancken et al. (2012) also achieved nearly full density using 250 W laser power and 1600 mm/s scanning speed. 3.4. Typical mechanical properties of SLM-ed Ti6Al4V alloy In the first place, depending on the porosity fraction caused by selected combination of process parameters and applied heat treatment, the mechanical properties of these materials differ from each other. After the applied heat treatment on specimens manufactured using different combinations of SLM process parameters, elongation at break can range from 1% to 13%, while the values of yield and ultimate tensile strength are quite similar and usually range from roughly 850 MPa to 1100 MPa (Tab. 3). According to ASTM F1472-02a (2002) standard, minimum ultimate tensile strength ( R m ), yield strength ( R p0.2 ) and elongation at break ( A ) requirements of Ti6Al4V alloy strips for surgical implant applications are, 924 MPa, 869 MPa and 10% respectively. Elongation at break criteria is still very difficult to achieve using SLM technology, especially in the as-built state of the Ti6Al4V alloy which microstructure usually consist of the acicular martensite inside prior β columnar grains. However, by using annealing heat treatment or hot isostatic pressing it is possible to increase the ductility of the material, at the expense of reducing its ultimate tensile and yield strength. Typical monotonic mechanical properties that can be achieved using different SLM process parameters and conditions on Ti6Al4V alloy are shown in Tab. 3.