PSI - Issue 8

Gianni Nicoletto / Procedia Structural Integrity 8 (2018) 184–191 Author name / Structural Integrity Procedia 00 (2017) 000–000

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A further advantage of the mini specimen geometry is also highlighted. The mini specimen geometry was developed for a plane cyclic bending loading condition applied by a modified Schenck-type machine, Nicoletto (2016). Fig. 5 shows how the specimen geometry allows the investigation of the unnotched fatigue behavior (cyclic tensile loading with load ratio R=0 applied on the flat surface) and of the notched fatigue behavior (cyclic tensile loading with R=0 applied on the notched surface). A first application of this feature will be presented at the end of the next section.

Fig. 5. Alternative bending loading for R=0 and different test conditions investigated.

Fig. 4. Specimen orientations with respect to build Type B (left) Type C (right).

3.2. Specimen fabrication details The Ti6Al4V ELI alloy powder used to fabricate the specimens was characterized by spherical powder particles of predominant diameter range from 25 to 45 µm. All fatigue specimens were fabricated using the Direct Metal Laser Sintering (DMLS) technology with layer thickness of 60 µm in a EOS M 290 system. This system uses a 400 W Yb-fiber laser unit with a wavelength of 1075 nm. Metal processing was in a protective Ar atmosphere and a chamber temperature of 80 °C. Process parameters used were according to system producer recommendation and system operator validation. After removal from build plate, all specimens were heat treated in a vacuum furnace as follows: 740 °C for 2 h then slow cooling in vacuum to 530 °C and final cooling to room temperature in Ar atmosphere. Metallographic and fractographic investigations are reported by Konečná et al (2017). 4. Influence factors on the fatigue behavior of DMLS Ti6Al4V 4.1. Testing method validation The mini specimen geometry and the fatigue test method is verified in Fig. 6 where two data sets for a custom heat-treated DMLS Ti-6Al-4V with as-built surfaces are presented. One set of Type C mini specimens, see Fig. 4, and a second set of standard rotating bending specimens were produced on the same SLM system and under identical conditions. Since the fatigue tests on the two types of specimens were conducted under different load ratios (i.e. R=0 for mini specimens and R=-1 for rotating bending specimens), data were made comparable using an equivalent stress amplitude  a,eq definition on the basis of the Haigh relationship for mean stress effect on fatigue, see Juvinall and Marshek (2012). When R=0, that is when the stress amplitude  a is equal to the mean stress  m the equivalent  a,eq is given by the following relation   a,eq =  a [R m /(R m -  m )] (1) where R m is the tensile strength of DMSL Ti6Al4V. Inspection of the plot of Fig. 6 shows that the heat treatment introduces a significant scatter, which was not present in the case of a previous heat treatment, Bača et al. (2016). Nonetheless, the trend curves for the two sets of