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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contribution is machine run time. Summing up all four contributions to production costs of a hypothetical 1-kg DMLS-Ti6Al4V batch of specimens, a total cost of € 2500 is estimated. Additional costs are associated to post-processing (i.e. heat treating, machining and surface finish processes). Here no cost of the specimen post-processing is considered as if tests were performed only on as-built specimens and the applied heat treatment is the standard one applied for part production (i.e. no special use of furnace). Specimen testing costs are typically relevant and are proportional to the testing time of a dedicated fatigue machine. Here the costs depend on total testing time with the role of test machine type (i.e. servo-hydraulic test rig the most expensive vs. rotating bending machine the least expensive) and number of specimens. A rough cost estimate of the high cycle fatigue testing of 15 specimens is € 3000. So the estimate of the total cost of the production and fatigue characterization of a batch of SLM specimens is €5500 per kg of specimen weight. So specimen dimensions are quite relevant as they impact material weight and AM machine run time costs.

Fig. 3. Specimen geometries used for fatigue testing of AM metals.

Fig. 2. AM metal fabrication cost breakdown.

3. Fatigue testing of SLM metals using mini specimens 3.1. Features of the mini specimen geometry

Consideration of all the factors driving up the fatigue characterization costs discussed in the previous section, motivated Nicoletto (2016) to propose the miniature specimen geometry shown in Fig. 3 along with the standard rotating bending and standard push/pull specimens. They have comparable minimum cross sectional area properties (5x5 mm 2 for the mini specimen, section modulus W = 20.8 mm 3 ; 6-mm-diameter for the rotating bending and W 21.2 mm 2 . If production cost is assumed proportional to the specimen volume, the comparison of Fig. 3 demonstrates that the volume of material to be produced can be drastically reduced (i.e. the volume of the mini specimen is approx. 1/7 of the rotating bending specimen and 1/78 of the push-pull specimen). Therefore, batches of numerous specimens can be cheaply built in SLM systems in a short time if the mini specimen geometry is adopted. A first successful validation of the mini specimen geometry for fatigue testing was reported by Nicoletto (2016) and Bača et al. (2016). A further validation of the methodology will be reported in the next section. In addition to the economic advantage, an additional advantage of the mini specimen geometry compared to the standard geometries is that an a-priori defined material surface is under test. Therefore, specimens can be oriented as desired with respect to the build direction to investigate anisotropic fatigue response. Fig. 4 shows that the long dimension of Type B and Type C specimens is oriented perpendicular and parallel to the build direction, respectively. The directional fatigue behavior of DMLS Ti-6Al-4V after a stress relief treatment was reported by Nicoletto (2016). A high temperature post fabrication heat treatment drastically reduced the directionality in fatigue DMLS Ti-6Al-4V, Bača et al. (2016).