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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To investigate the role of surface machining, rotating bending fatigue tests results (stress amplitude  a at R = -1) on the same heat treated DMSL Ti6Al4V material after machining are introduced in Fig. 6. Fig. 6 shows also that surface machining that removes a surface layer of significant depth results in considerably higher fatigue strength compared to manually ground surfaces and to the rough as-built surface. The knockdown factor associated to the as built surface is important and can be estimated C surf ≈ 0.35 If the fatigue results of Fig. 6 were introduced into the comprehensive plot of Fig. 1 from Li et al (2016), it would be found that the as-built data are close to the yellow line (i.e. the best performance for as-built & heat treated condition) and the machined data are close to machined SLM and to conventional Ti6Al4V data. 4.3. Directional notch fatigue factor The fatigue properties of unnotched SLM Ti6Al4V specimens with as-built surfaces have been recently under investigation, Li et al (2016), but, in practice, few metal AM components are expected to have simple geometries without any corners or radii that would act as stress concentrations. Therefore, the combined effect of a rough as built surface and a geometrical notch needs to be established to enable relevant fatigue predictions for structural parts. Recently Kahlin et al (2017) reported the fatigue behavior of notched SLM Ti6Al4V with as-built surfaces and quantified the notch fatigue factor for as-built tensile bar with a circumferential notch. Fig. 5 shows how the mini specimen geometry can be used to investigate the combined directional and notch fatigue effect. Here original evidence on this important influence factor obtained with mini specimens is presented. Two sets of heat treated Type B and Type C mini specimens made of DMLS Ti6Al4V were tested under R=0 ratio in the two testing modes specified in Fig. 5 (i.e. to quantify the unnotched and notched fatigue behavior). The semicircular notch in bending induces a mild stress concentration, i.e. S.C.F. K t = 1.56. All the test results for the two specimen orientation with respect to build and for the unnotched and notched condition are plotted in Fig. 8. The data appear well-behaved with a reduced scatter and apparently define trends which allow the experimental determination of the fatigue notch factor K f of the two fabrication directions.

Fig. 8. Notch fatigue behavior of two types of as-built & heat treated DMLS Ti-6Al-4V specimens (load ratio R=0; K t =1.56).

Adopting the classical definition of K f as the ratio of the smooth fatigue strength and notch fatigue strength, see Juvinall and Marshek (2012), Type B K f,B and Type C K f,C can be determined using the respective unnotched and notched responses yielding K f,C = 1.63 and K f,B = 1.36, respectively. These are possibly the first results of this kind where the notch surface quality that depends on specimen orientation is quantified. Interestingly, K f is directional, evidence of a technology-dependent effect, and comparable to the theoretical K t of the notch geometry.