PSI - Issue 17

Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138 – 145 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Material characterization

Metallographic specimens were prepared according to standard techniques and then observed using the Neophot 32 light microscope and Tescan LYRA 3 XMU FEG/SEM with EDX analysis system. Microstructure was analyzed after etching with Kalling's reagent (2 g of CuCl 2 , 40 ml of HCl, 80 ml of methanol).

2.3. Fatigue testing

A special miniature prismatic specimen geometry introduced by Nicoletto (2017) was adopted to investigate the directional effect of the as-built surface quality on the notch fatigue strength of as-built SLM Inconel 718. The present test methodology has been already extensively used on DMLS Ti6Al4V, Nicoletto (2018). Fig. 1 shows the side view of the specimen subjected to bending and the associated principal stress distribution obtained by elastic finite element analysis. The size of the nominal minimum section is 5 x 5 mm 2 , the radius of the semicircular notch is 2 mm and the specimen length is 22 mm. A mild stress concentration factor K t =  max,FEA /  nom = 1.63 was obtained using  nom = M/W and W is the section modulus of the notched section. The specimen loading is cyclic bending with a stress ratio R = 0 and a frequency of 20 Hz. Test run-out was fixed at 2x10 6 cycles.

Fig. 1. Side view of notched miniature specimen subjected to bending and elastic stress distribution.

Fig. 2 shows the different specimen directions with respect to build direction (i.e. Z directions) and their respective denomination.

Fig. 2. Orientation of the notched miniature specimens on the build plate and denomination (Z is build direction).

The surface quality of the notches is affected by the specimen orientation due to the layer-wise SLM fabrication. In addition, the direction of applied stress at the notch root may be parallel or orthogonal to the material layers. Namely, the semicircular notch geometry of Type B specimens is not influenced by the fabrication and the applied stress is parallel to the layers. The semicircular geometry of Type C specimen at its notch root is weakly influenced by the layer-wise fabrication and the applied stress is perpendicular to the layers. The circularity of the notch roots of