PSI - Issue 5

David Palousek et al. / Procedia Structural Integrity 5 (2017) 393–400 Vendula Kratochvilova et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 5. Comparison of S-N curves of SLM Al alloy (a) AlCu2Mg1.5Ni and its reference material; (b) AlCu2Mg1.5Ni and AlSi10Mg

Fig. 6. Comparison of S-N curves of SLM Cu alloy Cu7.2Ni1.8Si1Cr and its reference material

3.2. Acoustic emission AE records are shown on the Fig. 7 and 8. All materials have three fatigue stages (A – pre-initiation, B – initiation and C – post-initiation). The main difference is observed in their ratio to total fatigue lifetime. Also, the AE activity in SLM material is higher. The stage A at the beginning of loading is pre-initiation stage. This stage is characterized by significant AE activity due to changes in the microstructure, interaction and glide of dislocations and persistent slip band formation. The AE activity decrease in the next stage (B) and micro-cracks start to propagate here (initiation stage). The last stage (C) is the post-initiation stage. The AE activity again increase most. The micro-cracks are linked and long cracks are propagated here. The main difference is observed in ration of stage A of both Al and Cu alloys. This stage takes approximately 40 % of extruded AlCu2Mg1.5Ni total fatigue lifetime, respectively 65 % in case of AMPCOLOY® 644. Stage A of SLM material takes only about 25 %, respectively 16 % in case of Cu SLM material. Also, the ratio of stage C is different. While in extruded material it takes approximately 35 % in case of Al alloy and 10 % in case of Cu alloy, in SLM material it takes more than half of lifetime (55 % of Al alloy and 75 % of Cu alloy lifetime). Differences in AE signal suggest different mechanism of crack initiation and propagation. In the extruded material, the main crack spreads mainly by changes in the microstructure (slip band formation or dislocation). In the SLM material, the main crack spreads through the connection of micro-cracks between the voids in the structure.