Low cycle fatigue behavior of additively manufactured Ti-6Al-4V under non-proportional and proportional loading

Abstract

Experimental tests were conducted on additive manufactured Ti-6Al-4V titanium alloy to investigate the mechanical and crack properties under multiaxial cyclic loading. Selective Laser Sintering technique (SLS) was employed to fabricate four types of cylindrical hollow specimens. The typology of each specimen is defined by the orientation of the layers and by the application of a stress-relieving heat treatment after the production process. Stress-strain cyclic curves of the materials were obtained to investigate the material cyclic plastic behavior, that resulted independent of specimen variety. Strain-controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading paths were carried out on the specimens. Not heat-treated specimens exhibited a higher low cycle fatigue resistance both for proportional and non-proportional loading. Drastic initial softening was detected in the majority of the tests. Additional hardening was detected in part of non-proportional tests, which is atypical for this alloy. The mutual influence of applied load and microstructural characteristics on fatigue life are finally discussed.

Multiaxial fatigue strength under non-proportional loading of additively manufactured notched components of Ti-6Al-4V

“…2. In the case of PP, the final crack propagation rate could be connected to the layer orientation as previously observed on smooth specimens made of the same material [8].…”

“…In fact, the stress levels remain constant before a sudden drop which coincide with the crack propagation leading to the final failure. The slope of the final trait is slightly different between the vertical and horizontal specimens, indicating that the final crack propagation mechanism is different as reported also in [8]. In case of OP, an anomalous hardening behavior have been observed for the vertical specimens and the causes are still under investigation and may represent the cause of the earlier failure life exhibited by the vertical layered specimens in case of multiaxial loading.…”

“…Furthermore, such components are often subjected to multiaxial loading conditions (proportional or non-proportional) which further complicates the mechanical response of the material. In a previous work, the influence of heat treatment and layer orientation on uniaxial and multiaxial non-proportional low cycle fatigue have been investigated [8] and the heat-treatment has been found having the major influence on fatigue life compared to the layer orientation and voids. In the current literature, there are few works discussing the low cycle fatigue of AM notched components made of Ti-6Al-4V and further tests are necessary to define the material behavior also in relation to geometrical discontinuities.…”

The Influence of Heat Treatment on Low Cycle Fatigue Properties of Selectively Laser Melted 316L Steel

“…A different approach was suggested by Bressan et al [ 27 ], in which the authors analyzed an influence of stress-relieving heat treatment on LCF properties of titanium alloys. During their tests, it was observed early sudden material’s weakening, which was correlated with the formation of cracks in internal voids.…”

“…Based on two previously-mentioned research works [ 26 , 27 ], a significant influence of porosity in the material volume was stated. It is necessary to understand the reasons for porosity generation in AM parts [ 28 , 29 ].…”