PSI - Issue 7

Gianni Nicoletto et al. / Procedia Structural Integrity 7 (2017) 133–140 Gianni Nicoletto/ Structural Integrity Procedia 00 (2017) 000–000

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Fig. 7 Comparison of present data on SLM Ti6Al4V and fatigue behavior of conventional Ti6Al4V. Inspection of Fig. 7 shows that: i) the HIP process does not represent a source of improvement in the case of high quality of the SLM process because internal defects on which HIP has a positive effect are very limited in number and small (i.e. theoretical densities greater than 99.9 % are typical of an optimized SLM process on Ti6Al4V), ii) the HT1 treatment of SLM Ti6Al4V and subsequent surface machining achieves a fatigue strength comparable to that of conventionally produced Ti6Al4V, iii) the literature signals a sensitivity of the surface finish on the fatigue behavior of conventional Ti6Al4V, which is expected to play a role even in SLM Ti6Al4V. As a general comment on material defects and the SLM process, they may be internal, near-surface or surface breaking for example after machining. Internal defects can in principle be eliminated by a HIP treatment if oxides are not present thus preventing surface welding. The high temperature of the HIP treatment results in reduced mechanical properties compared to an optimized heat treatment (for example HT1). The as-built surface associated to the SLM process is intrinsically rough. Such roughness, on which HIPing has no influence, can be interpreted in terms of an equivalent surface defect to be used in fatigue life estimates according to damage tolerant approaches. 4. Conclusions In this paper, the influence of four post fabrication heat treatments in a vacuum furnace of Ti6Al4V produced with a state-of-the-art SLM system and process was investigated in terms of microstructure, tensile properties and fatigue behavior. The comprehensive study demonstrates that two heat treatments, one above and one below the β transus, are especially valuable for the associated high fatigue strength and tensile properties above the ASTM requirements. Due to the practical importance of the as-built surface condition, the present fatigue tests were mainly performed on as-built and heat treated specimens. For one heat treatment condition, the influence of surface machining was quantified and allowed the definition of an as-built surface knock-down factor in fatigue of 0.3. This factor is high but comparable to traditional process-affected (i.e. casting or forging) surface factors. The present fatigue results for SLM Ti6Al4V demonstrated that this laser-based additive manufacturing process achieves performances that are comparable to conventional processes when the surface conditions are similar. References ASTM F2924-14, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion, ASTM International, West Conshohocken, PA, www.astm.org (2014) Bača A. , R. Konečná, G. Nicoletto, L. Kunz, “Influence of build direction on the fatigue behaviour of Ti6Al4V alloy produced by direct metal laser sintering”, Materials Today: Proceedings 3 921-924 (2016).