PSI - Issue 38

Alok Gupta et al. / Procedia Structural Integrity 38 (2022) 40–49

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Author name / Structural Integrity Procedia 00 (2021) 000 – 000

1. Introduction Selective Laser Melting (SLM), a type of Additive Manufacturing (AM) process, is rapidly being adopted in the aviation industry due to its several advantages, such as the high material usage efficiency and light weight part designs in complex shapes [Gupta et al. (2021a)]. SLM products can individually be customized, with virtually no customization cost involved, for load bearing applications in aviation, medical and dental industries. High relative density (up to 99.9%), good mechanical properties of the SLM material and on-demand manufacturing of parts providing efficient supply chain are within the other benefits which contribute to attract the practitioners of these industries to adopt the SLM technology. Building a good understanding of fatigue performance of SLM parts and of mitigation actions required to reduce residual stresses, distortion levels and defects sizes (through process optimization and post build heat treatments) are some of the prerequisites for a successful implementation of SLM technology in the aerospace industry. Due to its high strength to weight ratio, the Ti-6Al-4V alloy remains a preferred material for manufacturing of aero-engine parts using the SLM technology [Baragetti et al. (2019)]. Brackets installed on an aero-engine are required to withstand cyclic loading conditions during operation, a dominant loading type responsible for the majority of failures [Gorelik (2017)]. The brackets are designed against the stringent low and high cycle fatigue loading conditions to demonstrate compliance against the certification requirement set-out by the European Aviation Safety Agency [European Union Aviation Safety Agency (2018)]. The studies thus far considering the fatigue performance of AM components offer limited scope and understanding, and do not fully support the smooth implementation of AM parts for load bearing applications [Gupta et al. (2021b)]. Although, both Brusa et al. (2017) and Leuders et al. (2017) have attempted to study the fatigue performance of AM components, their studies provide only minimum relevance to the end applications. Mardaras et al. (2017) suggested a method to perform fatigue assessment of AM parts, but again they did not present a test case to support the validation of their method. While brackets in aero-engine applications are designed to operate mostly in the elastic regime, they may experience small plastic strains during normal operation of an engine (mostly around geometric discontinuity features) under the maximum normal operating loads (limit loads), which may occur for low number of cycles (e.g. <3000 cycles). In certain extreme loading conditions, the brackets are likely to undergo a higher level of plasticity, but only for very small number of loading cycles (e.g. < 100 cycles) before an inspection and further maintenance actions are taken. In this paper, the Low Cycle Fatigue (LCF) performance of a weight optimized SLM Ti-6Al-4V bracket has been presented. Firstly, the Tensile and LCF fatigue data obtained from the coupon testing are discussed and then the LCF test results of the bracket are discussed. The loading levels for the LCF tests on the bracket were established based on the findings from the coupon test results. It was demonstrated that the bracket met its cyclic requirements for the displacement levels causing strain levels close to and higher than the material elastic limit. 2. Experimental Campaign 2.1. Material and Specimen Manufacturing The test specimens and the bracket were manufactured using the Ti-6Al-4V (Grade 23) Extra Low Interstitials (ELI) plasma atomized powder with a mean chemical composition of Ti Bal, Al 6.4, V 4.0, FE 0.19, O 0.12, N 0.02, H 0.002, C 0.02 (wt. %), which is in line with the ASTM-B348 (2019). The powder size was distributed between 15 – 45  m with mean of the distribution centered at 30  m. The test specimens and bracket were made using an EOSINT M280 SLM machine. The machine used a laser power of 170 W, operated in argon atmosphere and had a substrate of Grade 5 Ti-6Al-4V material which was preheated to 35  C. The layer deposition speed was kept at 150 mm/s, laser scan speed was 1250 mm/s and layer thickness used was 30  m. The specimens and bracket were removed from the substrate plate using the Wire-EDM cutting process post build. Subsequently, the build supports were removed, and aqua blasting was carried out to remove the loosely sintered powder particles and also to improve the surface finish. The specimens and bracket were stress relieved at 650  C for 3 hours in an argon atmosphere and were subsequently furnace cooled to room temperature. The dimensions of the tensile and LCF test specimens are shown in Fig. 1, both of which were built in the vertical orientation and were subsequently machined to produce threads at two ends to suit the grips of the test machines whilst