PSI - Issue 24

Giulia Morettini et al. / Procedia Structural Integrity 24 (2019) 349–359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

AM technologies, first developed in the late 1980s, started playing a major role among manufacturing techniques in the last decade, to that extent, in 2012, The Economist claimed Additive Manufacturing protagonist of the “Third Industrial Revolution” , Wohlers (2014), The Economist (2018). Conventional technologies (e.g. casting, forging, stamping, turning) require the designer to first evaluate carefully the whole production cycle, in order to assess the required steps needed to obtain the final component. Those analyses become more and more difficult as the geometrical complexity increases, leading to higher costs and, sometimes, to the impossibility of producing some demanding parts. The success of AM is based on its ability to produce virtually any complex geometry that the designer can imagine: undercuts, internally hollow parts... All this without the use of specific tools. The final component construction is achieved thanks to a layerwise-additive strategy starting from the CAD model. It is possible to process a lot of different materials classes, namely metals, polymers, ceramics, sand, wax, reducing sensibly the production of scraps, with respect to subtractive technologies. Powder Bed Fusion (PBF) is the most widely adopted AM technology among industry and scientific research and it takes advantage of a heat source, that could be a laser (as in SLM technique), or an electron beam (as in Electron Beam Melting (EBM)), that impinging on a powder layer, it results in material melting and subsequent solidification. AM is mainly advantageous, with respect to traditional technologies, and even if there has not yet been collected in the literature such a quantity of experimental data on fatigue behavior (even in conditions of multiaxiality) as done for conventionally produced metallic materials Morettini (2019); for low-volume and high added value production, EPMA (2018), the additive manufacturing technique turns out to be very interesting for industries such as aerospace and biomedical. Their application is still growing, involving industrial fields like automotive, energy and tool production, Frost & Sullivan (2016). A wider adoption of AM in general could be achieved through a deeper understanding of materials behavior under service conditions, and, in this frame, the herein work deals with fatigue performances. Focusing on metal alloys, the PBF manufacturing strategy results in unique material properties, both from microstructural, mechanical and finishing point of view, Shamsaei (2017) and Molaei (2018). Final microstructure is mainly affected by the peak temperature and subsequent solidification and cooling rates: these parameters are determined by the presence of a fast moving highly-focused heat source that instantly melts a small portion of the powder bed, causing the arise of high cooling and solidification rates, DebRoy (2018), Sames (2016), Megahed (2016). Metal alloys in the As-Built condition are characterized by oriented and metastable microstructures, and non-complete secondary phases precipitation. At the same time, highly localized thermal gradients, within the same layer and between subsequent layers, with high cooling rates cause the arise of tensile residual stresses inside the final component itself, resulting in distortions, lower mechanical properties and uneven failures due to non-homogeneous stress distribution. Residual stresses and proper microstructures can be restored through a proper heat treatment. Rough surfaces, in the As-Built condition, are characterized by a high frequency component, due to the presence of metal particles partially sintered on the surface, and a low-frequency component, i.e. the stair-stepping caused by the superimposition of melted layers, Fox (2016). Local asperities, together with potential microcracks on the surface, can act as crack initiator, sensibly lowering fatigue life of produced components. Real-life applications usually require to perform post-processing steps, e.g. machining or polishing, to meet project requirements. Additionally, when the process is not under control, it can happen that the produced part be affected by internal porosities and microcracks, further reducing tensile strength and fatigue limit. These internal porosities can be reduced by applying a Hot Isostatic Pressing (HIP) treatment, impacting on the cost of the final component. All of the listed phenomena, detrimental to fatigue life, are strongly dependent on selected process parameters – namely laser power, laser speed, hatching distance, scanning strategy – and part orientation inside the building chamber. Several studies have been published on the subject, but, to the best of authors ’ knowledge, the available literature on high-cycle fatigue analysis, on specimens produced with AM, is still limited, Wycisk (2015), Leuders (2013), Nicoletto (2016), Walker (2017), Benedetti (2018), Bača (2016). A major lack results from the absence of specific literature on As-Built samples which have not been subjected to heat treatment. In addition, the strong dependence of alloys performance on the process parameters, further widens the research field. In this context, this work aims to assess fatigue resistance of SLM manufactured Ti-6Al-4V samples in their As-Built condition, without post-surface and/or -heat treatments, to establish initial fatigue properties of the alloy and allow future comparison with conditioned samples, making it possible to quantify the effect of post treatments.