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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1. Introduction Powder bed fusion (PBF) is one of the seven AM technologies identified by ASTM norm and it is mainly aimed at producing parts using metal alloys such as Titanium, Ni-based super alloys, Cr-Co alloys and Al/Si alloys. Alternative acronyms within the same technology are: SLM (selective laser melting), SLM (direct metal laser sintering) where a laser is the energy source and EBM (electron beam melting) where the source is an electron beam, see Bandyopadhyay and Bose (2016). In the rest of the paper the SLM acronym will be used to specify the key role of the laser energy source on the powder bed process. The Ti6Al4V alloy is the most widely used Ti alloy in high-value-added industrial sectors such as aerospace, medical, energy, motorsport etc.. The SLM process, which is characterized by a layer-wise building process that enables the near net shape production of parts with a high geometrical complexity, is recently complementing traditional production routes of Ti parts, such forging and casting. The SLM process offers several additional advantages compared to conventional production techniques, such as reduction of production steps, a high level of flexibility and a high material consumption efficiency, see Srivatsan and Sudarshan (2016). To industrialize the SLM process as well as to design and qualify the SLM part, the quality of processed material has to be controlled and the mechanical properties determined and guaranteed. The SLM process is characterized by the build-up of thermal stresses, while the rapid solidification activates segregation phenomena and the development of non-equilibrium phases. Therefore, suitable post-production heat treatments should be defined to mitigate thermal stresses and to optimize the microstructure and the mechanical properties for the specific application. The influence of post fabrication heat treatments on static properties of SLM Ti6Al4V has been recently investigated by different authors, for example Vranken et al. (2012), Rafi et al (2013). However, the fatigue performance is often the critical design parameter for many high-value applications. Several investigations of the fatigue characteristics of Ti6Al4V materials produced with layered AM techniques have been published recently. Measured fatigue strength of as-built SLM Ti6Al4V was shown to be considerably lower than that of wrought, annealed material, for example Leuders et al. (2013), Edwards and Ramulu (2015), Mower and Long (2016). The influence of microstructural directionality on fatigue behavior of SLM Ti6Al4V was investigated by Bača et al (2016). Since surface machining of SLM Ti6Al4V specimens improves the measured fatigue strengths approaching corresponding conventional (wrought and machined) materials, the fatigue behavior in the presence of as-built surface is especially important. This contribution presents the results of a systematic investigation of the fatigue behavior of SLM Ti6Al4V in dependence of four different post-fabrication heat treatments. Tensile and fatigue specimens were produced with a state-of-the-art SLM system using optimized process parameters. Microstructures before and after heat treatment were examined using metallographic methods. Since one of the major challenges of the SLM technology is the poor fatigue behavior of its high roughness surfaces, the present fatigue tests were mainly performed using as-built specimens, although the influence the machined surface was quantified in one case. 2. Experimental details 2.1. SLM fabrication Ti6Al4V-ELI powder (Extra Low Interstitials) was used for specimen fabrication using an EOS M290 system with a laser power of 400 W, a spot size of 100 µ m and a layer thickness 60 µm. The Ti6Al4V powder had a particle size ranging from 15 to 45 µ m. The building platform was heated to 80 ° C during the job and the fabrication process was in an Argon gas atmosphere. A large set of cylindrical tensile and fatigue specimens were produced with axis all oriented in the build direction (i.e. axis Z). The laser motion in the individual layer was programmed on the base of a contour and core strategy. 2.2. Post fabrication heat treatments The as- built microstructure of SLM Ti6Al4V consists of a fine acicular martensite called α’ phase , see Vranken et al (2012). The associated mechanical properties are high yield and ultimate strength, but a relatively low ductility.