PSI - Issue 28

S. Raghavendra et al. / Procedia Structural Integrity 28 (2020) 517–524 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The recent advance in the manufacturing has made it feasible to produce complex geometries with great precision. Additive manufacturing (AM) techniques which involves layer-wise fabrication of materials to build complex structures has gained importance. AM process such as laser-based powder bed fusion (LPBF) and electron beam melting (EBM) are extensively used in the manufacture of metallic parts. Studies indicated that LPBF process has been able to produce geometries with greater accuracy with makes it suitable in manufacturing of cellular structures with strut thickness values down to 200-300 microns. Cellular structures are being studied extensively for application in aerospace, automobile, and bio-medical applications. The properties of these structures can be easily tailored depending on the loading conditions and required stiffness values. Implants made of titanium cellular structures are considered over the conventional solid implants. Titanium alloys have a high corrosion resistance and specific strength which make them suitable for bio-medical applications (Mark Long & Rack, 1998). Cellular structures are characterized by their unit cell size, strut thickness and porosity level: these parameters can be adjusted to match the properties of the surrounding bone in order to reduce the stress shielding phenomenon observed in dense and stiff implants. Also, development of bone tissue inside the lattice pores (bone in-growth) can be achieved, resulting in better long-term performances of implants. In the recent years, various studies have been carried on cellular structures using different unit cells such as diamond, cubic, BCC, gyroid (Ahmadi et al., 2015; Mahmoud & Elbestawi, 2017; Zaharin et al., 2018). Mechanical properties of these structure have been characterized mainly by compression testing (Lei et al., 2019; Zhang et al., 2016) and fatigue testing (Amin Yavari et al., 2015). The studies include the effect of cell types, relative density, graded porosity: the results indicate that the deformation mechanism can be bending dominated or stretching dominated according to the cell geometry. Increasing the relative density increases the stiffness and strength of the materials following the Gibson-Ashby power law in most cases (Lorna J. Gibson, 1997; Murr et al., 2010). Other studies also include the effect of heat treatment and LPBF process parameters on the mechanical performance of cellular structures. Heat treatment induces ductility in Ti6Al4V alloys but at the expense of strength (Dallago et al., 2018; Ghouse et al., 2017; Yuan et al., 2018). Process parameters play a major role in the formation of defects such as geometric variations, surface roughness and internal porosity which influence the static and fatigue properties of cellular structures (Kasperovich et al., 2016). The focus and novelty of this study is the comparison of the mechanical behavior of three regular lattice structures with that of a bone mimicking trabecular structure. The three investigated regular lattice structures have respectively a cubic, a star-shaped, and a X-shaped repeating unit cell, while the fourth has a random cellular structure. The cubic and star-shaped structures exhibit stretching dominated behavior due to the presence of vertical struts in the loading direction. The X-shaped and trabecular structures exhibit a bending dominated behavior. The samples were subjected to static testing to assess strength and stiffness in compression and tensile loading conditions. Compression-compression fatigue test was carried out to understand the effect of cell topology on the fatigue properties. 2. Materials and methods This section provides a description of the sample manufacturing, design and the experimental procedure followed. 2.1. Specimen design The samples were manufactured at the Lincotek Medical facility, Italy, by means of LPBF using a EOS machine equipped with a 400W laser and starting from a spherical biomedical grade Ti6Al4V powder with a particle size in the range 15–45μm. A layer thickness of 60μm was deposited at each step. After fabrication, all specimens were subjected to a heat treatment under proprietary conditions, in order to relieve the residual stresses generated during the LPBF process and to transform the martensitic as-built microstructure into a stable alfa/beta one.