The role of node fillet, unit-cell size and strut orientation on the fatigue strength of Ti-6Al-4V lattice materials additively manufactured via laser powder bed fusion
Introduction
Lattice materials typically feature a periodic porous architecture, which differs from that of traditional materials. Their properties are not only governed by the chemical composition of the constituent solid, but also by the topology and geometry of the unit cell [1], [2]. Low density, superior thermal insulation, high impact response and other properties have contributed to make cellular materials attractive to the aerospace, biomedical, automotive and naval industry [2], [3], [4], [5], [6]. For instance, stress-shielding, one cause of orthopaedic implant failure, is mitigated if a low modulus lattice with tailored porosity gradients is integrated at the interface with the bone tissue [7], [8], [9], [10]. These and other high-end applications require the attainment of a highly controlled architecture called to satisfy often strict and diverse design requirements. On this front, additive manufacturing (AM), a layer-by-layer process, has a clear advantage over traditional fabrication technologies [11], [12]. Powder bed fusion (PBF) is a subset of AM whereby a heat source is used to consolidate material in powder form to create three-dimensional (3D) objects. When the heat source is a laser, the process is named Laser-based Powder Bed Fusion (L-PBF) [13], [14].
Despite L-PBF has been so far successfully used to produce complex lattice architectures [12], [15], the properties of the printed parts can deviate significantly from those predicted assuming an ideal geometry and homogeneous properties of the base material [16]. Indeed, the manufacturing process markedly affects the microstructure of the base 3D printed material and as well as the lattice architecture with an outcome that impacts mechanical and biological performance [17], [18]. The quality of as-built L-PBF parts is typically influenced by high cooling rates and preferential grain growth direction caused by directional heat flow, which leads to the formation of low ductility metastable phases and anisotropicity, respectively [19], [20], [21], [22]. Fast cooling is also responsible for the occurrence of residual stresses that, besides altering the mechanical properties, can also cause distortions in the lattice [18], [23]. Pores can be often found in L-PBF parts, which can be spherical or irregularly shaped [24].
The as-built architecture of a lattice material, on the other hand, can feature several types of geometrical defects and irregularities [5], [25], [26], [27], [28], [29], [30]. Partially unmelted particles increase the surface roughness [17]. On a larger scale, the size of the melt pool significantly affects the geometry of the lattice, e.g. strut thickness, strut straightness, junction alignment and junction shape [23], [29], [31]. The melt pool extent is determined by the input energy of the scanning laser and by the local heat transfer properties of the solid/powder system [27], [32], [33]. While the input energy, expressed by the specific energy, depends on the machine parameters [24], the local thermal properties can be ascribed mainly to the spatial orientation of the solidified material, the solid/powder fraction and the packing density of the powder [27], [34]. Being the powder less conductive than solid, a larger melt pool is prompted to form in regions supported prevalently or entirely by powder [27], [28], [34], leading to an accumulation of the solid material on the overhanging parts of the lattice [35], [36]. Therefore, increasing the inclination angle of the struts to the build plane leads to a gradual decrease of the deviation from the nominal shape because the fraction of the melt pool supported by solid material grows [29], [37]. On the other hand, inclined struts can be affected by the staircase effect, which is the outcome of the consecutive stacking of discrete layers welded together with a small offset due to inclination [34], [38]. The complexity of the as-built/as-designed deviation is also amplified by solidification and cooling shrinkage [39].
In general, the mechanical performance of a lattice material is negatively affected by manufacturing defects, although the extent of such impact is highly variable. For instance, struts aligned with the direction of the applied load carry a higher fraction of the load and hence defects appearing on these struts have a major impact on the mechanical properties [17]. On the other hand, lattice materials suffer from an ample scatter in mechanical performance [40], [41].
The effect of manufacturing defects on the quasi-static mechanical properties (elastic moduli and strength) has been addressed by several studies [23], [30], [32], [42], [43]. A decrease of struts straightness (waviness) and node displacement causes a loss of stiffness and strength due to the rise of bending actions, particularly in stretching dominated lattices. Any deviations of the cross-section shape and size from the nominal geometry alter the load bearing area, e.g. the second moment of area, of a strut, possibly increasing or decreasing the modulus. Although to a lesser degree compared to the other morphological defects, internal porosity and surface roughness decrease the elastic modulus [40]. Conversely, these factors have a stronger effect on the yield stress and the ultimate tensile strength because they act as stress raisers, inducing premature plasticization and subsequent rupture [17], [40]. The relationship between build orientation of the struts and the manufacturing defects translates to a measurable effect on the mechanical properties of L-PBF lattices: the smaller the angle between the strut and the build plane, the lower is the strength [21]. A similar behavior was observed also in single struts [19].
In contrast to the elastic modulus and other monotonic properties, fatigue, a highly localized phenomenon, is very sensitive to the microstructural and morphological quality of a component, and thus it is extremely sensitive to the manufacturing process [44]. For example, the low fatigue strength of as-built L-PBF parts compared to machined or wrought parts is partly determined by the microstructure (less ductile), but even more by surface roughness [45], [46]. Similarly, the fatigue behavior of L-PBF lattices is negatively impacted by surface defects, such as semi-molten powder particles, as well as by irregularities in the strut cross-sections that act as notches [23], [47], [48], [49]. Furthermore, any geometric deviation that alters the stress distribution in the strut, such as geometric defects at the junction, acts as stress raiser [50], [51]. These imperfections have a detrimental impact on the fatigue properties. Fatigue cracks typically nucleate on the surface in proximity of defects and much less within the bulk of the solid, where internal cavities in L-PBF parts often appear [49], [52]. Ti-alloys, such as Ti-6Al-4 V, are especially sensitive to stress raisers due their high notch sensitivity [53], [54]. Besides stress raiser, residual stress also contributes to the fatigue behavior [46], [47], [55].
This work examines the fatigue response of Ti-6Al-4 V regular cubic lattices 3D printed via L-PBF. In particular, the focus is on the fully-reversed fatigue S-N curves with the goal of investigating the role of the lattice orientation to the printing direction, junction geometry and unit cell size. We use a combination of optical microscopy, electron microscopy and fatigue testing to examine four batches of 3D printed specimens each with selected inclination (0° versus 90°) to the build plane, junction geometry (wide fillet versus sharp fillet), and unit cell sizes (3 mm and 4 mm).
Section snippets
Specimen design and description
The unit cell examined in this work has a regular cubic topology of strut length L and strut diameter t0. At the junctions, the struts are joined by circular arc fillets of in-plane radius R (Fig. 1a). The rationale behind the choice of the cubic unit cell is its simplicity which translates into an easy-to-control relationship between the printing direction and the loading direction of the struts and the possibility of obtaining well-defined fillet radii at the strut junctions. The results here
Conclusions
This work has investigated the fatigue response of Ti-6Al-4 V regular cubic lattices 3D printed via L-PBF at given printing directions and for prescribed morphological characteristics of the samples. In particular, the focus has been on the role of the width of the fillet radius at the strut junctions and the orientation of the struts to the printing direction in determining the S-N curves and the fatigue failure behavior. It was observed that the fatigue resistance of the lattice was markedly
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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