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Additive manufacturing is an evolving technology for fabricating porous structures used in a broad array of applications, ranging from the aerospace industry to biomedical engineering. Porous titanium alloy (Ti6Al4V) structures play a major role in biomedical implants and are preferred over conventional solid implants because their properties can be tailored to obtain the stiffness required to avoid stress shielding and improve osteointegration. The mechanical properties of these structures are dependent on unit cell topology and overall porosity. In the present work, three open cellular configurations were studied, namely regular (square), irregular (skewed square) and fully random structures, at three different porosity levels. The samples were manufactured using the selective laser melting of spherical Ti6Al4V powder. The deviations of manufactured samples from as designed were assessed using morphological characterisations and porosity analyses. The mechanical characterisations of the samples included monotonic and cyclic tensile tests, along with conventional compression tests under monotonic and cyclic conditions. The results from the study indicate a clear deviation of thickness values from as-designed values. The effect of inclination of the strut with respect to the loading axis has been studied in compression samples. The off-axis loading in compression led to the asymmetry in the Young's modulus in compression and tension. These led to finite element modelling of structures in the elastic regime and its validation using Gibson–Ashby model for cellular structures.

Selective Laser Melting (SLM) is a technology that allows for the realization of metallic porous solids that cannot be produced with other methods. Nevertheless, a geometric discrepancy between the as-designed and as-built part is a well-known issue that can be critically important for biomedical metallic lattices with pore size and strut thicknesses of a few hundred microns. In practice, any geometric imperfection introduces a degree of uncertainty that can alter the mechanical properties of the as-built lattice. A quantitative relationship between the as-designed geometry and the real geometry is a very useful design tool because it makes possible to predict the final morphology of the lattice by considering the manufacturing errors: a more predictable geometry of the lattice structure means more predictable mechanical properties of the implant. In this work, we investigate the relationship between the as-designed strut thickness and the printed strut thickness for regular cubic cell lattices. The joints between the struts are filleted to reduce the stress concentration and increase fatigue resistance. The size of the unit cell is scaled to find the smallest radius which can be accurately reproduced. Moreover, we also studied the effect of the printing direction on geometrical accuracy

Additive manufacturing of industrially-relevant high-performance parts and products is today a reality, especially for metal additive manufacturing technologies. The design complexity that is now possible makes it particularly useful to improve product performance in a variety of applications. Metal additive manufacturing is especially well matured and is being used for production of end-use mission-critical parts. The next level of this development includes the use of intentionally designed porous metals - architected cellular or lattice structures. Cellular structures can be designed or tailored for specific mechanical or other performance characteristics and have numerous advantages due to their large surface area, low mass, regular repeated structure and open interconnected pore spaces. This is considered particularly useful for medical implants and for lightweight automotive and aerospace components, which are the main industry drivers at present. Architected cellular structures behave similar to open cell foams, which have found many other industrial applications to date, such as sandwich panels for impact absorption, radiators for thermal management, filters or catalyst materials, sound insulation, amongst others. The advantage of additively manufactured cellular structures is the precise control of the micro-architecture which becomes possible. The huge potential of these porous architected cellular materials manufactured by additive manufacturing is currently limited by concerns over their structural integrity. This is a valid concern, when considering the complexity of the manufacturing process, and the only recent maturation of metal additive manufacturing technologies. Many potential manufacturing errors can occur, which have so far resulted in a widely disparate set of results in the literature for these types of structures, with especially poor fatigue properties often found. These have improved over the years, matching the maturation and improvement of the metal additive manufacturing processes. As the causes of errors and effects of these on mechanical properties are now better understood, many of the underlying issues can be removed or mitigated. This makes additively manufactured cellular structures a highly valid option for disruptive new and improved industrial products. This review paper discusses the progress to date in the improvement of the fatigue performance of cellular structures manufactured by additive manufacturing, especially metal-based, providing insights and a glimpse to the future for fatigue-tolerant additively manufactured architected cellular materials.

Pitting is frequently encountered in machine elements, such as bearings, cams and gears, containing parts in relative sliding-rolling motion. Most of investigations on rolling contact fatigue have been carried out by means of numerical/analytical models in the frame of LFEM, but the interpretation of the numerical results is still controversial. The SIFs history is highly non-proportional and the intensity of the peaks is not expected to cause propagation, since they do not exceed the fatigue threshold usually found for long cracks in steels and cast irons. To shed light on this issue, this work is intended to reproduce experimentally the SIF-cycles, estimated by our previous model of RCF crack, by using an alternative non-standard procedure. The novel experimental apparatus is based on the exploitation of a kinked edge crack subjected to a compressive load distributed through the thickness. Changing the load position and intensity permits to induce different values and ratios of SIFs. It has been found that lubricant is strictly necessary to trigger crack propagation, which can be in form either of coplanar extension followed by branching or of branching only. Coplanar extension occurs when mode II SIF range exceeds a certain threshold, well below the typical fatigue threshold. Empirical evidences are in accordance with the tendency of the RCF cracks to do coplanar extension and to branch into the bulk material and toward the surface. Coplanar extension is more pronounced in the interior of the sample, while branching occurs only at the edges. The results indicate that our previous modelling of RCF crack is likely to underestimate the effective SIFs. Thus this work would open the way to improve the actual understanding and modelling of RCF.

Pitting is one of the causes of failure for mechanical components subjected to rolling contact fatigue. In the present article, a FE model is described in which a 2D half-space with an edge crack is affected by a travelling contact load produced by a cylindrical body. The contact load is not approximated by an analytical pressure distribution but the actual mating bodies are modelled. The presence of lubricant between the mating bodies and inside the crack is taken into account and its effect on the crack is simulated via hydrostatic elements. The lubricant is assumed to be entrapped into the crack by the external body when the latter covers the crack mouth, that is, the crack is sealed by the contact area and not by the contact between the crack faces (fluid entrapment mechanism). The pressure of the fluid is calculated via an iterative procedure by assuming that its volume stays constant inside the crack. Comparisons between this model and the alternative fluid pressurization mechanism have been made. The effects of the coplanar extension are investigated. The outcomes suggest that the fluid pressures inside the crack produced by the fluid entrapment mechanism tend to those of the fluid pressurization mechanisms as the crack becomes short.

Cellular materials are characterized by a complex interconnected structure of struts or plates and shells, which make up the cells edges and faces. Their structure can be advantageously engineered in order to tailor their properties according to the specific application. This aspect makes them particularly attractive for the manufacturing of bone prosthetics, where the elastic modulus of the implant should match that of the bone in order to avoid loosening due to the stress shielding phenomenon. In this regard, the ability to design a component with the desired mechanical response is crucial. For this reason, the present paper evaluates the stiffness of 2D cellular structures with variously arranged square cells. Specifically, two spatial arrangements are considered: the former one is a regular square cell honeycomb, while in the latter the square cells are staggered by a prescribed offset of half of the cell wall length. An analytical model based on classical beam theory is proposed to identify the effect of stretching and bending actions on the stiffness of a single cell by applying the periodic boundary conditions. The theoretical beam model is fitted on the results from a 2D Finite Elements model based on plane elements via an extensive parametric analysis. In this way, semi-analytical formulas are proposed to calculate the stiffness in large domains of the geometric parameters: wall thickness to edge length ratio in the interval [0.04,0.20] and fillet radius to edge length ratio in the interval [0,0.15].

Fully dense titanium alloy implants have long been used for the replacement and stabilization of damaged bone tissue. Nevertheless, they can cause stress shielding which brings to a loss of bone mass. Additive manufacturing (AM) allows obtaining highly porous cellular structures with a wide range of cell morphologies to tune the mechanical properties to match that of the patient’s bone. In this work, the fully reversed fatigue strength of cellular specimens produced by Selective Laser Melting (SLM) of Ti-6Al-4V alloy was measured. Their structures are determined by cubic cells packed in six different ways and their elastic modulus is roughly 3GPa to match that of trabecular bone. Part of the specimens was left as sintered and part treated by Hot Isostatic Pressing (HIP). The fatigue resistance of such AM parts can be affected by surface morphology, geometrical accuracy as well as internal defects. Micro X-ray computed tomography (CT) was used in this work to compare the geometry of the produced specimens with the CAD model and to carry out residual stress measurements using the Plasma FIB-SEM-DIC micro-hole drilling method.

Porous structures have great potential in the biomedical field because, compared to traditional fully dense implants, prostheses with a porous structure show reduced stress shielding and improved osseo-integration. Selective Laser Melting (SLM) made possible to obtain metallic cellular materials with highly complex structures characterized by a wide range of cell morphologies that allow to finely tune the mechanical properties of the implant. Nevertheless, there are still several issues to address: among others, detrimental residual stresses and the discrepancy between the as-designed and the manufactured geometry. Micro X-ray computed tomography (µCT) combined with the Finite Elements (FE) method permits to carry out in-depth investigations on the effect of the number and severity of defects on the mechanical properties. In the current study, the results of fatigue and quasi-static tests were compared with FE calculations based on the as-designed geometry and on the as-built geometry reconstructed from µCT scans. Both the elastic modulus and the fatigue resistance resulted strongly correlated with the number and severity of defects. Moreover, predictions of the mechanical properties based only on the as-designed geometry were shown not to be accurate. The importance of considering the limitations in accuracy of the manufacturing technique when designing load bearing lattice structures was highlighted.

Traditional implants made of bulk titanium are much stiffer than human bone and this mismatch can induce stress shielding. Although more complex to produce and with less predictable properties compared to bulk implants, implants with a highly porous structure can be produced to match the bone stiffness and at the same time favor bone ingrowth and regeneration. This paper presents the results of the mechanical and dimensional characterization of different regular cubic open-cell cellular structures produced by Selective Laser Melting (SLM) of Ti6Al4V alloy, all with the same nominal elastic modulus of 3 GPa that matches that of human trabecular bone. The main objective of this research was to determine which structure has the best fatigue resistance through fully reversed fatigue tests on cellular specimens. The quality of the manufacturing process and the discrepancy between the actual measured cell parameters and the nominal CAD values were assessed through an extensive metrological analysis. The results of the metrological assessment allowed us to discuss the effect of manufacturing defects (porosity, surface roughness and geometrical inaccuracies) on the mechanical properties. Half of the specimens was subjected to a stress relief thermal treatment while the other half to Hot Isostatic Pressing (HIP), and we compared the effect of the treatments on porosity and on the mechanical properties. Fatigue strength seems to be highly dependent on the surface irregularities and notches introduced during the manufacturing process. In fully reversed fatigue tests, the high performances of stretching dominated structures compared to bending dominated structures are not found. In fact, with thicker struts, such structures proved to be more resistant, even if bending actions were present.

Metallic lattice biomaterials can be very complex structures that are often impossible to be fabricated with other manufacturing technologies than additive manufacturing (AM). Residual stresses and geometric defects such as severe notches and distorted struts are inevitably introduced into the printed structures and these can affect the mechanical and biological properties. Micro X-ray Computed Tomography (µCT) has been proven to be a very powerful tool for accurately measuring the mismatch between the as-designed CAD model and the SLM structure. In this work, selective laser melting (SLM) Ti6Al4V lattices were measured using a metrological µCT system to identify and classify the geometrical distortions introduced by the printing process. The µCT measurements have also been used to build Finite Element (FE) models based on beam elements that make possible a quantification of the effect of these defects on the elastic modulus of the lattice by comparison with FE models based on the ideal geometry. Moreover, solid FE models of the junctions between the struts have been built by importing the CT data in Ansys® to calculate the stress concentrations caused by the severe notches.