Metallurgical and Mechanical Properties of Additively Manufactured Cellular Structures

Naturally occurring cellular materials are always optimized in terms of morphology, structural resistance, and functionality. The use of cellular materials is based on the application as well as the loading condition. Cellular materials are composed of an interconnected network of struts, plates, or repeating unit cells, forming edges or faces. The properties of these structures can be tailored according to the requirements by changing one or more of the parameters mentioned above. This makes cellular materials suitable for various applications ranging from aerospace to biomedical. In biomedical applications, these cellular materials can be used to manufacture porous implants to match the properties of the surrounding bone. They can also be used as coatings on solid implants to promote bone tissue ingrowth for better implant fixation. The production of these complex, porous implants using traditional manufacturing methods is a difficult task. However, the development of additive manufacturing processes such as Laser Powder Bed Fusion (LPBF) has made it possible to manufacture complex and intricate shaped cellular materials with minimum material wastage and considerable accuracy. Therefore, with the combination of the LPBF process and cellular materials design, it is possible to produce a wide range of cell topologies with customized mechanical properties depending on the implant location, material, and the needs of the patient. Titanium and its alloys such as Ti6Al4V have been used in biomedical applications due to their high strength to weight ratio, corrosion resistance, and good biocompatibility. Also, the LPBF process has been used to produce various Ti6Al4V components for various applications including cellular materials. The development of cellular materials for implants is dependent on the relative density, response of the unit cell to loading conditions, and the optimal pore size for bone ingrowth. Studies have been carried out to understand the behavior of the cellular materials under compressive loads since most of the implants experience compression loads during their operation. Nevertheless, the implants also undergo fatigue loading due to day-to-day activities and tensile loads when the implant is loose or when the host performs an extensive physical activity. Therefore, designing and studying the cellular materials for these loads is necessary to completely understand their behavior. Considering the pore size, studies have suggested that a pore size of ~ 800 µm is suitable to induce bone ingrowth after implantation. The cellular materials can be broadly classified into stretching and bending dominated. Stretching dominated cellular materials are characterized by high strength and stiffness while bending dominated structures are high compliant. This behavior of cellular materials is dependent solely on the unit cell topology. Therefore, the development of different types of cell topologies and their characterization is required to produce optimized fully porous implants. Also, the effect of the LPBF process on the designed parameters of the unit cell alters the obtained mechanical properties from the desired values. The present work aims at developing different Ti6Al4V cellular materials that can be potentially used for application in implants. A combination of different cellular materials can be used to develop completely porous implants or single cellular materials can be used as coatings for solid implants to induce osseointegration. A major portion of the work is focused on the mechanical properties of LPBF manufactured cellular materials characterized using static and fatigue tests. The study also investigates the discrepancy between the as-designed and as-built geometrical parameters of these cellular structures. Finite elements analysis and the Gibson-Ashby modeling has been employed to understand the difference between the as-designed and as-built properties. Another part of the study was focused on the effect of designed geometrical parameters on the as-built geometry of cellular materials. The aim was to develop a relationship between the as-designed and the as-built parameters. This thesis covers all the aspects mentioned in the above paragraph in detail. The research work has been provided in three different chapters (Chapter 2, 3, and 4) which are well connected to each other. Each chapter is composed of an abstract, introduction, materials and methodology, results and discussion, and conclusion. A conclusion on the complete research and the future scope is provided at the end. The first chapter introduces all the aspects concerned with the development of cellular materials for biomedical applications. Literature review on all aspects have been provided, ranging from the properties of the bone, cellular materials, manufacturing process for cellular materials, and the properties of bulk materials suitable for biomedical applications. In chapter 2, Ti6Al4V cellular materials with three different cell topologies namely cubic regular, cubic irregular, and trabecular have been investigated. The irregular specimens are obtained by skewing the junctions of the cubic regular configuration. Trabecular specimens are designed by randomly joining 4-6 struts at a node to mimic human trabecular bone. The three cell topologies were manufactured at three different porosity levels by changing their strut thickness and pore size. The cubic regular cells are considered due to their stable and simple configuration, while irregular and trabecular based specimens have shown promising results in the osseointegration according to the partner company. However, the mechanical properties of irregular and trabecular specimens play an important role in implant design. Therefore, all the specimens were subjected to compression test and as well as a novel tensile test under two different types of loading conditions, monotonic and cyclic to obtain their strength and stiffness. However, a misalignment in the struts with the loading direction in compression led to an asymmetric behavior between tensile and compression. Higher strength and stiffness values were observed under tensile loading, the results of which were in conjunction with the theoretical prediction from the Gibson-Ashby model. The experimental results indicated the irregularity tends to reduce the strength, stiffness and induce bending dominate behavior. Morphological analysis was carried out to obtain the discrepancy between the as-designed and the as-built thickness values. This led to the FE analysis of as-designed models to obtain the difference in the properties of as-designed and as-built cellular materials. Furthermore, as-built FE models were generated using morphological data to study the effect of strut defects and compare them with the experimental results. The next step involved comparing the experimental results with the FE analysis carried out tomography-based FE models. The last part of the study involved obtaining a relation between the as-designed and as-built Young’s modulus for cubic regular, cubic irregular, and trabecular specimens to create a reference database. The mechanical properties from the compression and tensile test of the highest porosity specimens were closer to the properties of human bone. The tensile tests were successful in predicting the mechanical properties accurately. These observations were the motivation to further study the effect of irregularity on various cell topologies, by subjecting them to static and fatigue loads. In chapter 3, seven different types of unit cells, three regular configurations, three irregular configurations, and one trabecular based unit cell. The unit cells used in the study consisted of regular and irregular configurations of the cubic-based, star-based, and cross-based specimens. These specimens were selected to have a comparison of properties from stretching dominate cubic specimen to bending dominated cross-shaped specimens and to study the effect of irregularity. Therefore, the specimens were subjected to and mechanical characterization using compression, tension, and compression – compression fatigue tests along with porosity and morphological analysis. The tensile specimens in this chapter were designed with a thicker transition at the ends, while compression specimens had uniform configuration throughout the specimen. FE analysis was carried on the as-designed configuration of these specimens to study the effect of transition and to compare the as-designed and tensile experimental results to understand the effect of decreased porosity on the failure mechanisms. Fatigue tests were carried under compression-compression load and failure mechanisms in different unit cells were captured. The results of the study indicated that the irregularity has a greater effect on the strength and stiffness of stretching-dominated cellular material and has a negligible effect on bending-dominated cross-based specimens. The trabecular specimens display excellent mechanical properties under static load with good strength, stiffness and sustain high strain values. The normalized S-N curves indicate a clear demarcation between the bending and stretching-dominated cellular materials. The FE analysis showed a similar failure location as compared to the experimental results despite the decrease in the porosity due to manufacturing. The morphological analysis showed the effect of strut orientation of the as-built thickness. The morphological analysis and the difference between the as-designed and as-built geometrical parameters show that an in-depth study on the geometrical deviation due to the LPBF process is necessary. The next chapter focuses on the geometrical deviation in LPBF manufactured cellular specimens and the parameters influencing this deviation. In chapter 4, cubic regular cellular materials with filleted junctions are studied for geometrical deviation and to obtain a relationship between the as-designed and as-built geometric parameters. Initially, nine different specimens with different strut thickness, fillet radius, and unit cell size were manufactured at three different orientations with respect to the printing plane. The main aim of this study was to devise a compensation strategy to reduce the geometrical deviation due to the LPBF process. A linear relation between the as-designed and as-built geometrical values is obtained, which is used for compensation modeling. Struts perpendicular to the building plane were uniform in cross-sections while horizontal and inclined struts had an elliptical cross-section. The internal porosity analysis has been carried out which indicates that the porosity at the junctions is lesser than the porosity at the junctions. The compensation strategy worked well for the second set of specimens produced using the same parameters, thereby reducing the geometrical deviation between the as-designed and the as-built parameters. Finally, the effect of filleted junctions, building directions, and compensation modeling on fatigue properties have been studied. Specimens with load-bearing struts printed parallel to the building plane had the lowest mechanical properties, while the specimens with struts inclined to the loading direction and building plane displayed excellent static and fatigue properties. The fillets at the junctions improve the fatigue resistance of the specimen by reducing the stress concentration. The printing direction and the presence of fillets influence the fatigue failure locations as well. Therefore, filleted junctions that can be reproduced well by the LPBF process can be used to reduce the stress concentration in cellular materials.