effect of the geometrical defectiveness on the mechanical properties of slm biomedical ti6al4v lattices

Abstract 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

Design and testing of additively manufactured lattice structures for musculoskeletal applications

Additive manufacturing (AM) methods present a new frontier in engineering, allowing the fabrication of porous lattice structures with tailored mechanical properties. AM structures can be made using bio-inert metals, creating controlled stiffness biomaterials. As bone formation is strain dependent, these AM biomaterials can be used in implants to optimise the strain in surrounding trabecular bone for peak bone formation. However, the behaviour of AM lattices varies and is subject to manufacturing constraints. The aim of this PhD was to investigate the mechanical behaviour of AM lattices, and maximise the clinical benefits of AM for musculoskeletal applications. Lattice architecture was shown to affect the anisotropy of an AM lattice biomaterial, increasing the stiffness in directions not often tested in the literature. The mechanical and morphological properties of individual struts within powder bed fusion (PBF) lattices were also shown to vary depending on the orientation of the struts to the build direction. The ultimate tensile strength of titanium alloy (Ti6Al4V) struts more than doubled when built at a low angle versus perpendicular to the build platform, and other properties were substantially lower than for the bulk material. Geometric imperfections were found for struts built at low angles. As such, a low stiffness modified stochastic lattice was designed and tested which avoided the problems found with struts built at low angles. The resulting lattice had improved stiffness isotropy and could be used for musculoskeletal applications, tuned to match the mechanical properties in local trabecular bone and enhancing bone formation.Open Acces

Joining of Dissimilar Materials

Material manufacturers and engineering structure designers are currently focusing new ways to exploit the benefits of light-weight, hybrid materials with improved properties at a low cost. The ability to join dissimilar materials is enabling the design engineers to develop light-weight and efficient automobiles, aircraft and space vehicles. The objective of this PhD research study was to produce alternative and efficient joining solutions for automotive and aerospace applications. The joining of dissimilar material was experimented to obtain light-weight Fibre Reinforced Polymer (FRP) sandwich composites, Al-foam sandwich (AFS) composites, hybrid dynamic FRP epoxy/polyurethane composites and the joining of Ti6Al4V alloy with and without surface modification to Ceramic Matrix Composite (CMC) and itself. The joining of Al-foam and Al-honeycomb to FRP skins was performed. The experimental results show that higher flexural properties can be achieved by replacing Al-honeycomb with low-cost Al-foam as a core material in the sandwich structures. Compared to FRP-honeycomb sandwich panels, FRP-Al foam sandwich panels display ~25 % and ~65 % higher flexural strength in a long and short span three-point bending tests respectively. AFS composites with complete metallic character, to withstand high-temperature application conditions, were produced by soldering/brazing techniques using Zn-based and Al-based joining alloys. A post-brazing thermal treatment was designed to recover the mechanical properties of AFS composites, lost during the soldering/brazing process. The microstructural analysis of the Al-skin/Al-foam interface revealed that the diffusion of joining materials into the joining substrates (Al-sheet and Al-foam) was achieved. Around 80% higher bending load before failure was observed when the AFS specimens produced with Zn-based joining alloys were subjected to flexural load compared to those produced with Al-based joining alloys. Hybrid dynamic Carbon Fibre Reinforced Polymer (CFRP) composites with enhanced impact properties were produced by exploiting the reversible cross-linking functionalities of dynamic epoxy and dynamic PU resin systems. By joining dynamic CFRP-epoxy and dynamic CFR-PU laminates, hybrid dynamic composite in three different configurations and a non-hybrid composite were obtained. The four dynamic composites were characterised for structural, thermal, flexural and impact properties. The damage initiation upon impact was observed at around 95% higher energy level in the hybrid configuration (CFRP-4), compared to the non-hybrid configuration. The hybrid configuration CFRP-3 responded with around 55% higher perforation threshold energy compared the non-hybrid configuration. Preliminary work on Adhesive joining of the Ti6Al4V alloy to itself was performed to analyse the effect micro-machining on adhesion and the effect of shape/design of micro-slots on an adhesive joint strength. Three types of micro-slots: V, semi-circle and U-shaped micro-slots were produced on Ti6Al4V sheet surface by using an in-house developed Micro-Electro-Discharge Machining (Micro-EDM) setup. Ti6Al4V alloy specimens with and without micro-machined surfaces were bonded together using a commercial epoxy adhesive. The Single Lap Offset (SLO) shear test results revealed that the micro-slot oriented perpendicular to the applied load displayed ~23 % higher joining strength compared to when the micro-slots were oriented parallel to the applied load. U-shaped micro-slots configuration displayed ~30 % improvement in the joint shear strength compared to the specimens with un-modified surfaces. The fractured surfaces analysis revealed mix (adhesive-cohesive) with cohesive dominated failure in bonded specimens with micro-machined surfaces compared to the as-received where pure adhesive failure was observed. The joining of CMCs (C/SiC and SiC/SiC) to Ti6Al4V alloy was experimented using active brazing alloy (Cusil-ABA) and Zr-based brazing alloy (TiB590) in a pressure-less argon atmosphere. The CMC-Ti6Al4V joint strength was further improved by modifying the surface of Ti6Al4V alloy using an in-house built Micro-EDM setup. Around 40% higher joining strength was recorded when the Zr-based brazing alloy was used as a joining material compared to the conventional active brazing alloy, Cusil-ABA. Improvement in the joining strength was noticed when the Ti6Al4V surface was modified prior to joining

Fatigue behaviour of PBF additive manufactured TI6AL4V alloy after shot and laser peening

Article number 106536Additive manufacturing (AM) of metallic parts is a relatively new manufacturing procedure. Many industry sectors, such as the aerospace or automotive sectors, have started to apply this technology to produce some elements, thus reducing costs and weight. Several metallic alloys have been employed for AM. Due to the high strength-to-density ratio, Ti6Al4V alloy is probably the alloy most used for AM in the aerospace industry. This alloy usually shows good static strength properties. However, the presence of internal defects and the surface roughness result in a fatigue strength that is clearly lower than that of materials produced by traditional pro cesses. Moreover, the scatter of the fatigue results is generally higher than in the case of wrought pieces. Different treatments have been proposed to improve the fatigue behavior by reducing internal defects and roughness or generating a favorable residual stress field. In this work, selected surface treatments were considered to improve the fatigue strength of AM parts, including shot and laser peening as well as a combination of shot peening plus chemical assisted surface enhancement (CASE®). Three groups of specimens, each with one of the surface treatments, were fatigue tested to compare the results produced by these treatments. The residual stresses, roughness and hardness produced by the treatments were analyzed. After testing, the fracture surfaces were also analyzed to better understand the fatigue process of the different groups of specimens. The results indicate that laser peening produced the best results, followed by shot peening plus CASE and shot peening. In all three cases, the fatigue strength was much higher than that of the reference group without surface treatment. It was also observed that all failures initiated from an interior defect in the shot peening plus CASE group, four out of six failures in the laser peened group, but only one failure in the case of shot peened group and none in the reference group. Failures of specimens with initiation from internal defects started from defects located deeper than the compressive residual stress layer produced by the treatments

Fatigue behaviour of PBF additive manufactured TI6AL4V alloy after shot and laser peening

Additive manufacturing (AM) of metallic parts is a relatively new manufacturing procedure. Many industry sectors, such as the aerospace or automotive sectors, have started to apply this technology to produce some elements, thus reducing costs and weight. Several metallic alloys have been employed for AM. Due to the high strength-to-density ratio, Ti6Al4V alloy is probably the alloy most used for AM in the aerospace industry. This alloy usually shows good static strength properties. However, the presence of internal defects and the surface roughness result in a fatigue strength that is clearly lower than that of materials produced by traditional processes. Moreover, the scatter of the fatigue results is generally higher than in the case of wrought pieces. Different treatments have been proposed to improve the fatigue behavior by reducing internal defects and roughness or generating a favorable residual stress field. In this work, selected surface treatments were considered to improve the fatigue strength of AM parts, including shot and laser peening as well as a combination of shot peening plus chemical assisted surface enhancement (CASE®). Three groups of specimens, each with one of the surface treatments, were fatigue tested to compare the results produced by these treatments. The residual stresses, roughness and hardness produced by the treatments were analyzed. After testing, the fracture surfaces were also analyzed to better understand the fatigue process of the different groups of specimens. The results indicate that laser peening produced the best results, followed by shot peening plus CASE and shot peening. In all three cases, the fatigue strength was much higher than that of the reference group without surface treatment. It was also observed that all failures initiated from an interior defect in the shot peening plus CASE group, four out of six failures in the laser peened group, but only one failure in the case of shot peened group and none in the reference group. Failures of specimens with initiation from internal defects started from defects located deeper than the compressive residual stress layer produced by the treatments

Energy absorption in lattice structures in dynamics: Nonlinear FE simulations

An experimental study of the stress–strain behaviour of titanium alloy (Ti6Al4V) lattice structures across a range of loading rates has been reported in a previous paper [1]. The present work develops simple numerical models of re-entrant and diamond lattice structures, for the first time, to accurately reproduce quasi-static and Hopkinson Pressure Bar (HPB) test results presented in the previous paper. Following the development of lattice models using implicit and explicit non-linear finite element (FE) codes, the numerical models are first validated against the experimental results and then utilised to explore further the phenomena associated with impact, the failure modes and strain-rate sensitivity of these materials. We have found that experimental results can be captured with good accuracy by using relatively simple numerical models with beam elements. Numerical HPB simulations demonstrate that intrinsic strain rate dependence of Ti6Al4V is not sufficient to explain the emergent rate dependence of the re-entrant cube samples. There is also evidence that, whilst re-entrant cube specimens made up of multiple layers of unit cells are load rate sensitive, the mechanical properties of individual lattice structure cell layers are relatively insensitive to load rate. These results imply that a rate-independent load-deflection model of the unit cell layers could be used in a simple multi degree of freedom (MDoF) model to represent the impact behaviour of a multi-layer specimen and capture the microscopic rate dependence

Biomanufacturing Technologies for Tissue Engineering

Il seguente lavoro di tesi ha come obiettivo lo studio e la realizzazione di device biomedicali realizzati tramite la manifattura additiva. La manifattura additiva sta avendo una forte crescita negli ultimi anni grazie soprattutto alla possibilità di realizzare facilmente geometrie complesse. Questa caratteristica permette di personalizzare i prodotti ad un costo competitivo. Inoltre, lo spreco di materiale viene ridotto moltissimo dal principio di fabbricazione. Tutte queste proprietà hanno fatto in modo che negli ultimi anni la manifattura additiva prendesse sempre più piede in campi come l’automotive, l’aerospace e il biomedicale. Questo lavoro di tesi è focalizzato sull’utilizzo di alcune tra le più diffuse tecnologie additive per la produzione di device biomedicali. In particolare, il lavoro si è concentrato principalmente sulla realizzazione di due modelli, il primo per lo studio dello sviluppo dei black floaters all’interno del corpo vitreo dell’occhio, il secondo per l’emulazione del comportamento dell’osso mandibolare durante la foratura per l’installazione di impianti dentali. Il modello dell’occhio è composto da due elementi principali, un supporto e un hydrogel. Il supporto serve a contenere e supportare l’hydrogel. Deve essere trasparente, biocompatibile facilmente manovrabile in laboratorio. La sua realizzazione è avvenuta tramite stereolitografia. L’hydrogel, invece, ha lo scopo di fornire un’ambiente 3D per la crescita e sviluppo delle cellule. Deve perciò anche lui essere biocompatibile e con adeguate caratteristiche meccaniche e di stampabilità. La struttura 3D è stata realizzata tramite material extrusion. Il modello di osso mandibolare è stato realizzato tramite fused filament fabrication. Il modello si compone di due parti, una parte esterna piena per emulare l’osso corticale, e una parte interna porosa per emulare l’osso trabecolare. Le prove di foratura sono state realizzate con un trapano dentistico agganciato a robot collaborativi. La ricerca ha infine toccato ulteriori due ambiti, lo studio delle proprietà di strutture lattice realizzate tramite laser based- powder bed fusion e la valutazione di diversi trattamenti di finitura superficiale. La tesi, dunque, ha la seguente organizzazione. Il capitolo 1 presenta un’introduzione sull’additive manufacturing e il bioprinting. Le tecnologie ed i materiali utilizzati sono brevemente descritti e sono riportati alcuni esempi di applicazione della manifattura additiva nel campo biomedicale. I capitoli seguenti, invece, riportano gli articoli pubblicati o in corso di pubblicazione riguardo alle diverse tematiche affrontate. Nello specifico, il capitolo 2 riporta la ricerca sulle strutture lattice e la loro realizzazione. I capitoli 3 e 4 comprendono gli studi relativi al modello dell’occhio. Il capitolo 3 si concentra sulla realizzazione del supporto, il 4 sulla formulazione e la valutazione dell’hydrogel. Il capitolo 5 approfondisce lo studio del modello per l’emulazione del comportamento dell’osso mandibolare a foratura mentre il capitolo 6, l’ultimo di questo elaborato, si concentra sui processi di finitura superficiale. Per concludere, la manifattura additiva include processi molto diversi tra loro, ma che presentano molti punti in comune come la flessibilità, libertà di progettazione e personalizzazione. Sfruttando queste proprietà è possibile realizzare oggetti su misura, soprattutto in campi come quello biomedicale dove la personalizzazione e la specificità sono fondamentali.The following thesis aims to study and to develop biomedical devices made through additive manufacturing. Additive manufacturing has been experiencing a strong growth in recent years, mainly due to its ability to easily realize complex geometries. This feature allows customization of products at a competitive cost. In addition, material waste is greatly reduced by the manufacturing principle. All these properties helped the recent years diffusion of additive manufacturing in fields such as automotive, aerospace and biomedical. This thesis focuses on the use of some of the most popular additive technologies for the production of biomedical devices. In particular, the work focused mainly on the fabrication of two models, the first to study the development of black floaters within the vitreous body of the eye, and the second to emulate the mandibular bone behavior during drilling for the installation of dental implants. The eye model consists of two main elements, a scaffold and a hydrogel. The scaffold contains and provides support to the hydrogel. It must be transparent, biocompatible easily handled in the laboratory. It is printed by stereolithography. The hydrogel, on the other hand, is intended to provide a 3D environment for cell growth and development. Therefore, it must be biocompatible and have adequate mechanical properties together with good printability. The 3D scaffold structure was made by material extrusion. The mandibular bone model was made by fused filament fabrication. The model consists of two parts, a solid outer part to emulate cortical bone, and a porous inner part to emulate trabecular bone. Drilling tests were performed with a dental drill attached to collaborative robots. Finally, the research covered two additional areas, the study of the properties of lattice structures made by laser-based- powder bed fusion and the evaluation of different surface finish treatments. The following thesis, therefore, has the following organization. Chapter 1 presents an introduction on additive manufacturing and bioprinting. The technologies and materials used are briefly described, and examples of additive manufacturing applications in the biomedical field are given. The following chapters, on the other hand, report published or forthcoming articles regarding the various topics mentioned above. Specifically, Chapter 2 reports the research on lattice structures and their fabrication. Chapters 3 and 4 include studies related to the eye model. Chapter 3 focuses on the fabrication of the support, and Chapter 4 on the formulation and evaluation of the hydrogel. Chapter 5 presents the study of the model for emulating the behavior of mandibular bone upon drilling, while Chapter 6, the last of this work, focuses on surface finishing processes. In conclusion, additive manufacturing includes various processes that are very different from each other but have many common points such as flexibility, freedom of design, and customization. By exploiting these properties, it is possible to make tailored objects, especially important in fields such as the biomedical one, where customization and specificity are a great added value

Development of a qualification procedure, and quality assurance and quality control concepts and procedures for repairing and reproducing parts with additive manufacturing in MRO processes

This MSc by Research is focused mainly on Quality Assurance (QA) and Qualification Procedures for metal parts manufactured using new Additive Manufacturing (AM) techniques in the aerospace industry. The main aim is to understand the state of the art of these technologies and the strong regulatory framework of this industry in order to develop correct QA/QC procedures in accordance with the certification process for the technology and spare parts. These include all the testing and validation necessary to implement them in the field, as well as to maintain their capability throughout their lifecycle, specific procedures to manufacture or repair parts, workflows and records amongst others. At the end of this MSc by Research, an entire Qualification Procedure for Electron Beam Melting (EBM) and Selective Laser Melting (SLM) for reproduction of an aerospace part will be developed and defined. Also, General Procedures, Operational Instructions, and Control Procedures with its respective registers, activities, and performance indicators for both technologies will be developed. These will be part of the future Quality Assurance and Quality Management systems of those aerospace companies that implement EBM or SLM in their supply chain

Enhancing Fracture Toughness of Ultrahigh Strength Aerospace Components made by Additive Manufacturing

Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) are an Additive Manufacturing (AM) technique that produces complex three- dimensional parts by adding layer upon layer of powder materials from bottom to top. Recently, AM has received a significant amount of press and is set to have an enormous impact such as decreasing the cost of production, fast and flexible, design freedom and increase the innovation opportunities. The powder base nature allows these techniques to process a variant of materials as well as produce complex composite parts and develop new materials system for Aerospace industries. The biggest problems in the process are limited surface quality and residual porosity in SLM and DMLS parts that are undesirable for some applications where fatigue resistance and high strength are essential. This research aims to improve the fracture toughness, ductility and fatigue of the metallic components, which is essential to be able to exploit the potential of the SLM and DMLS of these alloys for aerospace applications. In an additional development of the AM technology is not only limited to new machines but also processes, new materials, and methods, as it offers high mechanical properties and performance. This research focuses on DMLS and SLM of titanium and stainless steel alloys to investigate the effect of processes parameter and different build direction on toughness and fatigue crack growth property to change the physical and mechanical properties. Also, manipulate the process parameters and their effect on strength, fracture toughness and quality for both bulk and cellular lattice structure parts. The novelty in this study lies in using additive manufacturing process to evaluate the local failure mechanism of 316L bulk and cellular lattice structures made by SLM under uniaxial tension and three-point bending load. The effect of different build directions of the 316L lattice structure on the fracture toughness properties is compared to the Ashby and Gibson models. The findings demonstrate that the build direction does have an effect on the microstructure of parts, which subsequently has an effect upon mechanical properties and the surface quality of manufactured parts. Results found in this study will enable the designer to understand the important factors which affect the SLM and DMLS process and quality of final parts at different build direction. The comparison between micromechanics model and experimental results will help the designer to predict fracture toughness of AM cellular structures without the need of experimental tests. Finally, the results of mechanical properties of these bulk and lightweight parts will give a confidence to the designer to use and tailor their properties to specific applications