Additively manufactured Ti-6Al-4V thin struts via laser powder bed fusion: Effect of building orientation on geometrical accuracy and mechanical properties

Optimising β-Ti21S Alloy Lattice Structures for Enhanced Femoral Implants: A Study on Mechanical and Biological Performance

The metastable β-Ti21S alloy exhibits a lower elastic modulus than Ti-6Al-4V ELI while maintaining high mechanical strength and ductility. To address stress shielding, this study explores the integration of lattice structures within prosthetics, which is made possible through additive manufacturing. Continuous adhesion between the implant and bone is essential; therefore, auxetic bow-tie structures with a negative Poisson's ratio are proposed for regions under tensile stress, while Triply Periodic Minimal Surface (TPMS) structures with a positive Poisson's ratio are recommended for areas under compressive stress. This research examines the manufacturability and quasi-static mechanical behaviour of two auxetic bow-tie (AUX 2.5 and AUX 3.5) and two TPMS structures (TPMS 2.5 and TPMS 1.5) in β-Ti21S alloy produced via laser powder bed fusion. Micro-CT reveals printability issues in TPMS 1.5, affecting pore size and reducing fatigue resistance compared to TPMS 2.5. AUX 3.5's low stiffness matches cancellous bone but shows insufficient yield strength and fatigue resistance for femoral implants. Biological tests confirm non-toxicity and enhanced cell activity in β-Ti21S structures. The study concludes that the β-Ti21S alloy, especially with TPMS 2.5 structures, demonstrates promising mechanical and biological properties for femoral implants. However, challenges like poor printability in TPMS 1.5 are acknowledged and should be addressed in future research.

Feature size specific processing parameters for additively manufactured Ti-6Al-4V micro-strut lattices

The material properties of individual micro-struts are critical to the overall success of lattice structures. These properties can be significantly compromised by defects inherited from powder bed fusion processes. Among these defects, porous inclusions are well understood to have a detrimental effect on mechanical properties; posing a high risk to the implant under loading. While the majority of these defects can be avoided through optimisation of printing parameters, this has generally only been done for traditional bulk components with no in-designed porosity. Furthermore, a number of studies have observed changes in the frequency of such porous inclusions as feature size is reduced, indicating a size effect. This also suggests that the optimal parameters for bulk material are not necessarily translatable to the individual micro-struts which build the lattice. In this study, the relationship between parameter optimisation and feature size was investigated. Here, a higher energy density input was required for processing micro-strut lattices with an optimised relative density, than it was for bulk components. This could be attributed to faster rates of heat loss in micro-strut samples on account of their increased surface-to-volume ratio. The consequential improvement in mechanical properties was also assessed. An increase in both strength and stiffness could be largely attributed to an increase in the percentage volume of load bearing material, while improvements in failure strain were largely driven by minimisation of stress concentrations around the irregular pore morphologies. Fatigue properties did not improve beyond the effects of yielding. Rather, crack initiation was dominated by surface defects; which on account of their surface free energy, experience a much higher stress intensity factor.

Influence of building direction on physical and mechanical properties of titanium implants: A systematic review

The objective of the systematic review is to find an answer to a question: "What is the influence of the building direction of titanium implants produced by additive manufacturing on their physical and mechanical properties?" This review followed the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA 2020) and was registered in the Open Science Framework (OSF) (osf.io/rdc84). Searches were performed in PubMed, Scopus, Science Direct, Embase, and Google Scholar databases on February 17th, 2024. Articles were chosen in 2 steps by 2 blinded reviewers based on previously selected inclusion criteria: In vitro studies that evaluated the influence of the impression direction of titanium implants produced by additive manufacturing on their physical and mechanical properties were selected. Articles were excluded that (1) did not use additive technology to obtain the implants, 2) used surfaces other than titanium, 3) did not evaluate the direction of impression, 4) Studies with only in vivo analyses, clinical studies, systematic reviews, book chapters, short communications, conference abstracts, case reports, and personal opinions.). In the initial search, 581 results were found. Of this total, 108 were excluded for duplication and, after applying the eligibility criteria, 16 articles were included in the present review. The risk of bias was analyzed using the RoBDEMAT. The risk of bias was analyzed using the RoBDEMAT. In addition, the coefficient of interagreement of the reviewers (Cohen's Kappa) and the certainty of evidence by GRADE were analyzed. In general, different impression angles showed variations in the physical and mechanical characteristics of the groups evaluated, including roughness, tensile strength, hardness, and modulus of elasticity. While some impression orientations resulted in greater strength or hardness, others showed greater elasticity or lower surface roughness. These findings suggest that print orientation plays a significant role in determining material properties. It can be concluded that printing directions influence the physical and mechanical properties of titanium implants and the studies included showed that the 0°, 45°, and 90° directions are the most evaluated as they present lower probabilities of structural anisotropies and provide better results in their roughness, hardness, tensile and compressive strength.

Experimental validation of auxetic stent designs: three-point bending of 3D printed Titanium prototypes

Introduction

Numerical simulations have demonstrated the superior bending flexibility of auxetic stents compared to conventional stent designs for endovascular procedures. However, conventional stent manufacturing techniques struggle to produce complex auxetic stent designs, fueling the adoption of additive manufacturing techniques.

Methods

In this study, we employed DMLS additive manufacturing to create Titanium Ti64 alloy stent prototypes based on auxetic stent designs investigated in a previous study. These prototypes were then subjected to experimental three-point bending tests.

Result

The experimental results were replicated using a finite element model, which showed remarkable accuracy in predicting the bending flexibility of four auxetic stents and two conventional stents.

Discussion

Although this validation study demonstrates the promising potential of DMLS and other additive manufacturing methods for fabricating auxetic stents, further optimization of current stent design limitations and the incorporation of post-processing techniques are essential to enhance the reliability of these additive manufacturing processes.

Fatigue database of additively manufactured alloys

Fatigue is a process of mechanical degradation that is usually assessed based on empirical rules and experimental data obtained from standardized tests. Fatigue data of engineering materials are commonly reported in S-N (the stress-life relation), ε-N (the strain-life relation), and da/dN-ΔK (the relation between the fatigue crack growth rate and the stress intensity factor range) data. Fatigue and static mechanical properties of additively manufactured (AM) alloys, as well as the types of materials, parameters of AM, processing, and testing are collected from thousands of scientific articles till the end of 2022 using natural language processing, machine learning, and computer vision techniques. The results show that the performance of AM alloys could reach that of conventional alloys although data dispersion and system deviation are present. The database (FatigueData-AM2022) is formatted in compact structures, hosted in an open repository, and analyzed to show their patterns and statistics. The quality of data collected from the literature is measured by defining rating scores for datasets reported in individual studies and through the fill rates of data entries across all the datasets. The database also serves as a high-quality training set for data processing using machine learning models. The procedures of data extraction and analysis are outlined and the tools are publicly released. A unified language of fatigue data is suggested to regulate data reporting for the fatigue performance of materials to facilitate data sharing and the development of open science.

Bone marrow stromal cells are sensitive to discrete surface alterations in build and post-build modifications of bioinspired Ti6Al4V 3D-printed in vitro testing constructs

Current standards in bone-facing implant fabrication by metal 3D (M3D) printing require post-manufacturing modifications to create distinct surface properties and create implant microenvironments that promote osseointegration. However, the biological consequences of build parameters and surface modifications are not well understood. This study evaluated the relative contributions of build parameters and post-manufacturing modification techniques to cell responses that impact osseointegration in vivo. Biomimetic testing constructs were created by using a M3D printer with standard titanium-aluminum-vanadium (Ti6Al4V) print parameters. These constructs were treated by either grit-blasting and acid-etching (GB + AE) or GB + AE followed by hot isostatic pressure (HIP) (GB + AE, HIP). Next, nine constructs were created by using a M3D printer with three build parameters: (1) standard, (2) increased hatch spacing, and (3) no infill, and additional contour trace. Each build type was further processed by either GB + AE, or HIP, or a combination of HIP treatment followed by GB + AE (GB + AE, HIP). Resulting constructs were assessed by SEM, micro-CT, optical profilometry, XPS, and mechanical compression. Cellular response was determined by culturing human bone marrow stromal cells (MSCs) for 7 days. Surface topography differed depending on processing method; HIP created micro-/nano-ridge like structures and GB + AE created micro-pits and nano-scale texture. Micro-CT showed decreases in closed pore number and closed porosity after HIP treatment in the third build parameter constructs. Compressive moduli were similar for all constructs. All constructs exhibited ability to differentiate MSCs into osteoblasts. MSCs responded best to micro-/nano-structures created by final post-processing by GB + AE, increasing OCN, OPG, VEGFA, latent TGFβ1, IL4, and IL10. Collectively these data demonstrate that M3D-printed constructs can be readily manufactured with distinct architectures based on the print parameters and post-build modifications. MSCs are sensitive to discrete surface topographical differences that may not show up in qualitative assessments of surface properties and respond by altering local factor production. These factors are vital for osseointegration after implant insertion, especially in patients with compromised bone qualities.

Hybrid Ti6Al4V/Silk Fibroin Composite for Load-Bearing Implants: A Hierarchical Multifunctional Cellular Scaffold

Despite the tremendous technological advances that metal additive manufacturing (AM) has made in the last decades, there are still some major concerns guaranteeing its massive industrial application in the biomedical field. Indeed, some main limitations arise in dealing with their biological properties, specifically in terms of osseointegration. Morphological accuracy of sub-unital elements along with the printing resolution are major constraints in the design workspace of a lattice, hindering the possibility of manufacturing structures optimized for proper osteointegration. To overcome these issues, the authors developed a new hybrid multifunctional composite scaffold consisting of an AM Ti6Al4V lattice structure and a silk fibroin/gelatin foam. The composite was realized by combining laser powder bed fusion (L-PBF) of simple cubic lattice structures with foaming techniques. A combined process of foaming and electrodeposition has been also evaluated. The multifunctional scaffolds were characterized to evaluate their pore size, morphology, and distribution as well as their adhesion and behavior at the metal-polymer interface. Pull-out tests in dry and hydrated conditions were employed for the mechanical characterization. Additionally, a cytotoxicity assessment was performed to preliminarily evaluate their potential application in the biomedical field as load-bearing next-generation medical devices.

Mechanical and microstructural characterization of titanium gr.5 parts produced by different manufacturing routes

Since a few decades, the aircraft industry has shifted its preference for metal parts to titanium and its alloys, such as the high-strength titanium grade 5 alloy. Because of titanium grade 5 limited formability at ambient temperature, forming operations on this material requires high temperatures. In these conditions, a peculiar microstructure evolves as a result of the heating and deformation cycles, which has a significant impact on formability and product quality. On the other hand, additive manufacturing technologies, such as selective laser melting and electron beam melting, are increasingly being used and are replacing more traditional approaches such as machining and forging. Fundamental part characteristics such as mechanical and microstructural properties, geometric accuracy, and surface quality strongly depend on the selection of the manufacturing method. The authors of this paper seek to identify the strengths and limitations imposed by the intrinsic characteristics of different manufacturing alternatives for the production of parts of aeronautical significance, providing guidelines for the choice of the most appropriate manufacturing route for a given application and part design.

Additive Manufacturing of AlSi10Mg and Ti6Al4V Lightweight Alloys via Laser Powder Bed Fusion: A Review of Heat Treatments Effects

Laser powder bed fusion (L-PBF) is an additive manufacturing technology that is gaining increasing interest in aerospace, automotive and biomedical applications due to the possibility of processing lightweight alloys such as AlSi10Mg and Ti6Al4V. Both these alloys have microstructures and mechanical properties that are strictly related to the type of heat treatment applied after the L-PBF process. The present review aimed to summarize the state of the art in terms of the microstructural morphology and consequent mechanical performance of these materials after different heat treatments. While optimization of the post-process heat treatment is key to obtaining excellent mechanical properties, the first requirement is to manufacture high quality and fully dense samples. Therefore, effects induced by the L-PBF process parameters and build platform temperatures were also summarized. In addition, effects induced by stress relief, annealing, solution, artificial and direct aging, hot isostatic pressing, and mixed heat treatments were reviewed for AlSi10Mg and Ti6AlV samples, highlighting variations in microstructure and corrosion resistance and consequent fracture mechanisms.