Biomedical Applications of Titanium Alloys: A Comprehensive Review

Titanium alloys have emerged as the most successful metallic material to ever be applied in the field of biomedical engineering. This comprehensive review covers the history of titanium in medicine, the properties of titanium and its alloys, the production technologies used to produce biomedical implants, and the most common uses for titanium and its alloys, ranging from orthopedic implants to dental prosthetics and cardiovascular devices. At the core of this success lies the combination of machinability, mechanical strength, biocompatibility, and corrosion resistance. This unique combination of useful traits has positioned titanium alloys as an indispensable material for biomedical engineering applications, enabling safer, more durable, and more efficient treatments for patients affected by various kinds of pathologies. This review takes an in-depth journey into the inherent properties that define titanium alloys and which of them are advantageous for biomedical use. It explores their production techniques and the fabrication methodologies that are utilized to machine them into their final shape. The biomedical applications of titanium alloys are then categorized and described in detail, focusing on which specific advantages titanium alloys are present when compared to other materials. This review not only captures the current state of the art, but also explores the future possibilities and limitations of titanium alloys applied in the biomedical field

Tribological Properties of Nanometric Atomic Layer Depositions Applied on AISI 420 Stainless Steel

Atomic Layer Deposition ( ALD ) is a modern technique that Allows to deposit nanometric, conformal coatings on almost any kind of substrates, from plastics to ceramic, metals or even composites. ALD coatings are not dependent on the morphology of the substrate and are only regulated by the composition of the precursors, the chamber temperature and the number of cycles. In this work, mono- and bi -layer nanometric, protective low-temperature ALD Coatings, based on Al2O3 and TiO2 were applied on AISI 420 Stainless Steel in orderto enhance its relatively low corrosion resistance in chloride containing environments. Tribological testing were also performed on the ALD coated AISI 420 in order to evaluate the wear and scratch resistance of these nanometric layers and thus evaluate their durability. Scratch tests were performed using a standard Rockwell C indenter, under a variable load condition, in order to evaluate the critical loading condition for each coating. Wear testing were performed using a stainless steel counterpart, in ball-on-discconfiguration, in order to measure the friction coefficient and wear to confront the resistance. All scratch tests scars and wear tracks were then observed by means of Scanning Electron Microscopy (SEM) in order to understand the wear mechanisms that occurred on the sample surfaces. Corrosion testing, performed under immersion in 0.2 M NaCl solutions, clearly showed that the ALD coatings have a strong effect in protecting the Stainless Steel Substrate against corrosion, reducing the corrosion current density by two orders of magnitude.Tribological The preliminary results showed that ALD depositions obtained at low Temperatures have a brittle behavior caused by the amorphous nature of their structure, and thus undergo delamination phenomena during Scratch Testing at relatively low applied loads. During ball-on-disc testing, the coatings were removed from the substrate, in particular for monolayer ALD configurations, Which seem to have a lower toughness when compared to bi-layer configurations

Tribological Behavior of Thermal Spray Coatings, Deposited by HVOF and APS Techniques, and Composite Electrodeposits Ni/SiC at Both Room Temperature and 300 °C

The Both the thermal spray and the electroplating coatings are widely used because of their high wear resistance combined with good corrosion resistance. In particular the addition of both micro particles or nano‐particles to the electro deposited coatings could lead to an increase of the mechanical properties, caused by the change of the coating microstructure. The thermal spray coatings were deposited following industrial standards procedures, while the Ni/SiC composite coatings were produced at laboratory scale using both micro‐and nano‐sized ceramic particles. All the produced coatings were characterized regarding their microstructure,mechanical properties and the wear resistance. The tribological properties were analyzed using a tribometer under ball on disk configuration at both room temperature and 300oC. The results showed that the cermet thermal spray coatings have a high wear resistance, while the Ni nano‐composite showed good anti wear properties compared to the harder ceramic/cermet coatings deposited by thermal spray technique

A novel thermo-geometrical model for accurate keyhole porosity prediction in Laser Powder-Bed Fusion

When performing Laser Powder-Bed Fusion, undesired physical phenomena, such as balling, preballing and keyhole, must be avoided in order to achieve high-quality products. To date, keyhole-free process parameters can be identified either using demanding empirical methods or complex numerical simulations, while only a few analytical models can be found in literature for this purpose. In this work, state-of-the-art analytical models for predicting keyhole porosity were summarized and proved to be rather inaccurate because they are only based on thermodynamic principles, whereas they neglect the geometry and both the kinetics and kinematics of the keyhole cavity, which do also influence cavity collapse and porosity formation. Here an innovative physics-based semi-analytical model for predicting the formation of keyhole-related porosities was conceived, in which both thermodynamic and geometrical factors are taken into account. The proposed model was validated by performing single tracks experiments on Ti6Al4V according to a full factorial DoE on laser power and scanning speed. Produced samples were cross-sectioned and analyzed to evaluate keyhole porosity formation. The comparison between experimental data and model predictions confirmed the higher accuracy of the new model with respect to state of the art models. Besides improving the understanding of the keyhole phenomenon, the proposed model may provide a novel, effective and simple tool for fast process parameter optimization in industry

Effects of post-printing heat treatment on microstructure, corrosion and wet wear behavior of CoCrW alloy produced by L-PBF process

CoCr alloys are widely used as human implants because of both their superior corrosion resistance and superior mechanical properties (fatigue, wear resistance, etc.) respect to other metal alloys used in biomedical field. In particular, CoCrW alloys are used mainly to produce dental implants. In this study, the effects of thermal treatment on the corrosion resistance and wet wear resistance of CoCrW alloys produced via Laser-Powder Bed Fusion (L-PBF) were investigated, and the corrosion resistance and wet wear resistance of the L-PBF specimens were compared with those of the specimens obtained after forging. The heat treatment involved the solubilization of the alloy at 1150 °C in an Ar-saturated atmosphere, followed by furnace cooling. A detailed microstructural characterization of the L-PBF specimens was carried out using a light microscope and a scanning electron microscope in both the horizontal and vertical growth directions. Scanning Kelvin probe measurements were performed on the heat-treated specimens obtained by three-dimensional printing and forging. The void contents of the specimens were evaluated using the Archimedes’ method and image analysis. Vickers (HV2) hardness measurements were performed to evaluate the mechanical properties of the specimens. The corrosion properties of the specimens were evaluated by carrying out potentiodynamic tests in two different corrosive media (aqueous solution (9 g/L NaCl) at pH = 2 and 7). The corroded areas of the specimens were then examined using scanning electron microscopy (SEM). Finally, tribological tests were performed using the pin (Ti counter material)-on-flat configuration under dry and wet conditions, using the same corrosive environments as those used in the potentiodynamic tests and two different stroke lengths. The worn samples were characterized using SEM to investigate their wear mechanisms, and a stylus profilometer was used to determine the wear rates of the materials. The experimental results showed that the additively manufactured CoCrW L-PBF alloy had higher corrosion resistance than the wrought material. In addition, the additively manufactured material showed better dry and wet wear performances than the wrought material. Nevertheless, the heat treatment did not affect the properties evaluated in this study

Single tracks data obtained by selective laser melting of Ti6Al4V with a small laser spot diameter

Nowadays, advanced metal components with high geometrical complexity can be 3D printed by using the Selective Laser Melting (SLM) technology. Despite SLM resolution and accuracy are generally limited to some tenths of mm, it should be possible to produce finer and more precise details by applying lasers with a small spot diameter. However, to present date the data collected with small laser spot diameters are poor. In this work, experimental data describing the effects of laser power and scan speed on single track formation when applying a small laser spot diameter of 50 \ub5m on Ti6Al4V powder are reported. SEM images and the extracted geometrical data characterizing the obtained single tracks are provided here, as well as their microstructural analysis and microhardness measurements

Study of the Effect of L-PBF Technique Temporal Evolution on Microstructure, Surface Texture, and Fatigue Performance of Ti gr. 23 Alloy

Titanium alloys are widely used in various technological fields due to their excellent performance. Since the early stages of the 3D printing concept, these alloys have been intensively used as materials for these processes. In this work, the evolution of the performance of the 3D printing process has been studied by analysing the microstructure and the mechanical properties, fatigue and tensile, of the Ti gr. 23 alloy produced by two different models of Concept Laser M2 Cusing machines (an old model and a more recent one). The process parameters recommended by the manufacturer were adopted for each machine. Both microstructural and surface texture characterisations were carried out to better correlate the differences with the production process technique. For the same purpose, tensile tests and microhardness profiles were obtained, while the dynamic mechanical properties were evaluated by means of fatigue tests aimed at determining the fatigue limit of the material using a staircase approach. The mechanical tests were carried out on specimens with three different orientations with respect to the building platform, using two different SLM techniques. The fatigue behaviour was then analysed by evaluating the fracture surfaces and, in particular, the crack nucleation sites. By comparing the calculated fatigue values with the results of local fatigue calculations, an estimate of the residual stresses near the crack nucleation site was obtained. The results showed that the specimens produced on a newer machine had lower roughness (about 10%), slightly higher ductility, and a higher fatigue limit (10–20 MPa) compared to the specimens produced with the same material but on older equipment

Experimental and Numerical Investigation of Hot Extruded Inconel 718

Inconel 718 is a widely used superalloy, due to its unique corrosion resistance and mechanical strength properties at very high temperatures. Hot metal extrusion is the most widely used forming technique, if the manufacturing of slender components is required. As the current scientific literature does not comprehensively cover the fundamental aspects related to the process–structure relationships, in the present work, a combined numerical and experimental approach is employed. A finite element (FE) model was established to answer three key questions: (1) predicting the required extrusion force at different extrusion speeds; (2) evaluating the influence of the main processing parameters on the formation of surface cracks using the normalized Cockcroft Latham’s (nCL) damage criterion; and (3) quantitatively assessing the amount of recrystallized microstructure through Avrami’s equation. For the sake of modeling validation, several experimental investigations were carried out under different processing conditions. Particularly, it was found that the higher the initial temperature of the billet, the lower the extrusion force, although a trade-off must be sought to avoid the formation of surface cracks occurring at excessive temperatures, while limiting the required extrusion payload. The extrusion speed also plays a relevant role. Similarly to the role of the temperature, an optimal extrusion speed value must be identified to minimize the possibility of surface crack formation (high speeds) and to minimize the melting of intergranular niobium carbides (low speeds)

Strain-controlled fatigue loading of an additively manufactured AISI 316L steel: Cyclic plasticity model and strain–life curve with a comparison to the wrought material

Low cycle fatigue (LCF) regime was experimentally studied for a 316L steel additively manufactured by laser-powder bed fusion (L-PBF), a material widely used in sectors that require a reliable durability analysis. Material cyclic elastoplastic behavior is described by the Chaboche–Voce combined plasticity model, which displayed a great degree of accuracy. The fatigue life was modeled by both invoking the Manson–Coffin curve and other simplified models derived from static properties of the material; some of which showed remarkably good accuracy. A quantitative comparison with a wrought-processed 316L steel displayed a markedly different cyclic elastoplastic response but comparable fatigue strengths

The use of ALD and PVD coatings as defect sealants to increase the corrosion resistance of thermal spray coatings

Thermal spray coatings are widely used to improve the surface properties of materials, in particular the wear and oxidation resistance. Nevertheless, the corrosion resistance is slightly increased due to the fact that this type of coatings present some internal defects (pores, cracks) that allow the corrosive media to penetrate up to the substrate, that undergoes to corrosion degradation. The amount of these defects is strongly influenced by both the deposition technique and the material deposited. The aim of this work is to seal the internal porosities of the thermal spray coatings by the use of both PVD and ALD coatings or the combination of the two. The thermal spray coating analysed in this work is a pure alumina coating, deposited by Air Plasma Spray (APS) technique, that has been sealed with CrN coating, deposited by PVD (Physical Vapour Deposition) technique, and/or TiO2 coatings, deposited by ALD (Atomic Layer Deposition). The substrate used is a common medium C steel. The samples were then characterized in order to determine the microstructure (SEM+EDXS, light microscope) and the chemical composition (Rf-GDOES elemental profiling), that is important to determine the depth of penetration of the PVD and/or ALD coating inside the thermal spray deposit. Afterwards, a detailed electrochemical characterization in 3,5wt% NaCl aqueous solution was performed to verify the efficiency of the sealant treatment. In detail, a monitor in function of the time of the OCP potential was performed up to 24h of immersion time. In addition, potentiodynamic tests were performed using a 3 electrode electrochemical cell (CE: Pt wire, RE: Ag/AgCl). The same tests were then performed on the same samples that present an artificial defect produced by Rf-GDOES. The main goal of these tests is to determine the maximum depth of a defect that can allow the corrosive media to penetrate the thermal spray coating. Preliminary results showed that the use of PVD and ALD coatings as sealants can reduce the permeation of the corrosive media on the substrate