Ti-6Al-4V alloy has been used as biomedical implants for decades because of its superior mechanical properties, good biocompatibility, lack of allergic problems and good corrosion resistance. It is widely used as the tibial components in total knee arthroplastry and hip cup in total hip replacement. However, the mechanical properties of Ti-6Al-4V implant can be deteriorated due to corrosion pits. In the past decades, the rapid developments in additive manufacturing have broadened their applications in biomedical area due to the high geometrical freedom in fabricating patient-friendly implants. However, the high-localized thermal input and fast cooling rate during laser processing usually result in non-equilibrium phase with high residual stress. Therefore, it is necessary to apply proper post-treatments on the as-printed parts to ensure better properties. In this work, various post-treatments (e.g. post-heat treatments, hot isostatic pressing) were applied aim to improve the corrosion behavior of direct metal laser sintered Ti-6Al-4V parts. The effect of post-treatment temperature on the mechanical properties and corrosion behavior were examined experimentally. A discussion on factors influencing corrosion rate was presented, and the corrosion mechanism on the Ti-6Al-4V part in simulated body fluid was proposed. Based on the electrochemical measurement results, enhanced corrosion resistance was observed in the samples after high temperature HIPing at the annealing temperature (α+β region) of 799°C.

This research studies individual and combined effects of hydrogen gas exposure and low temperature on the tribological characteristics of Ti-6Al-4V. Experimental approaches include test system modification and tribological analysis. An existing ballon- disk tribometer was modified to allow liquid nitrogen to be constantly injected into an insulated test chamber to enable testing at low temperature. Twelve 3.8 cm diameter Ti-6Al-4V disks were manufactured and polished, then half were exposed to pure hydrogen gas at elevated temperature and pressure and the remaining disks were untreated. The testing was split in to four groups of three disks based on testing temperature and previous hydrogen exposure. A silicon nitride ball was used for all tests. Each group was tested at two normal loads, 10N and 20N, at the same linear speed. Group 1 was unexposed and tested at room temperature, Group 2 was unexposed and tested at low temperature, Group 3 was exposed and tested at room temperature and Group 4 was exposed and tested at low temperature. Average friction coefficients and the specific wear rate were calculated from the test data. Also high-resolution digital microscope imaging was used to observe and characterize the wear mechanisms of the four groups of samples. Results show that hydrogen exposure facilitated adhesive wear of the surface and that low temperature induced a slip-stick wear mechanism under higher loads, but not at lower loads and regardless of exposure to hydrogen gas. This research opens avenues for future investigation in effects of hydrogen and low temperature embrittlement on the tribological performance of materials. With the increasing interests in hydrogen energy, the present work established a foundation for future study.

L’objectif de cette thèse est l’étude expérimentale et la modélisation des évolutions microstructurale et des cinétiques de transformation de phase α + β → β → αII dans l’alliage de titane Ti-6Al-4V dans des conditions isothermes et non-isothermes. Dans cette optique, diverses techniques ont été utilisées (DSC, MEB, microscopie optique, résistivité électrique in-situ et DRX). L’équation de KM-JMA a été utilisée pour décrire les cinétiques de transformation de phase en conditions isothermes. Une approche basée sur le principe d’additivité est proposée pour adapter l’équation de KM-JMA pour les transformations de phases non-isothermes et est comparée à celle utilisant la notion de temps fictif. Les paramètres de cinétique de transformation de phase (n, k et Q) sont ainsi déterminés et interprétés sur la base des résultats de l’analyse métallographique réalisée. Les mécanismes des différentes transformations de phase ont été discutés en tenant compte de l’analyse microstructurale et des paramètres de cinétiques déterminés. Enfin la cinétique de transformation de phase βm → αII en condition isotherme a été simulée à l’aide du logiciel MatCalc et modélisée en utilisant la théorie de germination – croissance ainsi que l’équation de KM-JMA. / The aim of this thesis is to experimentally study and to model the kinetics of α + β → β → αII phase transformations in Ti-6Al-4V alloy for isothermal and non-isothermal conditions. To this purpose, several technics have been performed (DSC, SEM, optical microscopy, in-situ electrical resistivity and XRD). The KM-JMA equation has been used to describe the kinetics of phase transformations during isothermal holdings. An approach based on the additivity rule has been developed to adapt the KM-JMA equation to non-isothermal phase transformations and is compared to the one using the fictitious time. Kinetics parameters (n, k and Q) have been determined using this approach and discussed based on the microstructural analysis. Mechanisms of the α + β → β → α phase transformations have been discussed by considering the microstructural analysis and the kinetics parameters already determined. Finally, the βm → αII phase transformation kinetics in isothermal conditions has been simulated with the MatCalc software and modelled using both the nucleation – growth theory and the KM-JMA equation.

hree-­‐dimensional characterization of fully equiaxed Ti-­‐6Al-­‐4V has been performed using the recently developed TriBeam system. To date, no general model for full prediction of Ti-­‐6Al-­‐4V tensile properties as a function of empirical microstructural parameters exist. To develop such a model would require knowledge about the relevant deformation mechanisms and also detailed information about the microstructure of the material. As a thrust to develop such a model a large three-­‐dimensional dataset was collected, reconstructed and analyzed, in order to extract statistical information about microstructural parameters. It was found that the material contained heavily textured regions with α grain boundaries that have misorientation of 1° to 2° or less. The low grain misorientation angle required segmentation parameter optimization, which included the integration of a grain size filter and a thorough exploration of the segmentation parameter space. The α grains are shown to be volumetrically smaller in textured regions compared to bulk, while β grains showed an even grain volume distribution throughout the structure. The TriBeam system collected the dataset faster than any other available serial sectioning methods and is described in detail with its subsystems and operating conditions. The extracted statistical information is presented in detail numerically and visually in this report.

A friction stir weld (FSW) is a solid state joining operation whose processing parameters are currently determined by lengthy trial and error methods. To implement FSWing rapidly in various applications will require an approach for predicting process parameters based on the physics of the process. Based on hot working conditions for metals, a kinematic model has been proposed for calculating the shear strain and shear strain rates during the FSW process, validation of the proposed model with direct measuring is difficult however. Since the shear strain and shear strain rates predicted for the FSW process, are similar to those predicted in metal cutting, validation of the FSW algorithms with microstructural studies of metal chips may be possible leading to the ability to predict FSW processing parameters.

The work presented in this dissertation is concerned with the microstructures andsurface roughness of test slabs of Ti-6Al-4V produced by one of powder based AdditiveManufacturing (AM) technique namely Electron Beam melting (EBM). The effects of processparameters of a EBM system and geometry factors of a EBM build such as slabs’ thickness andheight etc. on the microstructure and the surface roughness of the EBM produced Ti-6Al-4Vhave being investigated. The processing parameters of the EBM system involved in the presentwork include beam current, scan speed, offset focus and scanning length etc. In this study threedifferent batches of samples were prepared. Microstructures of EBM built Ti-6Al-4V werestudied using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), TransmissionElectron Microscopy (TEM), and X-ray diffraction (XRD). Confocal microscopy and imageanalysis codes ImageJ’s routine SurfCharJ were used to quantify the surface roughness of the testslabs. The microstructures of EBM built Ti-6Al-4V in general consist of columnar grains of priorβ phase. Inside the columnar grains there is a typical (α+β) microstructure of titanium alloyscontaining Widmanstätten α platelets and rod-shaped β phase. Grain boundary α layer has alsobeen observed on the grain boundaries of prior b columnar grains. By using TEM, the β phase inEBM built Ti-6Al-4V has been identified as a rod-like structure located on the grain boundariesof the fine a grains and often grows up along the build direction. The size of the β rods is about200nm and the distance between the β rods is ranged between 0.5 -2μm. Chemical compositionsfor different phases have been measured by TEM/EDX and volume fraction of the β phase in theEBM Ti-6Al-4V has been determined to be 2.7%. The phase transformation sequence in EBMbuilt Ti-6Al-4V has been discussed according to processing history and microstructuresobserved. It has been observed that, the size, number and geometry of prior β columnar graindepend on the sample thickness and other process parameters setting. The diameter of columnargrain varies between 2-70μm. The increase in size, number and regularity of columnar grains hasbeen observed with increase in sample thickness, beam energy density and scanning length.While with increase in height of the build it decreases. The length and smoothness of α plateletsincreases with increase in diameter of prior β columnar grain. The β phase rods are unaffected bysample thickness and process parameter settings. The sample thickness and beam energy densityhas a strong effect on the surface roughness of the test slabs. The value of surface roughnesscoefficient Ra for different test slabs varies between 1-20μm. It is observed that the surfaceroughness increases of the test slab increases with increase in thickness of the slab and beamenergy density. The possible reasons for these variations in microstructures and surfaceroughness have been discussed. / <p>All papers in dissertation as manuscripts.</p>

En este trabajo de Tesis se estudiaron distintos recubrimientos biocompatibles sintetizados sobre la aleación Ti-6Al-4V, tanto de manera química como electroquímica, para mejorar el comportamiento frente a la corrosión en medios fisiológicos simulados. Las películas fueron caracterizadas mediante distintas técnicas de superficie y la resistencia a la corrosión fue evaluada empleando diferentes métodos electroquímicos. En primer lugar, se electrosintetizó potenciostaticamente polipirrol (PPy) con estructura microtubular sobre la aleación Ti-6Al-4V a partir de una solución que contenía salicilato de sodio como dopante. La presencia del polímero evitó la disolución activa de la aleación en saliva artificial modificada. En una etapa posterior y con el fin de desarrollar un recubrimiento antibacterial y anticorrosivo, se empleó la matriz polimérica microestructurada para la inmovilización de especies de Zn. Tanto el recubrimiento de PPy como el de PPy modificado con especies de Zn mostraron actividad antibacterial frente a S. aureus y aumentaron la protección frente a la corrosión en saliva artificial modificada. Por otra parte, se empleó la técnica de anodizado en modo galvanostático para sintetizar óxidos delgados en una solución de Na2MoO4. Se estableció el efecto de diferentes parámetros como el pH, el tiempo de anodización y la concentración del anión inhibidor sobre el comportamiento anticorrosivo, para luego ampliar el estudio a electrolitos que contenían NaH2PO4 y NH4VO3. Independientemente de la solución utilizada, se obtuvieron óxidos compactos y amorfos, sin poros ni grietas. La muestra anodizada en solución Na2MoO4 registró el mejor comportamiento frente a la corrosión en solución Ringer. Asimismo, se sometió a la aleación Ti-6Al-4V a un tratamiento químico con peróxido de hidrógeno. El recubrimiento resultante, constituido mayormente por TiO2 amorfo, presentó una estructura del tipo barro agrietado. El tratamiento redujo la cantidad de iones liberados desde la aleación al entorno fisiológico simulado tras los ensayos electroquímicos. La técnica, sin necesidad de un postratamiento térmico, mejoró el comportamiento frente a la corrosión de la aleación Ti-6Al-4V en solución Ringer. Por último, la aleación modificada mediante oxidación química en H2O2 se empleó como sustrato para la electrosíntesis de ZnO. Las condiciones empleadas en la síntesis dieron lugar a la formación de micro y nanoláminas de ZnO con capacidad bactericida frente a las bacterias S. aureus y S. mutans. Si bien la electrosíntesis de ZnO no brindó al sustrato propiedades anticorrosivas significativas, la combinación del tratamiento de oxidación química en H2O2 con la presencia del ZnO, permitió mejorar la protección con respecto a la del sustrato desnudo. / In this Thesis work, different biocompatible coatings chemically and electrochemically synthesized onto Ti-6Al-4V alloy were studied to improve the corrosion behavior in simulated physiological environments. The coatings were characterized using different surface techniques and the corrosion resistance was evaluated using different electrochemical methods. Firstly, microstructured polypyrrole (PPy) films were potentiostatically synthesized onto Ti-6Al-4 V alloy from an aqueous solution containing sodium salicylate as a dopant. The presence of PPy prevented the active dissolution process of the alloy in modified artificial saliva. In order to develop a coating that prevents corrosion and microbial growth on the alloy, the microstructured polymer matrix was later employed to immobilize Zn species. Both PPy and PPy films modified with Zn species showed antibacterial activity against S. aureus and increased the corrosion protection in modified artificial saliva. On the other hand, the anodization technique was employed under galvanostatic mode to synthesize thin oxide films from a Na2MoO4 solution. The effect of different parameters such as pH, anodization time and the inhibitor anion concentration on the corrosion behavior was established and the study was then extended to electrolytes containing NaH2PO4 and NH4VO3. Compact and amorphous oxides without pores or cracks were obtained regardless of the solution used. The sample anodized in Na2MoO4 solution showed the best corrosion behavior in Ringer solution. Furthermore, the Ti-6Al-4V alloy was subjected to chemical treatment with hydrogen peroxide. The resulting coating consisted mostly of an amorphous TiO2 oxide with a mud-cracked structure type. The treatment reduced the number of ions released from the alloy to the simulated physiological environment after the electrochemical tests. The technique, without the need for thermal post-treatment, was enough to enhance the corrosion behavior of Ti-6Al-4V alloy in Ringer solution. Finally, the alloy modified by chemical oxidation in H2O2 was used as a substrate for the electrosynthesis of ZnO. The conditions employed in the synthesis resulted in the formation of ZnO micro and nanosheets with bactericidal capacity against S. aureus and S. mutans bacteria. Although the electrosynthesis of ZnO did not confer significant anticorrosive properties to the substrate, the combination of the chemical oxidation treatment in H2O2 with the presence of ZnO improved the protection compared to that of the bare substrate

Friction stir welding (FSWing) is a solid state, thermo-mechanical process that utilizes a non-consumable rotating weld tool to consolidate a weld joint. In the FSW process, the weld tool is responsible for generating both the heat required to soften the material and the forces necessary to deform and consolidate the former weld seam. Thus, weld tool geometry, material selection, and process parameters are important to the quality of the weld. To study the effects of the weld tool geometry on the resulting welds, a previous study was conducted using varying degree taper, microwave-sintered tungsten carbide (WC) weld tools to FSW Ti-6Al-4V. Fully consolidated welds were down selected for this study to evaluate the resulting mechanical properties and to document the microstructure. X-ray diffraction (XRD) was used to compare the parent material texture with that in the weld nugget. The purpose of this study is to quantify the temperatures obtained during FSWing by interpreting the resulting microstructure. This information is useful in process optimization as well as weld tool material selection.

The current work focuses on three aspects of laser formed Ti-6Al-4V: an evaluation of the as-deposited and heat treated macro and microstructures and preliminary results obtained from a model developed to calculate the temperature profile resultant of the laser forming process. A "solution treat and age" heat treatment with a variable cooling rate was performed on the Laser Formed Ti-6Al-4V single line builds. Increasing the cooling rate decreases the acicular alpha grain size in the basketweave Widmanstätten alpha plus untransformed beta microstructure. Distinct features of the as-deposited macrostructure include: large columnar prior-beta grains that have grown epitaxially through multiple deposited layers; a well defined heat affected zone in the substrate; and the presence of "layer bands," a macroscopic banding present at the top of every layer except for the last three layers to be deposited. The nominal microstructure between the layer bands consists of acicular basketweave Widmanstätten alpha outlined in untransformed beta. The alpha grain width is smaller just above a layer band and larger just below a layer band. The microstructure of the layer band consists of larger colonies of acicular alpha outlined in untransformed beta. The gradient in the alpha grain size and presence of the layer band is due to thermal cycling as opposed to segregation effects which were ruled out using quantitative compositional analyses. Through analysis of the microstructural results the gradient in the nominal microstructure and formation of the layer band in layer n was caused by the deposition of layer n+2, and n+3, respectively. A thermal model has been developed to assist in the prediction and interpretation of the as-processed microstructure. The model is used to explain that the microstructural evolution of the layer bands and gradient microstructure in layer n is due to the deposition of layer n+2. The difference in the two analyses of microstructural evolution based on microstructural observations and thermal model results are due to differences in the parameter sets used to build and model the deposit. / Master of Science

The largest single cause of failure in fan and compressor components in the cold frontal sections of commercial and military gas turbine engines has been attributed to high cycle fatigue (HCF). Additionally, both high-cycle fatigue (HCF) and lowcycle fatigue (LCF) loadings are widely recognized as unavoidable during operation of these components and because the classic Linear Damage Rule (LDR) neglects to account for the synergistic interaction between these damage contributors, dangerous over predictions of lifetime can result. Combined low-cycle fatigue / high-cycle fatigue (HCF/LCF) loadings were investigated in smooth Ti-6Al-4V. The specimens were subjected to a variable amplitude block loading history comprised of completely-reversed (R = -1) tensioncompression overloads followed by constant-amplitude zero-tension (R = 0) minor cycles. Axial specimens were excised from forgings representative of turbine engine fan blade forgings, and consisted of approximately 60% primary α in a matrix of lamellar α + β. Data are reported for smooth specimens of Ti-6Al-4V subjected to both constant amplitude and variable amplitude loadings. The axial specimens were prepared according to two distinct specimen conditions: low stress ground and longitudinallypolished (LSG+LP) and stress-relieved and chemically milled (SR+CM) conditions. Significantly longer lives were observed for the LSG+LP specimen condition under both constant and variable amplitude loading, due to the presence of a beneficial compressive surface residual stress. The presence of this residual stress was confirmed by x-ray diffraction, and its magnitude was of the order of 180 MPa (~20% of the yield stress). In either specimen condition, no appreciable effect of periodic overloads on the life of subsequent minor cycles was observed. / Master of Science