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Prabhu, Avinash WImproving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control
Master of Science, The Ohio State University, 2014, Welding Engineering
Laser Engineered Net Shaping (LENS) is a solid freeform fabrication process capable of producing net shape, custom parts through a layer-by-layer deposition of material using a laser energy source. LENS based Ti-6Al-4V parts are currently being explored for applications to aerospace and biomedical implant applications. To achieve satisfactory mechanical performance in the components, homogeneity of the microstructure and the physical structure is important. This study explores methods of improving the fatigue life of Ti-6Al-4V LENS deposits through the creation of defect free components with appropriate microstructure. The work focuses on the impact of beta grain refinement and the elimination of lack of fusion porosity defects on the fatigue life of the alloy. Further, a model is developed to predict the epitaxial grain growth in LENS builds of Ti-64. This model is used to augment the prediction capability of a simultaneous transformation kinetics (STK theory) based model developed previously. The beta grain refinement in the fatigue properties of Ti-64 is achieved through addition of boron in amounts < 3 wt%. The addition of boron is found to refine the columnar beta structure typically observed in LENS deposited Ti-64. However, at high concentrations of boron, it was difficult to discern the prior beta grain size to visualize the extent of grain refinement. The boron alloying further causes a significant change in the structure of alpha laths, leading the shorter and thicker individual laths. Grain boundary alpha is not observed in the microstructure on addition of boron above a certain threshold. The addition of boron is observed to improve the fatigue properties of deposited Ti-64 samples. The Ti-64 LENS builds are observed to contain lack-of-fusion porosity in the lower regions of the deposit close to the substrate. The effect of process parameters namely the power, travel speed, hatch width, pre-heating, powder flow rate, substrate surface quality, and the hatch path on the amount of porosity is analyzed through trial experiments followed by a more detailed design of experiments approach. At a fixed power setting of 400W, the hatch width and powder flow rate are observed to have a significant influence on the extent of porosity present in the deposit. The hatch width has been further optimized through a CFD based simulation based approach leading to an increased material efficiency and reduced time of building. These builds are observed to show a columnar beta grain structure growing epitaxially from the base of the deposit and consist of primarily basketweave alpha. The fatigue properties are analyzed at the combination of parameters determined through experiments and are found to show an improvement over previously reported values by 3 times. The epitaxial grain growth seen in the LENS deposits is modeled using the classical grain growth equation. The prediction is observed to be sensitive to the atomic mobility, starting grain size, activation energy for grain growth and alpha dissolution temperatures. This grain growth information is incorporated into simultaneous transformation kinetics (STK theory) based microstructure model to improve the predictions of microstructure fraction in Ti-6Al-4V LENS builds.

Committee:

Wei Zhang (Advisor); Dave Farson (Committee Member); Sudarsanam Babu (Committee Member)

Subjects:

Materials Science

Keywords:

Ti-6Al-4V; LENS; Additive Manufacturing; Laser Additive Manufacturing; Grain growth modeling; Boron alloyed Ti-6Al-4V;

Lee, EunhaMicrostructure evolution and microstructure/mechanical properties relationships in α+β titanium alloys
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
In this study, the microstructural evolution of Timetal 550 was investigated. Timetal 550 showed two types of phase transformations (martensitic and nucleation and growth) depending on the cooling rate from the β region. The α phase initially precipitated at the prior β grain boundaries, and it had a Burgers OR with one of the adjacent grains. It was found that colonies could grow, even in the fast-cooled Timetal 550 sample, from the grain boundary α into the prior β grain with which it exhibited the Burgers OR. Three orientation relationships were also found between α laths in the basketweave microstructure. Microhardness testing demonstrated that fast-cooled Timetal 550 samples with basketweave microstructure were harder than slowly-cooled samples with colony microstructure. Orientation-dependent deformation was found in the colony microstructure. Specically, when the surface normal is perpendicular to the [0001] of α, the material deforms easily in the direction perpendicular to the [0001] of α. Fuzzy logic and Bayesian neural network models were developed to predict the room temperature tensile properties of Timetal 550. This involved the development of a database relating microstructural features to mechanical properties. A Gleeble 3800 thermal-mechanical simulator was used to develop various microstructures. Microstructural features of tensile-tested samples were quantified using stereological procedures. The quantified microstructural features and the tensile properties were used as inputs and outputs, respectively, for modeling the relationships between them. The individual influence of five microstructural features on tensile properties was determined using the established models. The microstructural features having the greatest impact on UTS and YS were the thickness of α laths and the width of grain boundary α layer, and the microstructural features having the greatest impact on elongation were the thickness of α laths and the prior β grain size. Nanoindentation testing found that the hardness of the individual grains was related to their orientations. The hardness values were highest near the [0001] stress axis, and they decreased as the stress axis deviated from [0001] orientation. Dislocation analyses indicated that the deformation in individual grains conformed to the Schmid factor analysis where slip primarily occurs on those slip systems where RSS (SF) values are highest.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Timetal 550; Ti-6Al-4V; microstructure evolution; modeling microstructure/tensile properties relatioships; nanoindentation

Collins, Peter ChancellorA combinatorial approach to the development of composition-microstructure-property relationships in titanium alloys using directed laser deposition
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The Laser Engineered Net Shaping (LENS™) system, a type of directed laser manufacturing, has been used to create compositionally graded materials. Using elemental blends, it is possible to quickly vary composition, thus allowing fundamental aspects of phase transformations and microstructural development for particular alloy systems to be explored. In this work, it is shown that the use of elemental blends has been refined, such that bulk homogeneous specimens can be produced. When tested, the mechanical properties are equivalent to conventionally prepared specimens. Additionally, when elemental blends are used in LENS™ process, it is possible to deposit compositionally graded materials. In addition to the increase in design flexibility that such compositionally graded, net shape, unitized structures offer, they also afford the capability to rapidly explore composition-microstructure-property relationships in a variety of alloy systems. This research effort focuses on the titanium alloy system. Several composition gradients based on different classes of alloys (designated a, a+b, and b alloys) have been produced with the LENS™. Once deposited, such composition gradients have been exploited in two ways. Firstly, binary gradients (based on the Ti-xV and Ti-xMo systems) have been heat treated, allowing the relationships between thermal histories and microstructural features (i.e. phase composition and volume fraction) to be explored. Neural networks have been used to aid in the interpretation of strengthening mechanisms in these binary titanium alloy systems. Secondly, digitized steps in composition have been achieved in the Ti-xAl-yV system. Thus, alloy compositions in the neighborhood of Ti-6Al-4V, the most widely used titanium alloy, have been explored. The results of this have allowed for the investigation of composition-microstructure-property relationships in Ti-6-4 based systems.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

combinatorial method; combinatorial approach; laser deposition; directed laser deposition; LENS; titanium; molybdenum; Ti-6-4; Ti-6Al-4V; Timetal 21S; composition; microstructure; property; relationships; neural network; fuzzy logic

Pilchak, Adam L.The effect of friction stir processing on the microstructure, mechanical properties and fracture behavior of investment cast Ti-6Al-4V
Doctor of Philosophy, The Ohio State University, 2009, Materials Science and Engineering

In recent years, friction stir processing has emerged as a solid state metalworking technique capable of substantial microstructure refinement in aluminum and nickel-aluminum-bronze alloys. The purpose of the present study is to determine the feasibility of friction stir processing and assess its effect on the microstructure and mechanical properties of the most widely used alpha + beta titanium alloy, Ti-6Al-4V. Depending on processing parameters, including tool travel speed, rotation rate and geometry, the peak temperature in the stir zone was either above or below the beta transus. The resulting microstructures consisted of either ~1 micron equiaxed α grains, ~25 micron prior beta grains containing a colony alpha + beta microstructure or a combination of 1 micron equiaxed alpha and fine, acicular alpha + beta. The changes in microstructure were characterized with scanning and transmission electron microscopy and electron backscatter diffraction. The texture in the stir zone was nearly random for all processing conditions, however, several components of ideal simple shear textures were observed in both the hexagonal close packed alpha and the body centered cubic beta phases which provided insight into the operative grain refinement mechanisms.

Due to the relatively small volume of material affected by friction stir processing, conventionally sized test specimens were unable to be machined from the stir zone. Thus, the mechanical properties were investigated using micropillar compression and microtensile specimens. The effect of friction stir processing on crack initiation resistance was assessed using high cycle fatigue tests conducted in four-point bend which put only the stir zone in maximum tension. The results indicated that at constant stress amplitude, there was greater than an order of magnitude increase in fatigue life after friction stir processing. In addition, the fatigue strength of the investment cast material was improved between 20 pct. and 60 pct. by friction stir processing. These improvements have been verified with a statistically significant number of tests.

Finally, the wide range of microstructures created by friction stir processing provided an opportunity to study the effect of underlying microstructure on the fracture behavior of alpha + beta titanium alloys. For this purpose, high resolution fractography coupled with quantitative tilt fractography and electron backscatter diffraction was used to provide a direct link between microstructure, crystallography and fracture topography. These techniques have been used extensively to study the early stages of post-initiation crack growth in Ti-6Al-4V, especially at low stress intensity ranges in the as-cast material. A limited number of experiments were also performed on Ti-6Al-4V specimens in other microstructural conditions to assess the generality of the detailed results obtained for the fully lamellar material. The results show that fracture topography depends strongly on the stress intensity range and microstructural length scale. In addition, many of the features observed on the fracture surface were directly related to the underlying crystallographic orientation.

Committee:

James Williams, PhD (Advisor); Mary Juhas, PhD (Advisor); Hamish Fraser, PhD (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

titanium; friction stir processing; fatigue; fractography; EBSD; Ti-6Al-4V

Yatnalkar, Ravi ShriramExperimental Investigation of Plastic Deformation of Ti-6Al-4V under Various Loading Conditions
Master of Science, The Ohio State University, 2010, Mechanical Engineering

Plastic deformation of 0.25” thick Ti-6Al-4V plate is investigated. Compression, tension and shear tests are carried out from quasi-static strain rates (10-4 s-1, 10-2 s-1) to high strain rates (up to 5x103 s-1) to study the strain rate sensitivity of the material. Tension and compression tests are carried out on specimens machined in different directions of the plate (at 0°, ±45°, 90° to the rolling direction of the plate in tension and at 0°, ±45°, 90° to the rolling direction and through the thickness of the plate for compression tests) to study the anisotropy effects. High temperature compression tests are carried out at 200° C, 400° C and 600° C to study the temperature effects on the plate. The setups required to perform all these tests and the theories behind the high strain rate tests are explained.

The results show the strain rate sensitivity, anisotropy and the temperature effects on the Ti-6Al-4V plate. Johnson – Cook material constants are found out from the experimental data and simulations are run to fit the Johnson Cook model to simulate various tests.

Committee:

Amos Gilat (Advisor); Brian Harper (Committee Member)

Subjects:

Mechanical Engineering; Mechanics

Keywords:

Ti-6Al-4V; Ti64; Experimental Mechanics; Dynamic Behavior of Materials.

Makiewicz, Kurt TimothyDevelopment of Simultaneous Transformation Kinetics Microstructure Model with Application to Laser Metal Deposited Ti-6Al-4V and Alloy 718
Master of Science, The Ohio State University, 2013, Materials Science and Engineering
Laser based additive manufacturing has become an enabling joining process for making one-of-a-kind parts, as well as, repairing of aerospace components. Although, the process has been established for more than a decade, optimization of the process is still performed by trial and error experimentation. At the same time, deployment of integrated process-microstructure models has remained as a challenge due to some of the reasons listed below: (1) lack of good process models to consider the laser-material interactions; (2) inability to capture all the heat transfer boundary conditions; (3) thermo-physical-mechanical properties; and (4) robust material model. This work pertains to the development of robust material model for predicting microstructure evolution as a function of arbitrary thermal cycles (multiple heating and cooling cycles) that can be integrated into a process model. This study focuses on the development of a material model for Ti-6Al-4V and Alloy 718. These two alloys are heavily used in turbine engines and undergo complex phase transformations, making them suited to developing a material model for laser metal deposition (LMD). The model uses simultaneous transformation kinetics (STK) theory to predict the transformation of one parent phase into several products. The model uses calculated thermodynamic properties of the alloys for portions of the respective transformation characteristics. Being a phenomenological model there are several user defined calibration parameters to fit the predicted output to experimental data. These parameters modify the nucleation and growth kinetics of the individual transformations. Analyses of experimental LMD builds are used to calibrate the material model. A Ti-6Al-4V build made on a room temperature substrate showed primarily colony alpha morphology in the bottom half of the build with a transition to basketweave alpha in the top half. An increase in hardness corresponding to the microstructural transition was observed. This sample had an average of 340 HV hardness. Analysis of the calculated thermal profiles at the location of the morphology transition showed a transition from cooling below the beta transus to cooling above the beta transus. The Ti-6Al-4V STK model was calibrated using the experimental data from this sample. The substrate of a second build was heated above the Ti-6Al-4V beta transus. This build showed predominantly basketweave alpha without a microstructural transition. Large prior beta grains (>1mm) were observed growing epitaxially from the substrate. These large grains promoted the basketweave formation. Hardness testing showed an average of 344 HV. Samples built in this way were also fatigue tested in the as built condition. Results show that they match previous builds that had been stress relieved. A third build was performed at room temperature on a substrate with large prior beta grains. This build showed basketweave morphology like the second build even though the substrate was not thermally controlled. The hardness for this build averaged 396 HV which is ~50 HV higher than the previous two. This build shows that it may be possible to produce better mechanical properties by controlling the beta grain size rather than heating the substrate. Eighteen Alloy 718 builds were made using proprietary processing conditions. All of these builds were analyzed for nano-scale γ’ and γ’’ precipitates. Two of the builds were similar but had different laser powers. The low laser power build did not show nano-scale precipitates. The higher power build did show small amounts (<3%) of nano-scale precipitates and a corresponding increase in hardness at their locations. The higher power build was used to develop the STK model for Alloy 718. Sixteen of these builds were part of a design of experiments and are referred to as DOE samples. Eight of them have a single layer while the other eight have multiple layers. They were examined for nano-scale precipitates. The amounts of precipitates were correlated to hardness values and thermal profiles.

Committee:

Sudarsanam Babu (Advisor); Wolfgang Windl (Committee Member)

Subjects:

Aerospace Materials; Materials Science; Metallurgy

Keywords:

Simultaneous Transformation Kinetics; STK; Microstructure Modeling; Laser Additive Manufacturing; Laser Metal Deposition; aerospace repair; Ti-6Al-4V; Inconel 718; Alloy 718; Additive Manufacturing; LAM; LMD;

Ngan, TiffanyEvaluation of the Response of Armor Alloys to High Temperature Deformation
Master of Science, The Ohio State University, 2014, Welding Engineering
High strength alloys, such as titanium alloys and steels have been widely used for armor applications. However, high strength materials have poor formability at room temperature and are prone to cracking during welding. It is necessary to develop alternative manufacturing methods that can replace conventional welding technologies. The main objectives of this project are: 1) development of a testing procedure for evaluation of the response of high strength alloys to hot induction bending, and 2) development of optimal process control windows for hot induction bending of three high strength materials: alloy Ti-6Al-4V and armor steels Armox 440 and ARL XXX. A testing procedure has been developed that combines hot ductility testing, high temperature straining using a GleebleTM thermo-mechanical simulator, high temperature straining followed by room temperature tensile testing, evaluation of response to tempering, phase transformation analysis, thermodynamic simulations, and metallurgical characterization. Hot ductility testing indicates a gradual increase in ductility of the three tested alloys as temperature increases. Strain rate has no significant effect on hot ductility of alloy Ti-6Al-4V and ARL XXX steel. During hot ductility testing, extensive void formation is observed in alloy Ti-6Al-4V between the starting temperatures of alpha and beta transformation and the recrystallization temperature, and in Armox 440 steel between the A1 and A3 temperatures. No voids were found above beta solvus temperature in Ti-6Al-4V and above the A3 temperature in Armox 440. Limited void formation occurs below the start of alpha and beta transformation in Ti-6Al-4V and below the A1 temperature in Armox 440. High temperature straining tests show that strain-induced porosity in Ti-6Al-4V can be avoided if strain is limited below 24% at 430 degrees Celsius and below 7% at 650 degrees Celsius. In Armox 440 and ARL XXX steels, voids were only observed in samples strained to failure. High temperature straining followed by room temperature tensile testing shows that in class 2 Ti-6Al-4V alloy highest ultimate tensile strength is obtained after 11% straining at 430 degrees Celsius. For the two armor alloys, straining above the A3 temperature provides room temperature mechanical properties that are closest to the original properties of two armor steels. Holloman-Jaffe parameters have been developed for evaluation of the effect of hot induction bending on hardness in Armox 440 and ARL XXX steels. A continuous cooling transformation diagram has been developed for the coarse grained heat affected zone of ARL XXX steel. The liquidus, solidus, A1 and A3 temperatures in this steel have been determined using single sensor differential thermal analysis. The reasons for solidification cracking in fillet welds of ARLXXX steel have been investigated using metallurgical characterization and thermodynamic simulations. The presented approach allows developing optimized processing windows for hot bending of high strength alloys. Process parameters developed in this research, in terms of optimal bending temperatures and strain ranges, can be applied to avoid defect formation and minimize loss of properties during hot induction bending of the three tested alloys.

Committee:

John Lippold (Advisor); Boian Alexandrov (Committee Member); David Phillips (Committee Member)

Subjects:

Engineering; Metallurgy

Keywords:

Armor alloys; High strength alloys; Ti-6Al-4V; Armox 440; ARL XXX; Hot induction bending; High temperature; Deformation; Strain-induced porosity; Void formation

Sheridan, Luke CharlesAn Adapted Approach to Process Mapping Across Alloy Systems and Additive Manufacturing Processes
Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering
The continually growing market for metal components fabricated using additive manufacturing (AM) processes has called for a greater understanding of the effects of process variables on the melt pool geometry and microstructure in manufactured components for various alloy systems. Process Mapping is a general approach that traces the influence of process parameters to thermal behavior and feature development during AM processing. Previous work has focused mainly on Ti-6Al-4V (Ti64), but this work uses novel mathematical derivations and adapted process mapping methodologies to construct new geometric, thermal, and microstructural process maps for Ti64 and two nickel superalloy material systems. This work culminates in the production of process maps for both Inconel 718 (IN718) and Inconel 625 (IN625) that were developed via both experimental and analytical data, and the tools used in the established process mapping approach have been thoroughly explored. This has resulted in a non-dimensional template for solidification behavior in terms of material solidification parameters and AM process parameters. The optimized non-dimensional approach presented here will increase the efficiency of future process map development and will facilitate the comparison of process maps across alloy systems and AM processes, laying the ground work for integrated AM feature control and evaluation of current and future materials for AM application.

Committee:

Nathan Klingbeil, Ph.D. (Advisor); Joy Gockel, Ph.D. (Committee Member); Raghavan Srinivasan, Ph.D. (Committee Member)

Subjects:

Engineering; Materials Science; Mechanical Engineering

Keywords:

additive manufacturing; Inconel; Inconel 718; Inconel 625; Ti-6Al-4V; process mapping; microstructure; melt pool; finite elements; closed-form process maps; solidification maps

Sweeney, Deborah May-KatherineELEVATED TEMPERATURE OXIDATION OF BORON MODIFIED Ti-6Al-4V
Master of Science in Engineering (MSEgr), Wright State University, 2008, Materials Science and Engineering
The addition of trace amounts (~0.1 wt%) of boron to titanium alloys refines theas-cast grain size by an order of magnitude from 2000μm to 200μm. The reduced grain size has potentially beneficial effects on the processibility of titanium alloys. Reports also indicate that the room temperature corrosion resistance of the boron containing alloys may be substantially greater than conventional titanium alloys. In this study, the effects of boron addition on oxidation resistance are investigated, since conventional titanium alloys have limited corrosion resistance in air above 650°C. The mass gain per unit surface area was measured during elevated temperatures exposed to air to compare the oxidation of the alpha-beta titanium alloy, Ti-6Al-4V with boron and without boron additions. These findings are presented in conjunction with the characterization of boron modified titanium alloys. Results include weight gain as a function of time and temperature, activation energy calculation, microstructural characterization of the oxide layer, measurement of oxide scale thickness and composition profiles. The experimental results are compared to literature values for similar experiments on conventional titanium alloys.

Committee:

Raghavan Srinivasan, PhD (Advisor); Sharmila Mukhopadhyay, PhD (Committee Member); Allen Jackson, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Metallurgy

Keywords:

boron; titanium; Ti-6Al-4V; oxidation

Subramanian, SethuramanA Study Of The Effects of Laser Shock Peening (LSP) On the Fatigue Life Of Ti-6Al-4V (ELI) Spinal Implant Rods
MS, University of Cincinnati, 2012, Engineering and Applied Science: Materials Science
Solid implant rods made of Ti-6Al-4V (ELI) are currently used in spinal implant devices. Due to the high stiffness of these solid rods, the implant devices are unable the meet the needs for increased flexibility over a wide range of human activities owing to changes in lifestyles and in human work environments, thereby, necessitating the need for more flexible rods. Hence, designing, developing, manufacturing and testing flexible spinal implant rods is the aim of this project. In this project, increased flexibility has been achieved by reducing the cross-section of the solid rods. Since reduction in cross-section leads to reduced load bearing capacity, the fatigue endurance level of the flexible rods is lowered. In order to increase the endurance levels of these flexible rods, Laser Shock Peening (LSP) has been used. As a part of this project, the Ohio Center for Laser Shock Processing of Advanced Materials and Devices (LSP Center) has been set-up at the University of Cincinnati through the Ohio State Third Frontier Wrights Projects funding. This project is a collaborative effort between the LSP Center and X-Spine Systems Inc., an implant devices manufacturer based in Miamisburg, Ohio. All the experimental and modeling work pertaining to this project were carried out at the LSP Center. To begin with, flexible rods were designed using finite element modeling. After considering models like flat rods and flute rods, the final design was chosen as grooved rods. Based on this design, grooved rods were machined by X-Spine Systems Inc. Also, a fatigue model to predict the effect of LSP on the fatigue life of the flexible grooved rods was developed. Based on the model, fatigue experiments were designed and carried out. Per requirements outlined by X-Spine Systems Inc, the flexible implant rods were required to demonstrate endurance level (run-out) for 5 million cycles in construct fatigue testing at -160N. This load is higher than the current industrial standard of -150N for solid rods in constructs. These tests were carried out per standard procedures recommended in ASTM F-1717 04. Results from the construct tests showed that the construct with grooved rods endured -160N for 5 million cycles. Static 4-point bending tests carried out on the flexible single rods showed a 10% reduction in stiffness as compared to the rigid solid rods. After successful construct testing, fatigue testing of flexible rods in 4-point bending showed that LSP improved endurance level by a small extent. To study the extent of LSP on the rods, residual stress measurements as a function of depth were performed on LSP treated rods. The results from residual stress measurement indicated that LSP gives rise to 5 X deeper compressive stress regimes as compared to conventional shot peening techniques. Fatigue fractured rods were observed in Scanning Electron Microscope (SEM) to study the fractographic features under different treatments and loading conditions. The overall results showed that flexible grooved rods meeting all the specifications were successfully developed and tested in this project. Results pertaining to these are presented and discussed.

Committee:

Vijay Vasudevan, PhD (Committee Chair); Dong Qian, PhD (Committee Member); Rodney Roseman, PhD (Committee Member)

Subjects:

Materials Science

Keywords:

laser shock peening;implant rods;Ti-6Al-4V;fatigue;;;

Loughnane, Gregory ThomasA Framework for Uncertainty Quantification in Microstructural Characterization with Application to Additive Manufacturing of Ti-6Al-4V
Doctor of Philosophy (PhD), Wright State University, 2015, Engineering PhD
The sampling of three dimensional (3D) mesoscale microstructural data is typically prescribed using simple rules, likely resulting in data under- or oversampling depending on the measurement(s) of interest. The first part of this work investigates one approach for determining a minimally sufficient sampling scheme for 3D microstructural data, using computer-generated phantoms of polycrystalline grain microstructures. Sources of error that are observed experimentally are modeled using phantoms, in order to determine the effect that errors have on the microstructural statistic(s)-of-interest. Minimally-sufficient sampling schemes are then established based on a required accuracy in the microstructural statistic(s). The characterization error modeling framework is subsequently demonstrated on experimentally-derived statistics from high resolution 3D serial sectioning data, in order to inform future experiments on the same material. The second part of this work lends the aforementioned approach to the additive manufacturing (AM) of Ti-6Al-4V. Statistical analysis and virtual modeling tools developed herein are used to analyze alpha and beta phase microstructures in two thin-walled Ti-6Al-4V samples. Ultimately, this research aims to provide a virtual modeling framework for analyzing uncertainty in microstructural characterization, and to produce an offering of novel solutions for addressing current issues associated with rapid qualification methods for AM of Ti-6Al-4V components.

Committee:

Nathan Klingbeil, Ph.D. (Advisor); Ramana Grandhi, Ph.D. (Committee Member); Raghu Srinivasan, Ph.D., P.E. (Committee Member); Michael Uchic, Ph.D. (Committee Member); Jaimie Tiley, Ph.D., P.E. (Committee Member); Peter Collins, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Materials Science; Mechanical Engineering

Keywords:

3D microstructural characterization; phantom microstructures; uncertainty quantification; integrated computational materials engineering; additive manufacturing; laser engineered net shaping; Ti-6Al-4V alpha beta;

Phillips, Peter LouisIntegrated Multiaxial Experimentation and Constitutive Modeling
Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Mechanical Engineering
Modern plasticity models contain numerous parameters that no longer correlate directly to measurements, leading to a lack of uniqueness during parameter identification. This problem is exacerbated when using only uniaxial test data to populate a three-dimensional model. Parameter identification typically is performed after all experiments are completed, and experiments using different loading conditions are seldom conducted for validation. Experimental techniques and computational methods for parameter identification are sufficiently advanced to permit real-time integration of these processes. This work develops a methodology for integrating multiaxial experimentation with constitutive parameter calibration and validation. The integrated strategy provides a closed-loop autonomous experimental approach to parameter identification. A continuous identification process guides the experiment to improve correlation across the entire axial-torsional test domain. Upon completion of the interactive test, constitutive parameters are available immediately for use in finite element simulations of more complex geometries. The autonomous methodology is demonstrated through both analytical and physical experiments on Ti-6Al-4V. The proposed approach defines a framework for parameter identification based on complete coverage of the stress and strain spaces of interest, thereby providing greater model fidelity for simulations involving multiaxial stress states and cyclic loading.

Committee:

Robert Brockman (Advisor); Steven Donaldson (Committee Member); Thomas Whitney (Committee Member); Andrew Rosenberger (Committee Member); Reji John (Committee Member)

Subjects:

Engineering; Mechanical Engineering

Keywords:

Autonomous Experimentation; Multiaxial; Torsion; Plasticity; Constitutive Model; Ti-6Al-4V; Finite Element Method Updating

Tiley, Jaimie S.Modeling of Microstructure Property Relationships in Ti-6Al-4V
Doctor of Philosophy, The Ohio State University, 2003, Materials Science and Engineering
Fuzzy logic neural network models were developed to predict the room temperature tensile behavior of Ti-6Al-4V. This involved the development of a database relating microstructure to properties. This necessitated establishing heat treatment processes to develop microstructural features, mechanical testing of samples, creating rigorous stereology procedures, developing numerical models to predict mechanical behavior, and determining trends and inter-relationships relating microstructural features to mechanical properties. Microstructural features were developed using a Gleeble™ 1500 Thermal-mechanical simulator. The system used computer controlled resistive heating equipment to provide rapid cooling and heating abilities. Samples were obtained from mill annealed plate material and both alpha + beta forged and beta forged materials. A total of 72 samples were beta solutionized and heat treated using different heating and cooling conditions. Rigorous stereology procedures were developed to characterize the important microstructural features. The features included Widmanstätten alpha lath thickness, volume fraction of total alpha, volume fraction of Widmanstätten alpha, grain boundary alpha thickness, mean edge length, colony scale factor, and prior beta grain size factor. Chemical composition was also determined using standard chemical analysis and microscopy techniques. The samples were tested for yield strength, ultimate tensile strength, and elongation at room temperature. The samples were imaged using various microscopy techniques. Results from the tests and the characterization were used to develop fuzzy logic neural network models to predict the mechanical behaviors and develop relationships between the microstructural features (using CubiCalc RTC™). Results were compared to standard multi-variable regression models. The fuzzy logic neural network models were able to predict the yield, and ultimate tensile strength, within acceptable error ranges with a limited number of input data samples. The models also predicted the elongation values but with larger errors. The models also provided trends detailing the relative importance of the input parameters and the inter-relationships between the features. Of particular importance, the models identified the importance of the Widmanstätten alpha lath widths, the mean edge length of the Widmanstätten alpha laths, the colony scale factor, and the prior beta grain size to the tensile behavior.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Ti-6Al-4V; fuzzy logic; neural networks; microstructure

Hammer, Jeremiah ThomasPlastic Deformation and Ductile Fracture of Ti-6Al-4V under Various Loading Conditions
Master of Science, The Ohio State University, 2012, Mechanical Engineering

Plastic deformation and ductile fracture of Ti-6Al-4V plate stock is investigated under multiple loading conditions. The objective of this study is to generate experimental data that can be used for the development and calibration of constitutive and failure models for numerical simulations of dynamic events. Plastic deformation is investigated at various strain rates, orientations, temperatures, and stocks. The stress state dependence of ductile fracture is also investigated.

Uniaxial tension, compression, and pure shear experiments are conducted at strain rates ranging from 1.0E-4 1/s to 8000 1/s. Specimens are fabricated from several sheet and plate stocks with thicknesses of 2.29mm, 3.56mm, 6.35mm, and 12.7mm. Compression and tension tests are conducted with specimens oriented in several different directions. These data show significant strain rate sensitivity in tension, compression and shear. Both plates exhibit anisotropic plastic deformation behavior in tension and compression. The response of each of the plates are significantly different for yield stress, flow stress, hardening, failure, and anisotropic effects.

Ductile fracture testing is conducted at various stress states, which are achieved with mechanical tests on various sample geometries subjected to various loading conditions. Tension tests are conducted on thin flat specimens, wide flat specimens and axisymmetric specimens with varying notch radii. Thin walled tube specimens are subjected to combined axial-torsional loading for additional states of stress. The results show that the stress triaxiality alone is unable to properly capture the failure characteristics of material. Digital image correlation is used to measure surface strains of the specimens. Parallel LS-DYNA simulations are used to determine the stress states and fracture strains. A fracture locus for Ti-6Al-4V is created in the stress triaxiality and Lode parameter stress space giving a more accurate description of the material fracture.

An experimental technique is introduced to measure full field strains using three dimensional digital image correlation at temperatures up to 800C. This test setup has been designed to be a straight forward, repeatable, and accurate method for measuring strains at high temperatures. Design hurdles included thermal gradients of air, speckle pattern adhesion, viewing window image distortion, camera calibration, and infrared light pollution of the camera sensor. For validation, the coefficient of thermal expansion for Ti-6Al-4V up to 800C is measured using the technique and compared to published values. Tests on Ti-6Al-4V were conducted in tension, compression, and torsion (shear). Experimentally measured coefficient of thermal expansion values correlate well with handbook values. The system performs well for each of the tests conducted here and gives substantially more data than standard methods.

Committee:

Amos Gilat (Advisor); Mark Walter (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Titanium; Hopkinson bar; Kolsky bar; digital image correlation; high temperature; ti-6al-4v; plasticity; anisotropic; mechanical engineering

Bontha, SrikanthThe Effect of Process Variables on Microstructure in Laser-Deposited Materials
Doctor of Philosophy (PhD), Wright State University, 2006, Engineering PhD
The ability to predict and control microstructure in laser deposition processes requires an understanding of the thermal conditions at the onset of solidification. The focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (laser power and velocity). The approach employs the well-known Rosenthal solution for a moving point heat source traversing an infinite substrate. Cooling rates and thermal gradients at the onset of solidification are numerically extracted from the Rosenthal solution throughout the depth of the melt pool, and dimensionless process maps are presented for both 2-D thin-wall and bulky 3-D geometries. Results for both small-scale (LENS) and large-scale (higher power) processes are plotted on solidification maps for predicting trends in grain morphology in laser-deposited Ti-6Al-4V. Although the Rosenthal predictions neglect the nonlinear effects of temperature-dependent properties and latent heat of transformation, a comparison with 2-D and 3-D nonlinear FEM results for both small-scale and large-scale processes suggests that they can provide reasonable estimates of trends in solidification microstructure. In particular, both the Rosenthal and FEM results suggest that changes in process variables could potentially result in a grading of the microstructure (both grain size and morphology) throughout the depth of the deposit and that the size-scale of the laser deposition process is important. In addition, the effects of a uniform distributed heat source on melt pool geometry and microstructure is investigated by superposition of the Rosenthal point source solution. In particular, the effect of beam width on melt pool length, melt pool depth, solidification cooling rates and thermal gradients is investigated. These results are also interpreted in the context of a solidification map to investigate the effect of beam width on trends in grain morphology in laser-deposited Ti-6Al-4V. Finally, transient effects near the free edge are investigated in both 2-D thin-wall and bulky 3-D geometries through thermal finite element analysis. Here the effect of transient melt pool behavior on solidification cooling rates and thermal gradients (and thereby the resulting microstructure) is investigated.

Committee:

Nathan Klingbeil (Advisor)

Keywords:

Laser Deposition; Ti-6Al-4V; Microstructure; Rosenthal Solution; Solidification Map; Finite Element Modeling; Transient Effects; Distributed Heat Source

Kuchi, Satish C.Effect of Finite Geometry on Solidification Microstructure in Beam-Based Fabrication of Thin Wall Structures
Master of Science in Engineering (MSEgr), Wright State University, 2009, Mechanical Engineering
With the advent of Rapid Protyping (RP) in the manufacturing industry, many newtechnologies came into development for quick fabrication of materials. One of the most versatile fabrication process among them is additive manufacturing. Laser based manufacturing (LBM) and electron beam manufacturing (EBM) processes use the energy from a laser or electron beam to build up structures layer by layer directly from powdered metals and wire feed stock. It has been determined from previous work that solidification cooling rate and thermal gradients are the important factors for controlling microstructure (grain size and morphology) and resulting mechanical properties of the deposit. The previous work was concerned with semi-infinite thin wall and bulky 3-D geometries where the thermal solution near the laser or electron beam is independent of the boundaries. The goal of the current study is to investigate the effects of finite geometry (finite length and height of the build) and associated non-steady state changes in process variables (beam power and velocity) on the thermal conditions controlling microstructure through parametric finite element modeling with ABAQUS. The results of this project will guide process designers in the additive manufacture of a variety of common thin walled geometries.

Committee:

Nathan Klingbeil, PhD (Advisor); Raghavan Srinivasan, PhD (Committee Member); Srikanth Bontha, PhD (Committee Member)

Subjects:

Design; Mechanical Engineering

Keywords:

Solidi¿¿¿¿¿¿cation; mdb; ¿¿¿¿¿¿nite; Ti-6Al-4V; Process Maps; thermal gradients; MICROSTRUCTURE

Barker, Samuel PaulKinetically-Controlled Nitridation of Titanium Alloys
Master of Sciences, Case Western Reserve University, 2010, Materials Science and Engineering

Traditional titanium nitridation techniques increase surface hardness by forming a thin layer of titanium nitrides (Ti2N and TiN) on the substrate surface. Such nitride-forming techniques successfully increase hardness and improve wear resistance, but can result in premature fatigue failure.

A novel method, known as kinetically-controlled titanium nitridation, is capable of diffusing nitrogen into titanium without the formation of undesirable nitrides by providing an extremely low nitrogen pressure. Controlled parameters have yielded nitrogen-stabilized α-Ti cases of (10-30) micrometers in thickness with as much as a three-fold improvement in surface hardness.

Nevertheless, diffusion-affecting features such as structural defects and crystallographic orientation play a significant role in non-uniform nitride nucleation – giving way to unwanted nitrides. Numerous experiments combined with surface analysis techniques such as XRD, XPS, SEM, XEDS, EELS, and TEM were utilized to more fully understand such causes of non-uniform nitridation, as well as methods to control them.

Committee:

Frank Ernst, PhD (Committee Chair); Gary Michal, PhD (Committee Member); Gerhard Welsch, PhD (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

Ti-6Al-4V; nitridation; titanium nitrides; case-hardening; nitrogen in solid solution; powder pack; getter; XRD; XPS; TEM; XEDS; EELS; TiN; twinned; Ti2N; nucleation; kinetically-controlled

Kullman, Nicholas AllenMetallurgical Characterization of Armor Alloys for the Development and Optimization of Induction Bending Procedures
Master of Science, The Ohio State University, 2011, Welding Engineering

A need exists to develop manufacturing technologies for armor alloys that can replace risk-sensitive welding. Armor materials have poor formability at room temperature and are prone to cracking even at modest bend radii. An Induction assisted bending process has been successfully performed using titanium alloy Ti-6Al-4V. The main objective of this work was to develop optimized procedures for induction assisted hot bending of titanium and steel armor. The response of several armor alloys to high temperature straining and the effect of induction bending on their mechanical properties and microstructure were evaluated. A methodology for characterization and testing of armor materials which supports the development of induction bending procedures was also developed. The results of this research have expanded the elevated temperature data for several grades of alloy Ti6-4 and for three armor steel alloys. The latter included single grades of rolled homogeneous armor (RHA), high hard armor (HHA) and Armox 440 steel.

Induction bent plates of grade 5 Ti6-4 were characterized using optical and scanning electron microscopy. Utilizing a mandrel radius of 50 mm, crack free induction bends were made in ½” thick grade 5 plates at temperatures as low as 427 °C. Using a 12.5 mm mandrel radius resulted in cracking at bending temperatures below 621 °C. The surface roughness of the plates may have provided stress concentration sites for crack initiation. Also, voids were discovered near the surface and in front of crack tips using optical and scanning electron microscopy. These voids provide evidence for a possible crack propagation mechanism. The depth of the cracks and the void locations were utilized to calculate the strain associated with these defects. This was used to generate a strain vs. deformation temperature plot that can be utilized by manufactures as a process development and control tool.

Heating of Armox 440 below the A1 temperature resulted in over tempering and a significant softening with a drop in hardness to 28 HRc from 47.6 HRc in the as-delivered condition. Straining of this still below the A1 temperature at 700 °C resulted in significant loss of room temperature tensile strength. These results illustrate the need for a post bend heat treatment of Armox 440 to restore the mechanical properties.

A methodology for characterization and testing of armor materials has been established, which supports the development of induction bending procedures that can be applied to other materials. This methodology includes: 1) hot ductility testing to determine the optimum temperature range for induction bending; 2) metallurgical characterization and hardness testing to evaluate the material response to elevated temperature exposure; and 3) high temperature straining followed by room temperature tensile testing and hardness testing to evaluate the effect of induction bending on mechanical properties. The test results are utilized to generate an optimal strain vs. temperature process control window for induction bending of the tested armor alloy.

Committee:

John Lippold, Dr. (Advisor); Boian Alexandrov, Dr. (Advisor); Suresh Babu, Dr. (Committee Member)

Subjects:

Engineering

Keywords:

Ti-6Al-4V; titanium armor alloys; armor steel; Armox 440; RHA; HHA; induction bending; hot bending; hot forming

Subramanian, RohitCOMPUTATIONAL FRAMEWORK TO ASSESS ROLE OF MANUFACTURING IN MATERIAL-DEFECT RELATED FAILURE RISK
Master of Science, The Ohio State University, 2014, Industrial and Systems Engineering
In the design of complex engineering systems, the failure risk is usually assumed to arise out of operational variations. Therefore, the risk from worse than normal operating conditions is incorporated into its estimated service life via safety factor approach while a product is being developed. However the failure risk due to variations in material properties, and processes used in manufacture is often lost because the objective quality specifications that are used to control those parameters, do not take into consideration a lot of factors that are hidden beneath the measurable properties. This leads to uncertainty in the performance of some systems, and is often the cause of their premature failure causing great damage to life and/or property. The primary variation in materials stems from the presence of material inhomogeneities, or defects that act as a source for micro-cracks and voids which eventually result in fatigue failure in the component. This thesis aims to evaluate the role of manufacturing in the risk of failure due to such discrete material defects, and explores computational models that can be used to quantify their effect. A methodology is proposed to take into consideration process changes, and how they impact material properties, and consequently performance characteristics. Overall, a framework is suggested, based on the tools deployed in this thesis, to characterize potential failure modes and estimate their impact on the behavior of complex systems.

Committee:

Rajiv Shivpuri (Advisor); Jerald Brevick (Committee Member)

Subjects:

Aerospace Materials; Industrial Engineering; Materials Science; Mechanical Engineering; Metallurgy; Systems Design

Keywords:

Multiscale Modeling; Hierarchical Materials; Failure Risk; Windmill Bearing Failure; AISI 52100 Steel; Ti-6Al-4V; Melt-related Defects in Titanium; Aeroengine Disk Failure; Multibody Modeling; Hard Alpha inclusions;

Blankenship, Alec MitchellElucidating the Role of Microstructure, Texture, and Microtexture on the Dwell Fatigue Response of Ti-6Al-4V
Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering
Ambient temperature dwell sensitivity is known to be deleterious to the fatigue response of near-alpha titanium alloys. Dwell fatigue refers to the presence of a sustained hold at peak stress as opposed to the continuous variation of normal cyclic fatigue loading. This reduction in failure life-times from dwell loading is attributed to early crack nucleation and faster crack propagation. The degradation is the result of plastic anisotropy on the microstructural scale along with tendency of titanium alloys to creep at low temperatures at stresses well below the 0.2% offset yield strength. Despite being the most widely used titanium alloy, Ti-6Al-4V has not been the subject of most dwell fatigue research. Generally, dwell sensitivity is microstructurally dependent and believed to only affect Ti-6Al-4V when severe crystallographic texture is present and under high peak stress loading. Recent studies, however, have suggested that small clusters of preferred crystal orientations, known as micro-textured regions (MTR), can have a significant effect on the dwell sensitivities in Ti-6Al-4V even without severe overall texture in the material. In this study, smooth-bar fatigue specimens were subjected to uniaxial fatigue at 20 Hz cyclic and 2-min dwell loading conditions under load-control at stresses representative of service conditions, until failure occurred. A reduction in specimen life-times by approximately a factor of three was observed under dwell conditions, which was less than for the highly susceptible near-a titanium alloys such as Ti-6Al-2Sn-4Zr-2Mo, where the dwell debit is often in excess of a factor of ten. Measurement of fatigue and dwell fatigue crack growth rates revealed a significant acceleration of the dwell crack growth rates in certain cases. Backscattered electron imaging and electron backscattered diffraction were utilized to quantify the interaction between the cracks and local microstructure. Though no correlation was found between crack growth rate and the local microstructure and crack trajectory, strong correlation was found between crack growth and the presence of grains with [0001] axes at small angles (<25º) with respect to the stress axis. These observations were further validated by serial sectioning to the fracture surface plane of a low life dwell fatigue specimen. The results showed that the accelerated growth rates, which produced a characteristic faceted fracture topography, occurred over a length scale consistent with the underlying microtextured regions.

Committee:

Raghavan Srinivasan, Ph.D. (Advisor); Adam Pilchak, Ph.D. (Committee Member); Joy Gockel, Ph.D. (Committee Member)

Subjects:

Materials Science; Mechanical Engineering

Keywords:

Titanium; Dwell; Fatigue; Fracture; microstructure; Micro-Texture; Crack Growth; Ti-6Al-4V; Texture

Holycross, Casey M.A Critical Assessment of the High Cycle Bending Fatigue Behavior of Boron-modified Ti-6Al-4V
Master of Science in Engineering (MSEgr), Wright State University, 2010, Materials Science and Engineering

Boron-modified Ti-6Al-4V alloys have shown increased performance in mechanical properties over unmodified alloys and are currently of interest for use in turbine engine applications. These alloys offer up to 40% increase in ultimate tensile strength, up to 30% increase in stiffness, and favorable damage characteristics while maintaining a ductility greater than 10%. These attractive properties are attributed to small additions of boron that refine the microstructure and form strong and stiff TiB whiskers. Previous research has found that these modified alloys compare favorably in fatigue. Samples machined from a powder-metallurgy forging with nominal composition Ti-6Al-4V-1B, were tested in fully-reversed bending at room temperature using a vibration based step test method to determine the 106 fatigue strength. This method simulates fatigue failure modes for high speed turbomachinery more accurately and produces high-cycle fatigue results much faster than traditional tensile testing methods. Results were compared with data generated in a similar fashion for Ti-6Al-4V, as well as traditional methods. Additionally, failure mode and damage characteristics were identified using fractographic analysis.

The fatigue strengths at 106 cycles compared poorly in comparison to both tensile and bending data for Ti-6Al-4V. This poor performance was attributed to inclusions of foreign material that were smaller than the theoretical maximum powder dimension. Fatigue damage characteristics were found to be consistent with previous research, with the most severe damage modes having little influence from the TiB whiskers.

Committee:

Raghavan Srinivasan, PhD (Advisor); Ravi Penmetsa, PhD (Committee Member); Seshacharyulu Tamirisakandala, PhD (Committee Member)

Subjects:

Aerospace Materials; Materials Science; Mechanical Engineering

Keywords:

fatigue; titanium; bending; Ti-6Al-4V; FEM

Li, Jr-HungJoint Zone Evolution in Infrared Brazed Ti-6Al-4V with Copper Thin Film
PhD, University of Cincinnati, 2005, Engineering : Materials Science
Joining of Ti-6Al-4V with thin film copper fillers was developed using an active infrared processing technique. The process includes rapid infrared heating of the specimen to 1125°C, and holding at that temperature for 60 seconds in flowing argon without vacuum, then followed by rapid cooling. The copper thin film filler was electroplated to various thicknesses: 1, 2, 4 and 8 µm. Excellent wetting between copper and Ti-6Al-4V was observed in all joints. High joint shear strengths were obtained for all joints. The highest joint strength, 616±10 MPa, was obtained with 2 µm filler thickness. This joint strength is significantly higher than the shear strength of pure copper, 174 MPa, or Ti-6Al-4V, 542 MPa. When the filler material was only 1 µm, the joint zone was not completely filled. X-ray diffraction analysis on the as fractured joint surface revealed the presence of intermetallic compounds when joined with 4 and 8 µm copper fillers but it disappeared when joined with 1 and 2 µm copper fillers. Compositions of the joining zone and the base materials near the joining zone were determined with an energy dispersive spectroscope (EDS) and electron microprobe analysis (EMPA) equipped on the SEM. Also, specimen were joined with 25 µm Cu at 1125°C with various holding time of 0, 60, 120, 180, 240, and 300 seconds to observe the joint zone evolution. The joint evolution follows by melting of Cu filler, dissolution of base materials into Cu liquid, saturation of Cu liquid and isothermal solidification of liquid phase.

Committee:

Dr. Ray Lin (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Active Infrared processing, Ti-6Al-4V, Joint shear strength, Intermetallic compounds, Joining affected zone, Widmanst&228;tten structure

Kuntz, Sarah LouiseFeasibility of Attaining Fully Equiaxed Microstructure through Process Variable Control for Additive Manufacturing of Ti-6Al-4V
Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering
One of the greatest challenges in additive manufacturing is fabricating titanium structures with consistent and desirable microstructure. To date, fully columnar deposits have been achieved through direct control of process variables. However, the introduction of external factors appears necessary to achieve fully equiaxed grain morphology using existing commercial processes. This work introduces and employs an analytic model to relate process variables to solidification thermal conditions and expected beta grain morphology at the surface of and at the deepest point in the melt pool. The latter is required in order to ensure the deposited microstructure is maintained even after the deposition of subsequent layers and, thus, the possibility of equiaxed microstructure throughout. By exploring the impact of process variables on thermal, morphological, and geometric trends at the deepest point in the melt pool, this work evaluates four commercial processes, estimates the range of process variables capable of producing fully equiaxed microstructure, and considers the expected size of the resultant equiaxed melt pool.

Committee:

Nathan Klingbeil, Ph.D. (Advisor); Joy Gockel, Ph.D. (Committee Member); Raghavan Srinivasan, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Mechanical Engineering; Metallurgy; Morphology

Keywords:

additive manufacturing; 3D printing; titanium alloy; Ti64; Ti-6Al-4V; equiaxed morphology; equiaxed beta grains; solidification maps; process variable control; process parameter control; power, velocity, preheat; Rosenthal; Hunts criterion curve equations

Jahadakbar, AhmadrezaThe Additively Manufactured Porous NiTi and Ti-6Al-4V in Mandibular Reconstruction: Introducing the Stiffness-Matched and the Variable Stiffness Options for the Reconstruction Plates.
Master of Science, University of Toledo, 2016, Mechanical Engineering
Mandibular reconstruction surgery is a part of treatment for cancer, tumor, and all the cases that involve segmental defects. One of the most common approaches for the reconstruction surgery is to resect the segmental defect and use a double barrel fibula graft to fill the resected region and recover the mandible’s normal functions, such as chewing. The grafted bone is connected to the host mandible, using the standard of the care Ti-6Al-4V fixation plates. The fixation plates are available in the form of prefabricated plates and also patient-specific plates in the market. Due to the high stiffness of the Ti-6Al-4V plates in comparison with the mandible bone and the grafted bone, the loading distribution on the whole reconstructed mandible will be different from a healthy mandible. The high stiffness fixation hardware carries a great portion of the loading and causes stress shielding on the grafted bone and the surrounding host bone. Based on the bone remodeling theory, the stress shielding on the cortical bone causes bone resorption and may lead to implant failure. A solution to reduce the risk of implant failure is to use a low stiffness biocompatible material for the mandibular fixation plates. We have proposed the use of stiffness-matched, porous NiTi fixation plates either in the form of patient-specific or prefabricated, instead of the standard of the care Ti-6Al-4V plates. NiTi is a biocompatible material that has a low stiffness in comparison with Ti-6Al-4V and also benefits from the superelastic feature. Superelasticity, which can also be found in bone tissues, allows the material to recover large strains (up to 8%) and increases the shock absorption. In this thesis, we have evaluated the use of proposed fixation hardware by comparing it with a healthy mandible and a reconstructed mandible using the standard method. To this end, first different models including a healthy mandible, a reconstructed mandible using patient-specific Ti-6Al-4V fixation hardware, a reconstructed mandible using stiffness-match patient-specific hardware, and several prefabricated fixation plates were prepared. After verification of the models, the cases of reconstructed mandibles were used to simulate different periods, including during healing, and post-healing periods. Also, different loading conditions including highest bite force on the first molar tooth, rest condition, and also highest bite force on a dental implant right in the grafted bone were simulated. Also, the theory of applying pretention to the fixation plates was evaluated using the finite element method. We also designed and evaluated a set of prefabricated fixation kits with various stiffness option. After all these finite element simulations and having the CAD files of the porous fixation plates, the possibility of fabrication of the proposed hardware, in both forms of patient-specific, and prefabricated plates was evaluated using selective laser melting.

Committee:

Mohammad Elahinia (Advisor); Mohammad Elahinia (Committee Chair); Mehdi Pourazady (Committee Member); Efstratios Nikolaidis (Committee Member)

Subjects:

Biomechanics; Biomedical Engineering; Materials Science; Mechanical Engineering

Keywords:

Mandibular reconstruction surgery, Finite Element, Stiffness-matched fixation hardware, Selective laser Melting, Ti-6Al-4V, NiTi, Superelastic NiTi, Stress shielding, Fabrication, Biomaterial, patient-specific fixation hardware, custom-made

Bohun, Michael H.Several Non-Destructive Inspection Methods Applied to Quantify Fretting Fatigue Damage in Simulated Ti-6Al-4V Turbine Engine Dovetail Components
Doctor of Philosophy (Ph.D.), University of Dayton, 2012, Materials Engineering
The objective of this research is to determine the ability of several Non-Destructive Inspection (NDI) methods to detect various levels of High Cycle Fatigue fretting fatigue damage induced in simulated Ti-6Al-4V dovetail engine components. To generate various levels of fretting fatigue damage; a specially designed dovetail specimen is utilized which more accuracy simulates the cyclic loading interaction between a compressor blade and disk of an aircraft turbine engine. All fretting fatigue tests were conducted with un-coated Ti-6Al-4V alloy at ambient temperature, at a load ratio of 0.1, and two 30 Hz cyclic load levels (10% and 30% of expected life). In addition, two microstructures (α+β, β-annealed) are utilized to determine their effect on fretting fatigue as well as the NDI signal response. To quantify the extent of fretting fatigue damage; mini-C specimens are extracted from the fretted dovetail specimens and “step-tested” to quantify the debit in fatigue strength. Specimens are heat-tinted after fretting fatigue to help qualify the extent of fretting fatigue damage and aid in crack initiation site identification using both optical microscope and the Scanning Electron Microscope (SEM). Three NDI techniques are used to qualify the extent of fretting fatigue damage in Ti-6Al-4V and relate this damage to the NDI signal response and the debit in fatigue strength. The NDI techniques utilized in this research include: White Light Interference Microscopy (WLIM), Wyle Lab Eddy Current Inspection System (ECIS) and the JENTEK Meandering Winding Magnetometer (MWM) Array. Note: these NDI techniques are used “as is” and were not modified for fretting fatigue detection. However, in the case of the WLIM, a fretting fatigue damage parameter methodology is utilized to specifically quantify the extent of fretting damage. In addition, the Scanning Electron Microscope (SEM), Auger Electron Spectroscopy (AES), and Knoop micro-hardness tester were utilized to investigate the compacted fretting fatigue layer beneath the fretting fatigue scar and determine the existence of either a tribologolically transformed structure (TTS) or hard alpha case (HAC). In general, all three NDI techniques were able to detect various degrees of fretting fatigue damage (i.e., fretting fatigue cracks). However, as fretting fatigue cycles increased, the white light surface measurement technique's ability to discern higher levels of crack damage is suppressed by the modification of the contact fretting surface features (i.e., particle compaction with reduced asperity heights with nano sized contact debris) to include debris filled pits and cracks that help promote the loss of surface fidelity. The WLIM damage parameter and elements of the JENTEK MWM signal did correlate with the Mode I Newman-Raju stress intensity factor. However, the detection of fretting fatigue damage beyond fretting fatigue cracking was not attempted and would require special calibration specimens of a specific damage type (i.e., TTS, HAC, multiple co-linear cracking or perpendicular, slanted or zig-zag cracking, or corrosion effects) to correlate the NDI signal response to that damage.

Committee:

Dr. Daniel Eylon, D. Sc. / Materials Engineering (Committee Chair); Dr. James A. Snide, Ph.D. / Materials Engineering (Committee Member); Dr. Terrence P. Murray, Ph.D. / Materials Engineering (Committee Member); Professor Gerald Shaughnessy, M.S. / Mathematics (Committee Member)

Subjects:

Aerospace Materials; Materials Science

Keywords:

Fretting Fatigue; Non-Destructive Inspection; Turbine Engine; Titanium Ti-6Al-4V; Eddy Current; White Light

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