Master of Science in Mechanical Engineering (MSME), Wright State University, 2018, Mechanical Engineering

As technology advances, the abilities of civilian and military vehicles, both air and ground, will undoubtedly increase as well. One of the main areas of improvement is in the electronics area. The new electronics are ever smaller, use ever higher amounts of electrical power, and require ever smaller temperature tolerances. This leads to the problem of effectively managing the increasing thermal loads and temperature tolerances on these systems. One electronic system that causes concern is a high energy pulse system (HEPS). These devices have very high thermal loads (100s of kW). On an air vehicle, where thermal management by legacy methods (i.e. fuel as the heat sink) is already problematic, a HEPS will certainly overload the thermal management system (TMS). HEPS performance must be understood and quantified more accurately to understand the design requirements of a TMS for this device. To aid in this understanding, the HEPS itself and a palletized system to thermally manage the HEPS will be modeled. Previous analysis of a cryogenic palletized HEPS contained a simplified power model for a HEPS that had a set efficiency and always gave a certain amount of optical power out and a certain amount of power dissipated as heat based on that set efficiency. The HEPS model developed and presented takes into account the temperature of internal HEPS components and changes the efficiency accordingly. The HEPS efficiency changes with component temperature to provide a better understanding of the consequences of not thermally managing a HEPS effectively. Along with the HEPS model, a cryogenic-based palletized TMS using Liquefied Natural Gas (LNG) for indirectly cooling the HEPS was modeled. Using LNG as a method of cooling is a possible alternative to using very large legacy systems (fuel as heat sink) to cool a HEPS. The architecture of this palletized system uses LNG to cool the heat loads. The LNG then becomes the fuel for the turbo-generator, which produces electrical po (open full item for complete abstract)

Doctor of Philosophy (Ph.D.), University of Dayton, 2010, Electro-Optics

The atmospheric transmission windows of 2-5 and 8-12 µm, coupled with organic and other chemical absorption lines occurring throughout this middle-infrared (mid-IR) wavelength region give rise to a wide variety of medical, scientific, commercial and military applications. Communications, remote sensing, IR countermeasures, laser surgery and non-invasive imaging are just a few of the drivers of high-power solid-state mid-IR laser development. These laser sources must be versatile enough to operate in a variety of temporal modes from continuous wave (CW) all the way to ultrashort pulse while still being widely tunable for wavelength agility. All of this is required at ever increasing power output levels while conforming to size, weight and power consumption limitations under harsh operating environmental conditions.Chromium-doped zinc selenide (Cr2+:ZnSe) lasers operating in the 2-3 µm region are excellent candidates to help fill these vital roles. As a transition-metal doped II-VI chalcogenide, Cr2+:ZnSe has a number of positive advantages over existing laser sources. Development and power scaling of these lasers however, has been hampered by thermal issues which have so far limited the ability of these lasers to be applied to systems-level development. This work presents research into the nature and mitigation of these critical thermal issues in development of versatile Cr2+:ZnSe laser sources. Advanced models for thermal and laser performance are developed and used to design optimally configured laser systems. Among other advances for this material, >10 W CW output from a Cr2+:ZnSe oscillator and master-oscillator / power amplifier systems producing multi-watt, widely tunable power levels are demonstrated.

Doctor of Philosophy, The Ohio State University, 2022, Physics

This thesis examines plasmas generated by and interacting with short pulse lasers (<50 fs) ranging from moderate to relativistic intensities (ie. a_0=eE_0/m_e ωc >1), with a particular focus on plasmas generated from initially ultrathin (<40 nm) solid density targets. Laser technology has made huge strides in developing high repetition rate high power lasers. However, more work is necessary to perform experiments at the full capacity of these lasers. This includes delivering a laser with sufficiently high pre-pulse contrast and protection from back reflections that are potentially fatal to the laser system. This work advanced plasma mirror theory and technology to address these problems by using transparent, free standing thin films. These same films also facilitated novel experiments exploring relativistic effects and novel light possessing orbital angular momentum. The thin films were implemented using the liquid crystal (LC) 8CB and a new film formation device, the spinning disk inserter, was fielded in an experiment for the first time. This work took place using three different laser systems in two countries. As many solid materials have damage thresholds between 10^12-10^13 W/cm^2 a laser's inherent pre-pulse contrast can become an issue when high intensities (>10^21 W/cm^2) are incident on the target. To improve the pre-pulse contrast, self-triggering plasma mirrors (PMs) are commonly used. However, these traditionally limit the collection rate of an experiment (a few shots per hour). Here we present a novel double plasma mirror (DPM) configuration utilizing ultrathin LC films formed in situ before every shot. The total reflectivity of the DPM system was >80%, and is the highest seen for any reported DPM system, and even exceeds that of many single PM systems. The contrast enhancement was measured to be 2-3 orders of magnitude and was the most comprehensive pre-pulse contrast measurement for a single or double PM reported in literature. For the first time (open full item for complete abstract)

Doctor of Philosophy (Ph.D.), University of Dayton, 2021, Electro-Optics

Mode-locked oscillators are the building blocks to generate ultrafast pulses which can then be used for many applications, including optical communication, metrology, spectroscopy, microscopy, material processing, as well as many applications in the healthcare industry. Mode-locked fiber oscillators are especially popular for their compactness, efficiency, and beam quality compared to their solid-state counterparts such as Ti: Sapphire lasers. Apart from their practicality, the mode-locked fiber lasers are an interesting object for studies, as they represent dynamically rich nonlinear systems. For ultrafast fiber oscillators at normal dispersion, a spectral filter is the utmost important optical component that determines the behavior of these systems in terms of the spectral bandwidth, pulse duration, central wavelength of the output spectra, multipulse dynamics, pulse structure as well as pulse velocity. Recently, there is a growing interest in fiber based spectral filters as they facilitate the construction of all-fiber laser cavities. This dissertation investigates the laser performance parameters by developing an all-fiber spectral filter and exploiting its characteristics. Especially, this dissertation reports the first experimental observation of dissipative solitons of the complex Swift Hohenberg equation. This is very important as it births multiple future projects related to implementing higher order spectral filtering in mode-locked fiber lasers. Although most of the ultrafast oscillators in this dissertation are built at 1 μm, ideas to build mode-locked lasers at visible wavelengths are also presented along with primary numerical simulation and experimental results. Finally, all the upcoming research directions are discussed in detail.

Doctor of Philosophy, The Ohio State University, 2021, Physics

This thesis explores the interaction of pulsed lasers with plasmas in the relativistic regime, i.e. when the kinetic energy of electrons in the laser field exceeds mc². Plasmas in such conditions can be created in the laboratory using petawatt class lasers and high-power lasers are rapidly being built around the world to explore this regime. The experimental capabilities needed to explore laser plasma interactions at solid density currently lag behind the capabilities of the lasers themselves, leading to possible gaps in exploring fundamental processes. In addition, exciting applications for intense laser-matter interactions require multiple technical breakthroughs to achieve long term goals. Here, improved target and plasma mirror technology that increases experimental capabilities in both the low and high plasma density regimes required for applications are presented, along with a novel study that exposes new characteristics of an often cited and important effect called relativistic transparency (RT). At low density, petawatt lasers can be used to drive Laser Plasma Accelerators (LPA). LPA's are currently capable of producing electron beams with multi-GeV energies from gas targets over acceleration lengths on the order of 10 centimeters, more than 100x shorter than conventional methods. The continued development of this scheme requires redirection of laser pulses near the laser focus, where the high intensity is certain to damage traditional optics. At high (solid) density, intensities often exceed 10^21 W/cm², and excellent pre-pulse contrast is essential for understanding many experiments because target damage thresholds are near 10^12 W/cm² (corresponding to a 10-9 contrast ratio). The use of a plasma mirror is a common method for increasing pulse contrast but traditionally operates at repetition-rates on the order of a few shots per hour. Presented in this work is the development and characterization of a new device called the Spinning Disk Inserter (SDI) (open full item for complete abstract)

Doctor of Philosophy, The Ohio State University, 2021, Materials Science and Engineering

Laser driven shockwaves are currently being used in an assortment of industrial applications and physics research. Although used in many studies, one of the most common and successful industrial applications is the process of laser shock peening (LSP). LSP has been a developing field of study since the 1970's but only experienced commercial success in the early 2000's. Despite the relatively long history, the physical impulses created by the process have been infrequently and incompletely investigated. This study was constructed to investigate the impulse loads created across the LSP tradespace parameters and evaluate how industry can better analyze LSP parameters and utilize the data in their own optimization. Using photon doppler velocimetry, peak pressures and magnitudes generated by LSP conditions are evaluated in titanium and aluminum alloys in this study. The studies are extended to be inclusive of opaque overlays on the target materials that act as thermal barriers and also modify the pressures generated. This data is critical to understanding and optimizing the LSP process for different material applications and LSP treatment purposes and has not been comprehensively investigated prior to this work. Extension of the pressure data to physical treatments was validated through measurements of residual stress with x-ray diffraction and simulation of the process with finite element simulations. Finite element studies were also used to define the converged boundaries for the newly defined impulse parameter space and demonstrated prediction of residual stresses in comparison to experimental datasets. Results of these studies are expected to provide additional understanding of the LSP process for both industrial use and extension to optimization studies of LSP treatments. It is the intent of these cumulative studies that a more thorough detailing of LSP impulse and simulation capabilities are available for those interested in evaluating the process.

Doctor of Philosophy, The Ohio State University, 2020, Physics

Like many problems in physics, the interaction between high intensity laser pulses and solid materials depends critically on the relative timescales of the drive (the laser pulse with finite duration) and the material response. This is especially true for Laser-Induced Damage and Ablation (LIDA) of solids, where femtosecond (1 fs = 10e−15 s) laser pulses can achieve extremely high energy densities since there isn't enough time for energy to diffuse away during the laser pulse like there is for picosecond (10e−12 s) and nanoseond (10e−9 s) pulses,for example. The pulse duration dependence of fs-LIDA for Near-Infrared (NIR) pulses less than 100 fs is less well-understood, especially in the Few-Cycle Pulse (FCP) regime (<10fs) where energy is deposited faster than almost all of the processes associated with the material response. In this thesis, the pulse duration dependence of LIDA of transparent dielectric material systems down to the FCP regime is studied using a well-established time-and space-resolved imaging technique as well as high-resolution depth-profiling. LIDA of dielectric solids has large application spaces in precision micro-machining andsurface patterning, as well as improving the LIDA performance of dielectric thin-film opticsto increase the output of high power laser systems. Practical multilayer thin-film opticsintroduce more complexity to the LIDA process due to thin-film interference, so I startedwith a study of FCP-LIDA of the simplest thin-film system: a single layer. I found that dif-ferences in LIDA between two film thicknesses are exacerbated by Few-Cycle Pulses (FCPs)relative to 100 fs pulses. I wrote a Finite-Difference Time-Domain (FDTD) simulation thatmotivates a possible mechanism for this, suggesting FCPs result in a more spatially non-uniform excitation of the films. My results show that the models I used must be extendedto more completely describe my experimental observati (open full item for complete abstract)

Master of Science in Engineering, University of Akron, 2020, Mechanical Engineering

This research concentrates on the development of a computational model to better understand the laser ablation of aluminum. When a laser interacts with its target, a plume consisting of gaseous target material, solid particles and droplets, and plasma is generated and limits the efficiency of successive laser pulses. This process is known as shielding of laser pulses by ablated multi-phase plumes. The amount of laser energy to reach the target is a function of properties of previously ejected plumes. Most existing work regarding the nanosecond-scale laser ablation of aluminum focuses on results of physical experiments. Benefits of computational models include cost reduction as well as the ability to test a wider range of plume and target characteristics. The commercially available software, ANSYS Fluent, is used to model this process. The development of a computational model of the laser ablation of water vapor appears first in the Thesis. The plausibility of using Fluent to simulate this process and the results produced are validated by comparison to published experimental data and prior computational analysis. Results are first confirmed using the generation of a single plume of material, followed by ejecting multiple plumes, stemming from multiple laser pulses. The shielding process is successfully implemented into Fluent. After validation, the model is adapted to analyze the laser ablation of aluminum. Aluminum is one of the most widely used materials in industry today and a better understanding of how to best manipulate this material could lead to industrial process improvements. Aluminum is a major structural material in air vehicles and laser weapons are explored to penetrate vehicle surfaces. Additional complications arising from aluminum ablation include the incredibly complex nature of aluminum multi-phase plume characteristics at high temperatures, such as density, pressure, and degree of ionization. Ablated plume properties must be approximated bas (open full item for complete abstract)

Master of Science (M.S.), University of Dayton, 2018, Electro-Optics

Cobalt-doped cadmium telluride (Co:CdTe) has been studied to identify its spectroscopic features and potential as a solid-state laser gain medium. Within the class of transition-metal-doped chalcogenide solid-state gain media, iron-doped zinc selenide (Fe:ZnSe) and chromium-doped zinc selenide (Cr:ZnSe) have emerged as practical sources of tunable mid-infrared radiation. Meanwhile, cobalt remains comparatively unexplored. Co:CdTe has an emission band within the 3 – 5 µm atmospheric transmission window and potential to fill the 3 – 3.8 µm spectral gap between Cr:ZnSe and Fe:ZnSe. A spectroscopic investigation of Co:CdTe was performed, and the temperature dependent emission and absorption properties were collected from 10 – 120 K. Cross-sections were calculated using the Fuchtbauer-Ladenburg and reciprocity methods. The optical amplification of a 3.8 µm Intraband Cascade Laser (ICL) was demonstrated by pumping Co:CdTe with a 2.8 µm continuous-wave Er-fiber laser. This is believed to be the first successful gain demonstration of a cobalt-doped chalcogenide material. A laser rate equation model was implemented to predict the optical gain and a thermal model was developed to analyze the temperature rise of Co:CdTe under continuous-wave pumping conditions. The primary aim of this work is to study the optical transitions between the ground state and first energy level of the crystal field, as well as evaluate the potential of Co:CdTe as a mid-infrared gain medium.

Doctor of Philosophy, The Ohio State University, 2017, Physics

A study of the fundamental mechanisms governing the femtosecond laser damage process is presented in this thesis. In particular, these studies are carried out at mid-infrared (IR) wavelengths, a regime that has not been investigated previously for femtosecond pulses. However, the increased interest in the interaction between mid-IR pulses and solid crystals, as well as the greater availability of short-pulse laser systems operating at these wavelengths, has made such studies paramount. Even beyond the practical applications of preventing laser damage in relevant laser systems, an understanding of the damage process offers insight into the fundamental mechanisms of mid-IR laser-interactions in general. This was achieved through measurements of the single-pulse laser induced damage and ablation thresholds of the semiconductors Si, Ge, and ZnSe across a range of wavelengths extending from the near-IR to the mid-IR. Additionally, the morphology of many damage sites was imaged through a wide variety of techniques. The results of these measurements were compared with existing theoretical models, testing their validity with low-bandgap semiconductors and in the mid-IR. These comparisons show that modifications to these models are necessary, particularly to account for effects that become especially relevant in the mid-IR. Such modifications are presented and shown to be in reasonable agreement with the experimental data. In addition to this, multi-pulse studies were performed in order to determine the nature of microstructure formation at mid-IR wavelengths. Laser induced periodic surface structures (LIPSS or ripples) were generated on Ge after irradiation with multiple mid-IR, femtosecond pulses at oblique incidence. Low spatial frequency LIPSS (LSFL) were observed for two types of laser polarization on the material surface. The quantitative and qualitative features of the LSFL were found to be consistent with a currently established model of their formation, after (open full item for complete abstract)

Doctor of Philosophy in Engineering, University of Toledo, 2010, Electrical Engineering

This dissertation is a study on the formation of conical tips with nanoscale sharpness on silicon films, as well as various microbumps or nano-sharp structures on metal films, as a result of localized single-pulse laser irradiation. Such conical tips with nanoscale structures are referred to as nanotips in this work. A Q-switched nanosecond-pulse Nd:YAG laser, emitting at its fourth harmonic of 266 nm, was employed. Irradiation of silicon-on-insulator, gold, platinum and other metal films, with thicknesses of several hundred nanometers, was studied in ambient air, low-vacuum or argon atmospheres. Individual circular laser spots, several micrometers in diameter, were used together with spot patterns generated through the use of a mask projection technique. The laser based formation method studied and developed during this research allows for the fabrication of nano-tips, and microbumps, in a single step, at far less expense when compared to traditional lithography based techniques. It also offers precise control over the location of these structures unlike other laser based techniques. In addition, it eliminates the use of expensive and complex femtosecond or excimer lasers as well as their respective optical systems, which in principle, can be used to fabricate similar structures. This laser based technique has proven capable of fabricating conical silicon tips in silicon-on-insulator films; where each of these tips is situated in the center of a circular depression with a depth, tens of nanometers below the original surface. The height of the tip apex is several hundred nanometers above the original surface with an estimated radius of curvature of 35-40 nm. Laser irradiation of gold films has produced microbumps with high-aspect-ratio protrusions with have heights of more than 3 µm above the original film surface, while the apex is estimated to have a radius of curvature of 5-10 nm. Irradiation of other metal films has produced various sized microbumps with and w (open full item for complete abstract)

Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

An electrically excited oxygen-iodine laser with a scaled-up electric discharge has been designed and operated. Singlet delta oxygen is generated in a capacitively coupled RF discharge sustained in the plenum of a M=3 nozzle. The discharge operates at pressures up to p0=120 torr, powers up to 4.5 kW, and flow rates up to 0.5 mole/sec of 0-15 % O2 in He. Singlet delta oxygen yields up to Y=7.5 % have been measured using absolute infrared emission spectroscopy. Small signal gain on the I*(2P1/2,F′=3)→I(2P3/2,F″=4) iodine atom transition in the supersonic flow has been measured by tunable diode laser absorption spectroscopy, up to γ=0.209 %/cm. The addition of NO to the laser mixture and injection of chilled helium to reduce the laser mixture temperature have been shown to enhance laser performance considerably. Other modifications, including the addition of argon to the laser mixture, chilling the main flow through the electric discharge, and dissociating iodine directly by electron impact in an auxiliary electric discharge have not resulted in further gain increase. Laser action has been achieved using a transverse resonator in the supersonic cavity, with laser output power up to PL=7.8 W.

Doctor of Philosophy, The Ohio State University, 2004, Mechanical Engineering

A new trans-rotational temperature diagnostic with ±50K accuracy has been developed for use in nonequilibrium, low temperature, monatomic gases seeded with carbon monoxide (CO). The scheme utilizes single-photon laser induced fluorescence (LIF) of CO under vibrationally-excited conditions in which single-photon transitions from the CO X1Sig+ ground electronic state to upper electronic A1Pi or D'1Sig+ states become accessible to a tunable, narrowband ArF excimer laser at 193 nm. Two vibrationally excited environments in which the chemistry is well understood were used as a testbed; an optically-pumped 3% CO/Ar plasma at 100 torr and a 4% CO/He d.c. glow discharge at 8 torr. For the optically-pumped CO/Ar plasma, a spatially-averaged temperature of 536±103 K (2sig) was obtained from rotationally resolved X1Sig+(v"=20)-D'1Sig+(v'=2) LIF excitation spectra. Temperature measurements pumping the X1Sig+(v"=7)-A1Pi(v'=1) 4th Positive (528±51 K) were also found to compare well with line-of-sight Fourier Transform-InfraRed (FT-IR) emission measurements (536±10 K). Spatially averaged FT-IR spectroscopy of the CO 1st overtone was used to verify a vibrational population of ~0.1% within the positive column of the CO/He d.c. glow discharge. The A-X (7,1) transition was pumped and subsequent (8,1) emission at 200.8 nm collected. Spectral peaks were assigned and used to determine a spatially averaged rotational temperature of 432±44 K on the discharge centerline. This was found to be in good agreement with FT-IR spectroscopy measurements (395±10 K). As a prelude to Planar-LIF (PLIF) temperature measurements, vibrationally-resolved emission from laser excitation of various rotational lines within the A-X and D'-X bands were used to investigate spectral interferences. This information was used to determine that a simple aqueous organic filter (urea) in the A-X case, or commercial glass filter (UG-11) in the D'-X case, are adequate for rejecting elastically-scattered radiation and extr (open full item for complete abstract)

Doctor of Philosophy, Case Western Reserve University, 2013, Materials Science and Engineering

Recent advances in minimally invasive surgical techniques and an increasing need to miniaturize medical devices has led to a surge in developing advanced manufacturing techniques. In order to meet the functional needs of such small devices as cardiovascular stents, guide wires, and needles, the use of new materials and delicate geometries has increased creating a new challenge for manufacturing and machining. Some applications often include fine details that are impossible to achieve with rotary tool machining. Laser machining is one tool harnessing an enormous potential for the manufacture of such finely detailed devices as well as providing a means for improving the local material effects as a result of processing. However, thermal damage caused by laser machining can affect the performance of the components. As devices continue shrinking in size, there is a greater need for “athermal” manufacturing methods that have no adverse effect on performance. In this study, Nd:YAG and femtosecond lasers with different pulse widths were used to machine AISI 316LVM biomedical grade wires. The mechanical behavior of these materials were evaluated in uniaxial tension, and in cyclic strain-controlled fatigue with the use of a flex tester operated to provide fully reversed bending fatigue. All the fatigue testing was conducted in air over a range of cyclic strains to determine both the high-cycle and low-cycle fatigue regimes. The effects of laser input energy and pulse width on surface quality, heat affected zone (HAZ), and subsequent mechanical response are reported Baseline fatigue data on 316LVM wires in the annealed and hard conditions revealed that the hard wires exhibited better high cycle fatigue behavior than exhibited by the annealed wires. However, the low cycle fatigue behavior of the annealed wires was better than that obtained on the hard wires. This was successfully modeled using the Coffin-Manson-Basquin approach. Mixed results were obtained on the fatigue (open full item for complete abstract)

Doctor of Philosophy, The Ohio State University, 2024, Mechanical Engineering

In this dissertation, a series of experiments were carried out to investigate the auto-ignition process of transient fuel jets and sprays issuing into high-temperature, environments. Novel high-speed imaging and laser diagnostic techniques were developed and applied to characterize mixing and turbulent flow conditions prior to and at the onset of ignition. In addition, this research examines the topology and dynamics of ignition kernels as they grow and transition into a stable flame. Research was carried out primarily in canonical atmospheric pressure experiments, but a new high-pressure spray test facility is developed in this work with preliminary measurements presented, demonstrating new experimental capabilities. Specific contributions of this dissertation include: (1) characterization of the transient mixing processes of variable-density atmospheric pressure jets both before and after ignition, (2) determination of the most probable mixing and turbulent flow conditions leading to local auto-ignition, (3) statistical evaluation of the dynamic growth and transport of ignition kernels, (4) construction and characterization of a novel high-pressure, high temperature spray and combustion facility, and (5) demonstration of high-speed mixture fraction measurements in non-reacting and reacting sprays at realistic thermodynamic conditions. First, a series of transient gas-phase fuel jets issuing into a high-temperature, vitiated environment at atmospheric pressure was investigated. A well-known jet-into-hot coflow configuration was utilized with the addition of a fast-acting solenoid valves to achieve pulsed fuel injection in an environment with well-defined boundary conditions. Four test conditions were studied to examine the effects of variations in jet Reynolds number, the fuel mixture composition, and coflow temperature. High-speed laser Rayleigh scattering (LRS) was performed at 10 kHz to measure the mixture fraction and temperature fields from fuel injection (open full item for complete abstract)

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

The work in this dissertation is focused on the development of new manufacturing technologies at the early stage. Two concepts are developed in the category of Impact Welding and two in the category of Metamorphic Manufacturing. Under the Impact Welding category two different welding processes are studied, the Vaporizing Foil Actuator Welding and the Augmented Laser Impact Welding processes. Both of these processes were demonstrated to produce impact welds between traditionally unweldable aircraft aluminum alloys which performed as well or better than comparable riveted joints without the need for the drilling of holes or removal of surface coatings. Additionally, basic engineering guidelines are established for the design of foils for the Vaporizing Foil Actuator Welding process and basic performance metrics are established for the Augmented Laser Impact Welding technique. Two new data analysis techniques were developed for the Augmented Laser Impact Welding process which were validated by the use of high-speed videography. Models of the impact conditions for both of these impact welding techniques were established. For the Augmented Laser Impact Welding process, a technique for accurately measuring the welding velocity during an impact event is developed and validated. Metamorphic Manufacturing refers to the agile use of deformation to create shapes and modify microstructure. In this area two concepts were developed where metallic components are transformed from one shape into a second more desirable and useful form. A device and process for bending medical fixation plates to match patient skeletal anatomy is developed. The method can make arbitrary controlled shapes and may save time in the operating room for reconstruction surgeries. The second concept is an approach for Robotic Blacksmithing, a process for incrementally transforming a malleable material into useful shapes by deformation. This concept was initially developed on a purpose-built desktop robotic (open full item for complete abstract)

Master of Science in Engineering, Youngstown State University, 2023, Department of Mechanical, Industrial and Manufacturing Engineering

This thesis presents a comparative analysis of the dissimilar laser welding of Ti6Al4V and Ni-Ti with two different interlayers, namely vanadium and niobium, using a continuous fiber laser welding machine. This study attempts to solve the problem associated with dissimilar welding of the Ni-Ti and Ti6Al4V with the use of interlayers specimen. The objective of this study is to improve the welding strength between Ni-Ti and Ti6Al4V in comparison to previous research and to investigate the effect of interlayer composition on the quality of the weld joint. The welding process was performed using identical laser power, welding speed, and focal position, and the quality of the weld joint was evaluated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) analysis, hardness testing, and tensile testing. The welding was successfully performed using both interlayers. The tensile strength of the welded samples with niobium interlayer was found to be 100 MPa greater than that of the samples with vanadium interlayer. Scanning electron microscopy images showed that the fracture occurred at the welding region interface between the Ni-Ti-interlayer in both cases due to the dendritic structure, which caused the region to be more brittle. Furthermore, the hardness of the Ni-Ti-interlayer interface was higher, resulting in brittle fracture at the same interface in both cases during tensile testing. The findings suggest that the use of niobium interlayer produces a higher quality weld joint with improved mechanical properties under the same laser welding parameters compared to the vanadium interlayer. These results are significant for designing the laser welding process and selecting the appropriate interlayer for specific applications. Further research can be conducted to optimize the laser welding parameters and explore the impact of different interlayer thicknesses on the welding behavior.

Doctor of Philosophy (PhD), Wright State University, 2022, Engineering PhD

High-power laser systems (HPLS) have wide-ranging applications in many prominent areas. HPLS use laser diodes to pump fiber gain media. Understanding the functionality of both components is critical for achieving effective HPLS operation. System optical efficiency is a function of diode junction temperature. As junction temperature changes, the wavelength spectrum of the diode output shifts causing optical power losses in the fiber gain media. Optical/thermal interactions of the dynamically coupled laser diodes and fiber gain media are not fully understood. A system level modeling approach considering the interactions between optical performance and component temperature is necessary. Four distinct models were created: Diode optical, diode thermal, fiber optical, and fiber thermal. Dynamically coupling these models together provided the capability to demonstrate how HPLS electro-to-optical efficiency changes when the laser diode pump spectrum shifts due to various levels of thermal management. Subsequent studies were done to determine which parameters across all four models had the most significant impact on laser performance from a designer's perspective. Next, a statistical surrogate model was created by varying these parameters to create a parameter space. Response variables of interest were then reduced to a single equation as a function of these important parameters across the parameter space, allowing for quicker exploration of the potential design space. Lastly, laser time to steady state and laser efficiency were employed to determine when a specific diode cooling method should be used to achieve the highest laser efficiency. Understanding the optical/thermal interactions of laser operation and exploring the impact of various thermal capabilities can provide better system design and optimization guidelines. Bridging the gap between the optical and thermal aspects of laser operation in pursuit of such understanding has been made possible by the re (open full item for complete abstract)

Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2022, Materials Science and Engineering

This work investigates processing parameters for laser powder bed fusion (LPBF) additive manufacturing (AM) to produce bismuth telluride coupons. AM provides the ability to fabricate complex geometries, reduce material waste, and increase design flexibility. The processing parameters for LPBF were varied in single bead experiments guided by analytical modeling to identify conditions that result in uniform beads. Coupons were built using these processing parameters and the cross-sections characterized using optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS). Porosity analysis using OM concluded most coupons had porosity levels less than 10% by area. SEM and EDS analysis revealed there were slight composition and microstructure variations throughout the cross-sections depending on the processing conditions. These results show that LPBF is a viable process for producing bismuth telluride coupons with low porosity. Investigations of the microstructure and composition of the coupons indicate further research opportunities.

Master of Science, The Ohio State University, 2022, Materials Science and Engineering

The process of laser impact welding utilizes impact welding and laser-driven flyers to form solid-state, metallurgical welds between similar or dissimilar metallic flyers and targets. With chemically augmented laser impact welding, stronger and thicker metal flyers and targets can be welded together. Using a high-powered laser, a laser pulse is shot through a transparent tamping layer onto a translucent layer of chemical liquid and the bare surface of a metallic flyer. The energy from the laser pulse detonates the chemical augment and the pressure created from the explosion is confined by the tamping layer. This pressure is directed towards the flyer that is then driven to velocities in the hundreds of meters per second within 20 microseconds. Under the correct conditions, high speed and acceptable impact angle between the flyer and target, jetting will occur. The jet cleans the surface of the flyer and target of oxides, and the two surfaces will form a solid-state, metallurgical bond. Using a chemical augment, thicker, stronger flyers and targets can be welded compared to unaugmented laser impact welding. With the chemical augment, a 3J, 8.1ns laser pulse can weld a 0.5mm Al2024-T3 flyer to a 0.5mm Al2024-T3 target. To explore the capabilities of chemically augmented laser impact welding, two chemical augments were used as candidates for the process. Various tamping materials and thicknesses were also investigated along with variance in the laser spot diameter. The velocities of flyers were measured using Photon Doppler Velocimetry and a thicker tamping layer produced higher velocities and larger deformations than thinner tamping layers did with the same parameters. The strength of the welds between 0.5mm Al2024-T3 flyers and targets were also measured using a tensile test. Over two-thirds of the welded samples failed by nugget pullout during these tensile tests, validating the strength of the welds formed. Micrographs of a welded sample were also collected to o (open full item for complete abstract)