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)

Committee: Glenn Daehn (Advisor); Antonio Ramirez (Committee Member); Boyd Panton (Committee Member); Enam Chowdhury (Committee Member) Subjects: Materials Science; Medicine; Robotics

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.

Committee: Jae Joong Ryu PhD (Advisor); Virgil Solomon PhD (Committee Member); Kyosung Choo PhD (Committee Member) Subjects: Materials Science; Mechanical Engineering; Morphology

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)

Committee: Glenn Daehn (Advisor); Boyd Panton (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2021, Welding Engineering

Laser-powder Bed Fusion (LPBF) Additive Manufacturing (AM) has brought many benefits to the manufacturing industry, such as increased freedom of design and capability to merge assemblies into fewer components; however, a handful of simple features are still avoided in AM design because they are notoriously difficult to fabricate without incurring defects. Features such as overhanging faces tend to see sharp increases in surface roughness and sub-surface porosity along with decreases in geometric accuracy due to over-penetration of the laser beam into the loose metal powder below. Solid supports can reduce these defects; however, they require difficult post-process removal, which in some cases is not possible, such as with features that are internal to a component. Other fine features, such as thin walls or columns, tend to be negatively affected by instabilities in the weld pool that are detrimental to surface quality and geometric accuracy. Because changes in the key laser processing parameters (laser power, travel speed, hatch distance, layer thickness) could negatively affect the components' bulk properties, an alternative solution to these issues lies in the composition of shielding gas used in the process. The shielding gas is a processing parameter that has shown to be advantageous in laser welding processes but has remained mostly overlooked in the LPBF industry. A review of engineering fundamentals and published literature showed that pure helium has a thermal conductivity several times higher than that of pure argon, and argon-helium mixtures have been reported to have thermal conductivity values even higher than that of pure helium. Few works have studied the effects of a higher thermal conductivity shielding gas on the LPBF process beyond single-layer experiments, but their results were promising and showed increased stability in the weld pool and plasma plume. In this work, a binary mixture of equal parts argon and helium was employed in a commercial L (open full item for complete abstract)

Committee: Antonio Ramirez (Advisor); Groeber Michael (Committee Member); Herderick Edward (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Engineering; Experiments; Fluid Dynamics; Gases; Materials Science; Mechanical Engineering; Mechanics; Metallurgy; Morphology; Scientific Imaging

Master of Sciences (Engineering), Case Western Reserve University, 2016, Materials Science and Engineering

Laser Hot-Wire (LHW) cladding is a wire-based, laser-assisted additive process of fusion joining. As the name suggests the filler wire is resistively heated prior to reaching the weld pool. The LHW process offers great benefits, relative to arc-based processes, in terms of high energy efficiency, excellent metallurgical control and high deposition rate. In work reported on in this thesis, two different material systems, Ti-6Al-4V alloy and the nickel-based superalloy 625, are experimentally evaluated through characterization of specimens created using the LHW process with a range of process parameters. Characterization includes chemistry of deposited metal, microstructure, selected mechanical properties, dimensions, and residual stress. Also, a rigorous analysis of energy efficiency was performed. All results are benchmarked relative to a laser/powder based additive manufacturing process. The result obtained in this work is anticipated to improve the understanding of the LHW process, expand its use to less common alloy systems, and promote its use as an industrially relevant form of additive manufacturing. The project that enabled this work is a collaboration between CWRU, Lincoln Electric, Alcoa Titanium & Engineering Products, and rp+m Incorporated.

Committee: James McGuffin-Cawley (Advisor) Subjects: Aerospace Engineering; Engineering; Materials Science; Mechanical Engineering

Master of Sciences (Engineering), Case Western Reserve University, 2015, Materials Science and Engineering

In this study, a number of established additive manufacturing processes were evaluated for their suitability repairing high-pressure die cast tooling. The processes included in this study are laser hot wire (LHW), electron beam freeform fabrication (EBF3), gas metal arc welding (GMAW), Laser Engineered Net Shaping (LENS®), and direct metal deposition (DMD). To determine each process' suitability, blocks of maraging 250 steel were deposited on H-13 base metal. The results show that the maraging deposits are capable of providing good strength (>160 ksi), toughness (>15 ft-lbs), and hardness (45 HRC) for die tooling applications, but care must be taken to limit the occurrence of defects, particularly porosity. Of the processes tested, the LHW, DMD, and LENS® processes had the best balance of deposit properties. However, additional work will be required to optimize the processing parameters for each process.

Committee: David Schwam (Advisor); John Lewandowski (Committee Member); Gerhard Welsch (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2006, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy, The Ohio State University, 2023, Welding Engineering

Collision welding or impact welding is a solid-state welding technique which enables unique materials joining opportunities. The high velocities and short timescales of the welding process lead to extreme peak collision pressures, high strain rates, low heat inputs, steep thermal gradients, and narrow thermo-mechanically affected zones (TMAZ). These features can be advantageous in the joining of traditionally unweldable materials such as 2XXX or 7XXX series aluminum alloys, and for the joining of dissimilar metals which would form unwanted phases under traditional fusion welding. However, these same features can also make impact welding challenging to control and characterize. Analytic process limits have been developed to calculate limiting flyer velocities and impact angles for welding success as functions of the weld member properties based on the physical mechanisms that enable joining. The maximum impact velocity for a weld is determined by a dynamic solidification cracking mechanism: reflected dynamic tensile stresses arrive to the nascent weld interface prior to full interface cohesion, which is mediated by the presence of interfacial melting. However, current analytic models for the upper limit of the collision welding window were developed for autogenous welds, which develop symmetric stress conditions at impact. In this thesis, we develop alternative formulations to the analytical upper limit of the welding window which better support dissimilar welding. Shock-informed calculation of the asymmetric stress and thermal partitioning between dissimilar weld members is achieved through the application of modified Rankine-Hugoniot relations. We compare the application of the shock-informed upper limit to the existing upper limits in the context of historical data. The shock upper limit is further validated experimentally through the use of The Ohio State University Impulse Manufacturing Laboratory's (IML) Laser Impact Welding (LIW) and Vaporizing Foil Actu (open full item for complete abstract)

Committee: Glenn Daehn (Advisor); Wei Zhang (Advisor); Eric Seiber (Committee Member); Boyd Panton (Committee Member); Avraham Benatar (Committee Member); Kevin Doherty (Committee Member) Subjects: Materials Science; Mechanics

Master of Science, The Ohio State University, 2023, Welding Engineering

High energy density welds have complex weld geometries due to the deep penetrations and low heat input associated with these processes, making it viable for many applications (aerospace, medical, defense, etc.). These complex geometries lead to difficulties in characterizing the transition between the conduction and keyhole weld mode geometries for this welding process. An ytterbium doped fiber laser with a beam diameter of 0.6 mm was used to make partial penetration autogenous weld on the nickel-based alloy, Inconel 690. Laser powers ranged from 600W – 2800W and travel speeds ranged from 5 mm/s – 150 mm/s to analyze characterization techniques of the weld's formation through conduction to keyhole mode. Six characterization techniques were attempted, with the final technique having two methods compared. First, the depth-to-width measurement were analyzed for a sudden increase as the power increased for welds made at the same travel speed. Evidence for a shift in depth-to-width ratios as the weld geometries transition were seen. A change in convexity of the weld pool measurements using ImageJ software proved a viable method for characterizing the weld mode. Uniformity was seen along the weld root's penetration was seen for longitudinal cross-sections for keyhole and conduction mode weld geometries. Weld roots within the transition of these modes showed nonuniform weld root penetrations. Inline Coherent Imaging was used to see if any notable changes of the weld's formation through the keyhole profile. Finally, analysis of the solidification rate and subsequent solidification parameters was done using a traced solid liquid interface and cell spacing measurements. These results were inconclusive as a current method of characterization, but solidification rate values were compared to those measured through cellular grain growth analysis. Cellular grain growth analysis showed that both plan and longitudinal weld cross-sections are needed when analyzing the solidifica (open full item for complete abstract)

Committee: Carolin Fink (Advisor); Boyd Panton (Advisor); Wei Zhang (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2021, Welding Engineering

High energy density welding processes are used for many industrial applications, such as aerospace and defense, and certain joint configurations and welding situations render post-weld inspection techniques difficult. Due to the inability to confirm penetration, there is a continual industrial need to improve prediction capabilities to aid in parameter selection. It is also necessary to understand the microstructural evolution to modify phase fractions and mitigate weldability related concerns such as solidification cracking or reduced corrosion performance. Using an ytterbium doped fiber laser, partial penetration laser beam welds were produced on a six stainless steel (SS) alloys and Ti-6Al-4V. The WRC-1992 Cr/Ni equivalencies for the SS alloys ranged from 1.7 to 2.8. These alloys included a commercially available 304L stainless steel and a commercially available 2205 duplex stainless steel. The other four SS alloys were experimental compositions. From an industrial perspective regarding weld feature prediction, the two alloys of interest include 304L SS and Ti-6Al-4V. Therefore, analysis was performed to relate weld geometry characteristics to process parameters for these two alloys. The objective of the weld geometry analysis was to develop a neural network model that can predict weld geometry characteristics, most importantly penetration. The best results were produced with individual neural networks developed for each weld feature and compiled into a comprehensive algorithm. An inline coherent imaging (ICI) or depth measurement system was also incorporated during LBW to further understand keyhole evolution and weld pool development. Process parameters influenced the location of the keyhole relative to the process beam and the overall shape and width of the vapor capillary. Transverse and longitudinal view sections were evaluated to determine solidification behavior, and electron backscatter diffraction (EBSD) patterns were used extensively to determine ph (open full item for complete abstract)

Committee: John Lippold (Advisor); Xun Liu (Committee Member); Carolin Fink (Committee Member); Boyd Panton (Advisor) Subjects: Engineering; Materials Science

Master of Science, The Ohio State University, 2020, Welding Engineering

High energy density welds are often used in critical applications involving a wide range of structural materials. In most cases, both laser and electron beam welding may be considered for these applications and the ability to use both processes to make comparable welds in terms of both weld profile and microstructure provides considerable process selection flexibility. In this study, autogenous, partial penetration welds on 304 SS, 304L SS, and Ti-6Al-4V were made using both fiber laser and electron beam processes. The main variables of interest, power and travel speed, were varied independently. Beam characterization was performed to determine parameters necessary for similar welding conditions between the two processes. Overfocused electron beams produced a more Gaussian distribution than underfocused beams. Laser beam characterization showed a slight increase in sharp spot size with increasing power, likely due to machine capabilities. Welds were made using sharp focus for laser welds and both a sharp, deflected beam and an overfocused beam for electron beam welds. Depth of penetration varied substantially between process conditions, but a similar trend between processes was observed when comparing area of the fusion zone, suggesting a similar melting efficiency. For defocused electron beam welds, increases in voltage yielded a dip in penetration for increasing power. This was likely due to complications with the machine or the diagnostic tool resulting in a narrower beam at lower voltages. A reduction in melting efficiency was observed in Ti-6Al-4V laser welds as compared to EB welds, likely due to vaporization effects, material properties, or both. Analysis of the depth of penetration for 304L and Ti-6Al-4V laser welds at varying powers and travel speed yielded predictive process maps. Stainless steel alloys showed consistent microstructures with work found in literature for pulsed laser welding. Limited metallographic analysis for Ti-6Al-4V welds was conducte (open full item for complete abstract)

Committee: John Lippold (Advisor); Boyd Panton (Advisor); Carolin Fink (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2019, Materials Science and Engineering

Laser impact welding is a relatively new technology that combines the existing technologies of impact welding and laser-driven flyers to create solid state welds between a thin metallic flyer and a target. A high power laser pulse is fired through a transparent confinement layer and onto an ablative layer that is directly attached to the flyer. This ablative layer absorbs the laser energy and is vaporized creating a plasma that is in turn confined by the transparent confinement layer directing the pressure into the flyer. This drives the flyer to velocities of hundreds to thousands of meters per second in under a microsecond. When the flyer impacts the target if the velocity is high enough and the angle between the two is sufficient, the two surfaces of the materials will be cleaned of their oxides, which in turn allows for a solid-state metallurgical bond to be made between the flyer and target. By controlling the laser parameters, much thicker flyers than previously reported can be welded. Using a 24J 65ns pulse, a 305µm 3003Al flyer was welded to 6061-T6Al. These welds were extremely strong and failed through the 3003Al flyer in all cases when peeled. To further push laser impact welding towards a scalable industrial process, liquid backing layers such as water and glycerin solution were studied for their effect on flyer impact velocity. Impact velocity was measured using Photon Doppler Velocimetry and it was determined that while both are suitable for welding, water would be the preferred backing in an industrial application due to its ease of use and low cost. Potential applications of laser impact welding, such as beverage can manufacturing, are also outlined and discussed.

Committee: Glenn Daehn (Advisor); Xun Liu (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2019, Welding Engineering

High Entropy Alloys (HEA) are a new class of alloys that was introduced in the early 2000's. These alloys are composed of five or more elements in near equiatomic ratios, with no single base element. HEAs have gained a lot of attention due to their unique or superior properties as compared to conventional alloys. However, there has been little attention paid to the welding metallurgy and weldability of HEAs. However, welding for manufacturing and repair is a key issue for structural engineering applications. This work aimed to establish an initial understanding of the welding metallurgy of HEAs, and identify any potential weldability issues with regard to weld cracking susceptibility in fusion welds. The outcomes of this initial evaluation were used to develop a methodology for rapidly screening the large compositional space of HEAs in order to find promising alloy compositions for weld applications, and ultimately to implement weldability in the early stages of HEA development. The most commonly studied equiatomic AlCoCrCuFeNi HEA was determined to have very poor weldability, due to the positive mixing enthalpy of copper and a high hardness microstructure promoted by aluminum. An improved weldability was achieved by modifying the composition to Al0.5CoCrCu0.1FeNi. Pulsed laser welding was shown to eliminate HAZ liquation cracking for AlCoCrFeNiTi HEA and reduces softening in the HAZ of Al0.5CoCrCu0.1FeNi HEA. Refractory HEA AlMo0.5NbTa0.5TiZr showed a very high susceptibility to porosity and brittle fracture, but a unique fusion zone microstructure with no cracking. A high-throughput screening based on thermodynamic modeling and experimental testing was developed in order to identify HEA compositions with promising weldability, and quickly reject alloy compositions with detrimental properties towards weldability.

Committee: Carolin Fink (Advisor); Antonio Ramirez (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2018, Welding Engineering

Past research has correlated preferential vaporization during autogenous laser rewelding of Type 304L stainless steel with a shift from primary ferrite to primary austenite solidification and an increase in solidification cracking susceptibility. In the present study, custom Type 304L alloys with systematically varying levels of manganese and commercial Type 304L alloys were subjected to various reweld cycles to understand preferential element vaporization and the evolution in composition, solidification mode, and solidification cracking susceptibility during laser rewelding. Rewelding was performed using both continuous wave and pulsed laser operation and the welding process parameters were varied to determine the effect of welding parameters on the vaporization behavior of an alloy. A combination of traditional and advanced techniques were used to characterize the rewelded samples. In particular, bulk weld metal composition analysis was performed using arc-spark optical emission spectroscopy, and local composition analysis was performed using electron probe microanalysis. Characterization of solidification mode and microstructure was performed using light optical microscopy and scanning election microscopy techniques. It was shown that vaporization is a strong function of equilibrium vapor pressure and weld pool temperature and that preferential vaporization of given alloying elements from the weld pool is dependent on the volatility of the element and the activity of the element in the weld pool. The experimental results indicated that preferential vaporization results in a decrease in the chromium, manganese, and copper content and an increase in the nickel, silicon, and iron content of the weld metal relative to the base metal with successive rewelding. Further, preferential vaporization results in a decrease in the chromium-nickel equivalent ratio and a shift from primary ferrite solidification to primary austenite solidification when a critical co (open full item for complete abstract)

Committee: Antonio Ramirez (Advisor); Zhang Wei (Committee Member) Subjects: Engineering; Materials Science

Master of Science in Engineering, Youngstown State University, 2017, Department of Mechanical, Industrial and Manufacturing Engineering

Additive manufacturing is opening new opportunities to produce parts in many industries. This thesis explores the Additive Manufacturing (AM) of copper parts with a Renishaw 250AM for resistance welding machines. The Renishaw 250AM uses a 200W ytterbium fiber laser to create Selective Laser Melting (SLM) parts. AcuPowder 150A was selected out of three copper powders. A series of prints were completed and optimal parameters were chosen. AM copper tensile samples at different orientations and laser exposure times were created to test mechanical properties. Sintered SLM copper samples were also tested and were discovered to be stronger and more dense than non-sintered SLM parts. Electrode and electrode adapters for INV-.5 resistance spot welders were printed and the machining process was recorded. Six weld test groups were obtained by using a combination of conventional and AM electrodes and electrodes adapters were completed and weld schedules were developed. The mechanical properties of the welds were investigated with shear tensile tests and U tensile tests. Groups 1 and 2 were control groups with conventionally made secondary circuits with C110 and RWMA Class 2 electrodes. Groups 3 and 4 contained mixed AM and conventional part systems. Groups 5 and 6 were fully printed systems. Group 6 contained sintered electrodes. The ultimate weld force, current density, and electrode wear were recorded. Depending on the type of pull test, the mixed system welds corresponded within 5-7% of groups 1 and 2. The fully printed systems had more inconsistences due to the increase in electrical resistance based on the porosity and surface roughness of the SLM parts. The numerical modeling of the heat transfer in a typical resistance spot welder electrode/ electrode adapter cooling water system was investigated using Solidworks Flow Simulation. Two models using different internal geometries were compared to maximize the cooling rate of the electrode. The determined results s (open full item for complete abstract)

Committee: Virgil Solomon PhD (Advisor); Hazel Marie PhD (Committee Member); Jae Joong Ryu PhD (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering

Master of Science, The Ohio State University, 2017, Welding Engineering

The medical device industry has stringent requirements for reliability, first time quality, biocompatibility, and process capability for its components and devices. The requirement of biocompatibility limits the materials available for use in long term implant applications and often requires the joining of exotic dissimilar metals. This provides a significant challenge when creating a robust weld schedule. This thesis contains two case studies in dissimilar metal welding on the small scale. The first chapter is focused on characterization of cross wire platinum to niobium micro resistance spot welds. The chapter details how small changes in processing conditions and material can have a profound effect on microstructure, defects, and mechanical properties. Transmission electron microscopy, nanoindentation, and micro pillar compression experiments were used to optimize the properties and characterize the resulting structure, improving the robustness of the joint. The second chapter utilized statistical methods to improve the microstructure and properties of a bulk metallic glass (Vitreloy 105) to titanium laser weld. A definitive screening design (DSD) experiment was performed to understand the effect of 11 different factors on the properties of a laser weld. The results showed pulse width, use of a zirconium filler metal, offset of the laser beam from the joint center, and pulse shape to be the prominent factors controlling the hardness and modulus of the weld.

Committee: Antonio Ramirez (Advisor); Carolin Fink (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2012, Welding Engineering

Recently, a new heat treatment process (flash processing) has been shown to take low-alloy steel and create steels with advanced high strength steel (AHSS) level properties that are better than most available martensitic AHSS. This steel has also shown better ballistic protection capability than currently available high hard, rolled homogenous, aluminum and titanium armors. The unique process works by rapid induction heating into the austenite phase field and subsequent quenching in less than ten seconds. The resultant mixture of carbides, bainite, and martensite allows for the mechanical properties it achieves. However, the steel has not been evaluated for its weldability, which could limit both the ballistic and mechanical property advantages that it has over currently used materials. A well documented decrease in strength, ductility, and toughness in the welds occurs when the heat-affected zone (HAZ) becomes softer compared to its initial state. The goal of this research is to examine the effect of different welding conditions on flash processed steel microstructure and resultant mechanical properties. Low heat input gas metal arc welding (GMAW) was performed as an initial process that is typically done for joining armor steels. A comparison to a currently used high hard armor steel showed lower hardness and a larger softened region softening in the flash process HAZ. To further understand the microstructure evolution for both steels, HAZ physical simulations were conducted to examine the effect specific peak temperatures for a given heating and cooling rate. It was found that flash processed steel softened to 170HVN (from base metal hardness of 540HVN) when intercritically heated close to the Ac3 temperature. High hard steel softened to 290HVN (from base metal hardness of 530HVN) when heated below the Ac1. The reasoning was found that flash process steel transforms to the soft ferrite microstructure when intercritically heated with more ferrite forming as the (open full item for complete abstract)

Committee: Suresh Babu (Advisor); John Lippold (Committee Member) Subjects: Engineering; Materials Science

Master of Science, The Ohio State University, 2009, Mechanical Engineering

Nickel-Titanium (NiTi) is a shape memory alloy that, depending upon composition, can exhibit shape memory or superelastic properties, recovering up to 8% deformation. Utilizing the shape memory effect it is possible to use NiTi as an actuator replacing traditional mechanical systems with a light-weight system using a fewer number of moving parts. In addition to strain recovery, NiTi undergoes significant changes in its material properties, including elastic modulus and electrical resistivity. With these changes in material properties, it is possible to create NiTi based transducers. Currently, NiTi is limited to niche applications due primarily to difficulty in machining and joining NiTi to traditional structural materials. The goal of this thesis is to develop and characterize consistent methods of creating adaptive structures using NiTi. The research presented consists of two parts; the first deals with the development and characterization of cost-effective methods of joining NiTi and common aluminum and steel alloys. Laser welding, tungsten inert gas welding, and ultrasonic soldering were used to create joints between NiTi and itself, aluminum 2024, O1 tool steel, and 304 stainless steel. Where applicable, joints were subject to mechanical testing and analysis using optical microscopy. The second part explores the development and characterization of NiTi/Al metal matrix composite transducers constructed using Ultrasonic Additive Manufacturing (UAM), a low temperature solid-state process also referred to as ultrasonic consolidation. An aluminum UAM matrix was first characterized through mechanical testing and analysis using optical microscopy. Using UAM, aluminum matrix composites with embedded NiTi wires were created with up to a 13.4% NiTi cross sectional area ratio. The composites were tested to characterize their stiffness as a function of temperature. A model was also developed using the Brinson constitutive model in order to predict the stiffness and strain (open full item for complete abstract)

Committee: Marcelo Dapino PhD (Advisor); Somnath Ghosh PhD (Committee Member) Subjects: Engineering; Mechanical Engineering

Master of Science, The Ohio State University, 2009, Welding Engineering

Penetration depth fluctuations of single mode fiber laser partial penetration welds with long focal length optics of 150mm were studied. It was found that welds having high aspect ratios and penetrations greater than 0.381mm were subject to the spiking discontinuity. Deeper welds contained spikes of greater magnitude. It was found that several methods could improve partial penetration welds containing severe spiking. Sinusoidal laser power modulations of 10% and 25% reduced the rate of occurrence and the magnitude of the spiking at frequencies between 900Hz to 3kHz; however, it was found that modulation was not robust and lacked repeatability. High travel speed welding at 400-450mm s-1 significantly reduced the magnitude of spiking compared to similar penetrations made at slower travel speeds. Spiking was not completely suppressible only increasing travel speeds but was found to be much more predictable and repeatable than modulation techniques. Laser beam circular oscillation techniques repeatably and robustly eliminated spiking from the penetration profile of single mode fiber laser welds at oscillation diameters of 0.1mm, 0.2mm and 0.35mm with corresponding frequencies of 2kHz, 1.5kHz, and 1kHz respectively. Decreasing the f# of the lens by reducing the focal length to 80mm also was found to decrease the magnitude of spiking and increased the threshold penetration depth for the formation of spiking as compared to 150mm focal length optics. A series of experiments was conducted to compare the fit up tolerability to gap, mismatch, and edge break between a traditional long focal length optic setup and a shorter f-θ galvanometer setup. It was found that even with the added bead width provided by the galvanometer, gap still remains a very difficult fit up variable to overcome. Edge Break fit up condition was improved from 0.076mm to 0.254mm using oscillation diameters of 0.2mm and 0.35mm. Mismatch fit up condition was improved from 0.076mm to 0.635mm using oscillati (open full item for complete abstract)

Committee: Dave Farson PhD (Advisor); Sudarsanam Suresh Babu PhD (Committee Member) Subjects: Engineering

Master of Science, The Ohio State University, 2009, Welding Engineering

With the ever increasing power and performance of solid-state lasers, autogenous laser welding is becoming more practical for thick-section welding applications. High-power, high-beam-quality fiber lasers can produce high aspect ratio (depth/width) welds at productive travel speeds with minimal distortion. However, autogenous laser welding can produce undercutting or other geometric stress concentrations at the weld toes.Through the design of custom optics, a laser beam can be directed to produce custom power distributions at the work, which can allow the fusion profile of a weld to be optimized for particular applications. By deflecting a portion of the laser power to trail the weld pool, the weld toes can be remelted to smooth stress concentrators and improve fatigue performance. This paper discusses the design and testing of a custom multi-spectral zinc sulfide beam shaping optic with a 10-kW IPG fiber laser. In this research, laser welding parameters were developed for single-pass conventional welding. Feasibility trials were then conducted to prove the concept of smoothing the weld toes. To simulate the concept of a custom three-spot optic, three passes of a conventional optic were tested. Finally, a custom optic was designed and tested to evaluate welding and smoothing the weld toes with a single-pass solution. Weld toe angles in stainless steel were improved from 125 to 163 degrees by welding with the custom optic instead of conventional optics.

Committee: Charles Albright PhD (Advisor); David Farson PhD (Committee Member) Subjects: Engineering; Optics