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.

Committee: Joy Gockel Ph.D. (Advisor); Henry D. Young Ph.D. (Committee Member); Raghavan Srinivasan Ph.D., P.E. (Committee Member) Subjects: Engineering; Mechanical Engineering

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 Science (M.S.), University of Dayton, 2020, Electro-Optics

This work explores new capabilities of recently emerged adaptive multi-beam laser power sources to optimally shape laser power spatial distribution at powder material during metallic laser powder bed fusion additive manufacturing. Conventional laser additive manufacturing (LAM) systems use a highly localized laser beam for powder material melting resulting in strong temperature gradients inside the heat affected zone (HAZ) leading to formation of columnar material grain structure having highly anisotropic mechanical properties. Beam shaping with multi-beam laser power source provides opportunities for on demand and location-specific altering grain structure from columnar to equiaxed resulting in more isotropic mechanical properties of LAM fabricated parts. In this work we perform numerical simulations of theLAM process using a reduced complexity analytical heat transfer solution in order to optimize multi-beam configurations leading to the desired transitioning from columnar to equiaxed grain morphology. The beam shaping optimization was performed using a stochastic parallel gradient descent optimization of the introduced performance metrics. The results demonstrate the possibility to significantly increase the fraction of equiaxed grains in the solidified powder material using optimal positioning and laser power control of multiple laser focal spots during LAM.

Committee: Mikhail Vorontsov (Committee Chair); Chenlong Zhao (Committee Member); Victor Kulikov (Committee Member) Subjects: Optics

Doctor of Philosophy, University of Akron, 2024, Mechanical Engineering

Processing-related defects such as porosity, residual stress, and surface roughness are the primary impediments to the widespread adoption of additive manufacturing in high-performance aerospace structures, primarily in applications where fatigue is an area of concern. Strengthening the surface through an emerging surface treatment approach has the potential to mitigate these defects and subsequently improve the surface quality, as well as increase the fatigue strength of the additively manufactured components. The core objective of this research work was to employ a severe surface plastic deformation (SSPD) process to improve the surface and fatigue properties of additively manufactured Ti-6Al-4V alloys with a particular emphasis on directed energy deposition (DED) re-paired and fully produced electron beam powder bed fusion (EB-PBF), via combination of laser heating (LA) and ultrasonic nanocrystal surface modification (UNSM). Laser heating plus ultrasonic nanocrystal surface modification is an innovative mechanical sur-face treatment tool, and it has been demonstrated as an interesting laser-based mechanical surface treatment technology to induce thicker deformation layer on the surface using low energy input, impact load, low amplitude, and high ultrasonic frequency, leading to enhancement of the microstructure features, surface strength, and resultant mechanical properties of metallic materials. Physical and mechanical characteristics changes in target materials were investigated using optical (OM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), profilometry, and a hardness tester. The results revealed that the proper thermal and impact energies of the applied surface treatment was effective in inducing higher plasticity flow and promoted greater surface grain refinement. Strengthening of metallic alloys through grain refinement is evidenced by achieving maximum strength, a phenomenon referred to as the Hall-Perch principle. In particular, the s (open full item for complete abstract)

Committee: Gregory Morscher (Advisor); Yalin Dong (Committee Member); Jun Ye (Committee Member); Wieslaw Binienda (Committee Member); Manigandan Kannan (Committee Member) Subjects: Aerospace Materials; Materials Science; Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2023, Engineering PhD

The as-printed surfaces of parts produced through laser powder bed fusion are significantly rougher than surfaces produced through traditional manufacturing processes. This increased roughness can have a significant impact on mechanical properties, with perhaps the most notable detriment in the fatigue life of the part. Therefore, the as-printed surface roughness in additively manufactured materials must be studied more extensively to determine its impact on fatigue performance. This work investigates the surface roughness of additively manufactured specimens through the investigation of processing parameters and their effects on surface roughness in metal additive manufacturing. Furthermore, the relationships between as-printed surface roughness and fatigue behavior in AM materials are studied using both axial fatigue testing and vibration bending fatigue testing of nickel superalloy 718. Following the results of these initial tests, this work also examines the relationships between sharp corners and fatigue life in as-printed additively manufactured alloy 718. Results suggest that both rougher surfaces and sharp corner geometries cause decreases in fatigue life, thus providing guidelines for the design of additively manufactured components under fatigue loading conditions.

Committee: Nathan Klingbeil Ph.D. (Committee Co-Chair); Joy Gockel Ph.D. (Committee Co-Chair); Luke Sheridan Ph.D. (Committee Member); Henry D. Young Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Computing and Information Systems, Youngstown State University, 2023, Department of Computer Science and Information Systems

Additive Manufacturing (AM), particularly LASER Powder Bed Fusion (LPBF), has gained prominence for its flexibility and precision in handling complex metal structures. However, optimizing L-PBF for intricate designs involves analyzing over 130 process parameters, leading to prolonged duration and increased costs. This thesis proposes a novel approach by harnessing statistical and machine learning algorithms to predict manufacturability issues before the printing process. By performing a comparative analysis of the intended design with the machine produced result, the study introduces two machine learning and one artificial neural network (ANN) algorithm to forecast the printability of new designs accurately. This innovative method aims to reduce or eliminate the need for iterative printing, reducing productivity costs and optimizing the LPBF additive manufacturing process.

Committee: Alina Lazar PhD (Advisor); John R. Sullins PhD (Committee Member); Hunter Taylor PhD (Committee Member) Subjects: Computer Engineering; Computer Science; Engineering; Information Science; Information Systems; Information Technology; Materials Science; Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2023, Mechanical Engineering

Additive Manufacturing (AM) methods are promising in applications where complex part geometries, exotic materials and small lot sizes are required. Aerospace manufacturing stands to use AM methods extensively in the future, and frequently requires temperature- and corrosion-resistant alloy materials such as Inconel 718. However, the microstructural evolution of Inconel 718 during additive manufacturing is poorly understood and depends on part size and shape. We studied the microstructure of Inconel 718 parts manufactured by Laser Powder Bed Fusion in order to further elucidate these dependencies. Microstructural analysis, SEM imaging, EBSD scans and Microhardness testing were performed.

Committee: Henry D. Young Ph.D. (Advisor); Dino Celli Ph.D. (Committee Member); Raghavan Srinivasan Ph.D., P.E. (Committee Member) Subjects: Materials Science; Mechanical Engineering

Doctor of Philosophy, University of Toledo, 2022, Engineering

At the early stages of research on additive manufacturing (AM), most of the research efforts were spent on optimizing the process parameters to obtain practical parts with minimum to no defects, while the performance of such components remained a lesser priority. Although the physical properties are the key factor in manufacturing, the final thermo-mechanical properties are of importance when it comes to applications. From a metallurgical point of view, the thermo-mechanical properties of materials are linked to their microstructure. Thus, for altering or enhancing the mechanical performance of a component, it is necessary to tailor/alter the microstructure. On the other hand, the microstructure is directly influenced by the manufacturing process, thus it creates Process-Microstructure-Property-Performance (PMPP) linkage. Besides the capability of making the components with complex shapes, additive manufacturing also provides high flexibility to control the process compared to conventional manufacturing methods by leveraging a large number of parameters (i.e., heat source power, scan speed, etc.) that allow better control over the process and the microstructure. Thus, additive manufacturing not only is able to fabricate complex geometries that may not be possible with conventional methods but also empowers the enhancement of the material properties. It is well reported that crystallographic orientation plays a key role in the thermomechanical behavior of the single crystal NiTi-based SMAs. That notable effect has also been observed in strongly textured SMAs. Such a high dependency of NiTi-based SMAs on crystallographic texture highlights the importance of developing an approach to control the AM process and obtain the preferred crystallographic textures. On the other hand, despite the growing demand for SMAs rotary actuators, the existing literature focuses mostly on assessing the compression and tension behaviors of AM-fabricated NiTi SMAs, while the torsional beha (open full item for complete abstract)

Committee: Mohammad Elahinia (Committee Chair); Othmane Benafan (Committee Co-Chair); Behrang Poorganji (Committee Co-Chair); Meysam Haghshenas (Committee Member); Ala Qattawi (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2022, Mechanical Engineering

Residual stresses developed during additive manufacturing (AM) can influence the mechanical performance of structural components in their intended applications. In this study, thermomechanical residual stress simulations of the laser powder bed fusion (LPBF) process are conducted for both simplified (plate and cube-shaped) geometries as well as five complex lattice geometries fabricated with Inconel 718. These simulations are conducted with the commercial software package Simufact Additive©, which uses a non-linear finite element analysis and layer-by-layer averaging approach in determining residual stresses. To verify the efficacy of the Simufact Additive© simulations, numerical results for the plate and cube-shape geometries are analyzed for convergence and compared to experimental residual stress results available in the literature. Numerical residual stress results are subsequently compared for the five complex lattice geometries. Results suggest that lattice geometry can play a significant role in the distribution and magnitude of residual stresses, which may be significant in some applications.

Committee: Nathan Klingbeil Ph.D. (Advisor); Joy Gockel Ph.D. (Committee Member); Anthony Palazotto Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2022, Mechanical Engineering

Refractory metal alloys in the tungsten molybdenum rhenium ternary system were additively manufactured using laser power bed fusion. Four ternary alloys with varying concentrations of tungsten, molybdenum, and rhenium were manufactured and manufactured again with an addition of 1 wt% hafnium carbide. Samples were heat treated to heal cracks, reduce porosity, and reduce inhomogeneity. Material microstructure was characterized before and after heat treatment using microscopy, energy dispersive x-ray spectroscopy, and electron backscatter diffraction mapping. Mechanical testing was conducted on both three-point bend specimens and compression specimens, resulting in maximum bending strengths of 677.86 MPa, and maximum compression 0.2% yield strengths of 583.88 MPa for the strongest composition. The ternary alloy samples exhibited less porosity, less cracking, more refined grains, and higher strengths. The hafnium carbide doped samples exhibited more cracking and porosity, larger grains, and lower overall strengths.

Committee: Nathan Klingbeil Ph.D. (Advisor); Daniel Young Ph.D. (Committee Member); Ryan Kemnitz Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering; Metallurgy

Doctor of Philosophy, University of Toledo, 2022, Engineering

Superelastic (SE) Nitinol (NiTi) could be a great candidate for a wide range of applications in the biomedical and aerospace industries. Despite its unique properties, fabrication still remains a challenge and is of high interest. To address the limitations, laser-based powder bed fusion (LPBF) additive manufacturing (AM) has been developed and used for the fabrication of superelastic NiTi components. However, in most SE applications NiTi components undergo cyclic loadings. There is, however, limited work on the fatigue life of NiTi components fabricated with LBPF. In general, different parameters starting from the powder preparation to process parameters, build conditions, and post-processing directly affect the microstructural and mechanical properties of the LBPF fabricated NiTi parts. After providing an introduction chapter, this work is organized into three papers, each forming one chapter of the dissertation, and there is a summary and conclusion as a final chapter. In the second chapter, the effect of build orientation on the fatigue behavior of the LBPF fabricated NiTi parts was evaluated. NiTi dog-bone samples with three different build orientations were fabricated and used to investigate the monotonic tensile and fatigue behavior of the material. In addition to the mechanical experiments, fracture surfaces of the monotonic and fatigue samples were evaluated, and different types of defects were assessed. It was shown that the samples fabricated on their edge has a low level of scattering in comparison to the other sample fabricated horizontally or in 45-degree. Since internal defects and unwanted porosities were recognized as a major cause of the fatigue failure, the remelting process was proposed as a potential solution to improve the parts' relative density and reduce internal pores. In the third chapter, to achieve a set of optimized process parameters for the remelting process, selective laser remelting with different process parameters was designe (open full item for complete abstract)

Committee: Mohammad Elahinia (Committee Chair); Mohammad J. Mahtabi (Committee Co-Chair); Mohamed Samir Hefzy (Committee Member); Ala Qattawi (Committee Member); Meysam Haghshenas (Committee Co-Chair) Subjects: Materials Science; Mechanical Engineering; Mechanics

Doctor of Philosophy in Materials Science and Engineering, Youngstown State University, 2022, Materials Science

Additive Manufacturing (AM) has changed the manufacturing world by opening doors to develop structures that were either not possible before or extremely complex with regular manufacturing. This work investigated Powder Bed Fusion AM which allows the creation of complex structures for metal parts using high performance materials. Industries such as aerospace have seen benefits in using AM, not only in designing but replacing parts for aging aircrafts. Polyaryle Ether Ketones (PAEK) materials, in a Fused Filament Fabrication (FFF) system, were used because its mechanical performance is close to that of aluminum and the applications for aerospace and biomedical industries. Printing of PEEK and PEKK can be difficult due to the high melting point required of the materials. The recent availability of soluble support has allowed the printing of lattices with overhanging features. Density can be optimized to have a balance between weight and strength for aerospace structures and implants. 3D printing has been applied in a variety of ways, from printing with material extrusion printers and stopping the print to embed electronics in the structure and then resuming the print, to printing components in vat photopolymerization (VPP) printers in a jigsaw-like way, with cavities to fit electronics and overmold together, which will leave the components inside the structure. A lot of research has been done on wearable electronics, for things like cortisol, H2S, haptic feedback, pressure sensors and others, due to the demand for smaller electronics. The flexibility provided by AM, allows for these two well studied technologies to be combined into one, 3D printed wearable electronics.

Committee: Snjezana Balaz Ph.D. (Advisor); Holly Martin Ph.D. (Committee Member); Donald Priour Ph.D. (Committee Member); Clovis Linkous Ph.D. (Committee Member); Christopher Hansen Ph.D. (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Polymers; Technology

Master of Science (M.S.), University of Dayton, 2021, Mechanical Engineering

To numerically simulate and predict the plastic deformation of aerospace metal alloys during extreme impact events (e.g., turbine engine blade-out and rotor-burst events, and foreign object damage), accurate experimental knowledge of the metal's hardening behavior at large strains is requisite. Tensile tests on round cylindrical specimens are frequently used for this purpose, with the metal's large-strain plasticity ultimately captured by an equivalent true stress vs. equivalent true plastic strain curve. It is now well known that if axial strain is measured using an extensometer, the equivalent true stress-strain curve calculated from this measurement is valid only up to the onset of diffuse necking. That is, once the strain field heterogeneously localizes in the specimen gage (onset of necking), extensometers, which average the strain field over the gage section, are unable to capture the local strain at the site of fracture initiation. Thus, a number of approaches have been proposed and employed to correct the post-necking hardening response. One commonly-used technique is an iterative approach commonly referred to as finite-element model updating (FEMU). This approach involves inputting a suite of candidate post-necking equivalent true stress-strain curves into finite-element software. The true stress-strain curve that produces the best agreement between simulation and experiment is ultimately adopted. In this document, a novel variation of this iterative approach is presented, aimed at decreasing computational expense and iterative effort with a better first guess that bounds this fan of prospective true stress-strain curves. In particular, we use local surface true (Hencky) strain data at the fracture location in an approximate analytical formula to generate a first guess curve and upper bound on the candidate true stress-strain fan of curves. To assess its performance and robustness, the proposed approach is verified using experimental data for a menu of ae (open full item for complete abstract)

Committee: Robert Lowe Ph.D. (Advisor); Luke Sheridan Ph.D. (Committee Member); Dennis Buchanan Ph.D. (Committee Member); Jeremy Seidt Ph.D. (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Engineering; Mechanical Engineering; Mechanics

Master of Science, University of Toledo, 2021, Mechanical Engineering

NiTi Shape memory alloys (SMAs) have recently gained increased interest due to their unique features such as super elasticity and shape memory effect. There is an ongoing research effort on practical Additive Manufacturing (AM) processing to arrive at optimized AM process parameters to develop functional NiTi devices with minimal post-processing. Laser Powder Bed Fusion (LPBF) is a promising free-form AM method for fabricating this material. It is well established that LPBF AM can be used to tailor the properties of NiTi SMAs by adjusting various process parameters. Modeling approaches are cost and time-effective tools to expedite the study of process parameters optimization for AM fabricated parts. Since the LPBF process deals with ii thermal cycling over the material, thermal modeling of the LPBF process is an essential step of the process and material optimization. That is why it is essential to have a comprehensive thermal understanding of the phenomena using a thermal model to open the door for NiTi optimization. More specifically, melt pool geometry is an essential thermal parameter that affects the resulting material properties. In this thesis, a thermal model has been developed to predict the melt pool size during the LPBF of NiTi. Macroscale physics has been selected for the modeling framework utilizing COMSOL Multiphysics® to develop a thermal model for LPBF melt pool size of NiTi alloy. The Two main stages that are considered to model the melt pool are 1) thermal/melt pool modeling of single laser pass on a NiTi substrate and 2) thermal/melt pool modeling of single layer pass over a single layer of NiTi powder with a thickness of 30μm spread on a NiTi substrate. The model was calibrated in the first stage of modeling the thermal process over the substrate. Through matching the experimental and modeling results, the thermal properties were calculated. These properties were then used to calculate powder thermal properties in the second stage of simulat (open full item for complete abstract)

Committee: Mohammad Elahinia Dr. (Committee Chair); Meysam Haghshenas Dr. (Committee Member); Ala Qattawi Dr. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2021, EMC - Mechanical Engineering

We present a powder-scale computational framework to predict the microstructure evolution of metals in Powder Bed Fusion Additive Manufacturing (PBF AM) processes based on the Hot Optimal Transportation Meshfree (HOTM) method. The powder bed is modeled through Discrete Element Method (DEM) as discrete and deformable three-dimensional bodies by integrating statistic information from experiments, including particle size and shape, and powder packing density. Tractions in Lagrangian framework are developed to model the recoil pressure and surface tension. The laser beam is applied to surfaces of particles and substrate dynamically as a heat flux with user-specified beam size, power, scanning speed and path. The linear momentum and energy conservation equations are formulated in the Lagrangian configuration and solved simultaneously in a monolithic way by the HOTM method to predict the deformation, temperature, contact mechanisms and fluid-structure interactions in the powder bed. The numerical results are validated against benchmark tests and single track experiments. Various powder bed configurations, particle size distributions, laser powers and speeds are investigated to understand the influence of dynamic contact and inelastic material behavior on the deformation, heat transfer and phase transition of the powder bed. The formation of defects in the microstructure of 3D printed metals, including pores, partially and un-melted particles, are predicted by the proposed computational scheme.

Committee: Bo Li (Committee Chair); Yasuhiro Kamotani (Committee Member); John Lewandowski (Committee Member); Ya-Ting Liao (Committee Member) Subjects: Mechanical Engineering

MS, University of Cincinnati, 2020, Engineering and Applied Science: Mechanical Engineering

Metal additive manufacturing (AM) has a variety of applications ranging from gas turbine engines to biomedical implants. By not being constrained to subtractive manufacturing limitations, additive manufacturing gives engineers greater design freedom than conventional manufacturing methods. Of particular interest has been Laser-Powder Bed Fusion (L-PBF), as it enables printing fine features with up to 99.9% dense material. One issue that has hindered the adoption of additively produced metal parts is that supports are typically required to anchor the printed part to the build plate as to resist warping caused by residual stress accumulation, as well as to prevent shifting due to lateral forces generated by the powder recoater. Additionally, the supports provide a path for heat conduction to allow the area of the part being printed to cool. However, the presence of supports increases the labor and cost of the manufacturing process once the parts are removed from the printer by requiring post-processing steps to be added including an extra thermal treatment, cutting of the part from the build plate, and removal of the supports. With recent advancements to the L-PBF process and printer technology, supports are no longer required for certain parts such that the parts are being supported solely by the powder bed itself, which is more commonly referred to as free-floating. One advancement is that the newest generation of machines have a non-contact recoater, thus eliminating lateral forces being applied to the part. An additional criteria is that the part being printed must initiate from a single point to avoid deformation due to residual stresses. Beyond the single point initiation, however, the geometrical constraints prior to process breakdown are not well understood. The common shapes generated when printing free-floating parts are cones, tetrahedrons and pentahedrons. Therefore, these standard geometries with varying overhang angles and filet radii were chosen to (open full item for complete abstract)

Committee: Ashley Paz y Puente Ph.D. (Committee Chair); Jing Shi Ph.D. (Committee Member); Sarah Watzman Ph.D. (Committee Member) Subjects: Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2020, Mechanical Engineering

The presence of defects within laser powder bed fusion (LPBF) parts can lead to reduced mechanical properties and life of components. Because of this, the ability to detect these defects within the parts is critical before the part is subject to its intended loading. Normally the parts are subjected to a quality analysis once they are completed however, this process is typically expensive and time consuming. A solution for these problems is to sense the creation of defects and pores in the parts in-situ, while the part is being fabricated. One proposed method of in-situ monitoring is visible spectroscopy to identify defects based on the light intensities during prints. In this work, in-situ spectroscopy intensities and ex-situ computed tomography defect data are compared for different processing parameters and two LPBF builds to determine correlation. Results show that changes in the signals from the spectroscopy occur for different conditions of processing parameters and geometries.

Committee: Joy Gockel Ph.D. (Advisor); Nathan Klingbeil Ph.D. (Committee Member); John Middendorf Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering

Master of Science in Engineering, Youngstown State University, 2020, Department of Mechanical, Industrial and Manufacturing Engineering

Laser Powder Bed Fusion (LBPF) allows for unparalleled freedom in design to manufacture complicated structures in high performance materials. Due to the advancement of additive manufacturing technologies, 3D printed parts have moved from the R&D phase to the development phase, with the aerospace industry having adapted this technology to cater small batch replacement parts mainly for an aging fleet of aircraft. The goal of this research was to systematically generate defects, mainly lack-of-fusion defects, to understand the mechanical and corrosion behavior of these defects in parts that are susceptible to flight criticality and safety criticality. This work investigated the influence of the main Selective Laser Melting process parameters (laser power, travel velocity, hatch spacing, and layer thickness) on the defect characteristics using an AlSi10Mg alloy. Five studies were conducted to analyze, evaluate, and compare the defect nature, including linearity, in different orientations and builds. Seventy-seven sample coupons were manufactured from two parameter development builds and image analysis was performed using Image J and Photoshop. Optical microscopy and X-ray CT imaging were the methods used for defect detection. The results showed that for decreasing energy density, the defect density and defect size increase which results in the decrease of the % average relative density, for the set of process parameters investigated, with lack-of-fusion defects predominantly forming at energy densities below 35 J/mm3. Following defect characterization, the effects of each of the four major process parameters were interpreted using a DOE approach with the help of regression and ANOVA testing. Hatch spacing proved to be the most significant process parameter affecting the defect density, while the layer thickness showed the most significant effect when predicting the average defect dimensions and the ratio of defect length to height for the set of process parameters (open full item for complete abstract)

Committee: Holly Martin PhD (Advisor); Brett Conner PhD (Committee Member); Hojjat Mehri PhD (Committee Member) Subjects: Aerospace Materials; Materials Science; Metallurgy; Statistics

Master of Science in Mechanical Engineering (MSME), Wright State University, 2020, Mechanical Engineering

In additive manufacturing (AM), it is necessary to know the influence of processing parameters in order to have better control over the microstructure and mechanical performance of the part. Laser powder bed fusion (LPBF) is a metal AM process in which thin layers of powdered material are selectively melted to create a three-dimensional structure. This manufacturing process is beneficial for many reasons; however, it is limited by the thermal solidification conditions achievable in the available processing parameter ranges for single-beam processing methods. Therefore, this work investigates the effect of multiple, coordinated heat sources, which are used to strategically modify the melting and solidifying in the AM process. The addition of multiple heat sources has the potential to provide better control of the thermal conditions, thus providing better control of the microstructure of the additively manufactured parts. To model this, existing thermal models of the LPBF process have been modified to predict the thermal effects of multiple coordinated laser beams. These computational models are used to calculate melt pool dimensions and thermal conditions throughout the LPBF process. Furthermore, the results of the simulations are used to determine the influence of the distance between the coordinated laser beams. The predictive method used in this research provides insight into the effects of using multiple coordinated beams in LPBF, which is a necessary step in increasing the capabilities of the AM process.

Committee: Joy Gockel Ph.D. (Advisor); Nathan Klingbeil Ph.D. (Committee Member); Ahsan Mian Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2020, Engineering PhD

In many structural applications void-like defects cause significant performance debits which call for component redesign or post-processing to account for or remove the defects. For laser powder bed fusion (LPBF) processes, it has been shown that many of these features and their size and shape characteristics are controllable through LPBF process parameter manipulation. For design efforts, however, it is necessary to understand the direct influences of processing on the formation of porosity and the role that individual pores and porosity distributions have on the properties and performance of AM components. Additionally, design criteria must be established to facilitate implementation of AM components into structurally critical applications. To this end, the investigations that have been performed here relate the AM material processing of alloy 718 to the pore structure, crack growth properties and fatigue performance. This dissertation first explores the influence of four key process parameters and scan strategies on the formation and characteristics of porosity distributions in AM material. Then, based on the porosity distributions observed via non-destructive inspection techniques, a crack-growth based life prediction method was developed to accurately predict fatigue lives of AM components. Additionally, fatigue limit models were modified based on experimental data to explore the interactions of defect size and applied stress with respect to both finite and "infinite" fatigue life which enables defect tolerant design for components manufactured via AM. Finally, a novel compliance-based method for crack initiation detection was developed and used to assess some of the assumptions made in the prior investigations. The connections made through the work presented herein link AM processing to potential design requirements which will facilitate faster, safer design efforts for implementation of AM components into structurally critical applications.

Committee: Joy E. Gockel Ph.D. (Advisor); Nathan W. Klingbeil Ph.D. (Committee Member); Ahsan Mian Ph.D. (Committee Member); Onome Scott-Emuakpor Ph.D. (Committee Member); Anthony Rollett Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering; Mechanics; Metallurgy