Mode of access: Internet via World Wide Web. Available at http://hdl.handle.net/1947/9655. / "September 2008" "AR-014-283" "DSTO-TR-2177" Includes bibliographical references.

The design and material properties of rails and projectiles are critical to the success of the Navy railgun. This thesis addresses the design, fabrication, and testing of a scalable square bore electromagnetic railgun. This railgun is designed to permit series augmented operation, and incorporates disposable rail liners to facilitate investigating the suitability of variousrail materials. A series of shots has demonstrated performance consistent with theoretical modeling, including significant performance enhancement as a result of both slotted rail geometry and augmentation over solid rail and un-augmented configurations. A capacitor based stored energy supply input of 35 kJ resulted in a measured velocity of 294 m/s for an 11.4 gram projectile. Suggestions are provided for future power supply configurations, rail materialsand surface treatments, and a variety of armature geometries.

This thesis investigates the use of laser shock peening (LSP) to improve mechanical properties, electrochemical behavior, and stress corrosion cracking (SCC) resistance in laser powder bed fusion (LPBF) stainless steel. The thesis begins by introducing metal additive manufacturing and reviews the current technological frontiers of LSP before elucidating the fundamentals behind the imaging, experimental, and theoretical frameworks used in the subsequent chapters. The experimental work is roughly divided into two parts; the first half is dedicated to study of the plasticity response augmentation by LSP in anisotropic stainless steel. The prevalence of back stress hardening occurring in anisotropic metal parts causes reduced fatigue life under random loading. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to AM metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back-stress hardening. This discussion is initially applied to rolled and annealed 304L stainless steel which is shown to exhibit material anisotropy. Back stress is extracted from hysteresis tensile testing for treated and untreated samples. Analysis of plasticity response by orientation imaging microscopy (OIM) and finite element analysis (FEA) describes back stress and residual stress development during tensile testing and LSP treatment. The research indicates LSP's potential to address manufacturing design challenges caused by yield asymmetry due to back stress and is thus next applied to additively manufactured 316L. The microstructure and texture in additively manufactured metal lead to anisotropic hardening behavior. Comparison of LSPed and as-built LPBF samples shows LSPed samples processed along the build direction demonstrate significant back-stress reduction. Electron backscatter diffraction (EBSD) illuminates grain morphologies' role, while crystal plasticity finite element (CPFE) modeling reveals mechanisms underlying back-stress reduction across different build orientations and crystal planes.In the second half of the thesis, LSP’s effect upon LPBF 316L material performance in corrosive environments is investigated. This effort begins with analysis of LSP’s improvement to electrochemical and wetting behavior of as-built LPBF surfaces. The corrosion performance of LPBF stainless steel varies between studies and build parameters, thus motivating the search for postprocessing methods that enable wetted surface applications. The study examines electrochemical properties before and after LSP, measuring pitting potential, electrochemical impedance, contact angle, surface free energy, and surface finish. LSP imparts surface improvement which is attributed to morphology and chemistry alterations as well as compressive residual stress. LPBF stainless steel is also particularly susceptible to SCC due to surface-level tensile residual stress. The final study demonstrates LSP's ability to enhance SCC behavior in LPBF stainless steel by increasing time to crack initiation. Analyses of residual stress, texture, dislocation distribution, hardness, microstructure, and fracture surfaces are conducted to understand the mechanisms underlying SCC improvement. Dynamic crack modeling supports observed outcomes, linking residual stress and failure modes to LSP's effects. This work highlights LSP's potential as a versatile tool for enhancing the performance and reliability of LPBF stainless steel components in demanding engineering applications. Further, it identifies the key relevance of the anisotropy of LPBF material structure to mechanical behavior and also to the effectiveness of LSP surface processing.

Dissimilar metal joining was performed with the main goal being maximization of the strength of the joined samples, but because of some potential applications of the dissimilar joints, analyzing their corrosion behavior also becomes crucial. Starting with materials that initially have suitable corrosion resistance, ensuring that the laser processing does not diminish this property is necessary. Conversely, the laser shock peening processing was implemented with a complete focus on improving the corrosion behavior of the workpiece. Thus, many commonalities occur between these two manufacturing processes, and this thesis goes on to analyze the thermal and mechanical influence of laser processing on materials’ corrosion resistances. Brittle intermetallic phases can form at the interfaces of dissimilar metal joints. A process called autogenous laser brazing has been explored as a method to minimize the brittle intermetallic formation and therefore increase the fracture strength of joints. In particular, joining of nickel titanium to stainless steel wires is performed with a cup/cone interfacial geometry. This geometry provides beneficial mechanical effects at the interface to increase the fracture strength and also enables high-speed rotation of the wires during irradiation, providing temperature uniformity throughout the depth of the wires. Energy dispersive X-ray spectroscopy, tensile testing, and a numerical thermal modelling are used for the analysis. The material pair of nickel titanium and stainless steel have many applications in implantable medical devices, owing to nickel titanium’s special properties of shape memory and superelasticity. In order for an implantable medical device to be used in the body, it must be ensured that upon exposure to body fluid it does not corrode in harmful ways. The effect that laser autogenous brazing has on the biocompatibility of dissimilar joined nickel titanium to stainless steel samples is thus explored. While initially both of these materials are considered to be biocompatible on their own, the laser treatment may change much of the behavior. Thermally induced changes in the oxide layers, grain refinement, and galvanic effects all influence the biocompatibility. Nickel release rate, polarization, hemolysis, and cytotoxicity tests are used to help quantify the changes and ascertain the biocompatibility of the joints. To directly exert a beneficial influence on materials’ corrosion properties laser shock peening (LSP) is performed, with a particular focus on the stress corrosion cracking (SCC) behavior. Resulting from the combination of an applied load on a susceptible material exposed to a corrosive environment, SCC can cause sudden material failure. Stainless steel, high strength steel, and brass are subjected to LSP and their differing corrosion responses are determined via cathodic charging, hardness, mechanical U-bend, Kelvin Probe Force Microscopy, and SEM imaging. A description accounting for the differing behavior of each material is provided as well as considerations for improving the effectiveness of the process. SCC can occur by several different physical processes, and to fully encapsulate the ways in which LSP provides mitigation, the interaction of microstructure changes induced by LSP on SCC mechanisms is determined. Hydrogen absorbed from the corrosive environment can cause phase changes to the material. Cathodic charging and subsequent X-ray diffraction is used to analyze the phase change of sample with and without LSP processing. Lattice dislocations play an important role, and transmission electron microscopy helps to aid in the analysis. A finite element model providing spatially resolved dislocation densities from LSP processing is performed.

High energy pulse laser peening (HEPLP) is a manufacturing process, in which a strong shock wave is produced and induces high pressures on the surface of the target material. Generally, this process is used to improve material properties such as the hardness and fatigue life. First a 2D multi-physics model for the process was investigated, which simulates the pressure induced on the surface of the target material. The model can be coupled with commercial finite element software, such as ABAQUS, to more accurately simulate the HEPLP process to find stresses and deformations on the surface. Next two novel applications using the HEPLP process were investigated. The first, laser shock bending is a sheet metal micro-forming process using HEPLP to accurately bend, shape, precision align, or repair micro-components with bending angles less than 10°. Negative bending angle (away from laser beam) can be achieved with the high-energy pulsed laser, in addition to the conventional positive laser bending mechanism. In this thesis, various experimental and numerical studies on aluminum sheets were conducted to investigate the different deformation mechanisms, positive and negative. The experiments were conducted with the sheet thickness varying from 0.25 to 1.75 mm and laser pulse energy of 0.2 to 0.5 J. A critical thickness threshold of 0.7-0.88 mm was found that the transition of positive negative bending mechanism occurs. A statistic regression analysis was also developed to determine the bending angle as a function of laser process parameters for positive bending cases. The second application studied used HEPLP to imprint complex two-dimensional (2D) patterns dental implant material of cpTi. Pure titanium (commercial pure cpTi) is an ideal dental implant material, without the leeching of toxic alloy elements. Evidence has shown that unsmooth implant surface topologies may contribute to the osteoblast differentiation in human mesenchymal pre-osteoblastic cells, which is helpful to avoid long-term peri-abutment inflammation issues for the dental implant therapy with transcutaneous devices. Studies have been conducted on the grit blasted, acid etched, or uni-directional grooved Ti surface, however, for these existing approaches the surface quality is difficult to control or may even damage the implant. The strong shock wave generated by HEPLP is used to press a stainless steel grid, used as a stamp, on Ti foils to imprint a 2D pattern. In this study, the multiple grid patterns and grid sizes were applied to test for cell-attachment improvements. Then, the cell culture tests were conducted with the patterned surface to investigate the contribution of these 2D patterns, with the control tests of the other existing implant surface topography forming approaches. The micro-patterns proved successful in increasing the cell-attachment, increasing the number of cells attaching to the material and also contributing to the cell-growth within the grooved areas.

No description available.

No description available.

Additive Manufacturing (AM) has revolutionized manufacturing processes by enabling the realization of custom products with intricate geometric features that were either too complex or even intractable for subtractive manufacturing processes. Yet, functional surfaces generated in AM have to be often finish machined because of their relatively inferior roughness. The first phase of this research worked around this limitation by tailoring the topography of an AM surface in-process to entail textures that further enhance certain functionalities in a process called Additive Texturing (AT). In this context, the Selective Laser Melting (SLM) process ability to realize intricate surface microfeatures was explored experimentally, evaluating its geometrical limitations. Utilizing such limitations, various patterns comprising pillars, channels, and re-entrant structures were printed to control the wetting behaviour of SLM stainless steel. AT's efficacy is demonstrated in its capability to generate hydrophobic AM surfaces with water contact angles exceeding 140°. Similarly, other texturing patterns comprising dimples, linear, V-shaped, and X-shaped grooves were investigated to tailor the tribological response of textured surfaces under dry sliding conditions. Evidently, a specific wear rate and coefficient of friction reduction of 80% and 60%, respectively, demonstrated another potential for AT. The undesirable tensile Residual Stresses (RS) that inevitably accumulate during the SLM process's rapid heating and cooling cycles were investigated in the second phase of this research. Laser Peening (LP) was utilized to post-process the printed samples to eliminate the initial tensile RS and induce near 500 Mpa compressive RS. Moreover, the LP parameters were explored and optimized to enhance RS, surface roughness, hardness, and wear resistance. / Thesis / Doctor of Philosophy (PhD)

No description available.