266 research outputs found

    Damage, contamination and surface treatment of electrical discharge machined materials

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    Electrical discharge machining (EDM) is a manufacturing process capable of machining electrically conductive materials regardless of their mechanical properties. It finds extensive usage across the aerospace, automotive, medical implant and mould/die industries, and is particularly useful for the micro-machining of precision components with complicated shapes. The surface integrity of materials machined by EDM is typically poor, and reduced service life is often expected as a result of surface properties. For example, reduced fatigue performance can result due to the presence of surface cracks as well as porosity, high surface roughness and tensile residual stress. Increased surface area due to surface cracks, porosity and surface asperities also inhibits corrosion performance. This thesis explores from a fundamental perspective the damage and contamination occurring in the surfaces of materials machined by EDM, and investigates the use of a novel surface modification technique, pulsed electron beam irradiation, to improve the most damaging surface property; surface cracking. A transmission electron microscopy (TEM) study was conducted on the surface of single-crystal silicon, which is a chemically and crystallographically homogenous material. For the first time, porosity, contamination and cracking were observed at a scale not visible to conventional imaging techniques such as SEM and optical imaging. The study suggested that conventional microscopic techniques such as SEM and optical microscopy are not sufficient to characterise recast layers created by EDM, and the properties of materials machined by the process are in fact determined by phenomena occurring at the nano-scale. The mechanism behind the movement of material between electrodes was investigated in this thesis. The flushing process in EDM is used to take machined material away from the machining region, and this material is not expected to reattach to electrode surfaces. Using the observation of single discharges and elemental analysis, the mechanism of attachment was determined to be a two-stage process, whereby material ejected at the end of discharge on-time is resolidified in the discharge gap by a successive discharge, which causes its fusion into the opposite electrode surface. This information is critical to the avoidance, or the deliberate deposition of foreign material on a workpiece. Pulsed electron beam irradiation was demonstrated as a rapid and simple method of repairing surface cracks induced by the EDM process. A 4 µm depth of surface cracks created by EDM of stainless steel could be completely eliminated in a pore-free layer. Only a small section of recast layer remained unaffected. The cathode voltage parameter was identified as key to increasing the depth of the remelted layer in future developments of the process. Roughness was at the same time reduced from 3.06 µm to 0.89 µm Sa value. A predominantly austenitic graded nanostructure with grain size down to 6 nm was characterised using TEM and XRD. Such structures have implications for improved mechanical properties via grain boundary strengthenin

    Damage, contamination and surface treatment of electrical discharge machined materials

    Get PDF
    Electrical discharge machining (EDM) is a manufacturing process capable of machining electrically conductive materials regardless of their mechanical properties. It finds extensive usage across the aerospace, automotive, medical implant and mould/die industries, and is particularly useful for the micro-machining of precision components with complicated shapes. The surface integrity of materials machined by EDM is typically poor, and reduced service life is often expected as a result of surface properties. For example, reduced fatigue performance can result due to the presence of surface cracks as well as porosity, high surface roughness and tensile residual stress. Increased surface area due to surface cracks, porosity and surface asperities also inhibits corrosion performance. This thesis explores from a fundamental perspective the damage and contamination occurring in the surfaces of materials machined by EDM, and investigates the use of a novel surface modification technique, pulsed electron beam irradiation, to improve the most damaging surface property; surface cracking. A transmission electron microscopy (TEM) study was conducted on the surface of single-crystal silicon, which is a chemically and crystallographically homogenous material. For the first time, porosity, contamination and cracking were observed at a scale not visible to conventional imaging techniques such as SEM and optical imaging. The study suggested that conventional microscopic techniques such as SEM and optical microscopy are not sufficient to characterise recast layers created by EDM, and the properties of materials machined by the process are in fact determined by phenomena occurring at the nano-scale. The mechanism behind the movement of material between electrodes was investigated in this thesis. The flushing process in EDM is used to take machined material away from the machining region, and this material is not expected to reattach to electrode surfaces. Using the observation of single discharges and elemental analysis, the mechanism of attachment was determined to be a two-stage process, whereby material ejected at the end of discharge on-time is resolidified in the discharge gap by a successive discharge, which causes its fusion into the opposite electrode surface. This information is critical to the avoidance, or the deliberate deposition of foreign material on a workpiece. Pulsed electron beam irradiation was demonstrated as a rapid and simple method of repairing surface cracks induced by the EDM process. A 4 µm depth of surface cracks created by EDM of stainless steel could be completely eliminated in a pore-free layer. Only a small section of recast layer remained unaffected. The cathode voltage parameter was identified as key to increasing the depth of the remelted layer in future developments of the process. Roughness was at the same time reduced from 3.06 µm to 0.89 µm Sa value. A predominantly austenitic graded nanostructure with grain size down to 6 nm was characterised using TEM and XRD. Such structures have implications for improved mechanical properties via grain boundary strengthenin

    Measurements of differential production cross sections for a Z boson in association with jets in pp collisions at root s=8 TeV

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    Search for leptophobic Z ' bosons decaying into four-lepton final states in proton-proton collisions at root s=8 TeV

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    Search for black holes and other new phenomena in high-multiplicity final states in proton-proton collisions at root s=13 TeV

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    Search for high-mass diphoton resonances in proton-proton collisions at 13 TeV and combination with 8 TeV search

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    Search for heavy resonances decaying into a vector boson and a Higgs boson in final states with charged leptons, neutrinos, and b quarks

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    Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model

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    We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the J-statistic, and by analyzing data from Advanced LIGO's second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=3.47×10-25 when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering. © 2019 American Physical Society

    Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background

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    The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society

    Search for Gravitational Waves Associated with Gamma-Ray Bursts Detected by Fermi and Swift during the LIGO-Virgo Run O3b

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    We search for gravitational-wave signals associated with gamma-ray bursts (GRBs) detected by the Fermi and Swift satellites during the second half of the third observing run of Advanced LIGO and Advanced Virgo (2019 November 1 15:00 UTC-2020 March 27 17:00 UTC). We conduct two independent searches: A generic gravitational-wave transients search to analyze 86 GRBs and an analysis to target binary mergers with at least one neutron star as short GRB progenitors for 17 events. We find no significant evidence for gravitational-wave signals associated with any of these GRBs. A weighted binomial test of the combined results finds no evidence for subthreshold gravitational-wave signals associated with this GRB ensemble either. We use several source types and signal morphologies during the searches, resulting in lower bounds on the estimated distance to each GRB. Finally, we constrain the population of low-luminosity short GRBs using results from the first to the third observing runs of Advanced LIGO and Advanced Virgo. The resulting population is in accordance with the local binary neutron star merger rate. © 2022. The Author(s). Published by the American Astronomical Society
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