162 research outputs found
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Solid Freeform Fabrication An Advanced Manufacturing Approach
Solid freeform fabrication (SFF) is the production of freeform solid objects directly from a
computer model without part-specific tooling or human intervention. SFF has been realized in the
last ten years through the merging of several previously distinct technologies: computer science,
mechanical design, controls, high-energy beam technology and materials science and engineering.
Their combination has produced over a relatively short time..frame numerous SFF methods. The
value of SFF to the commercial sector is usually articulated in terms of reduced time to market
(prototyping), low production "one-of-a-kind" parts and patterns for casting. The purpose of this
introductory paper is to describe briefly some of the approaches to SFF as a background for the
articles included in this proceedings.Mechanical Engineerin
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Solid Freeform Fabrication of Silicon Nitride Shapes by Selective Laser Reaction Sintering (SLRS)
SelectiveLaser ReactionSinteringis a variation ofselective laser sintering (SLS) that incorporates
anjn~situreaction underthe·scannedbeamtofabricate shapes from materials not directly accessible by
traditional SLS. Thispaperclescribesaninvestigation into the production of silicon nitride (Si3N4) shapes
by lasersinteringsiliconpowderinanammonia (NH3) atmosphere. The effect of gas pressure and the
importance of gas/laserinteractionsarediscussed. Single and multiple layer shapes are fabricated. The
material is analyzed by x~ray diffraction spectroscopy (XRDS) for phase content and scanning electron
microscopy (SEM) for macrostructure. Data is presented that demonstrates conversion rates from silicon to
silicon nitride on the order of 85%.Mechanical Engineerin
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Al2O3 Precursor Evaluation for SALD Joining
SFF technology is not limited to the creation of components and objects. Freeform
technology can be a mechanism for joining of materials. By defining the space between two
objects as the boundary for a free-form object it is possible to form a three dimensional joint fill.
Selective Area Laser Deposition or SALD, has been used for free form and joining of carbides
and nitrides but has the potential to make other classes of ceramics. By selecting a metal-organic
precursor and an oxidizer, an oxide can be created by CVD gas decomposition. In this study such
oxide joint fill precursors are studied, particularly for alumina.Mechanical Engineerin
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Selective Laser Sintering of Alumina-Boron Oxide Composites
The selection of an optimum composite system for selective laser sintering (SLS) is based on
materials properties such as the melting point and the wettability between the components in the
composite powder. The alumina-boron oxide composite system is attractive for SLS because the
presence of the low melting component B203 (melting point 4500 C) can enhance sintering. A
better wetting of solid alumina powder by molten boron oxide can also aid densification process.
The alumina-boron oxide conlposite system has been investigated by SLS and selective laser
reactive sintering (SLRS). The role of boron oxide content as a binder, laser power density, and
secondary heat treatment on the microstructure and mechanical properties is discussed.Mechanical Engineerin
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Modeling of Selective Area Laser Deposition for Solid Freeform fabrication
The results of a theoretical study of the selective area laser deposition process used for Solid
Freeform Fabrication (SFF) from gas phase is presented. We show how the deposition profile of
carbon deposited via pyrolytic laser chemical vapor deposition using acetylene as the source gas
can be computed by taking into account heat transfer, reaction, and mass transfer processes inside
the reactor. The two dimensional representation of the related experimental variables are used to
describe the substrate temperature, carbon deposit, and acetylene concentration in the process. The
parameters describing these processes are estimated.Mechanical Engineerin
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Densification Behavior of SLS Processed Al2O3/Al Composite
Production of structurally sound parts by any rapid prototype technique is essential, because
fully functional features are necessary where application testing is required. In the present work,
a powder blend of A1203/AI (3:1 by weight) was mixed with ammonium dihydrogen phosphate
and subjected to selective laser sintering (SLS) using a C02 laser. An attempt has been made
to increase the powder bed density by introducing vibration to the part cylinder. These SLS
processed preforms were then subj ected to a secondary heat treatment in a hydrogen
atmosphere and to hot isostatic pressing. Densification behavior of these Al20 3/Al composite
preforms is discussed.Mechanical Engineerin
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Solid Freeform Fabrication by Selective Area Laser Deposition
Laser chemical vapor deposition is capable of selective area deposition of thin fums at high
spatial resolution, and in the present work this advantage was used to perform solid
freeform fabrication (SFF). The pyrolytic selective area laser deposition of carbon is
studied as a function of the scanning speed, the laser power, and the diameter of the focal
spot on the substrate, at different pressures of the acetylene precursor in a gas phase SFF
system. Carbon rods and rings have been made. SEM and Raman microprobe were used to
characterize the deposits.Mechanical Engineerin
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Development of Nanocomposites for Solid Freeform Fabrication
Nanocomposites in which the constituents are mixed on a nanorneter scale can
provide important advantages in the Selective Laser Sintering (SLS) and Selective Laser
Reactive Sintering (SLRS) processes. The larger surface area and grain boundaries in the
nanocolnposites compared to that in the conventional microcomposites are expected to
enhance the solid state diffusion during laser irradiation as well as during any other
subsequent processes. Our strategy is to design and develop nanocomposites in which one
nanosize cOlnponent has a lower melting point than the other nanosize component, either of
which can serve as the matrix phase. The nanoscale dispersion of the low melting
component can aid the sintering process during SLS or SLRS. Nanocomposite powders of
AI203-COOx, Ab03-NiO, A1203-CO and A1203-Ni have been synthesized by sol-gel
processing and are evaluated by SLS.Mechanical Engineerin
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Fundamentals of Liquid Phase Sintering During Selective Laser Sintering
One of the advantages of the Selective Laser Sintering (SLS) process is that a variety of materials
can be processed. However, the goal of being able to produce fully dense metal parts with no post
processing has been elusive. Using Selective Laser Sintering to produce metal parts with full
density without post processing poses a challenge since both the processing conditions and the
metal system must be controlled. This article describes two metallurgical mechanisms by which
loose metal powder beds could be sintered to nearly full density using a scanning laser beam. The
mechanisms are particle rearrangement during liquid phase sintering (LPS) and in-situ infiltration.
Some of the particles, when heated by the laser radiation, melt and form a liquid. Ifthis liquid has
certain physical properties (e.g., low viscosity and high surface tension) and wets the other solid
particles, then the SLS process can in theory produce dense layers by either mechanism. The
purpose of this study is to determine the process and material selection parameters required to
achieve fully dense parts during direct Selective Laser Sintering of metal.Mechanical Engineerin
Quasi Stable Black Holes at the Large Hadron Collider
We adress the production of black holes at LHC and their time evolution in
space times with compactified space like extra dimensions. It is shown that
black holes with life times of hundred fm/c can be produced at LHC. The
possibility of quasi-stable remnants is discussed.Comment: 4 pages, 3 figures, typos removed, omitted factors included, accepted
for publicatio
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