554 research outputs found
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SFF-Oriented Modeling and Process Planning of Functionally Graded Materials Using a Novel Equal Distance Offset Approach
This paper deals with the modeling and process planning of solid freeform fabrication
(SFF) of 3D functionally graded materials (FGMs). A novel approach of representation and
process planning of FGMs, termed as equal distance offset (EDO), is developed. In EDO, a
neutral arbitrary 3D CAD model is adaptively sliced into a series of 2D layers. Within each
layer, 2D material gradients are designed and represented via dividing the 2D shape into
several sub-regions enclosed by iso-composition contours. If needed, the material
composition gradient within each of sub-regions can be further determined by applying the
equal distance offset algorithm to each sub-region. Using this approach, an arbitrary-shaped
3D FGM object with linear or non-linear composition gradients can be represented and
fabricated via suitable SFF machines.Mechanical Engineerin
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A Theoretical Model for Optimization of SALD Parameters
This paper addresses the need to conduct theoretical work concerning an economical way
of Solid Freeform Fabrication rendering by using selective Area Laser Deposition (SALD). The
part in SALD rendering process is formed layer by layer that, in turn, is composed of stripes of
material produced in the path of a laser. There are three situations in which such a stripe can be
rendered: a) alone, b) with one neighbor on one side, and c) with neighbors on both sides.
Residual thermal stresses in the part are expected to be affected by how a stripe is rendered.
Furthermore, the residual thermal stress and the mechanical property of the part are also dictated
by other processing variables such as laser scanning patters, laser input power, scanning speed,
scanning spacing, deposition temperature, gas precursor pressure, intrinsic thermal conductivity
and mechanical properties of the rendered material. A theoretical approach is proposed to address
the minimization of residual thermal stresses and rendering times and the maximization of the
strength of the part. It is proposed that such multiple optimizations that are dictated by many
decision variables can be solved by minimizing and/or maximizing object functions dePending on
the design criteria for each attribute of the rendered partMechanical Engineerin
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Solid Freeform Fabrication of Artificial Human Teeth
In this paper, we describe a solid freeform fabrication procedure for human dental
restoration via porcelain slurry micro-extrusion. Based on submicron-sized dental porcelain
powder obtained via ball milling process, a porcelain slurry formulation has been developed. The
formulation developed allows the porcelain slurry to show a pseudoplastic behavior and
moderate viscosity, which permits the slurry to re-shape to form a near rectangular cross section.
A well-controlled cross-section geometry of the extrudate is important for micro-extrusion to
obtain uniform 2-D planes and for the addition of the sequential layers to form a 3-D object.
Human teeth are restored by this method directly from CAD digital models. After sintering,
shrinkage of the artificial teeth is uniform in all directions. Microstructure of the sintered teeth is
identical to that made via traditional dental restoration processes.Mechanical Engineerin
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Comparisons Between Thermal Modeling and Experiments in Laser-Densified Dental Powder Bodies
A three-dimensional thermal finite element model including the effect of the powder-tosolid transition has been developed to investigate the transient temperature distribution during
laser densification of dental powder bed for the layer-by-layer fabrication. The model
encompasses the effects of the temperature- and porosity-dependent thermal conduction and
radiation as well as the temperature-dependent natural convection. The simulation result is
compared with the experiments which establish the temperature dependence of the dental
porcelain microstructure and utilize this dependence to construct the temperature distribution
profile. It is found that the trend of the simulation result matches the experiments very well.The authors gratefully acknowledge financial support provided by
the National Science Foundation under Grant Nos: DMI-9908249 and DMI-0218169.Mechanical Engineerin
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Stereometric Design for Desk-Top SFF Fabrication
Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly from
computer based solid models described by STL (Stereo Lithography) or VRML (Virtual Reality Modeling
Language) files generated by Computer-Aided Design (CAD) systems. Most of the SFF processes produce parts
by building them layer by layer using a row by row pattern, though it is possible to build the part using other
patterns. The SFF technology represents a challenge to designers who, in addition to making decisions
concerning optimum shape and functionality of the entire part, have'to take under consideration several other
manufacturing factors. These factors cover a wide range of technical issues such as Computer-Aided Design
model generation, part description and model slicing files, laser path files, precision of part design, rendering
patterns, manufacturing tolerances, thermal expansion and residual stress phenomena.
This paper investigates the effect of rendering patterns on the integrity, material characteristics and
mechanical properties of the parts prepared by a desk-top SFF device using diode lasers. Fe - Bronze (Cu - Sn)
premixed metal powders were used as the starting material. The particle size was about 100 /lm to 200 /lm.
Density, tensile strength and microstructure of the parts prepared using different rendering patterns were
characterized. The results were analyzed to seek optimal rendering patterns. It was noticed that the samples
were strong along the laser scanning direction, while they were weak perpendicular to the scanning direction.
These results suggest that the laser scanning patterns should be designed to minimize the warping and maximize
the strength of the part in the direction depending on the part's function.Mechanical Engineerin
High energy and high power primary Li-CF batteries enabled by the combined effects of the binder and the electrolyte
Several effective methods have been developed recently to demonstrate
simultaneous high energy and high power density in Li - carbon fluoride
(CF) batteries. These methods can achieve as high as 1000 Wh/kg energy
density at 60-70 kW/kg power density (40-50 C rate) in coin cells and 750 Wh/kg
energy density at 12.5 kW/kg power density (20 C rate) in pouch cells. This
performance is made possible by ingenious nano-architecture design, controlled
porosity, boron doping and electrolyte additives. In the present study, we show
that a similarly great performance, 931 Wh/kg energy density at 59 kW/kg power
density, can be achieved by using a polyacrylonitrile binder and a LiBF4
electrolyte in Li - graphite fluoride coin cells. We also demonstrate that the
observed effect is the result of the right combination of the binder and the
electrolyte. We propose that the mechanistic origin of the observed phenomena
is an electro-catalytic effect by the polyacrylonitrile binder. While our
proposed method has a competitive performance, it also offers a simple
implementation and a scalable production of high energy and high power primary
Li-CF cells.Comment: 13 pages, 8 figures, 1 tabl
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Control of the Cross Section Geometry of Extruded Dental Porcelain Slurries for Rapid Prototyping Applications
This study investigates the dependence of the cross section geometry of extruded dental
porcelain slurries on the rheological property of the slurry and the extrusion conditions. It is
found that a pseudoplastic slurry is a basic requirement for obtaining extruded lines with
rectangular cross sections. The cross section geometry of the extrudate is also strongly affected
by extrusion parameters including the extrusion nozzle height, nozzle moving speed, and
extrusion rate. Proper combinations of these extrusion parameters are necessary in order to
obtain extrudates with near rectangular cross sections. The results obtained have been explained
in terms of the interactions among the rheological properties of the slurry, the shear rate imposed
on the slurry during extrusion, the wettability of the slurry on the substrate, and the forced flow
of the slurry during extrusion.The authors gratefully acknowledge financial support provided by
the National Science Foundation under Grant Nos: DMI-9908249 and DMI-0218169.Mechanical Engineerin
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SALDVI of SiC into Metal and Ceramic Powders
Selective Area Laser Deposition Vapor Infiltration (SALDVI) is the SFF technique using
gas phase precursors to locally infiltrate a powder bed into a desired shape. Experiments were
performed with a CO2 laser and the silicon carbide forming gas precursor Si(CH3)4. This paper
will report on the microstructural aspects of SiC into a variety of metal and ceramic powders
including Mo, SiC, ZrO2, and WC.The authors acknowledge the support for this research by the Office of Naval Research
(grant #N00014-95-1-0978).Mechanical Engineerin
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