623 research outputs found
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Selective Laser Sintering and Post Processing of Fully Ferrous Components
Commercially available steel for indirect SLS (LaserFormtm A6 tool steel) is normally
post-process infiltrated with a copper-based material. While such parts have high thermal
conductivity necessary for short- and medium-run injection molding dies, they are weakened by
the second phase with limited high temperature stability. This paper deals with a modification to
the commercial process whereby a low-melting-point cast iron is substituted for the copper alloy
infiltrant. A predictive model is presented that describes the part equilibrium solid fraction at the
infiltration temperature as a function of the green density and infiltration temperature. In an
experimental study, green parts were fabricated using LaserFormtm A6 tool steel powder. They
were then heated in vacuum to drive off the binder and infiltrated with ASTM A532 white cast
iron. During infiltration, an equilibrium state is established between the solid SLS steel part and
liquid cast iron associated primarily with carbon diffusion from the cast iron into the tool steel.
The equilibrium state is governed by the carbon content of the steel and cast iron, the relative
density of the steel part prior to infiltration and the infiltration temperature. In some cases guided
by Ashby densification maps, pre-sintering of the tool steel green part was performed to increase
the initial relative density of the solid metal.Mechanical Engineerin
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Selective Laser Sintering of Cu-Pb/Sn Solder Powders
Solid Freeform Fabrication (SFF) as an overall approach in the
manufacturing field has been applied to very different areas and on
different materials systems including polymers, ceramics, metals and
vapor phase. Even though it is clear that polymers have enjoyed a rapid
and successful advance in terms of applications, research in the other
areas have shown potential applications as well.
In this paper, Cu-Sn/Pb solder and Cu-Sn blends have been used as
starting material for the Selective Laser Sintering (SLS) manufacture of
SFF objects. The general properties of the objects thus produced are
presented. In particular, density and microstructural characterizations
results are discussed..Mechanical Engineerin
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Selective Laser Sintering of Binary Metallic Powder
A selective laser sintering technique has been used to process metal powders and
powder blends. Precursor powders include copper, tin, a 70Pb-30Sn solder and their blends.
Excessive balling due to surface free energy effects occurred in single layer tests when the
laser fluence was sufficient to cause melting of monolithic tin or solder. Improvements in
single layer quality were obtained using copper-solder powder blends in a reducing
atmosphere. The binary powder layers were characterized metallographically and the effect of
processing parameters such as laser fluence and scan speed were assessed. Post-process
annealing improved interparticle wetting and part strength. The influence of ZnCl2 flux was
investigated when present as a coating in copper-solder blends. Multiple layer tests were
performed on the most promising powder blends and the results are presented.Mechanical Engineerin
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Fabrication of Electrically Conductive, Fluid Impermeable Direct Methanol Fuel Cell (DMFC) Graphite Bipolar Plates by Indirect Selective Laser Sintering (SLS)
Graphite bipolar plates are highly desirable due to their properties of high electrical conductivity
and low weight but are associated with prohibitive machining costs arising from poor mechanical
properties. Bipolar plates made by indirect Selective Laser Sintering (SLS) offer the advantages
of complex part production in shorter times at lower cost. Due to the nature of the SLS process,
the as-produced (green part) plates are porous and possess low electrical conductivity which can
be improved by carbonizing the phenolic resin binder at high temperatures (brown part). It has
been found that the electrical conductivity increases significantly (> 200 S/cm) with a
corresponding increase in pyrolyzing temperature which correlates well with literature on the
carbonization of phenol formaldehyde resins. The brown parts are subsequently infiltrated with
low viscosity (~5 - 10 cps) cyanoacrylate to seal up the open pores, rendering the plates fluid
impermeable.Mechanical Engineerin
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Laser Polishing of Silica Rods
Lasers have been widely used in surface modification. In this research a CO2 continuous
wave laser has been used to polish the slot surface of the silica rods. The strong absorption of the
lO.6 um C02 radiation by the silica surface promotes the softening of a very thin layer of material
that flows under the action of surface tension. As a result, a mirror smooth glassy surface has been
formed which decreases the surface roughness without any substantial change in the surface
geometries. The effect of laser to surface inclination angle on the requisite power requirement was
assessed experimentally and theoretically. With laser beam scanning controlled by a computer aided design (CAD) database without specific tooling or human intervention, reliability and
reproducibility of this process have been greatly improved compared to conventional fire polishing.
The potential use of laser polishing as a post-processing step for freeform-fabricated parts is very
promising.Mechanical Engineerin
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An Initial Assessment of Infiltration Material Selection for Selective Laser Sintered Preforms
High-temperature infiltration is an important process that is used to add strength to skeletal
microstructures. In this study, particulate metal matrix composites (MMCs) are fabricated.
MMCs are applied in a wide variety of industries, including military, aircraft, tooling and
automotive. In this paper, various materials for infiltrating selective laser sintered (SLS) silicon
carbide and titanium carbide preforms are considered based on fundamental knowledge of SLS
and infiltration mechanics. Proposed infiltrant materials systems include an aluminum-silicon
alloy infiltrant and a silicon carbide preform, ductile iron infiltrated into a titanium carbide
preform, and commercially pure silicon infiltrated into a silicon carbide preform. The first two
infiltrants are considered because they add ductility to the brittle silicon carbide or titanium
carbide part, thus broadening the range of applications. They also will model a broader field of
possible infiltrants, including magnesium and iron-based materials, (e.g., steel). Silicon is
investigated because it adds strength to silicon carbide, is robust at high temperatures, and has a
comparable coefficient of thermal expansion. Presented is a feasibility assessment of these
systems based on infiltration theory.Mechanical Engineerin
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Solidification Morphology Analysis of SLM of Cu Powder
The solidification morphology analysis of fine Cu powder melted by a raster
scanned energy beam from a focused Nd:YAG laser is presented here. The powder was
processed inside of sealed chamber where it was subjected to a high vacuum cycle. The
laser fusion process consisted raster scanning a narrow rectangular pattern with a high
density of scanning lines, the chamber was purged with inert gas during the process. Up
to a 3.3 mm/s laser travel speed and maximum laser power level of 240 W were used to
melt a 2 mm thick bed of loose powder. The resulting solidified ingots were separated
into categories based on their shape integrity. Metallographic analysis by means of
optical microscopy and scanning electron microscopy was performed on the cross section
and longitudinal section of the ingots with homogeneous surface and complete shape
integrity. Characterization revealed an elongated columnar grain structure with a grain
orientation along the direction of the laser travel direction, some degree of porosity was
observed too in some of the specimens. It was observed that grains diameter ranged from
10 to 100 µm and contained a two phase eutectic microstructure of copper and it oxides.
Oxygen content was accounted from a 5.5 up to 8.1 atomic percent, a small percentage of
chlorine was present, too. A 2 to 8 percent variation in the Vickers microhardness values
were found between the different specimens when measured along the longitudinal
section. These HV values corresponded to approximate 20-25% cold rolled oxygen free
copper (80-90 HV). The ingots thus produced suggest that a multilayer structure from Cu
powder could be build by the SLM process having sufficiently adequate compositional,
microstructure and mechanical properties for functional applications.Mechanical Engineerin
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Improvement of Electrical Conductivity of SLS PEM Fuel Cell Bipolar Plates
Previous work in this research demonstrated the feasibility of fabrication of proton
exchange membrane (PEM) fuel cell bipolar plates by an indirect selective laser sintering
(SLS) route. Properties of the SLS bipolar plate, such as flexural strength, corrosion
resistance and gas impermeability, etc. are quite promising and satisfactory. However,
initial results showed that there was still room for the improvement in electrical
conductivity. This paper summaries the strategies investigated in an effort to increase the
electrical conductivity, among which are: (1) infiltration of brown parts with conductive
polymer (2) addition of a liquid phenolic infiltration/re-curing step prior to final sealing
and (3) reduction of glassy carbon resistivity by curing process parameter control. Results
show that the electrical conductivity value may be improved from 80 S/cm to around 108
S/cm, which is equivalent to a 35% jump, when the phenolic infiltration/re-curing step is
applied before final epoxy sealing.Mechanical Engineerin
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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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Fabrication of PEM Fuel Cell Bipolar Plates by Indirect SLS
The paper presents a new manufacturing technique involving Selective Laser
Sintering (SLS) for proton exchange membrane fuel cell (PEMFC) bipolar plate
fabrication. A material system for bipolar plate fabrication was identified to satisfy both
the cell performance requirement and SLS operation restriction. Carbonization and liquid
epoxy infiltration are subsequently performed following the completion of SLS green
bipolar plate. The finished SLS bipolar plate showed impressive surface finish and
mechanical strength, and a single fuel cell was assembled with two SLS end plates and
membrane electrode assembly (MEA) in between. Various physical property tests were
performed with positive results. Fuel cell performance (voltage vs. current density,
voltage vs. time, etc.) will be assessed in the near future.Mechanical Engineerin
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