103 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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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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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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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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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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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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Mechanics of the Selective Laser Raster-Scanning Surface Interaction
In recent years, the use of a high power laser beam actuated by fast speed
scanning mirrors has opened up novel selective laser raster-scan processing venues as
extremely rapid motion and high overlapping of the beam can be attained. This permits
distribution of laser energy precisely over geometric patterns such as rectangles, circles,
triangles etc. The surface thermal history at any given point under such processing was
estimated using an analytical solution for the 1D, semi-infinite, surface flux boundary
condition heat conduction problem together with linear superposition theory. Presented
here is the comparison of the thermal histories of different selective laser surface
processes previously implemented, namely: laser surface polishing of flat surfaces, laser
induced cementation of cylindrical surfaces and direct laser single layer masked
deposition. It was possible to verify that in laser induced cementation, long-width and
short-length scanned regions provided low average temperature and low heating rate with
spaced out temperature peaks, whereas for direct laser single layer deposition in which a
narrow-width – long-length region is scanned, the heating rate and peak temperature are
higher and the peaks are squeezed. The analysis also provided ways to estimate the
Andrew’s number associated with a raster-scan process for the sake of comparison with
single-beam processes having a given number value. Understanding the influence of scan
geometry and overlapping on the selective raster-scan processing provides a method to
tailor the surface peak temperature as well as the heating and cooling rates, affecting the
solidification or sintering conditions and therefore the mechanical properties of the parts
obtained.Mechanical Engineerin
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Design and Freeform Fabrication of Deployable Structures with Lattice Skins
Frontier environments—such as battlefields, hostile territories, remote locations, or outer
space—drive the need for lightweight, deployable structures that can be stored in a compact
configuration and deployed quickly and easily in the field. We introduce the concept of lattice
skins to enable the design, solid freeform fabrication (SFF), and deployment of customizable
structures with nearly arbitrary surface profile and lightweight multi-functionality. Using
Duraform FLEX® material in a selective laser sintering machine, large deployable structures are
fabricated in a nominal build chamber by either virtually collapsing them into a condensed form
or decomposing them into smaller parts. Before fabrication, lattice sub-skins are added
strategically beneath the surface of the part. The lattices provide elastic energy for folding and
deploying the structure or constrain expansion upon application of internal air pressure. Nearly
arbitrary surface profiles are achievable and internal space is preserved for subsequent usage. In
this paper, we present the results of a set of experimental and computational models that are
designed to provide proof of concept for lattice skins as a deployment mechanism in SFF and to
demonstrate the effect of lattice structure on deployed shape.Mechanical Engineerin
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Experimental Study of Snap-Fits Using Additive Manufacturing
A snap-fit is a mechanical joint system whose mating parts exert a cam action, flexing until one
part slips past a raised lip on the other part, preventing their separation. The use of snaps in
additive manufacturing (AM) is an approach for assembling components of parts too large to
build in one piece in AM. There are broadly two types of snap-fits possible to encounter,
permanent and non-permanent, depending on the design geometry. An experimental study
was carried out to evaluate the mating/dismounting force for snap-fits regarding several
geometrical parameters for additive manufacturing. The design chosen for this study has been
established from the start to work on only one design. The parameters chosen for experimental
investigation were the mating angle, the separation angle and the inner diameter of the mating
part. All in all, fifteen pairs were designed and additive manufactured for evaluation. The force
required to insert and separate the snap components was recorded and compared to the value
based on a derived equation.Mechanical Engineerin
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