Selective laser sintering: A qualitative and objective approach
Selective laser sintering: A qualitative and objective approach
This article presents an overview of selective laser sintering (SLS)
work as reported in various journals and proceedings. Selective laser
sintering was first done mainly on polymers and nylon to create
prototypes for audio-visual help and fit-to-form tests. Gradually it
was expanded to include metals and alloys to manufacture functional
prototypes and develop rapid tooling. The growth gained momentum with
the entry of commercial entities such as DTM Corporation and EOS GmbH
Electro Optical Systems. Computational modeling has been used to
understand the SLS process, optimize the process parameters, and
enhance the efficiency of the sintering machine.
INTRODUCTION
Selective laser sintering (SLS) is a powder-based layer-additive
manufacturing process generally meant for rapid prototyping1 and rapid
tooling.2-6 Laser beams either in continuous7 or pulse mode8 are used
as a heat source for scanning and joining powders in predetermined
sizes and shapes of layers. The geometry of the scanned layers
corresponds to the various cross sections of the computer-aided design
(CAD) models or stereolithography (STL) files of the object. After the
first layer is scanned, a second layer of loose powder is deposited
over it, and the process is repeated from bottom to top until the
artifact is complete.
Selective laser sintering is also known as solid freeform
fabrication, layer manufacturing technology, rapid prototyping
technology,9 desktop manufacturing, and selective metal powder
sintering.10,11 It is called selective laser reactive sintering12-14
when it utilizes the chemical reaction of the components of the mixture
in the presence of a laser beam and selective laser melting (SLM),4
direct metal laser sintering (DMLS),5,15 or direct metal laser
remelting16,17 when the complete melting of the powder prevails over
the solidstate sintering.
Selective laser sintering has been used to make models for design
testing, patterns for investment casting, and small lots of functional
parts. This process has also been used in injection molding,18 rapid
tooling for electrical discharge machining electrodes,19,20 polymer
molding,21 sand casting molds,22 zirconia molds for titanium
castings,23 bio-medical applications,23-25 leadzirconate-titanate (PZT)
parts,26 and sheet metal parts.27
In the past, the status of SLS has been reviewed relative to other
rapid prototyping methods,1 laser sintering of metals along with the
role of phosphorous in the binding mechanism,28 post-processing of
laser-sintered parts,29 direct selective laser sintering of metals,30
the progress of rapid tooling using this method,6 and the materials
used and their suitability for being processed.31 However, a
substantial amount of work has been reported after publication of these
earlier reviews warranting the need for another comprehensive,
integrated, and referenced approach.
MATERIALS
A wide range of materials have been used in SLS.31 A number of those
materials make SLS superior to other techniques32 for rapid prototyping
when the characteristics of materials are dependent on the type of
process. The materials include wax, cermet, ceramics, nylon/glass
composite, metal-polymer powders, metals, alloys or steels,7,9
polymers,33,34 nylon, and carbonate.35,36
Polycarbonate powders36-38 and Bishphenol-A polycarbonate39,40 have
been initially used as the starting materials for both experimentation
and modeling in SLS. Hitherto, a number of metal systems (e.g., Fe-Cu,
Fe-Sn, Cu-Sn),10,11,41 metals (e.g., Al, Cr,4 Ti,42 Fe, Cu),43 ceramics
(Al^sub 2^O^sub 3^, FeO, NiO, ZrO^sub 2^, SiO^sub 2^, CuO),12 and
alloys (e.g., cobalt-based, nickel-based,42,44 bronze-nickel,19
pre-alloyed bronze-nickel,28 Inconel 625, Ti-6A1-4V,45 stainless
steel,4 gas-atomized stainless steel 316L,9,16,17 AISI 1018 carbon
steel,46 high-speed steel,7,47 precoated foundry sand,22 and alumina
with polymer binder)48 were tested for laser sintering. The results
demonstrated that any material can be combined with a low-melting-point
material that will serve as glue. Researchers tested the use of a
sacrificial polymer binder that is commonly used in conventional
sintering in SLS and found a wide range of materials that can be laser
sintered without sacrificial binder as compared to other rapid
prototyping techniques.31
The use of proprietary or special materials such as Cibatool-Express
2000,3 rapid steel, Truform, Protoform, Duraform,18,50 and direct
steel49 for making prototypes is rising, and improvements to product
quality have been reported as a result.5,6,51,52
BINDING MECHANISMS
When laser heat energy is absorbed by the materials, the powders
bind46,53 through the following mechanisms: viscous-flow binding,
curvature effect, particle wetting,54 solid-state sintering,
liquid-phase sintering, and true melting.1,10,11 Viscous-flow binding
is dominant in materials with the appropriate temperature-dependent
viscosity while the curvature effect is the driving force in
nano-crystalline materials.14
Laser sintering takes place in a short time interval (of the order
of milliseconds)-insufficient time for binding to take place due to
solid-state diffusion. Therefore, the joining of the powders is caused
either by melting one of the low-melting-point components of the powder
or completely melting the whole mass. Sintering by melting part of the
powders is the most prevalent practice and is accomplished by using the
powder systems of a combination of low-and high-melting-point
components. In this case, the laser beam heats the powder bed locally,
inducing melting only of the low-melting-point solid, which then wets
and binds high-melting-point components. Interparticle wetting, which
can be done by this two-phase approach, is necessary to prevent
"balling phenomena."14
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