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Annals of the CIRP Vol. 56/2/2007 -730-
doi:10.1016/j.cirp.2007.10.004
Consolidation phenomena in laser and powder-bed based layered
manufacturing J.-P. Kruth1 (1), G. Levy2 (1), F. Klocke3 (1),
T.H.C. Childs 4 (1)
1 K.U.Leuven, Division PMA, Leuven, Belgium 2 FHS - University
of Applied Sciences St. Gallen, Switzerland
3 Fraunhofer, Institute for Production Technology IPT, Aachen,
Germany 4 University of Leeds, School of Mechanical Engineering,
UK
Abstract Layered manufacturing (LM) is gaining ground for
manufacturing prototypes (RP), tools (RT) and functional end
products (RM). Laser and powder bed based manufacturing (i.e.
selective laser sintering/melting or its variants) holds a special
place within the variety of LM processes: no other LM techniques
allow processing polymers, metals, ceramics as well as many types
of composites. To do so, however, quite some different powder
consolidation mechanisms are invoked: solid state sintering, liquid
phase sintering, partial melting, full melting, chemical binding,
etc. The paper describes which type of laser-induced consolidation
can be applied to what type of material. It tries to understand the
underlying physical mechanisms and the interaction with the
material properties. The paper demonstrates that, although SLS/SLM
can process polymers, metals, ceramics and composites, quite some
limitations and problems cause the palette of applicable materials
still to be limited. There is still a long way to go in tuning the
processes and materials in order to enlarge the applicability of
LM. This is not surprising if one compares it to the decades of
R&D work devoted to tuning processes and materials for hot or
cold forming, metal cutting (e.g. development of free machining
steels), casting and injection moulding (including powder injection
moulding: MIM, CIM, etc.).
Keywords Rapid Prototyping and Manufacturing, Selective Laser
Sintering (SLS), Selective Laser Melting (SLM)
1 INTRODUCTIONLayered manufacturing (LM) goes back to the late
1980s, early 1990s [95, 98] with a clear breakthrough in 1994 at
which time machine sales took off exponentially: see Figure 1
[216]. Today, distinction is made between Rapid Prototyping (RP)
and Rapid Manufacturing (RM) [49, 121,164].x RP means the
production of prototypes, visual design
aids, touch, feel, fit and assembly test parts, etc., that are
used in the product development phase and are not meant to be
equivalent to real production parts at all levels.
x RM means the production of functional parts that are meant to
be used as real production parts (end products) and should meet the
various basic requirements put to such production parts. Rapid
Tooling (RT) may be considered in this context as a sub-category of
RM, i.e. production of functional tool components produced by
layered manufacturing. [41,153, 181, 208]
Many layered manufacturing techniques exist today, the most
popular being: photo-polymerisation (Stereolithography (SLA) and
its derivates), ink-jet printing (IJP), 3D printing (3DP), Fused
Deposition Modelling (FDM), Selective Laser Sintering or Melting
(SLS/SLM and EBM 1 ) and to a lesser extend Laminated Object
Manufacturing (LOM and similar sheet stacking processes) and laser
cladding (LC) processes [98]. The importance of these technologies
is confirmed by a recent study of NACFAM (National Council for
Advanced Manufacturing, USA) that has identified Rapid
Manufacturing as the most innovative and potentially disruptive
manufacturing technology to emerge within the next 3 to 5 years
[139]. Many of those techniques are however limited to Rapid
Prototyping as they do not allow common engineering materials to be
processed with sufficient mechanical properties (polymers, metals,
ceramics, and composites thereof). In the prospect of Rapid
Manufacturing, SLS/SLM seems to be the most versatile process,
capable of 1
Selective Electron Beam Melting (EBM) may be considered as a
variant of this category.
processing engineering polymers, metals, ceramics and a wide
range of composites. In order to cover this wide range of
materials, the laser processing of powder materials in SLS/SLM
calls on various consolidation mechanisms: the binding of polymers,
metals, ceramics and their mixtures or composites varies
substantially with initial powder composition, the final aimed
material composition and the aimed material structure (aimed
porosity or density, microstructure, properties, etc.). Some
process variants may apply post-processing after SLS/SLM: in such
case, the selected layer consolidation mechanism should account for
the post-process as well: e.g. need for open porosity if
post-infiltration is applied.
Figure 1 : Yearly additive system unit sales worldwide [216]
This paper gives a survey of powder consolidation mechanisms
applicable in SLS and SLM for various types of materials.
2 CLASSIFICATION
2.1 Types of powder consolidation mechanism (binding
mechanism)
Laser-based consolidation of 3D parts from layers of powder
material pre-deposited on a build platform is commonly referred to
as Selective Laser Sintering (SLS) or Selective Laser Melting
(SLM). The distinction between SLS and SLM is rough, vague and does
not cover all
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types of consolidation. Various papers describe possible ways to
consolidate material with a laser [18, 126]. A more detailed
classification of consolidation mechanisms is given in Figure
2.Solid State Sintering (SSS) is a consolidation process occurring
below the materials melting temperature: diffusion of atoms in
solid state (volume diffusion, grain boundary diffusion or surface
diffusion [82, 156]) will create necks between adjacent powder
particles that will grow with time (Figure 3) [66, 204]. This
binding mechanism is only rarely applied in layer manufacturing
(LM) as diffusion of atoms in solid state is slow and not very
compatible with the desired high laser scan speed that should yield
process productivity and economic feasibility. However, it is one
of the consolidation mechanisms suited for consolidating ceramic
powders [121]: see 2.2. SSS diffusion sintering is sometimes
applied in post-processing of metallic or ceramic parts after
initial (partial) laser consolidation using solid state, liquid
phase or chemical binding [121]: see 4.
Figure 3 : Typical neck formed between two stainless steel
powder grains obtained with Nd:YAG laser [204]
Liquid Phase sintering (LPS) and partial melting include a
number of binding mechanisms in which part of the powder material
is melted while other parts remain solid. The liquefied material
will spread between the solid particles almost instantaneously as
it is driven by intense capillary forces. This allows much higher
laser scan velocities than for SSS. The material that melts may be
different from the one remaining solid: the former low melting
point material is then often called the binder, while the high
melting point material is generally called the structural material.
Different ways exist to bring those distinct binder and structural
materials together:
x mixture of two-component powder (i.e. separate binder and
structural powder particles),
x using composite powder particles that have a micro composite
structure containing the structural and binder material, or
x using coated particles where the binder material is applied as
a coating around the structural material. The binder material
(coating) will preferentially absorb the incident laser radiation.
This enforces the intended melting of the binder.
When a different binder and structural material is applied, the
two materials may differ substantially: e.g. polymer binder versus
metallic structural material, metallic binder vs. ceramic
structural material, low-melting metal (e.g. Cu) vs. high melting
metal (e.g. steel or Fe). Examples and references are given in next
sections. In all cases, the binder may be permanent (i.e. remain as
part of the final product) or may be sacrificial (i.e. removed
during a de-binding cycle). In both cases, the green part coming
out of the SLS machine might be porous (and become even more porous
after de-binding) and might require a post-densification process:
the latter being generally either a furnace infiltration process or
a furnace post-sintering process (thermal sintering, HIP or other).
In most cases, thermal post-sintering without applying pressure is
not feasible: the porosity of the green SLS part is still too high
(25-50%) to initiate further densification by thermal heat only.
Alternatively, this process may be used to produce porous
structures [122, 168, 185].Partial melting is also possible even in
cases where no clearly distinct binder and structural materials are
used. SLS parameters are then adjusted to only partially melt the
powder particles, which may be a single phase material or a mixture
of different powders but without distinct binder powder material
(particles): 1. When the heat supplied to a powder particle is
insufficient to melt the whole particle, only a shell at the
grain border is melted. The core of the grain remains solid. This
way the molten material will form necks between the particles and
act as a binder between the nonmolten particle cores. This binding
mechanism can arise as well with metals as with polymers, although
the consolidation of polymer powders may also result from other
mechanisms (consolidation at the glass transition temperature,
which is lower than the melting temperature, polymer chain
rearrangement and cross-linking).Such partial melting phenomenon
and the resulting liquid neck formation were modeled at EPFL,
Lausanne, [53]. Using a simple thermal model, skin and core
temperatures of the powder particles were
Figure 2 : Laser-based powder consolidation mechanisms
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calculated. This way the minimal pulse energy to fully melt the
particle can be calculated. Below this value, the core temperature
never exceeds the melting temperature and only partial melting is
obtained.
2. Powders consisting of multiple phases or a mixture of
different powder particles can be classified as partial melting
when they are only partially molten. For example, the University of
Leuven experimented with a Fe-Fe3P-Ni-Cu powder mixture, aiming at
the production of fully dense parts [93]. The resulting part
clearly contains unmolten Fe particles (see Figure 37).None of the
4 powders can be identified as a distinct binder material. The
addition however of a melting point lowering additive like Fe3P or
Cu3P is favorable in making the process more energy efficient: see
details in 3.2.2, section B2.
3. Partial melting may also occur when using a single material
powder having a bi-modal distribution: small particles are melted,
while larger ones remains solid.
Liquid phase sintering and partial melting may have advantage
over full melting (see below), even when aiming at full dense
parts. The former allows the laser consolidation speed (scan speed,
scan spacing) to be increased drastically. The resulting economic
benefit has to be considered against the disadvantage of the need
for post-densification. Comparative studies demonstrated that molds
made from PA-coated steel that are post-infiltrated with copper may
not only be more cost effective [103], but also technically may
perform better due to the beneficial properties of copper [106,
208].
Full melting is a third major consolidation mechanism, often
applied to achieve fully dense parts without need for any
post-process densification. Major progress has been achieved
towards full melting of metal powders up to densities of 99.9% by
applying modern laser sources and optics yielding high energy
densities in the spot [105].Dedicated SLM machines are now offered
by 4 German vendors [58, 103]. As compared to early day SLS
machines equipped with CO2 lasers, these machines apply solid state
lasers: diode pumped Nd:YAG, fibre or disc lasers [150].Full
melting has the main advantage to produce almost full dense
products in one step, but also has drawbacks that require careful
process control: x The high temperature gradients and densification
ratio
during the process yield high internal stresses or
partdistortion (from 50% powder porosity to 100% density in one
step) [6, 134].
x The risk of balling and dross formation in the melt pool 2 may
result in bad surface finish.
Chemical induced binding is a fourth main consolidation
mechanism. Today it is not commonly used in commercial LM
equipment, but it turns out to be a feasible consolidation
mechanism for polymers, metals and ceramics. IPT-Aachen has applied
chemical binding for laser sintering SiC ceramic powder [88].
Chemical induced binding of metals could e.g. be invoked for
sintering Al powder by making the Al react with the N2 atmosphere
used commonly in SLS machines, thus creating an AlN binder phase
holding the Al particles together. Such mechanism has been used by
the University of Queensland, be it during post-sintering in a N2
furnace, rather than during the initial SLS phase itself, at which
stage a sacrificial polymer binder was used to consolidate
2 Those melt pool phenomena have been studied within the CIRP
STC-E Working Group on Melt pool phenomena in SLS/SLM/LC that met
in Leuven (Jan. 2004), Aachen (Oct. 2004) and Leeds (Sept. 2005).
They are dealt with in 2.4 and 3.5.
the green Al part [172]. Other (potential) examples of Selective
Laser Reactive Sintering were investigated at the University of
Texas at Austin [11]. Some investigators tried to apply
self-propagating high-temperature synthesis (SHS) in which a low
laser power induces a self-propagating exothermal reaction capable
of self-heating and consolidating the powder in a controllable way
[107,177,182]. This allows reducing the laser power significantly.
The metallic liquid phase stays longer and the accuracy of the
produced piece will be higher. Shi et. al. applied a SHS process to
laser sinter metallic materials (Ni-Al, Al-Ti, Ni-Ti) [176]. The
Osaka Sangyo University tried to produce cermets (Al2O3-Cu),
ceramics (MoSi2),and intermetallics (TiAl) by SLS induced chemical
reaction [80]. This will be discussed in 3.4.7.
2.2 Type of consolidation mechanisms versus materials
The applicable type of consolidation is basically dependent on
the material being processed. Table 1 gives an overview of what
material can be basically consolidated with the various binding
mechanisms.
2.3 Influence of laser type Different types of lasers are used
for powder-bed based RM: CO2, Nd:YAG (lamp pumped, diode pumped, or
Q-swithed) [60], fibre lasers, disc lasers, Cu-vapour lasers [76].
The type of laser has a large influence on the consolidation of
powder particles, because: x laser absorption of various materials
greatly depends
on the laser wavelength: e.g. high absorption of polymers and
oxy-ceramics at 10.6 m wavelength of CO2 laser; high absorption of
metals and carbo-ceramics at 1 m wavelength of Nd:YAG, fibre and
disc lasers [105, 196];
x the possible consolidation mechanism highly depends on
features like energy density: e.g. SLM of metals and ceramics
requires a high energy density that is more easily achieved with
fibre and disc lasers (high beam quality, i.e. high
focusability);
x laser mode (continous, pulsed, Q-switched,) also has a large
influence on consolidation (see end of 2.4).
Several researchers investigated the influence of different
laser types on the absorption and consolidation of powders in
SLS/SLM. Some did it experimentally [100,114, 156], others by
simulation [114].
2.4 Problems in consolidation There are two main general
problems in consolidation. One, applicable to all process variants
other than the full melting of metals, stems from the short
material heating times caused by the scanning laser beam, relative
to the time required for consolidation, and the fact that it is
only temperature effects and gravity and capillary forces that can
provide the driving force (there is no mechanical pressure, as in
moulding processes). It can lead, as has already been written, to
porosity in parts. Post-processing is then required if a pore-free
material is needed. For polymer processing, it means that melt
viscosity must be kept less than some critical value. For example
Nylon-12 (PA 12) with an in-process viscosity K | 100 Pas can be
fully densified under surface tension driving forces J | 30 mN/m
(J/K | 0.3 10-3 m/s), but polycarbonate (viscosity |5000 Pas, and
similar surface tension) can not [28].The other problem, important
for the full melting of metals, is quite the opposite. Here the
viscosity K is about 1 to 5 mPas [16], the surface tension J about
1 to 2 N/m [12] and consequently J/K | 100 to 1000 m/s. The problem
is not time for flow, but controlling the flow [87]. The pool
of
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molten metal must wet the previously processed metal below it
(except on the rare and sometimes problematic occasions that it is
an overhang). And when it solidifies, its upper surface must be
flat enough to enable a next layer of powder to be spread over it.
Both materials properties and processing variables can influence
these things. As far as wetting the material below is concerned,
contamination, usually by oxygen, and in that case either as oxide,
impurities within the powder or as trapped gas in the powder bed,
is a main materials cause of consolidation problems. For example,
the surface tension of pure liquid iron near its melting point is
almost 2 N/m, but 0.02 wt.% of oxygen reduces this towards 1 N/m
[12,110]. Liquid iron will not spread out over an iron oxide
covered surface. Even in the absence of such contamination, there
may be a problem of a liquid metal wetting its solid form, if the
solid has almost the same temperature. In this homologous wetting
case, there is no driving force for wetting. It has been suggested
on the basis of experimental studies on a stainless steel [35] that
it is necessary for the previously processed material to be
re-melted by the currently melted material. The excess energy (and
hence the higher peak temperatures in the melt pool) required for
this will have consequences for the quality of the upper surface of
the melt pool. When temperature gradients are created in a melt
pool, there is the potential for convective motions in the pool to
reduce those gradients. Temperature gradients through the depth of
the pool, causing density gradients, and hence buoyancy forces, are
probably of no importance to the millimeter or less melt pool sizes
of SLM (compared to the much larger sizes that can make buoyancy
effects relevant to welding [137]), although see [142]. But
temperature gradients in the surface, coupled with a temperature
dependent surface tension, can cause rapid motions (tens of cm/s or
more), known as thermo-capillary or Marangoni flow. The equation
below defines the dimensionless Marangoni number Ma.
ad w d dT wMdx w dT dx wJ N J N
K { K
(1)
It is the ratio of the speed by which a surface temperature
gradient dT/dx may be reduced by convection or conduction (w is the
linear size of the pool, N is its thermal diffusivity) [137]. For
typical ferrous material property values, also supposing an initial
temperature gradient dT/dx to be due to the difference between
liquidus and solidus temperatures over the half-width of the melt
pool, and w = 1 mm, Ma can be calculated to be of the order 1000.
Marangoni flow dominates. When dJ/dT is negative, as it naturally
is with decreasing surface energy from the liquidus to the
vapourisation temperature, the Marangoni flow is from high to low
temperature, that is to say from the centre to the edge of the melt
pool, and a flattened or dished surface is created.
3HM = hardmetals
But with oxygen contamination of steels, and dependent on
sulphur and nitrogen content too, dJ/dT can be positive [83, 159].
Then the melt pool becomes humped. Again, such effects are clearly
seen in welding [127, 128], as well as in SLM [156, 158].In the
previous paragraphs, different effects at the under and top side of
the melt pool have been considered. But in SLM there is a leading
edge to the melt pool too, where the surface, subject to the
surface tension induced flow, is advancing into powder that at some
distance ahead of the melt front will certainly still be below the
solidus temperature. The wetting or not of this solid powder and,
if it is wetted, its dragging or not into the melt pool, are
further complications of the consolidation process that have not
been systematically studied. It may be that the compromises needed
to balance good wetting or remelting of previously melted material,
with the top surface of the melt pool freezing sufficiently flat,
and the powder bed not being disturbed too much ahead of and around
a currently melting track, will put a large constraint on metals
and powder preparations that are suitable for processing by SLM.
Much basic work remains to be done in this area. The consolidation
flows and the materials dependent phenomena accompanying them and
discussed above differ from flow instabilities in a pool that can
arise because of its shape. Long thin melt pools are known to break
up into balls, called balling and commonly described as due to
Rayleigh instabilities [3, 19, 74, 87,135, 136, 157]. For a free
melt pool, as would describe a single melted track in a deep powder
bed, balling is predicted for pools (imagined to be cylindrical) of
length to diameter ratio greater than S. Balling as this ratio is
approached is reported by [27, 156, 157]. That work also showed
length to diameter ratio increasing with laser scan speed. Thus in
this case, instability limits processing to a low scan speed. A
melt pool on a solid substrate can also be unstable, depending on
its contact behaviour with the solid [38, 169]. A limiting length
to diameter ratio of 2.1 has been reported [96], also creating an
upper limit to the laser scan speed. Most balling studies have been
performed in conditions of single line or raster scanning powder
beds when the laser-caused temperature rise has essentially been
due to the current line scan. At higher scanning velocities and/or
shorter vector lengths, however, previously scanned tracks are not
yet solidified when a new track is being scanned. Thus, the actual
width of the melt pool is enlarged, since it now spans multiple
scan tracks [135,136]. Therefore, the length-to-width ratio may
actually be reduced (even if the melt pool length enlarges),
resulting in a more stable behaviour compared with a single track
scanned with the same parameters. Mercelis [135]showed that melt
pool dimensions may depend much more on the geometry of the 3D part
and the 2D scanning path, than on the laser power and scanning
velocity. Therefore, also the scan track instability cannot be
Solid State Sintering Liquid Phase Sintering Partial melting
Full melting Chemical
Polymers No Yes (e.g. PS) Yes (e.g. PA) Seldom (e.g. partial
cross linking PMMA)
Metals Seldom Yes (many kinds) Yes (steel, Ti...) Yes (e.g.
Alu)
Cermets/HM 3 No Yes (e.g. WC-CO) No Yes (e.g. Al2O3 Cu)
Ceramics Yes (Phenix) Yes (e.g. SiC) Yes (e.g. ZrO2) Yes (e.g.
SiC)
Other composites No Yes No Yes
Table 1: Consolidation mechanisms versus materials
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examined without taking into account the scanned geometry. That
brings this section finally to the influence of process variables
on problems in consolidation. First is the influence of laser type
(see also 2.3). The higher the laser absorption into the powder
bed, the less scope there is for variation in absorption, and the
easier is the process to control. Pulsed laser beams and smaller
beam diameters may lead to smaller, more stable melt pools, and in
some cases pulsing may result in plasma formation and recoil forces
adding a mechanical component to consolidation phenomena [97, 144,
157]. Smaller layer thickness reduces the need for over-heat if
re-melting the previous layer is important. Thinner layers means
finer powder sizes. These in turn are easier to melt due to their
higher surface-to-volume ratio. Processing changes that lead to
smaller melt pools also seem to lead to easier consolidation. It
may be that a final problem in consolidation is that it cannot be
scaled up to faster processing speeds.
3 DISCUSSION OF MATERIAL CLASSES In this section the
consolidation phenomena are discussed per material class:x
Polymers: i.e. substance with a high molecular weight
which is made of many repeating smaller chemical units or
molecules.
x Metals: i.e. substance whose atoms are connected by metal
bonds.
x Ceramics: inorganic and non-metallic materials. x Cermets and
hard metals: i.e. composites in which
ceramic particles are bound in a metal matrix (binder). x
Composites: mixtures of different materials which
result in an inhomogeneous compound. Cermets and hard metals may
be considered a special class of composites.
3.1 Polymers Even though polymer is the most processed type of
material in SLS/SLM, the consolidation phenomena invoked for
polymers are probably still amongst the least understood or at
least the least described in literature [81].This explains why
commercial applications of SLS today are limited to a small number
of polymers: mainly polyamide (PA 12 and PA 11), and some
polycarbonate (PC) [48], polystyrene (PS) and variants of those
[175].
Figure 4: Thermoplastic polymers (Red = material used in
SLS)
In literature there is no agreement on the nomenclature of the
physical state of parts after laser powder processing. Laser
consolidation of polymers normally involves meltingof
thermoplastics (partial-SLS or full-SLM). A clear distinction
should be made between (Figure 4):
x (semi-)crystalline thermoplastics x amorphous thermoplastics.
When heated up from very low to very high temperatures, all those
thermoplastic materials will change from a hard (solid and glassy)
structure to a softer (tough leathery or rubbery, solid or
non-pourable) structure and finally turn into a viscous flowing
melt. Materials however differ in the way and the temperatures at
which those transitions occur. Semi-crystalline polymers have a
glass transition temperature Tg that is below or around room
temperature (-100 to 50C) and a distinct melting temperature Tm
that is above 100C (between 100 and 400C) at which a significant
volume change happens. Amorphous polymers do not depict a clear
melting temperature range. They have a glass transition temperature
Tg that lies around 100C and above which the material will
gradually evolve to a leathery, rubbery and finally liquid state as
temperature increases, without clear transitions. Figure 5indicates
the glass transition range ('Tg) and melting range ('Tm) for three
important semi-crystalline polymers (PE, PP, PA 6) and indicates
for three amorphous polymers (PS, PC, PMMA) the glass transition
temperature ranges ('Tg) and the temperature range in which the
transition to leathery material goes quickly ('Tf).This last range
('Tf) can physically not be defined exactly.
Figure 5 : Phases and transition temperatures of some
polymers
Notice also that the Tg and Tm values largely depend on the
molecular weight (MW) of the polymer. This explains why there might
be a significant difference in SLS processability between a low and
a high molecular weight PA or PE. The temperature transitions and
melting range of polymers can be observed by recording a DSC plot
by Differential Scanning Calorimetry analysis. It measures the
difference in the amount of heat required to increase the
temperature of a sample and a reference as a function of
temperature. Both the sample and reference are maintained at very
nearly the same temperature throughout the experiment. Figure 6
gives the DSC plots of Duraform PA 12 and PA 6 [166]. The plots
illustrate the much smaller melting range (small width of peak
around 187C) and the smaller melting temperature (Tm=187C) of PA 12
as compared to PA 6 (Tm=223C). This explains, besides viscosity
differences (see 2.4 and 3.1.1), why PA 12 is more prone to SLS
than PA 6 (or PA 66 having Tm= 262C and a wider peak than PA 12)
[212].The laser consolidation of (semi-)crystalline polymer powder
will happen by heating above their melting temperature Tm.
Semi-crystalline materials have a highly ordered molecular
structure with sharp melt points. They do not gradually soften with
a temperature increase but rather remain hard until a given
quantity of heat is absorbed and then rapidly change into a viscous
liquid.
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The molten polymer flows in between the powder particles,
forming sintering necks. With enough heat, the complete layer is
fully molten and overlaps to the previous layer. As the molten
polymer cools down below Tm,polymer crystals nucleate and grow,
recreating regions of ordered molecular chains (crystallites) mixed
up with disordered amorphous regions. Above Tm, the polymer depicts
a relatively low viscosity, as compared to amorphous polymers,
which favours the rate and amount of consolidation. The densities
obtained will be close to full density and the mechanical
properties will be close to those of moulded polymers. However, the
freezing of the polymer at Tm coincides with an important shrinkage
(phenomenon not occurring with amorphous polymers), that may induce
geometrical inaccuracies and distortion of the part (Figure 13). A
good way to prevent this is to preheat the polymer powder to a
temperature slightly below its melting temperature and keep it
there for a certain time after consolidation [26,197, 212].
Figure 6 : DSC plot of PA 12 (Duraform) and PA 6
The consolidation of amorphous polymer powder
r semi-crystalline polymers we have rather
thermal behaviour of polymers is amplified
occurs by laser heating above the glass transition temperature,
at which the polymer is in a much more viscous state than
semi-crystalline polymers at similar temperature. Amorphous
polymers have a randomly ordered molecular structure. They do not
have a sharp melt point but instead soften gradually as the
temperature rises. The viscosity of these materials changes when
heated, but they seldom are as easy flowing as semi-crystalline
materials. The flow and sintering rate will be less, resulting in a
lower degree of consolidation, higher porosity, less strength, but
also a lower shrinkage that is favourable in cases of SLS of
patterns for producing mould for moulding or casting (i.e. Indirect
Rapid Tooling, investment casting patterns, etc.). In several
applications post-infiltration of the pores is applied to
consolidate the part (see 4). In conclusion fofull melting
consolidations whereas for amorphous polymers we rather see a
partial melting binding mechanism.This complexin SLS processing due
to several factors. The costly pulverization either by cryogenic
milling or precipitation-process (Figure 7) limits the availability
of polymer powders. The pulverization can influence also the
consolidation process: the powder layers thermal characteristic
differs.
The material is in powder form with a certain particle size
distribution (Figure 8) and the induced energy, heating the fine
powder particles, may lead to evaporation or disintegration which
in turn disturbs the process. It shields off the laser window and
the IR heating elements. In short, this has a negative influence on
the process consistency and the economy of the process. Further the
fine particles may create either a dusty atmosphere or coagulations
clustering preventing the homogeneous deposition of layers, thus
preventing good layer consolidation.
a
b
Figure 7 : Powder particle shapes REM, a) precipitation, b)
Cryogenic milling
A variety of commercially available technical polymers with
excellent properties for SLS have been tuned by blending or by
special design of the chain length (analogy to molecular weight
ranges). Often they result in a multi-peak DSC diagram. SLS can
handle only one fixed controlled consolidation temperature or
several very close peaks (typically 5-10 C difference), which means
that polymers with more than one tight melting range are not
processable with SLS. Figure 9 demonstrates the DSC curve of a
commercial pulverized blend with no real full melting SLS
processability, due to two different melting points at 178C and
185C. We can associate seven main SLS polymer powder materials with
different application fields. These material families can also be
attributed to a specific consolidation mechanism as discussed in
2.The four main thermoplastic SLS applications and the related
thermoplastic material are:
a. SLS of polymer parts: semi-crystalline polymers (3.1.1)
b. SLS of investment casting patterns: amorphous polymers
(3.1.2)
c. SLS of metal or ceramic parts using a sacrificial polymer
binders: thermally degradable amorphous polymers (3.1.3)
-
-736-
d. SLS of reinforced or filled semi-crystalline
eric material parts (3.1.5) in the racing
mosetting materials (3.1.7)T
polymers for highly loaded parts (3.1.4)SLS today also allows
production of parts using:
e. Flexible elastomf. Polymer-polymer blends (used
industry) (3.1.6)g. Ther
hose different material categories are discussed in more detail
below.
Figure 8 : Typical particle size distribution of Duraform PA 12
(Source FHSG St. Gallen)
Figure 9 : Commercial pulverized blend with no real ltcurve
(right)
[151,
on is then obtained by keeping the powder bed
ay be raised to compensate for that, but only in a narrow range
and with limited effectiveness (see also 3.1.8).
a
SLS processability: (left) merecrystallisation curve
3.1.1 Semi-crystalline polymers Today, the production of
prototypes (RP) and functional parts (RM) by SLS is basically
limited to nylon, i.e. polyamide (PA) [7, 22, 24, 26, 84, 102,
218]. Quite some research is going on using other
(semi-)crystalline thermoplastics: polyethylene (PE) [155, 165],
PEEK152], PCL [148, 203, 214]. and PEEK are of special interest as
biocompatible polymers [151, 192, 214].The fundamental reason why
PA sinters well, while more difficulties are experienced with other
polymers is not totally clear yet. Some semi-crystalline polymers
are able to crystallize much faster than others, as a result of
their chain structure. The rate of crystallization is minimum near
Tg and Tm, and reaches a maximum between those two temperatures.
Crystallization rate should however be kept relatively slow
(relative to the vertical build rate) in order to avoid part
distortion due to freezing shrinkage. Good crystallizatiat high
temperature for a sufficient long time after sintering.To obtain
fully dense parts, the melt viscosity should be low enough to allow
complete consolidation within the time scale of the process. Pure
polymers with melt viscosity in the range of few tens to few
thousands poise (e.g. nylon and waxes) can be processed
successfully and reach near full density [11]. However, even for
PA-12, the SLS part still contains small amounts of porosity and
the
process is something between partial and full melting. The
densities obtained in SLS of PA 12 are just slightly below those of
compression moulded parts (0.95-1.00 versus 1.04 g/cm3), yielding
very similar mechanical propertiesunder compression, but somewhat
lower tensile strength and notched Izod values that are sensitive
to small voids. The melt viscosity is linearly related to the
molecular weight (MW). For good interfusion of the polymer chains,
sufficient to provide particle necking and layer to layer adhesion,
a low melt viscosity is desirable. However, low viscosity results
in high shrinkage and poor part accuracy. Thus an optimum MW range
exists but is not easily controllable, neither at the polymer
production nor during aging in process. The thermal degradation
over time when re-using the same powder repeatedly, results in the
decay of the MFI (Mold Flow Index) hence a drop in powder
flowability and a rise in melt viscosity preventing proper
sintering with reasonable quality: the surface gets an unsmooth
orange peel texture (Figure 10b) after a certain number of runs
(Figure 10a). A drop in mechanical properties can be observed as
well. The consolidation temperature m
b
Figure 10 : a) The rise in viscosity of re-used PA 12 material,
b) resulting orange peel texture on SLS part
To achieve functionally strong prototypes, all powder bed based
processes need to work with polymers that have molecular weights as
high as possible [155]. High molecular weight can lead to
difficulty in placing and forming the material because of the high
viscosities involved. Increasing the molecular weight of the
polymer
oxidation resulting in de-polymerization). Typically
powder can reduce shrinkage in the sintered part and thus
improve the dimensional accuracy. PA and other semi-crystalline
polymer powders for SLS are normally supplied without additives.
Some additives may be added to favour the powder fluidity and
powder spreading, but those have no influence on viscosity and
sinterability. Atmospheric control in the SLS chamber is very
critical in terms of temperature (control of sintering and
shrinkage), humidity (powder fluidity) and oxygen content (to avoid
thermal degradation of the polymer by
Cooling curve
Heating curve
Orange peel surface texture
-
-737-
polymers like PE and PP are more prone to rapid oxidation than
aromatic backbone polymers like PET
e to the purity differences compared to the first powder.
a
(semi-crystalline) or PC (amorphous). Several researchers
investigated SLS of biocompatible semi-crystalline thermoplastics
like PEEK and PCL (Poly--caprolactone) [9, 31, 148, 152, 170, 192,
214].PCL is a biodegradable linear polyester, while PEEK is not
biodegradable. A polymer has never one specific molecular weight,
but consists of a mixture of molecules with different chain
lengths, and so, different molecular weights. The mean molecular
weight and the molecular weight distribution are important
properties of a polymer. Typically SLS of PCL with MW of 50.000
g/mol demonstrated good laser sinterability [214], while PCL with a
MW of 40.000 g/mol could not be laser sintered well [203] due to
big distortions caused by shrinkage. This shrinkage is due to the
lower molecular weight of the second PCL powder, and du
b
Figure 11 : a) PA 12 Powder, b) tensile break cross section
showing some air voids (Source FHSG-St.
Gallen)
break cross section
oulded parts concerning
makes them well suited for investment casting patterns.
a
Powder particle morphology also influences the laser
consolidation. Figure 11a shows powder (PA 12) with a near to
spherical shape whereas Figure 12a shows irregular shaped PS
powder. When no pressure is applied during consolidation, voids and
air traps remain in the solid part. This reduces the effective
cross section and the tensile strength compared to moulded parts
from the same polymer (10-20% reduction). Figure 11b (PA 12) and
Figure 12b (PS) depict the tensiledemonstrating partial melting
regime.
Comparison between SLS and mstrength is discussed in 3.1.8 .
3.1.2 Amorphous polymers Amorphous polymers with strongly
temperature-dependent viscosities (or high activation energy for
viscous flow) have been readily processed using SLS [11,14, 75,
141]. Typical examples are polycarbonate (PC) and polystyrene (PS).
Depending on the powder size and MW, they are generally pre-heated
to a temperature near or even above the glass transition
temperature Tg and will
melt as the temperature further rises above Tg. They mainly have
the advantage that their volumetric shrinkage, when cooling down,
does not show a jump, as it does for semi-crystalline polymers: see
Figure 13. This fact provides high accuracy with little distortion.
However, amorphous polymers show a lower level of consolidation
(higher residual porosity) and hence do not reach the strength of
their moulded equivalent. The higher degree of porosity, low
thermal shrinkage/expansion and high accuracy
b
gure 12 : a) PS Powder (CastForm), b) Tensile breaFi kcross
section demonstrating partial melting regime
us polymers for 3.1.3 Thermally degradable amorphoSLS of metal
or ceramic parts
Various SLS applications make use of a powder combining a
degradable polymer binder and a structural material (metal or
ceramic) that produces a green part in which the structural
particles are bound into a degradable polymer matrix [77]. These
green parts are further processed in a furnace to burn out the
polymer (de-binding) and to consolidate the remaining polymer-free
metal or ceramic part by post-sintering, infiltration, or HIPing:
see 4. The polymers used as the sacrificial binder in those
applications are different from the polymers described in the
previous and subsequent sections. They should allow de-binding,
i.e. thermal depolymerization. Polymers used for this purpose are
PMMA and copolymers MMA-BMA that can be de-binded in a furnace at
temperatures around 350-450C. They should not be water soluble [11,
p. 112] and should have sufficient strength, even when heated up in
the de-binding furnace, in order to avoid that the green part would
collapse before some solid state sintering can consolidate the
metallic or ceramic particles together. The polymer must also be
fully de-bindable (i.e. depolymerised and sublimated) and should
not yield excessive contamination of the remaining part with carbon
residuals (important e.g. for steel parts). If the strength of the
polymer in the green stage is insufficient, the green part can be
consolidated by cross-linking the binder. This can be done by
impregnating the green part with a water soluble thermosetting
acrylic emulsion and drying it in an oven at 50C, before it goes to
the de-binding furnace [11, p. 139]. Alternatively, a cross-linker
can be added into the degradable polymer
-
-738-
coating of the powder [11, p. 139]. This will eliminate the step
of impregnating the green part. When using sacrificial binders,
care should be taken because depolymerization could also occur
during SLS [11 p. 108, 82, 88]. Indeed, these polymers are
deliberately designed to depolymerise as an aid to their thermal
removal in post-SLS processing. Polymers based on copolymers of
n-butyl methacrylate (BMA) and methyl methacrylate (MMA) have been
observed to also depolymerise during SLS processing [200, 201].
Figure 13 : Comparison of relative volume between amorphous and
semicrystalline polymers [11]
a
b
cFigure 14 : SLS of AW glass ceramic with sacrificial
binder (a) Powder mixture (b) Green part (c) Brown part
The polymer binder may be added to the structural material in
two ways: mixing [34, 123] or coating [171,172, 199, 217]. Typical
volume of binder in the powder is 5% for mixed powders [34] as well
as coated ones [11,
rates three stages in the SLS
e just enough to give the
still is not totally dense and may need further densification to
convert the brown part into a final one.
p136]. The latter typically corresponds to a 5 m polymer coating
on a 55 m 1080 carbon steel grain [200].Figure 14 [108]
demonstprocess of Apatite-Wollastonite (A-W) glass ceramic with 5
wt% MMA-BMA binder: a) powder mixture before processing (white
particles are
A-W glass ceramic, grey particles are MMA-BMA), b) highly porous
part after SLS processing (The few
polymer connections ar'green' part sufficient strength to be
transferred to the post-treatment furnace)
c) 'brown' part after polymer debinding and 1 hour firing at
1150C. During firing, A-W is partly melted. The part
Figure 15 : Glass-nylon powder mixture (Source: K.U.Leuven)
Figure 16 : SEM micrograph of HA particle distribution in HDPE
matrix [165]
Figure 17: PA-12 GF One of the most popular glass beareinforced
HTD materials (Source FHSG St. Gallen)
ds
binders described in
3.1.4 Semi-crystalline reinforced polymer for metal, ceramic or
glass reinforced plastic parts
Today there exist several SLS powders aimed at production of
reinforced polymer parts. Applications include parts made from
glass reinforced PA [28, 30], Cu filled PA [20], Al filled PA [1,
46], SiC-PA [59, 76], HA-HDPE, i.e. hydroxyapatite reinforced high
density polyethylene [165], HA-PA [69, 166], Polysiloxane-SiC [54].
Unlike the sacrificial polymer
-
-739-
3.1.3, the polymers used here should withstand thermal
degradation and should be durable. The initial powder may be a
mixture of polymer particles and reinforcement beads. Figure 15
shows such a mixture of spherical PA and glass particles: on the
right side of the picture, the glass particles are colored blue
with Prussic ink for better visibility. Alternatively, the single
powder particles may already be a composite consisting of a polymer
matrix (PA or PE) containing filler particles [165,166] (see
particles[20].
Figure 18 : New PA powder with a high-aspect ratio filler
a
Figure 16) or a polymer coated filler
(Source FGHS St.-Gallen)
bFigure 19 : a) PA 12 and 30% Al blend; b) a tensile break
turned out to be better
heat concentration on the filler has a negative influence on
only (for PA 6-8 times depending on judgment
s, strength
ls for Shore A hardness and elongation at break.
a
cross section with defects due to inhomogeneity (Source FHSG St.
Gallen)
Figure 17 shows the fracture surface of a part made with a
popular commercial glass filled nylon powder. One can see that the
connection between the glass beads and PA binder was not perfect,
that many glass beads have been pulled out and that the PA was
stretched during breakage, resulting in a rough spongy surface.
Figure 18 shows a new reinforced SLS powder having elongated filler
beads (patent pending). This new powder that the traditional GF-PA
powder: tensile strength x 1.8; E-modulus x 1.3; elongation x 3.3.
Aluminium filled PA 12 often shows coagulation, since mixing the
aluminium and PA powder particles isnt always very succesfull. This
causes segregation during layering due to the differences in size
and specific weight (Figure 19). This fact is even more striking in
PA 12-Cu. The inhomogeneous solid with voids creates mechanical
defects and deteriorates the tensile strength. The local
the binder polymer causing a very short recycle ability of 2-3
timescriteria).
3.1.5 Elastomeric materials Elastomeric polymers have a
structure with long chains and only few cross-links between them.
Below the glass-transition temperature they are brittle, above they
are very elastic. When the density of cross-links enlargeand
stiffness also enlarge, and ductility lowers. A new elastomer
powder material for the SLS process was recently released [120].
Figure 20 demonstrates this polyester based elastomer (patent
pending). The most significant properties in the thermoplastic
material selection were considered and compared with reference to
elastomeric materials as Neoprene, EPDM and natural rubber. Figure
21 gives a comparison for the different elastomeric materia
bgure 20 : Polyester based elastomer a) Green par
r sintering at low hardness b) Infiltrated wFi t
afte ith polyurethane (Source FHSG St. Gallen)
relation to commonly HSG St. Gallen)
Figure 21 : Developed elastomeric material for SLS in used
elastomers (Source F
0
25
50
75
100
0 1 2 3 4 5 6 7 8Material
Shor
e A
Har
dnes
s
0
250
500
750
1000
Elon
gatio
n %
Shore A (low) Shore A (high) Elongation %
-
-740-
Sintaflex not infiltrated
15W13W11W5W
9W7W
0
20
40
60
80
100
100 150 200 250 300 350Elongation (%)
Shor
e A
Har
dnes
s
Sintaflex infiltrated
5W 7W 9W 11W13W 15W
0
20
40
60
80
100
100 150 200 250 300 350
Elongation (%)
Shor
e A
Har
dnes
s
Figure 22 : Hardness Shore A vs. Elongation [%] for infiltrated
and not infiltrated parts processed with different laser powers
(Source FHSG St. Gallen)
a
Sintaflex: E module vs. LS Laser Power
0
5
10
15
0 5 10 15 20Laser Power [W]
E M
odul
us [M
Pa]
not infiltratedinfiltrated
b
Sintaflex: Rupture at break Rb vs. E Module
0
2
4
6
0 2 4 6 8 10 12E Modulus [MPa]
Rb
[Mpa
]
RbRb (inf.)
Figure 23 : (a) The elastic modulus [MPa] against laser power
for infiltrated and not infiltrated parts
(b) The tensile strength Rb [MPa] at ruptures against elastic
modulus (Source FHSG St. Gallen)
The influence of laser processing parameters on Shore A hardness
and ductility is shown by Figure 22. Clearly the parts processed
with higher laser power become more ductile and have a higher Shore
A hardness. The influence of laser processing parameters on elastic
modulus E and the tensile strength Rb at rupture is shown by Figure
23.
3.1.6 Polymer blends Polymeric blends offer an alternative mean
to obtain SLS parts with specific structure and properties,
permitting the development of new applications [174]. Most
polymeric
blends are multiphase systems and, therefore, their properties
largely depend on their microstructure [162,215]. Salmoria, for
instance, used blends of PA and HDPE (blend ratios of 80/20, 50/50
and 20/80 wt%) to achieve dedicated properties. Depending on the
blend ratio, different phases and micro-structures were observed
using SEM, EDX and XRD analysis [162].FH St.Gallen experimented
with powder blends (PA12 with additions of PA11 or Polyester)
mainly to enhance the elongation at break. Recently, a company from
Austin started commercializing a mixture of PA12 and PA11 that
demonstrates two DSC melt peaks at 185C and 187C respectively. The
process is set to only melt the lowest peak. This clearly suggests
a liquid phase sintering consolidation process.In a US patent [40]
the following is claimed: A particle for use in selective laser
sintering (SLS), including a core (1) formed from at least one
first material, and at least partial coating (2) of the core (1)
with a second material (further components are optional), the
second material having a lower softening point than the first
material, wherein the softening point of the second material is
lower than approximately 70C. The coating (2) generally contains a
polymer, preferably a thermoplastic polymer, e.g. a polyvinyl
acetal, preferably a polyvinyl butyral. It may consist of alloys
with a low softening point which are used e.g. in fuses. Moreover
saturated linear carboxylic acids with a chain length of 16(e.g.
heptadecanoic acid, melting point 60-63C.) or polymers in the
broadest sense may also be suitable. The softening point of the 2nd
material of approximately 70C or below, allows laser sintering to
be carried out at significantly lower temperatures compared to
particles which have been used hitherto, and therefore also allows
a significantly lower temperature difference between irradiated
particles and standard room temperature. Tests have shown that the
lower maximum temperature difference also improves the temperature
homogeneity of the building space as a whole.
3.1.7 Thermosetting materials Thermosetting polymers can be used
in different steps of the SLS process. These materials can be used
as an infiltrant [47], because it is a relatively inexpensive path
to a fully dense, stiff, net shape, polymer matrix composite part.
In addition this is a very useful intermediate step for fully
functional materials, as critical surfaces and tolerances may be
achieved easily at this stage. An example of infiltration with
thermosetting polymers is the production of metal-epoxy molds via
SLS indirect processing [11, p143]. In this process, SLS is used to
form green mold cavity inserts from metal powder that is coated
with fusible thermoplastic binder. In subsequent steps, the binder
is thermally removed and the metal powder is oxidized to form a
porous metal/ceramic cavity that shows little shrinkage and
generally excellent retention of geometry, relative to the green
part. The cavity is then strengthened and sealed by infiltration
and cure of an epoxy tooling resin. It is also possible to produce
metal parts by laser sintering a mixture of metals with
thermosetting polymers [123].When a thermosetting material is
exposed by a laser source, the thermosetting material turns into a
viscous liquid instantly. With for instance SLS of epoxy resin
mixture with iron powder, the polar groups (e.g. epoxy group) in
the molecule of the resin are activated simultaneously. The liquid
flows penetrate the pipes made from pores and wet metal particles.
As such, they make bridges from one particle to another. The
binding effect, depicted in Figure 24, is dominated by the
interfacial characteristic between the resin and the iron.
-
-741-
Iron surfaces commonly attach some active hydrogen atoms because
of its attraction of some molecule such as H2O and HCl due to its
high polarity. Hydrogen bonds occur between the electronegative
oxygen atoms in polar groups in resin molecules and the
electropositive active hydrogen ones on iron surfaces. Iron
particles are strongly bonded because hydrogen bond attraction is
more intensive than that of inter-molecular action existing on the
interfaces between iron surfaces and other nonpolar polymers. The
resin viscosity is lowered at higher temperature (i.e., laser
energy) and its viscous liquid can spread easily. Thus many more
iron particles surfaces can be attached by resin. However,
degradation of resin, induced by excessive laser energy, might
occur and reduce the bonding capability.
Figure 24 : Binding mechanism for epoxy resin to iron
particles
Injection vs. SLS Materials
0
4000
8000
12000
16000
0,0 100,0 200,0 300,0 400,0Elongation [%]
Tens
ile m
odul
us [M
Pa] Injection SLS
c
23
45 6
8 9
ab
d e f
1
7
0
50
100
150
200
250
0,0 50,0 100,0 150,0 200,0 250,0 300,0 350,0 400,0
Elongation [%]
Tens
ile s
tren
gth
[MPa
] Injection SLS1
2
3
4
10
67
8
9
a
c d e f
Figure 25 : Commercial available SLS materials (yellow) in
comparison to some injection polymer materials (in blue)
3.1.8 Properties of molded and SLS polymers Although great
efforts have been made and some results were achieved, the reasons
for the relative few material options are many. Some are surely of
interest to this paper about consolidation, however when dealing
with polymers other phenomena have to be considered as to enhance
the binding models.
By the end of the day we want to have nearly the choice as e.g.
in injection moulding. The obvious way is to compare the
differences and compensate for deficits. Figure 25 shows all the
commercial available SLS materials (in yellow) in comparison to
some injection polymer materials (in blue): the comparison is shown
for tensile strength, tensile modulus and elongation. We find some
deficits. While some SLS materials have comparable properties to
molded ones, none of them is able to reach the highest values of
tensile modulus and strength, nor elongation. The low elongation
capability can be explained by the many short binding necks
creating high stiffness with little elongation. A further
interesting finding indicated by Zarringhalam [218] is the boundary
between the loose powder and the solid part (see Figure 28, which
shows the microstructure of a part cross section). Under certain
conditions unconsolidated particles may remain in the solid part.
This can be overcome easily by proper energy intake settings,
correct scan strategies like cross scanning (in X, Y, in X&Y
and so on) or multi times scanning of the same layers if required.
The boundary conditions may be partially improved by the so called
out-line scanning imposing a sharper border line between loose
powder and consolidated part. A powder with slow recrystallization
rate, as depicted by non-overlapping or slightly overlapping
endothermic and exothermic peaks during the heating and respective
cooling phases of the DSC analysis (Figure 26), might result in
nearly fully dense parts with minimal distortion, while highly
overlapping peaks will yield bad results (Figure 27) [212].
Figure 26 : DSC plots (heating and cooling) of new reinforced PA
12 powder containing elongated rather
than spherical beads for better strength
Figure 27 : DSC plots (heating and cooling) of IP60 powder
showing overlap between melting and recrystallization peaks [US
patent 5,648,450]
In contrast to metals, one has to be aware about the thermal
deterioration of the powder material. The recyled material has a
limited usability as demonstrated for PA 12. The long exposure to
heat leads to chain growth, a rise in
-
-742-
MW and so a rise in viscosity. This causes nonconstant
consolidation conditions and a shift in the melting temperature.
The empirical melting temperature ramps up as a function of build
height and age of the powder. In the extreme this can lead to
creation of patterns on the surface known as Orange Peel because of
its texture (see Figure 10b). This can be improved by mixing with
virgin material (about 70% virgin 30% reused).
Melt Flow Index (MFI) is flow in grammes that occurs in 10
minutes through a standard die of 2.095 0.005 mm diameter and 8.000
0.025mm in length when a fixed pressure is applied to the melt via
a piston and a load of total mass of 2.16 kg at a temperature of
190C (some polymers are measured at a higher temperature, some use
different weights and some even different orifice sizes). MFI is an
assessment of average molecular mass and is an inverse measure of
the melt viscosity; in other words, the higher a MFI, the more
polymer flows under given test conditions. Hence the MFI of a
polymer is vital to anticipating and controlling its processing.
Generally, higher MFI polymers are used in injection moulding, and
lower MFI polymers are used with blow moulding or extrusion
processes.
When polymer powder is reused in SLS, the MFI is already about
three times lower than the virgin powder (Figure 29). This leads to
higher viscosity, and sometimes the quality of the product becomes
unacceptable.
Figure 28 : Boundary between loose powder and solid part
-20
0
20
40
60
80
1 6 11 16 21 26 31 36 41 46 51 56 61 66
Build (run)
MFI
QAMFILinear (MFI)
Virgin
Interior quality
Recycled
Figure 29 : Melt flow index of powder as a function of number of
builds in which powder is used (Source
FHSG St. Gallen)
Figure 30 : Tensile strength and elongation of PA-GF polymers as
function of number of build in which non-
refreshed powder is used
With PA 12, process stability and repeatability in the
consolidation process can be obtained by keeping the MFI within a
constant range and never working with virgin powder [119]. This
phenomenon is amplified for the glass filled PA 12 material,
leading to a rapid decline in mechanical properties, a clear
evidence of different consolidation conditions (Figure 30).The
aging and the resulting change in consolidation phenomena and final
component properties are very minor with amorphous materials like
PS and elastomeric polyester based material. To conclude on
polymers, Table 2 gives an overview of some commercial available
SLS polymers with their most important properties concerning the
SLS process.
Material
Tg (Glass transition
temperature)(C)
Tm (Melt temperature)
(C)
MW(molecular
weight) (g/mol)
PA 6 50 230
PA 12 (41) 184 9.600
PCL -65 -60 58 60 40.000
80.000
PEEK 143 340
Table 2 : Overview of commercial available polymers with most
important properties for SLS
3.2 MetalsToday metals are the second most used material type in
SLS/SLM. Almost all consolidation mechanisms listed in Figure 2 can
be applied to consolidate metallic parts [164].
3.2.1 Solid state sintering (SSS) Theoretically SSS can be
applied for consolidating metal parts by SLS [204]. Its application
is however limited for two reasons: x The process is difficult to
control: the process transits
between no consolidation (at lower energy levels) and
partial/full melting (at higher energy levels) within a very narrow
process window (range), making it difficult to adjust the process
parameters for real SSS.
x SSS requires a long interaction time between laser beam and
powder particles. This calls for slow scanning velocities and makes
the process not
Un-moltenparticle core
Un-moltencompleteparticlestuck to edge
Fullymoltenparticle(no core)
-
-743-
economically viable. Tests performed at K.U.Leuven with steel
powders indicated that scanning speed in solid state SLS needs to
be limited [204], where those scanning speed may reach about 1500
mm/s in SLM and about 10000 mm/s in liquid phase SLS with Laserform
[103].
Early SSS laser sintering tests with steel were performed by Van
der Schueren [95, 204]. Gusarov has applied and modelled laser SSS
of Ti powder [66, 67]. The laser interaction time was of the order
of magnitude of 5 seconds, as compared to 0.1- 0.3 ms with other
binding mechanisms. Tolochko sintered Ti teeth with a static
defocused Nd:YAG laser beam [194]. The Ti parts obtained were dense
in the core, i.e. in the laser spot (high energy density yielding
SLM), but had a porous shell (lower energy yielding SSS). No
post-infiltration was aplied to remove this porosity, as it
favoured osteo-integration of bone into the Ti tooth. Lanzetta has
experimented with gold powders [111]. He succeeded to thermally
sinter pure Au spherical powder ranging between 5 and 40 m in size
and then to infiltrate the obtained skeleton with binary gold
eutectics based on silicon, germanium and tin, with a melting point
as low as 278 C. The initial thermal sintering is not yet
laser-based, but this is the logical next step. This laser
sintering could be done by solid state sintering or by liquid phase
sintering (partial melting) as described further on (see 3.2.2,
subsection B1).
3.2.2 Liquid phase sintering and partial melting This category
unites many different kinds of technologies. Most of these
techniques combine a structural material remaining solid throughout
the process and a binder material being liquefied. In some cases,
however, the solid and the liquid phases result from the same
material. The
first group of technologies is characterized by a clear
distinction between the binder and structural materials. A.
Different binder and structural materials Technologies in this
category can be further divided into three groups according to the
type of powder grains that are used (Figure 2).A1. Separate
particles These technologies use different binder and structural
particles [4]. The structural material (in this case, a metal,
alternatively a ceramic as discussed below) should generally have a
higher melting point than the (metallic) binder material (Use of
polymer binders has been discussed in 3.1). The binder particles
are usually smaller than the structural ones, in order to
facilitate their preferential melting. However, preferential binder
melting may be counteracted by the higher reflectivity or lower
laser absorption of the metallic binder material (typically Cu or
Co) as compared to the structural material (metal or ceramic). In
some cases, this might even lead to a reverse melting process, in
which the structural particles will melt prior to the binder grains
[105, 204].The combination of small binder particles and larger
structural particles has the additional benefit of better packing
with small pores, favoring fast spreading of the molten binder by
capillary forces and fast rearrangement of the particles. Generally
a green part is produced which is still porous and brittle.
Therefore, a post treatment consisting of a furnace post-sintering,
Hot Isostatic Pressing (HIP) or an infiltration with a low melting
point material (metal, alternatively epoxy or other) is usually
necessary: see 4. To obtain sufficient mechanical properties, most
commonly infiltration is applied because of its effectiveness and
relative ease.
a) WC and Co powder mixture b) mech. alloyed WC-Co powder
c) sintered powder mixture d) sintered mech. alloyed WC-Co
powder
Figure 33 : WC-Co powder mixture and sintered parts (source:
University of Leuven)
Many different material combinations have been tested in the
past. At the University of Leuven, metal-metal composites were
tested as well as metal-ceramic composites [64, 100, 101, 109,
112]. Some examples of metal-metal composites are Fe-Cu and
Stainless Steel-Cu. As for the metal-ceramics combinations (see
also 3.3), WC-Cu, WC-Co, WC-CuFeCo, TiC-Ni/Co/Mo, ZrB2-Cu and
TiB2-Ni were tested. Figure 31 and Figure 32show cross sections of
Stainless Steel-Cu and WC-Co green parts (see also Figure 33 a and
c). A2. Composite particles Composite powder particles contain both
the binder and the structural material within each individual
powder grain. The powder may be obtained by mechanically alloying a
mixture of two different powders, causing powder particles to be
repeatedly milled, fractured and welded together.
Figure 31 : LPS of Stainless Steel-Cu powder mixture (a:
nonmolten steel particle, b: molten Cu, c: porosity)]
100 mP
Figure 32 : LPS of WC-Co powder mixture; top: before
infiltration (a: nonmolten WC particle, b: molten Co, c:
porosity); bottom: after infiltration with copper [112]
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Figure 33 a shows an initial powder mixture of WC and Co
particles. Mechanical alloying results in WC-Co powder particles
with a micro grain composite structure in which the two different
phases (WC and Co) can still be identified (see Figure 33 b). Such
composite particles yield a higher SLS green density and a better
surface roughness than a mixture of separate WC and Co powders
(compare Figure 33 c and d) [112]. Figure 34shows an example of a
WC-Co injection-moulding insert.
Figure 34 : Bronze infiltrated WC-Co injection moulding insert
(source: University of Leuven)
A3. Coated grains A third possibility to combine a binder and a
structural material is to coat the structural material with the
binder phase. This ensures that the laser radiation hitting the
powder particles is preferentially absorbed by the binder material
that is to be melted. Moreover a more effective bonding of the
structural particles is realized since the binder material already
surrounds all structural particles. Coated powders exist both with
metal and polymer binder coatings.The University of Leuven
experimented with Cu coated steel powder which turned out to be
successful despite the fact that the powder coating process was
hard to control [204].Figure 35 shows a cross section of an SLS
part made from commercially available polymer coated stainless
steel powder (LaserForm), after polymer debinding and infiltration
with bronze.
Figure 35 : Bronze infiltrated Laserform ST 100 part
At the University of Queensland, an aluminium LPS process has
been developed in collaboration with 3D Systems [171, 172, 173].
Since the strong oxide layer on the grains prohibits direct
sintering of Al powder, an indirect procedure was developed. Nylon
coated Al particles are fused together using the LPS mechanism.
Next, the nylon binder is de-binded in a furnace with N2atmosphere.
A small amount of magnesium added to the powder mixture makes it
possible to form aluminium nitride instead of aluminium oxide when
heating up to 540 C. The aluminium nitride acts as a rigid
skeleton, which is necessary during the infiltration phase. The
infiltrant being used is a ternary eutectic aluminium alloy
(Al-13.8Si-4.7Mg), having a lower melting point than the basic
aluminum alloy. Relative part densities up to 95 % were obtained.
Figure 36 shows the different processing steps. B. No distinct
binder and structural material Technologies in this group do not
exhibit a clear distinction between binder and structural phases
(even in cases where different powders are mixed). Rather than the
distinction between binder and structural material, there is a
distinction between molten and nonmolten
material areas. Therefore Partial Melting is a better name for
these technologies than Liquid Phase Sintering. B1. Single phase,
partially molten When the heat supplied is insufficient to
completely melt powder particles, only a shell or small particles
will melt. This way the molten material acts as a binder between
the nonmolten particle cores [184, 195].The partial melting
phenomenon was modeled by Karapatis [51, 52, 53, 82]. Using a
simple thermal model, skin and core temperatures of the powder
particles were calculated. This way the minimal pulse energy to
fully melt the particle can be calculated. Below this value, the
core temperature never exceeds the melting temperature and only
partial melting is obtained. B2. Fusing a powder mixture Powders
consisting of multiple kinds of powder particles can be classified
in this group when they are only partially molten [64, 220]. For
example at the University of Leuven experiments were done with a
Fe-Fe3P-Ni-Cu powder mixture, aiming at the production of full
density parts [93].The addition of a melting point lowering
additive like Fe3Pis favorable in making the process more energy
efficient. For instance, alloying pure Fe with a small amount of P
lowers the melting point of pure Fe (1538C) to the eutectic
temperature of the Fe-P system (1048C, see Figure 38). Moreover,
the dissolution of P in Fe also has the benefit of a lower surface
tension of the melt, resulting in a better wetting behavior. Ni is
added for its marked strengthening effect. A micrograph of the
structure obtained after partial melting of this powder is given in
Figure 37. A closer look at the microstructure reveals that not all
the powder particles are molten. The final part consists of a low
melting point P-rich phase (no. 3), a high melting point phase with
no significant amount of P (no. 2), some remaining porosities (no.
4) and some remaining unmolten Fe powder particles (no. 1).
Therefore, the process cannot be called full melting and the name
partial melting is preferred. Notice that, even though no full
melting occurs, almost full density is achieved.
50 mP 50 mP
20 mP
a b
cFigure 36 : Steps in Aluminium SLS: a) green part
(nylon binder in black, not visible) b) aluminium nitride
skeleton surrounding the aluminium grains, c) infiltrated
part
Figure 37 : Micrograph of multiphase steel powder: 1) unmolten
Fe particle, 2) high melting P-poor phase, 3)
low melting P-rich phase, 4) pores [93]
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Figure 38 : Phase diagrams of Fe-P and Cu-P showing the
temperature lowering effect of the added P
EOS Direct Metal Laser Sintering process (DMLS) also belongs to
this category [45]. Figure 39 and Figure 40show the difference in
porosity obtained with 50 m and 20 m DMLS powders. In both cases,
shot peening enhances surface quality and the density of the top
layer. Different parameter sets are used for the parts skin and
core in order to obtain a rigid and hard shell and at the same time
a higher building speed (by fast scanning of the core).
3.2.3 Full melting Since a couple of years, there is a big move
in the processing of metal powders from laser sintering or partial
melting towards full melting (SLM) [25, 27, 94, 97, 99,146, 147,
156, 204] , The major benefits and drawbacks of SLM are given in
Table 3.The binding mechanism invoked in full melting is largely
driven by the fluid behaviour of the melt which is related to:x
Surface tension (a/o Raleigh instabilities) x Viscosity x Wetting x
Thermocapillary effects (a/o Marangoni convection) x Evaporation x
Oxidation Changes of viscosity across the melt pool, due to changes
of viscosity between the liquidus and solidus temperature, may
largely influence the shape of the tracks and the resulting
smoothness and density of the parts obtained [158, 163].K.U.Leuven
did an extensive study on the effect of alloying elements on
binding and melt pool stability in SLM of ferro materials [158,
163]. Some major conclusions
are:x Increased oxygen content leads to an enlarged melt
pool size due to exothermal oxidation of Fe. This deteriorates
the surface quality.
x Adding carbon (steel powder 99.1% Fe, 0.1% O, 0.8% C versus
99.9% Fe, 0.1% O) induces spherical pores, caused by entrapment of
CO or CO2 gas bubbles. A reduction of the oxygen content in the SLM
part is observed, together with a depletion of carbon. Carbon
ameliorates the surface roughness of single layers, while worsening
it on 3D parts.
200 mP
100 mP
Figure 39 : DMLS Direct-Metal 50V-2 grain size 50 m, porosity
10-15%: a and c before shot-penning, b and d after shot peening (a
and b: cross section, c and d: top
view) (Source EOS [46])
a b
c
1 mm
Figure 40 : DMLS Direct-Steel 20V-2 grain size 20 m, porosity ~
5%, in the ZX plane: left before and right after
shot-penning, cross section (Source IPT)
x De-oxidizers like Si and Ti do not improve the process. On the
contrary, they lead to a larger melt pool and higher tendency to
balling, resulting in bad surface quality. They reduce the amount
of spherical pores, but increase the amount of irregular pores.
x Cu does not have a large effect on the melt pool behavior,
however when added in pure form, it reduces the absorption of laser
powder due to its high reflectivity.
The University of Liverpool studied the effect of small
additions of boron, in the form of iron boride, to steel [23].
Full Melting Benefits Drawbacks Material choice No distinct
binder and melt phases; hence,
the process can produce single material parts (e.g. Steel, Ti or
Al alloys), rather than producing a composite green parts which
might not be desired
Not suited for well controlled composite materials (e.g.
WC-Co)
Production steps (time, cost)
Elimination of time consuming and costly furnace post-processes
for debinding (in case of polymer binder phase), infiltration or
post-sintering
The laser powder processing needs higher energy level: i.e. high
laser power, good beam quality (more expensive laser) and smaller
scan velocities (longer build times)
Part quality Better suited to produce full dense parts (even
over 99.9%) in a direct way, without post infiltration, sintering
or HIPing
SLM suffers more from melt pool instabilities (low quality of
down facing surfaces, higher upper surface roughness, risk of
internal pores) and higher residual stresses (common need to build
and anchor part on solid base plate, risk of delamination,
distortion when removing base plate).
Table 3 : Benefits and drawbacks of SLM
d
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The boron is aimed to replace interstitial carbon in steel,
which does have a negative effect on SLM processibility. The
addition of boron did not have such deleterious effect on
processibility and to some extend increased the part density.
Addition of Ti to the Fe-FeB powder confirmed the negative effect
of Ti on processibility, even though it improved the bonding of the
samples to the base plate and reduced the extreme hardness. Notice
that the results reported are not always consistent and may differ
in view of the machine or laser used, the layer or part height,
etc. Decades of research might still be needed to develop
appropriate metallurgical understanding and tuned ferro materials
for SLM, in a similar way as done in the past for hot or cold
working steels, free-machining steels, high speed steels, etc. It
is not surprising that development and commercial release of ferro
powders for SLM happens at a very low rate, and that their number
to date is so limited (mainly Fe-based powders and stainless steel;
very few tool steels), not withstanding the large research effort
deployed [35, 73,145].The move from SLS to SLM represents a major
advance in rapid manufacturing of non-ferrous metal parts, which to
date could not be well processed using SLS solid state sintering,
liquid phase sintering or sacrificial polymer binders [109, 138].
SLM of powders from Ti or CoCr alloys has entered industrial
practice, mainly in the field of medical applications: e.g. for
dental crowns, copings and frameworks [2, 15, 85, 104, 113, 188].
Those powders can be melted more easily to almost full density than
ferro powders (Figure 41). This figure demonstrates that the
density of Ti6Al4V is highly repeatable and controllable through
the processing parameters up to densities of 99.98% [205]. SLM of
Al might be the next material reaching industrial application [43,
173]. The University of Liverpool tested SLM of Cu [149], while the
work of Lanzetta [111] might be a first step towards SLM of
gold.
3.2.4 Chemically induced consolidation Metals and hardmetals
(cermets) can also be consolidated chemically. In situ Cu-based
composites can be synthesized by reaction between elemental Ti and
C powders in Cu. Using a CO2 laser, Cu can be melted with the help
of Ti and the reaction heat of the TiC formation [125].The example
given above shows a possible way to apply chemical bonding in SLS
of Al powder. In this case, reaction of Al with the N2 atmosphere
of the SLS chamber can be used to create an AlN binder phase that
binds the Al particles together (Figure 36b). To date however, this
chemical induced binding mechanism has only been invoked during
furnace post sintering [176].
3.3 Cermets and hardmetals Cermets and hardmetals, being
composites in which ceramic particles are embedded in a metal
matrix (binder), can be readily processed by liquid phase SLS using
a mixture of ceramics particles that remain solid throughout the
process and metal particles that are melted by the laser [36, 39,
206, 210]. In most of the cases, the sintered part will not reach
full density and a post-infiltration of the porous green part will
be needed (see Figure 32) [132].Many researchers investigated
liquid phase SLS of common WC-Co cemented carbide (hardmetals)
using either a mixture of WC and Co powder particles or composite
WC-Co powder particles [112, 132]. Some examples and details,
originating from K.U.Leuven, have been given under 3.2 Metals,
subsection A (see Figure 32 to Figure 34). Researchers also
investigated cermets like WC-Cu, WC-CuFeCo, TiC-Ni/Co/Mo, ZrB2-Cu
and TiB2-Ni. More examples are given in 3.4 ceramics.
The Fraunhofer Institute of Production Technology (IPT) worked
with different compacted, respective ground, WC-Co powder
particles: see Figure 42. A higher mean density could be achieved
when applying the spherical particles of the compacted WC-Co
powder, in comparison with the irregular particles of the ground
WC-Co powder. The metallographical analysis of laser sintered WC-Co
structures showed a structure which is comparable with
conventionally sintered hard metal. The distribution of the WC
particles is even and dispersed within the cobalt matrix (Figure
43). This effect could be investigated because of a rearranging of
the WC particles within the fluid cobalt phase. The viscosity of
the liquid cobalt phase and the wetting behavior of the WC
particles with the cobalt binder are sufficient for forming an
almost dense structure. Inhomogeneities, caused by the layered
structure, were not identified. The maximally achieved relative SLS
(green) density when applying WC-Co 75-25 powder was 78.3% and 68%
in case of WC-Co 88-12. The energy input per unit area is
particularly important in terms of localized material densification
(localized liquid phase sintering). In case of a constant energy
input per unit area, a higher relative density was achieved by a
fast scan velocity (vS) and an increased laser power (PL) in
comparison with a slow scan velocity and a low laser power.
Identical energy input per unit area resulted in an 10% increase of
the relative density by the selection of a high vS-PL combination.
Figure 44 shows parts made of WC-Co 88-12. Other researchers
applied a sacrificial polymer binder(MMA-BMA) to consolidate
ceramic particles (e.g. SiC) into a green part [11, p. 111]. This
green part has then to be polymer debinded in a furnace, after
which a further furnace consolidation might be applied (mostly
infiltration with copper or bronze). An example of production of
cermets by chemical induced consolidation is reported in 3.4.7,
where Al2O3cermet is produced by making CuO react with Al. Free Cu
is added to control the exothermal reaction [80].
Figure 41 : Density of SLM processed titanium with different
scan spacing and scan speed
3.4 CeramicsBesides polymers and metals, ceramics is another
important group of materials with many applications in the field of
mechanical engineering. Therefore the qualification of ceramics for
rapid manufacturing technologies is a prior objective of worldwide
research activities. This is an ambitious challenge, because the
specific properties of most ceramics conflict with
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technological material requirements for SLS/SLM (e.g. low
thermal conductivity, high melting point etc.). By now,
technologies of laser and powder bed based layer manufacturing
using ceramics are not so well-investigated as compared to the
technologies using polymers or metals. However there are some
research activities which differ in various technological aspects.
A differentiation can be made by the type of material, the type of
consolidation and the necessity of post-processing of the
considered technology.
3.4.1 Types of Technical Ceramics Ceramic materials are
inorganic and non-metallic. Technical ceramics means ceramic
products made for technical applications. Usually they are shaped
from the "green body" at room temperature and acquire their typical
properties during a long sintering process at high temperatures. In
laser and powder bed based layered manufacturing of ceramics the
very short time of heat impact is the most critical point. The
different sintering mechanisms (described in 2) are more or less
time-dependent. In accordance with their chemical composition the
technical ceramic materials can be divided into three main
groups:
x silicate ceramics x oxide ceramics x non-oxide ceramics
Silicate ceramics are multi-phase materials; the fundamental
ingredients are clay, kaolin, as well as feldspar and soapstone as
silicate carrier. Further ingredients such as alumina and zirconium
silicate (ZrSiO4) are added to achieve specific properties. These
materials, obtained from natural raw materials, combine the basic
electrical, mechanical and thermal properties of technical
ceramics. The predominating types of powder consolidation
mechanisms are liquid phase sintering, partial melting and full
melting. Amongst the materials of silicate ceramics are:
x technical porcelain x steatite x cordierite x
mullite-ceramic
Oxide ceramics consist of at least 90% of single phase and
single component metal oxides. These materials are glass-phase low
or glass-phase free. Synthetic raw materials with a high level of
purity lead to an even structure with very good properties at a
very high sintering temperature.Various raw materials from mining
areas in the whole world form the basis for the production of
high-quality oxides and mixed oxides. The predominating types of
powder consolidation mechanism are solid state sintering, partial
melting and full melting. Following materials belong to the group
of the oxide ceramics materials:
x aluminium oxide x magnesium oxide x zirconium oxide x
aluminium titanate x piezo ceramic
Non-oxide ceramics are material compounds of silicon and
aluminium with nitrogen or carbon. In general non-oxide ceramics
demonstrate a high share of covalency bonding which provides them
very good mechanical properties, even when being used at high
temperatures. The predominating types of powder consolidation
mechanisms are chemical induced binding, partial melting and
full melting. Amongst non-oxide ceramics are:
x Carbide x Nitride
Figure 42 : WC-Co powder particle
Figure 43 : Distribution of WC particles in Cobalt matrix
Figure 44 : Parts made of WC-Co 88-12.
Selective laser consolidation of ceramic powders has already
been investigated for various materials and various applications:
e.g. micro SLS of SiC powder [154,187], sintering of transparent
Ta2O5 dielectric ceramics [78, 79], production of SiO2 investment
casting shells [88],production of biocompatible medical implants
[50, 115,160, 202, 213], SLS of bismuth titanate ceramics [131],SLS
of radiation detectors using bismuth titanate (Bi4Ti3O12)
ferroelectric ceramics [130] and bismuth germanate ceramics
(Bi4Ge3O12) [129] etc. Other examples are given below.
3.4.2 SLS of ceramics using polymer binder One way to powder
consolidation is the use of a sacrificial polymer binder [8, 71,
72, 140]. The used powder material consists of a ceramic and
polymer powder mixture or of polymer coated ceramic particles. The
polymer fraction of the material melts within a short time and at
low temperatures and binds the ceramic particles in a polymer
matrix. The mechanical properties of such ceramic parts are not
sufficient, so that in most cases a post-consolidation process
combined with a debinding, e.g. furnace treatment, is
necessary.
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The University of Liverpool has developed the SLS of
SiC/Polyamide Composites [76]. In this process, the powder is a
mixture of SiC and PA particles. Unlike the majority of the
previous research in this area, the polymer remains an integral
part of the final product rather than being removed in downstream
processes. The evaluation of the mechanical properties of the
sintered composite parts is therefore an integral part of the
research. The materials used were FEPA (Federation of the European
Producers of Abrasives) standard silicon carbide grits individually
blended with Duraform Polyamide which is based on nylon 12. SEM
micrographs of the fracture surface of SLS parts are shown in
Figure 45.
Figure 45 : SEM micrographs of the fracture surface of SLS parts
build at 8 Watts laser power [76]
Other polymer coated ceramic powders are the sand powders
developed by DTM-3D Systems (SandForm) as well as by EOS GmbH
(Direct Croning Process) and that have found industrial application
for some decades now [21, 183, 193]. The powders offered are
standard Si or Zr sands, coated with a small phenolic layer,
responsible for fusing the sand grains together. These powders are
used to produce moulds and cores for metal casting. Stabilized
zirconia powder in mixture with a 80:20 PMMA-BMA copolymer binder
was used at the University of Texas in Austin to produce shells for
titanium casting by SLS, because of its low reactivity and its
thermal shock resistance. A human femur head was reproduced
starting from laser-scanned data, out of Ti-6Al-4V to demonstrate
the possibility of the technology [71].
3.4.3 Liquid phase SLS of ceramics If multi-phase ceramics are
used for SLS, the dominating powder consolidation mechanism is the
liquid phase sintering. It always occurs if one phase of the
material is already molten while the others are still solid. A
great advantage of the liquid phase sintering is that the molten
phase can bind the solid phase in a matrix structure within a short
time [117, 124]. Unlike solid state sintering where the diffusion
process takes a long time, the SLS process can run faster. The
Fraunhofer IPT began its investigations into ceramic laser
sintering in 1997 using laser sinter equipment with maximum 100 W
laser power (CO2 laser) [88]. Initial tests performed on different
ceramics such as zirconium silicate or aluminium silicate showed
that the zirconium silicate was more suitable. Fundamental
experiments were performed using ground powders of differing
fineness and mixtures of standard gri