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Laser microsintering of tungsten in vacuum
Robby Ebert, Frank Ullmann, Lars Hartwig, Tino Suess, Sascha Kloetzer, Andre Streek, Joerg
Schille, Peter Regenfuss, Horst Exner
Hochschule Mittweida, Technikumplatz 17, 09648 Mittweida, Germany
ABSTRACT
Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum
level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In
addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring
during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse
the experimental outcomes. The dependence of sinter density on the parameters is discussed.
1. INTRODUCTION
Laser microsintering is a relatively recent process for fabricating microstructured components from metals and ceramics
[1-3]. By using short, high-intensity pulses, even metals with a high melting point such as e.g. tungsten can be machined.
The achievable resolution of this approach is better than 30 µm. This method, therefore, is currently the only one that
permits direct fabrication of free-formed microstructures with undercuts.
Tungsten is difficult to work mechanically. µMIM is the only method available at present for fabricating microparts. It is
used predominantly for making microstructural components for the illuminant industry. Due to the material’s unusual
physical and chemical properties, new manufacturing methods could be exploited to develop further fields of application
in the high-temperature range, in x-ray technology, in microsystem engineering and in medical engineering.
Up to now, laser microsintering of tungsten has usually been carried out in a protective gas environment. Our
investigations, carried out under different vacuums, are intended to provide further information about the mechanism
involved and about possible approaches for influencing the sintered structures.
2. EXPERIMENTAL PRINCIPLES
An existing laser microsintering facility was used for the investigations [4]. It consists of a Nd:YAG laser (Table 1), a
modulator for controlling the pulse, a scanner for beam deflection and focusing, and a vacuum sintering chamber for
performing the experiments under defined conditions.
Tab. 1: Laser parameter.
wavelength
pulse duration
pulse frequency
beam radius in focus
λ [nm]
t [ns]
f [kHz]
ω0,86 [µm]
1064
180
8
11 µm
The variable parameters included the incident laser power P, which was varied between 0.8-2.4 W and consequently the
pulse energy Wp ranged between 0.1-0.3 mJ, the pulse power PP between 555-1667 W, the pulse interval a between 4575 µm and the vacuum pressure p between 0.01-500 mbar. Helium was used as residual gas.
In order to adjust the intensity, the incident beam spot was defocused vertically in all the experiments by 175 µm away
from the powder’s surface, which leads to a calculated beam radius ω86(z = -1,75*10-6) of 17.5 µm at the powder’s
surface.
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Each layer was irradiated with a regular pulse pattern at equal pulse intervals over an area of 1000x1000 µm2. Relatively
uniform irradiation was achieved gradually by rotating the pulse pattern by 139° and performing 8 passes per layer. A
pulse interval of 45 µm produced 3950 pulses per dispensed layer area (1 mm2); a 60 µm interval produced 2222 pulses,
and a 75 µm interval 1422 pulses respectively.
The variation in power and pulse intervals led to variation in the mean laser radiation fluence Em incident on the surface
being sintered, ranging between 14.2-118.5 J/cm2. The intensity IF at the focus varied between 1.46 and 4.38 x 108
W/cm2, and the intensity IO at the powder’s surface between 0.58 and 1.74x108 W/cm2.
The effective laser power is subject to some uncertainty in terms of an undefined dirt contamination of the chamber’s
coupling pane. The sintering process can cause a coating of the pane with material. This results in decreased laser power
incident on the process site. At higher laser powers, in turn, self-cleaning of the pane may occur.
3. THE POWDER DISPENSING
The experiments were carried out with tungsten powder supplied by Goodfellow. The measured bulk density was 2.8
g/cm3. The grain shape is polyhedral. The powder has a strong agglomeration tendency. The manufacturer specifies a
grain size < 1 µm. Our own measurements showed that there is a maximum in the number of particles at a grain size of
300 nm (Fig. 1), and a maximum in terms of volume percentage at 2 µm.
Fig. 1: Number of particles against particle size for Goodfellow tungsten powder < 1 µm.
The powder’s layer generation was performed with a special cylindrical dispenser. In this way, it was possible to achieve
a relatively dense, uniformly thin layer. Before the dispensing, the carrier platform was lowered by 1 µm at a time.
To determinate the thickness of the resulting dispensed powder d a mathematically description of the cyclical process of
powder dispensation and lowering of carrier platform is given below with following variables:
d
dZ
ρ
ρP
ρS
-
thickness of dispensed powder layer
lowering of carrier platform
density
bulk density of powder
density of sintering body
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ρP
= DP
ρ
-
relative powder density
ρS
ρ
-
relative density of sintering body
-
density coefficient.
= DS
DP
ρP
=
= D
DS
ρS
In general assuming no powder losses during laser sintering process (Δm = 0) it is in force:
d1 = dz
d2 = dz + d1(1-D),
d3 = dz + d2(1-D),
d4 = dz + d3(1-D),
can be transformed in
can be transformed in
can be transformed in
d2 = dz (1 + (1-D)0(1-D))
d3 = dz (1 + (1-D)1(2-D)),
d4 = dz (1 + (1-D)2(3-D) + D (1-D))
d5 = dz (1 + (1-D)3(4-D) + D ((1-D) + 2(1-D)2))
d6 = dz (1 + (1-D)4(5-D) + D ((1-D) + 2(1-D)2+3(1-D)3))
…
For k > 2 a polynominal series can be arranged as follows:
dk = dz (1 + (1-D)k-2(k-1-D) + D ((1-D) + 2(1-D)2+3(1-D)3+…+k-3(1-D)k-3))
k
dk = dz (1 + (1-D)k-2(k-1-D) + D
∑( n − 3 )( 1 − D )
n −3
).
3
For 0 < D < 1 and k gravitating infinite summing up of the dispensed powder thickness d is in force:
∞
d = dz (1 + 0 + D
∑( n − 3 )( 1 − D )
n −3
).
3
∞
Because of the partial sum
∑ nx
0
n
=
x
( x − 1) 2
dispensed powder thickness can be summarized as follows:
d = dz (1 + D
d = dz (
d=
1− D
)
(1 − D − 1) 2
D +1− D
)
D
dz
.
D
Considering the mathematical derivation above, powder layer thickness d is inversely proportional to powder density
factor D and proportional to the lowering of carrier platform dz. With tungsten powder bulk density and the relative
density of the sintered body approximated to 0.7, the density coefficient can be calculated to D = 0.2078. Respectively a
constant dispensed powder thickness of d = 4.81 µm can be expected after infinite dispensing and sintering cycles. The
calculated powder layer thickness is shown in Fig. 2; after 20 dispensing and sintering cycles a constant is reached.
During the laser sintering process the powder layer will be compacted to a 1 µm thick sintering layer. In general, after
some tens dispensing cycles the supplied powder height always manifests itself as a sintered layer thickness.
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powder layer thickness (µm)
6
5
4
3
2
1
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
number of dispensing and sintering cycles
Fig. 2: Calculated tungsten powder layer thickness vs. the number of dispensing und sintering cycles (lowering
per dispensing cycle dz = 1 µm, rel. powder bulk density DP= 0.14545, rel. sinter density DS = 0.7, Δm = 0)
In all 176 experiments carried out, layer dispensing took place 400 times after lowering the active space by 1 µm. Hence,
assuming no mass losses and a relative laser sintering density of DS = 0.7, this would imply a built-up height of
396.19 µm (total dispenser height 400 µm – powder layer thickness 4.81 µm + sinter layer thickness 1 µm).
4. THE LASER MICROSINTERING PROCESS
The use of high pulse powers and intensities in laser microsintering brings about a very complex process [5]. When the
laser pulse irradiates the powder’s surface, some of the powder is melted by absorbed beam energy, and in turn part of
that material is vaporised. The result is that partly vaporised powder particles and separate molten material are
accelerated into the powder bed through recoil during vaporisation [6, 7].
Thus far, energy absorption and dissipation have remained largely unexplored. Our own previous experiments have
established that the intensity required for sintering depends very strongly (up to a factor of 20) not only on the powder’s
grain size but also on its size distribution and on the grains’ shape. It is conjectured that in dependence on the proportion
of nanoscale powder particles present and on the particles’ shape (sharp edges and vertices), optical near-field effects
such as e.g. the local field amplification at nanoparticles play a part [8].
The literature contains a temperature estimate for laser microsintering, applicable to metal particles with a diameter of
10 µm and a laser beam focus diameter of 100 µm [9]. At the intensity used, the temperature rise with a single pulse was
too low for sintering. By using overlapping pulses, however, it proved possible to bring the material to melting.
The intensities required to reach the boiling point, across different particle sizes with diameters ranging between 0.05 and
5 µm, were calculated with the help of a temperature field program [10], as a first approximation for the effect of powder
grain size on a laser microsintering process utilising single pulses. The program takes into account both the melting and
the vaporisation energy. Individual tungsten particles were calculated; which were assumed to be freely present in the
space, without taking into consideration heat conduction into the powder bed or into the agglomerate such as occurs
during a real sintering process. The calculated intensities can thus be regarded as an estimate for the lower limit. The
particle shape was cylindrical, with the diameter and height set to be equal. The laser pulse duration was 100 ns and the
laser beam radius 10 µm. The absorption rate was set to 0.41. Particles with a diameter > 2 µm require an almost
constant intensity of 5 - 6*107 W/cm2 in order to reach the vaporisation temperature at the surface (Fig. 3). In particles
with a diameter below 2 µm, the required intensity decreases almost linearly with particle size.
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7
7
2
intensity (10 W/cm )
6
5
4
3
2
1
0
0
1
2
3
4
5
6
particle diameter (µm)
Fig. 3: Calculated intensity to reach the vaporisation threshold as a function of particle
size in tungsten, with a laser wavelength of 1064 nm
When the powder’s particle size distribution includes nanoscale components, it would therefore be difficult to find
parameters under which the powder could be processed uniformly, since the nanoparticles would vaporise completely
even at relatively low intensities of < 1*107 W/cm2. But since the temperature gradients that are present between the
powder particles in the real powder bed are dissipated through heat conduction and radiation, processing of powders that
exhibits a wider range of grain sizes is possible also.
The calculated intensities reflect the experimentally used parameters, taking the boundary conditions into consideration.
Simultaneously with the powder’s heating, and as a function of the pressure and of the type of residual gas present, a
process plasma is formed which exhibits a far greater impact duration (> 100 µs) than the laser pulse itself. The plasma
expands and initially exerts a pressure of up to 100 MPa on the powder’s surface and on the molten material produced
[6], after which it leaves behind a temporary vacuum. As a result of the processes taking place, one obtains a horizontally
and vertically cross-linked stable structure with a density of 40-95%.
5. RESULTS AND DISCUSSION
The experiments were analysed in terms of the morphology of the fabricated sintered structures and the achieved sintered
structure height, using polished sections; selected specimens also underwent REM investigations. An attempt was made
to use optical methods to determine the exact sinter density. This led to a very high degree of variance between different
methods, such that no absolute values can be stated. Nevertheless, it was possible to observe a number of trends that
depend on the parameters chosen.
At the lowest laser power of 0.8 W, it was only with a pressure of 500 mbar and a pulse interval of 45 µm that a
complete layer buildup could be observed, which nonetheless led to a structure with low density and visible cracks (Fig.
4, left).
At lower pressures or longer pulse intervals, the buildup of the sinter structure is broken off at small heights (Fig. 5). In
other words, complete layer buildup with a laser power of 0.8 W required, in addition to an intensity of 0.58*108 W/cm2
at the powder’s surface and a pulse interval of 45 µm, also a corresponding residual gas pressure of > 100 mbar. At the
next investigated power of 1.2 W, however, a complete sinter structure was generated at all pulse intervals and pressures
other than 0.01 mbar. From a power of 1.5 W upwards, complete generation was generally possible at all pressures.
Based on visual assessment, sinter density depends on laser power, pulse density and pressure.
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P = 0.8 W
P = 1.2 W
P = 1.5 W
P = 1.8 W
P = 2.1 W
P = 2.4 W
h = 381 µm
h = 391 µm
h = 368 µm
h = 336 µm
h = 327 µm
h = 314 µm
Fig. 4: Macro images of polished sections, section width 200 µm, p = 500 mbar, a = 45 µm
p = 0.01 mbar, P = 0.8W, a = 45µm
p = 500 mbar, P = 0.8W, a = 75µm
Fig. 5: Macro images of polished sections of incomplete sinter structures
It can be seen clearly that at a pressure of 500 mbar, the sinter density increases with an increasing applied laser power
(Fig. 4). Since frequency and pulse duration were always held constant, this means that the pulse energy, the pulse power
and ultimately the intensity were increased in equal measure and thus affect the sintering density (Table 2).
Tab. 2: process parameter
laser power
P [W]
pulse energy
Wp [µJ]
pulse power
PP [W]
intensity in focus
IF [108 W/cm2]
intensity at the
powder’s surface
IO [108 W/cm2]
0.8
100
555
1.46
0.58
1.2
150
833
2.19
0.88
1.5
187.5
1042
2.74
1.10
1.8
225
1250
3.28
1.31
2.1
262.5
1458
3.83
1.53
2.4
300
1667
4.38
1.74
An increase in sinter density at smaller pulse intervals was also observed at this pressure (Fig. 6, left). At a pressure of
0.01 mbar, however, this was no longer observed (Fig. 6, right).
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p = 500mbar
p = 0.01 mbar
a = 75µm
a = 60µm
a = 45µm
a = 75µm
a = 60µm
a = 45µm
h = 329 µm
h = 343 µm
h = 336 µm
h = 218 µm
h = 222 µm
h = 215 µm
Fig. 6: Macro images of polished sections, section width 200 µm, P = 1.8 W
Furthermore, it was established that the carrier platform was abraded at the start of the build-up processes (Fig. 7). This
can be explained inter alia by the fact that aluminium, the platform material used for technical reasons, has a
considerably lower evaporation temperature than tungsten. As regards parameter variation, it was observed in particular
(as expected) that higher power contributed to higher platform abrasion.
p = 0.01 mbar,
a = 45µm, P = 1.8 W
p = 0.01 mbar,
a = 75µm, P = 1.8 W
p = 500mbar,
a = 45µm, P = 2.4 W
p = 500mbar,
a = 75µm, P = 2.4 W
Fig. 7: Macro images of polished sections, section width 300 µm
The pulse interval had no clear effect in this regard. At low pressure, a shorter pulse interval led to higher platform
abrasion (highest measured value was 67 µm). Sometimes the sintered body’s lower boundary areas were strongly
rounded off (Fig. 7, p = 500 mbar, a = 45 µm, P = 2.4 W). This cannot be explained logically by the procedure used,
since the same areas were always irradiated successively.
The REM images of polished sections in Fig. 8 show that at moderate power and low pulse intervals, the sinter density
increases with pressure. It is estimated that when the pressure varies from 0.01 mbar to 500 mbar, it increases
approximately between 10 and 20%.
Additionally, the built-up height of the sinter cubes as a function of pressure was determined from images of polished
sections (Fig. 9). In the pressure range of 1 to 500 mbar there is a linear increase in the built-up height, whereas before
that the built-up height increased in the direction of even lower pressures, such that a local minimum in built-up height
was observed around a pressure of 1 mbar. The pulse interval had no significant effect on the height.
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p = 0.01 mbar
p = 1 mbar
p = 10 mbar
p = 100 mbar
p = 500 mbar
Fig. 8: REM images of polished sections, section width 50 µm, P = 1.8 W, a = 45 µm
400
45µm
350
60µm
built-up high (µm)
300
75µm
250
200
150
100
50
0
0,001
0,01
0,1
1
10
100
1000
chamber pressure (mbar)
Fig. 9: Measured height of the sintered body as a function of chamber pressure, P = 1.8 W, Parameter Pulse interval a
As a further result, at a pressure of 500 mbar the built-up height decreased with increasing laser power (Fig. 4, measured
with a microscope), but again it was independent of pulse interval (Fig. 6, left).
Correspondingly the powder layer calculation (Capt. 3), the height of the sintered body was expected to 396µm. Even
with a assumed sintering density of 100% the height of the sintering body should not sink below 394 µm. However,
calculated sintering height is in opposite to measured values in Fig. 9 and reduction in height seems to be mainly caused
by ablation of both, tungsten powder or sintering layer during laser sintering built-up process.
In consequence equation to calculate the powder layer thickness is modified with Δm > 0. It was assumed that the
dispensed powder and the layer already sintered are reduced by a fixed amount during each sintering cycle, since it can
be assumed that after a start-up phase the abrasion occurs relatively uniformly.
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powder layer thickness (µm)
60
50
40
30
20
10
33
31
29
27
25
23
21
19
17
15
13
11
9
7
5
3
1
0
number of dispensing and sintering cycles
Fig. 10: Calculated powder layer thickness against the number of dispensing and sintering cycles (active space lowering per
dispensing cycle 1 µm, rel. tungsten powder bulk density DP= 0.14545, rel. sintering density DS =0.7, max. powder
abrasion 20 µm, max. sinter layer abrasion 5 µm)
The estimated values were 20 µm for the abraded powder layer thickness per irradiation cycle and 5 µm for the abraded
sinter layer thickness. These values were estimated from previous experience. Since the layer thicknesses present during
the first dispensing cycles are not yet sufficient, 90% abrasion was estimated for each powder layer thickness < 20 µm
and sinter layer thickness < 5 µm. Considering the calculation, the equilibrium between material application, abrasion
and sintering is reached after ca. 30 dispensing cycles at a powder layer thickness of 49 µm (Fig. 10). The height of a
sintered part would be accordingly 352 µm and correlates to measured height obtained after sintering process at 1.8W
laser power and 500mbar chamber pressure (Fig. 9). Therefore the ratio of body height losses caused by powder or
sintering layer ablation can only be approximated, currently experimental data are not available.
450
built-up high (µm)
400
350
300
250
200
150
100
50
0
0
10
20
30
40
50
60
70
80
90
100
abrasion (µm)
Fig. 11: Calculated built-up height as a function of abrasion during the build-up process (400 dispensing
cycles, active space lowering dz =1 µm each time, rel. tungsten powder bulk density DP= 0.14545,
rel. sintering density DS =0.7, max. powder layer abrasion = 20 µm, max. sinter layer abrasion = 5µm)
Accordingly the calculation the sintered part’s built-up height decreases linearly with the abrasion as shown in Fig. 11.
The abrasion of the dispensed powder layer goes into the calculation directly, and the abrasion of the sinter layer with
1/D (here 4.81).
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Additional abrasion of the substrate at the start of the sintering cycle as it occurs in practice, e.g. at a thickness of 50 µm,
means that the equilibrium between powder dispensing, ablation and sintering is reached as early as after 10 dispensing
cycles, with the sintered part’s height remaining unaffected.
The measured built-up heights as a function of the parameters and the calculations of layer heights, indicate that
pressure- and power-dependent abrasion mechanisms exist during the build-up process. Essentially, these are direct
vaporisation of powder and sinter material through the effect of the laser pulses, and indirect abrasion by the generated
process plasma.
The process plasma consists predominantly of material vapour plasma, and to some extent of residual gas plasma. The
observations established that changing the type of residual gas affects the process plasma. Without material vapour, no
residual gas plasma can be ignited under normal pressure at the intensities used.
The threshold for generating the process plasma drops with pressure, since the mean free path of the particles increases.
At lower pressure, a smaller field strength is needed to accelerate the particles up to their ionisation energy, since longer
acceleration paths can be travelled without collisions. The laser focus has a diameter of 22 µm in the region of its
greatest intensity, ca. 175 µm above the powder’s surface. A mean free path of 22 µm is present at a pressure of
2.2 mbar. This was confirmed by optical measurements of the built-up height, where the smallest value of 175 µm was
recorded at 2 mbar chamber pressure due to highest ablation (Fig. 9). This is likely to be caused by the strong plasma
formation in this pressure range.
At pressures < 1 mbar, the field strength needed to generate a residual gas plasma increases once again. Thus, plasma
intensity drops again in this pressure range. This is consistent with the experimental results, according to which the sinter
height increases slightly once again below a pressure of 1 mbar (Fig. 9).
Generally speaking, the effect of process plasma increases with increasing laser power, such that ever greater
vaporisation of the powder and sinter materials takes place and the built-up height decreases continuously. According to
the model of the process, the sintering product’s compaction should increase at the same time.
With decreasing pressure, however, a more complex behaviour can be observed. At a pulse interval of 45 µm, the
proportion of vaporised material increase, i.e. the built-up height decrease, and at the same times the sinter density also
(Fig. 6 and 8). This is attributed to the effect of the plasma, contrary to the behaviour observed thus far at a pressure of
500 mbar.
At a pulse interval of 75 µm, in contrast, although the proportion of vaporised material increases again (with a maximum
at 1 mbar), the sinter density increases also, as judged visually (Fig. 12). This would correspond to the current model of
the process.
p = 500mbar
p = 100 mbar
p = 10 mbar
p = 1 mbar
p = 0.01 mbar
h = 329 µm
h = 303
h = 232
h = 186 µm
h = 218 µm
Fig. 12: Macro images of polished sections, section width 200 µm, P = 1.8 W, a = 75 µm
This behaviour could indicate that the plasma’s expansion at 500 mbar is too low to result in sufficient compaction at a
pulse interval of 75 µm. At lower pressures, an increasing expansion of the plasma leads to higher compaction.
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At a pulse interval of 45 µm, in contrast, apparently the plasma’s overlapping is so great already that reducing the
pressure, with the associated expansion of the plasma, has no effect.
p = 0.01 mbar
p = 1 mbar
p = 10 mbar
p = 100 mbar
p = 500 mbar
Fig. 13: REM images of the sintered body’s surface, section width 50 µm, P = 1.8 W, a = 45 µm
It was also observed that different sinter structures are formed as a function of chamber pressure (Fig. 13). At low
pressures it was possible to note fine-grained, smooth structures on the sintered body’s surface. As a result of the thick
powder layers that need to be penetrated due to the low built-up heights, the laser pulses no longer impact the surface
with such a high energy. The appearance of the resulting structure is as though the material had been sprayed on. It is
open-pored, and therefore can be infiltrated or re-sintered. At high pressures, in contrast, smelt-like, relatively dense and
ragged structures are formed. The laser beam’s impact points can be recognised. Due to the thick necks and the closed
pores, it is unlikely that these structures could be re-sintered and infiltrated.
6. SUMMARY AND PROSPECTS
Mathematical calculations show that after some ten dispensing cycles a constant thickness of dispensed powder layer d
will be obtained correlating to the density coefficient D and the height of periodical space lowering of the carrier
platform dz. Furthermore sintering thickness is influenced by laser ablation and vaporization of powder layer or sintering
body. The degree of particle vaporisation during laser microsintering depends on particle size, applied laser power and
residual gas pressure. Despite relatively large vaporised layer thicknesses, after a few dispensing cycles the result is a
constant dispensed layer thickness. The exhibited effects in terms of sinter density are as follows:
- at 500 mbar:
o higher power leads to greater abrasion and higher sinter density
o higher pulse density does not lead to greater abrasion but does lead to higher sinter density
- in vacuum:
o the abrasion is generally higher
o higher laser power leads to slightly greater abrasion and correspondingly higher sinter density
o higher pulse density does not lead to greater abrasion nor to higher sinter density
The sinter structure can be influenced by the variation in chamber pressure. A finer adjustment of the laser parameters
will be necessary in future, in order to minimise the proportion of vaporised material. The possibility exists of building
up bodies with partially modified structural properties. The resulting open porosity at low chamber pressures permits
subsequent infiltration of the tungsten body by other materials.
The investigations will continue with shorter pulses (6-10 ns, 250 fs) and even lower chamber pressures for a better
plasma elimination caused by the remaining gas. Furthermore it will be examined to what extent post-processing
(furnace sintering, infiltration) leads to modified properties (density, sinter structure, strength) of the sintered tungsten
body.
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ACKNOWLEDGEMENTS
The authors wish to thank BMBF for sponsoring the Innoprofile project Rapid microtooling using laser-based methods
(Ministry ref. no. 03IP506), A. Fischer and F. Ospald for providing the temperature field calculation program and H.
Fritsch for assistance to calculate the dispensing powder layer thickness.
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