Laser joining of ceramics in liquid phase

Laser joining of ceramics in liquid phase
Horst Exner, Anne- Maria Reinecke, Peter Regenfuß, Maren Nieher
Laser Institut Mittelsachsen e.V. at Hochschule Mittweida (FH),
University of Applied Sciences
Abstract
The only way to join ceramic bodies- even with complicated geometries- effectively without
deterioration of their excellent properties is by a process with a liquid or a liquid like phase or
a diffusion bonding process.
We report the successful employment of laser beams for the joining of ceramic bodies to
each other and to metal specimens.
Our two-laser-beam welding method, an additive-free process in liquid phase, allows to join
bodies of fusible ceramics, e.g. of alumina parts, resulting in a strength of 85 % of the bulk
material.
Furthermore for the first time we present results on laser welding of alumina with niobium.
By this method parts of various shapes can be joined in only a few minutes without furnaces.
The achieved results in combination with the general advantages of laser material processing,
like narrow seams, high process flexibility, high productivity and a high degree of automation, proof this technology ideally suited for industrial application.
Introduction
Ceramics are materials usually produced by a special sintering process. Depending on their
composition this leads to special qualities like high temperature resistance, extreme hardness,
high chemical resistance and a lower density compared to metals. These excellent properties
are the reason for the application of technical ceramics in the vast fields of electronics, automotive industry, aerospace, chemical industry and so on.
Currently no technology exists which, within reasonable economical limits, produces joints of
satisfactory quality between ceramic parts and preserves the excellent properties of the material (1-5).
Metal brazing and adhesive bonding principally reduce the thermal and chemical stability of
the ceramic system (6,7,8). These disadvantages originate from the presence of an additional
material (glue or solder) with completely different properties than those of the ceramic. This
means, a critical weak point is generated at the joint. Furthermore, metal brazing usually is
possible only after the improvement of the wettability for the solder by metallisation of the
ceramic surface. This process is time consuming and very expensive.
On the other hand high quality joints can be achieved by diffusion welding (9). The joining
mechanism is based on diffusion processes at high temperatures. This means, diffusion welding needs a process time of about one hour. The preparation of the material is very expensive
(requires a high surface smoothness) and a high bearing pressure is necessary (therefore it is
not suitable for joining small parts). In addition both, diffusion welding and brazing, demand
a vacuum atmosphere.
Experimental set-up
Alumina specimens (α- Al2O3) and metal parts with a length of 20 … 30 mm, a width of 5 or
10 mm and a thickness of 0.7 … 1.2 mm were used in our experiments.
Because of the ratio of thermal conductivity to thermal expansion of alumina, a fast and locally restricted energy input by a laser beam will normally generate cracks in the material.
Thus the material has to be preheated to minimise the thermal shock effect of the welding
laser beam. To overcome the disadvantages of a furnace process, in our case the preheating is
performed by a second laser beam (Figure 1) (10).
fibre
Figure 1: Experimental set-up
CO2- laser beam
A continuous wave (cw) 600 W CO2- laser
beam scans the surface of the material. Because of its very high absorption at the wavelength of 10.6 µm, the material gets heated
focussing optic
within seconds. When the necessary preheating temperature is achieved the cw 1.2 kW
Nd: YAG laser beam starts to weld the parts
Nd: YAG- laser beam
together. It penetrates about 0.8 mm deep into
the alumina (11). This means that for the low
ceramics
thickness of our specimens the generation of a
melting bath for welding is nearly independent of the thermal conductivity (12,13). By welding the ceramic to metals argon was used as
shielding gas on the upper and lower side to avoid oxidation.
scanning mirrors
Table 1: Properties of the used material
alumina
niobium
steel 1.3981
purity %
96,0
99,9
melting point °C
2050
2468
1460
boiling point °C
3530
3300
3023
thermal conductivity W(mK)-1
27
52
17
thermal expansion coefficient 10-6 K-1
7,5
7,5
6,1
Ni: 29,0
Si: 0,2
C: 0,03
Co: 18,0
Mn: 0,2
Fe: 52,57
The surface temperature of the alumina was adjusted by a pyrometer controlled power application of the preheating laser. The preset for the emission value was 0.75.
The surface as well as cross-sections of the welded seams were examined by optical and
scanning electron microscopy (SEM). In the case of alumina specimen bonded to each other
the strength of the joint was determined by a 4-point- bending test.
Results & Discussion
Welding of alumina with alumina
In the case of alumina we found that 100 % crack-free bonded materials were generated with
a preheating temperature of 1500 °C. Investigations of the cross sections of the welded seams
showed minimum porosities for a preheating temperature of 1600°C. Higher preheating temperatures resulted in a higher porosity. To increase the efficiency of the process the time for
preheating and cooling was minimised to a limit which still guarantees a crack free result.
Our final results are: Up to 1400 °C a heating rate of about 30 Ks-1 can be applied. Above this
temperature obviously a morphological rearrangement takes place and around 1500°C the
critical temperature is reached and the highest stresses occur. So it was necessary to decelerate
the increase of temperature to reduce these stresses until we arrived at equilibrium. Above
these point we could raise the temperature again at a constant rate and the welding can be performed with the second laser beam (Nd: YAG). Altogether, the complete preheating process
takes at least 100 seconds.
After completion of the welding controlled cooling is recommended in the temperature range
above 1500°C. Below that temperature no cracks occurred during further cooling in an environmental atmosphere.
The maximum acceptable temperature gradients perpendicular to the seam were determined
as 70 x 10³ Km-1. That means, at distances further than 20 mm off the weld the temperature is
already below 500 °C. Therefore conventional metal clamps suffice as a holding device.
Figure 2: Cross section of a cw laser welded
Al2O3 seam (butt welding)
100 µm
The laser welded specimens in figure 2 shows
a very homogeneous structure. Average grain
size is about five times that of the initial material. The packing is dense and approaches that
of the base material. Furthermore it shows a
concentration of pores at the former border
liquid / solid. They arise from vaporisation of
impurities, and/or from agglomeration of
pores inherent in the initial material.
Path energy is often considered as a process parameter. It is defined as the ratio of laser power
over welding velocity. However, even at constant path energy, different solidification structures may be obtained. The heat gradient arises from the balance of heat flow in the material
which is reached as a function of both, the rate of laser energy transfer into and the dissipation
of it inside the material as well as the loss of heat due to thermal conductivity.
On one hand, if the heat gradient is shallow, the solidification of the bath starts at its borders
by the formation of columnar crystals oriented toward the centre. Impurities will concentrate
in the regions that solidify at the lowest temperatures. Simultaneously a contraction occurs at
both solidification fronts and leads to hot cracks which are similar to those observed in metal
welding. On the other hand at too high laser powers and velocities heat dissipation takes too
much time. This results in welding seams with big grains, up to 100 µm, in the centre. Impuri-
ties will be mostly vaporised. The borders of the seam show columnar crystals up to a length
of some 100 µm.
Figure 3: SEM view of the surface
morphology of an optimal welding seam
At a well-balanced ratio between energy
input and energy losses due to thermal
conductivity
solidification
in
a
homogeneous and nearly isotropic manner
(Figure 3) is possible. The cross-section of
these joints is comparable to those shown in
figure 2. The crystal growth is limited to the
threefold of the original crystal size. These
joints are also gas-tight. The quality shown
is the optimum that could be achieved
experimentally.
The strength of the bonded specimens was determined by a 4- point- bending method. Two
pieces measuring 30 x 7 x 0.8 mm³ were welded together in the cw mode.
The resulting strength (σ) of the weld was compared with the virgin material (see table 2).
1
F/N
m
σ/ MPa
σrel/%
cwwelded
34
11
183
85
Virgin
Alumina
51
3,9
191
100
Table 2:
Strength of the welded alumina
Fracture probability .
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0
cw
10
Referenz
20
30
40
50
60
70
Fracture load, N
Figure 4: Fracture probability vs. fracture load
The investigation of more than 20 specimens allows a statistical evaluation. For an industrial
application the relation between fracture probability and fracture load is of special importance
(figure 4). The slope of the curves is a measure for the predictability of a material failure. The
cw welded alumina shows a reproducible smaller scatter of fracture load compared to virgin
alumina. It is probable that the concentration of pores at the transition from the seam to the
base material weakens the material and induces a local failure point corresponding with the
higher inclination of the curve.
Welding of alumina with metals
In this part of the report we can present the first results and parameters which enable to weld
these materials of absolutely different composition. Within a window of parameters: a welding power of 100…140 W and a welding velocity of 0,45…0,65 mms-1, the materials get
welded completely in a relatively tight manner. Because of nearly the same thermal expansion
coefficients, no cracks occurred during the cooling process. Nevertheless, as a result of insufficient shielding gas atmosphere, the niobium showed superficial changes. These are due to
oxidation of the highly reactive metal. The development of a black, compact layer, sometimes
superimposed of a loose whitely yellowish layer, was already reported by other authors and
analysed as Nb2O5 and NbO (14). At lower preheating temperatures between 1250…1500°C
(at the beginning of the investigations due to the cooling effect of the shielding gas) big crystals of a size up to 1,5 mm could be obtained within the black compact layer (figure 5).
scin-deep
oxidised niobium
niobium
crystals
welding seam
welding seam
alumina
alumina
1 mm
a)
b)
Figure 5: formation of crystals on the surface of niobium after the welding process
In figure 5 a) and b) welding joints between Al2O3 and niobium with different surface oxidation are presented.
The SEM view (figure 6) of a cross section through the joint zone yields the following information: As expected from the good mutual wetabilities (15), a joint resulted. At the peak temperatures of welding, considerable masses from both specimens had merged into at least one
liquid phase forming the volume of the seam. During solidification the system had passed
through several phase transitions. Concluding from the number and arrangement of the solid
phases a composition gradient across the seam persisted throughout the whole process. Presently we are investigating the compositions and congruency of the various precipitates, taking
into account the incomplete equilibration of the system during the process. Reactions between
the two metals niobium and aluminium , between their oxides or between each metal with
each of the oxides have been reported (14,16,17). Between pure niobium (bright area in Fig.
6, left side) and pure alumina (dark area in Fig. 6, right side) two new regions, separated by a
sharp border line, were generated. In the region adjacent to the Niobium side of the border
line (left) black round spots were identified as alumina balls incorporated in particles of
nearly pure Niobium.
The contrast between the grey areas and pure niobium suggests a higher portion of heavier
weight elements than detected. In the region on the alumina side (right), niobium could be
detected in the interstices between the alumina crystals which had acquired the threefold of
the initial crystal size. For this area the notation “cermet” is appropriate, as it denotes a compound consisting of at least one ceramic component along with at least one metal component.
Figure 6: SEM view of the cross section of the welding zone between niobium (light) and
Al2O3 (dark)
Figure 7 displays the hardness of the joint which is considered a measure of its quality:
The greyscale of the background of the figure corresponds to different regions. Within the
pure niobium region (white) we determined a hardness of about Hv= 250 HV(0,1) corresponding to that of the initial material. Within the light grey region it arises to about Hv= 950. This
points to an embrittlement or a measurement on the fine distributed alumina areas.
The hardness becomes drastically higher within the middle grey region with a maximum of
Hv= 2332. At a distance of 3 mm from the seam the hardness approaches that of the initial
alumina.
vickers hardness, HV
.
2500
greyscales
Background
Elements
white
light grey
middle grey
dark grey
Nb
Nb, Al, O
Al, O, Nb
O, Al
2000
1500
1000
500
0
-4
-3
-2
-1
0
1
2
3
4
distance, mm
Figure 7: Hardness HV (0.1) along the transition from alumina to niobium across the joint
5 mm
Nevertheless, these first results encourage to further
investigations of laser beam welding of alumina to
metals. Therefore, to remove from the very expensive
metal niobium, our future work will be related to the
welding of alumina to steel. Our first results allow
optimism (figure 8); they will be discussed and
presented later.
Figure 8: Photo of a welding seam between alumina and
steel 1.3981 (perm alloy)
Summary
For the first time extensive investigations are reported on the bonding of alumina to itself and
to metals, by laser radiation, with our two-laser-beam method (Nd: YAG and a CO2- laser
beam).
The preheating of the material, necessary to achieve crack free joints, was performed by the
CO2- laser beam. This technology is very well suited for laser welding processes. Particularly
when compared with the alternative method of preheating in a furnace, the advantages are:
high processing speed
high flexibility, processing under conditions of normal atmosphere
temperature fields can be generated and varied very quickly
controlled temperature gradients instead of overall heating save energy
the assemblies can be clamped by conventional devices
direct observation of the process is possible.
On the other hand, in the case of alumina-alumina joints, material thickness is presently limited to about 2-4 mm because of its low thermal conductivity.
It could be established, that the wavelength of the Nd: YAG laser beam as well as the continuous mode are prerequisite for a homogeneous solidification structure of the welded seam.
The reasons are the absorption behaviour of alumina and the energy profile typical for a fibre
guided laser beam. The high quality of the joints between alumina is confirmed by a bending
strength of 85% of that of the base material.
The above results give rise to expectations that the properties of the joints at high temperatures and/or in a corrosive atmosphere are close to those of the base material.
Furthermore, the first experiments to join alumina with metals, turned out very promising, and
will be investigated and reported further.
The authors thank the Saxon Ministry of Science and Art SMWK (Contract No.: 4-7542.5/)
for the financial support of a national innovation scholarship.
References
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