Weldability of Tungsten and Its Alloys
Tungsten and its alloys can be successfully joined by gas tungsten-arc welding,
gas tungsten-arc braze welding, electron beam welding and by chemical vapor deposition
BY
N . C. C O L E ,
R. G. G I L L I L A N D
ABSTRACT. The weldability of tungsten
and a number of its alloys consolidated
by arc casting, powder metallurgy, or
chemical-vapor deposition (CVD) techniques was evaluated. Most of the materials used were nominally 0.060 in. thick
sheet. The joining processes employed
were (1) gas tungsten-arc welding, (2)
gas tungsten-arc braze welding, (3) electron beam welding and (4) joining by
CVD.
Tungsten was successfully welded by
all of these methods but the soundness
of the welds was greatly influenced by
the types of base and filler metals
(i.e. powder or arc-cast products). For
example, welds in arc-cast material were
comparatively free of porosity whereas
welds in powder metallurgy products
were usually porous, particularly along
the fusion line. For gas tungsten-arc
(GTA) welds in V ] 0 in. unalloyed tungsten sheet, a minimum preheat of 150°
C (which was found to be the ductileto-brittle transition temperature of the
base metal) produced welds free of
cracks. As base metals, tungsten-rhenium
alloys were weldable without preheat,
but porosity was also a problem with
tungsten alloy powder products. Preheating appeared not to affect weld porosity which was primarily a function
of the type of base metal.
The ductile-to-brittle transition temperatures (DBTT) for gas tungsten-arc
welds in different types of powder metallurgy tungsten were 325 to 475° C, as
compared to 150° C for the base metal
and that of 425° C for electron beamwelded arc-cast tungsten.
Braze welding of tungsten with dissimilar filler metals apparently did not
produce better joint properties than did
other joining methods. We used Nb, Ta,
W-26% Re, Mo and Re as filler metals
in the braze welds. The Nb and Mo
caused severe cracking.
Joining by CVD at 510 to 560° C
N. C. COLE and G. M. SLAUGHTER are
with Metals and Ceramics Div., Oak Ridge
National Laboratory, Oak Ridge, Tenn.:
R. G. GILLILAND is with the University
of Wisconsin. Milwaukee.
Paper presented at the American Welding
Society 49th Annual Meeting, Chicago,
April 1-5. 1968.
A N D G. M .
SLAUGHTER
eliminated all but a small amount of
porosity and also eliminated the problems associated with the high temperatures necessary for welding (such as
large grains in the weld and heat-affected zones).
Introduction
Tungsten and tungsten-base alloys
are being considered for a number of
advanced nuclear and space applications including thermionic conversion
devices, reentry vehicles, high temperature fuel elements and other reactor
components. Advantages of these materials are their combinations of very
high melting temperatures,
good
strengths at elevated temperatures,
high thermal and electrical conductivities and adequate resistance to corrosion in certain environments. Since
brittleness limits their fabricability,
the usefulness of these materials in
structural components under rigorous
service conditions depends greatly
upon the development of welding
procedures to provide joints that are
comparable in properties to the base
metal. Therefore, the objectives of
these studies were to (1) determine
the mechanical properties of joints
produced by different joining methods
in several types of unalloyed and alloyed tungsten; (2) evaluate the
effects of various modifications in heat
treatments and joining technique; and
(3) demonstrate the feasibility of
fabricating test components suitable
for specific applications.
Materials
Unalloyed tungsten in 1 / 3 6 in.
thick sheets was the material of most
interest. The unalloyed tungsten in this
study was produced by powder metallurgy, arc casting and chemical-vapor
deposition techniques. Table 1 shows
the impurity levels of the powder metallurgy, CVD and arc-cast tungsten
products as received. Most fall within
the ranges nominally found in tungsten
WELDING
but it should be noted that the CVD
material contained more than the normal amounts of fluorine.
Various sizes and shapes of tungsten and tungsten alloys were joined
for comparison. Most of them were
powder metallurgy products although
some arc-cast materials were also
welded. Specific configurations were
used to determine the feasibility of
building structures and components.
All materials were received in a fully
cold worked condition with the exception of the CVD tungsten, which was
received as-deposited. Because of the
increased brittleness of recrystallized
and large-grained tungsten the material was welded in the worked condition
to minimize grain growth in the heataffected zone. B e c a u s e o f t h e
high cost of the material and the
relatively small amounts available, we
designed test specimens that used the
minimum amount of material consistent with obtaining the desired information.
Procedure
Since the ductile-to-brittle transition
temperature (DBTT) of tungsten is
above room temperature, special care
must be used in handling and machining to avoid cracking 1 . Shearing
causes edge cracking and we have
found that grinding and electrodischarge machining leave heat checks
on the surface. Unless they are removed by lapping, these cracks may
propagate during welding and subsequent use.
Tungsten, like all refractory metals,
must be welded in a very pure atmosphere of either inert gas (gas tungsten-arc process) or vacuum (electron
beam process) 2 to avoid contamination of the weld by interstitials. Since
tungsten has the highest melting point
of all metals (3410° C ) , welding
equipment must be capable of withstanding the high service temperatures.
RESEARCH
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["CARTRIDGE HEATER
.-THERMOCOUPLE
CARTRIDGE HEATER
Fig. 1—Automatic welding apparatus. A (left)—apparatus incontrolled atmosphere chamber (arrow points to preheating
fixture; B (right)—schematic of preheating fixture
Results for Unalloyed Tungsten
General Weldability
Gas Tungsten-A rc Welding — In
gas tungsten-arc welding of V 1 6 in.
thick unalloyed sheet, the work must
be substantially preheated to prevent
brittle failure under stress induced by
thermal shock. Figure 2 shows a typical fracture produced by welding without proper preheating. The large grain
size and shape of the weld and heataffected zone are evident in the fracture. Investigation of preheating temperatures from room temperature to
540° C showed that preheating to a
minimum of 150° C was necessary for
consistent production of one-pass butt
welds that were free of cracks. This
temperature corresponds to the DBTT
of the base metal. Preheating to higher temperatures did not appear to be
necessary in these tests but material
with a higher DBTT, or configurations
that involve more severe stress concentrations or more massive parts,
may require preheating to higher temperatures.
The quality of a weldment depends
greatly upon the procedures used in
fabricating the base metals. Autogenous welds in arc-cast tungsten are
essentially free from porosity, Fig.
3A, but welds in powder metallurgy
tungsten are characterized by gross
porosity, Fig. 3 ( b ) , particularly along
the fusion line. The amount of this
porosity, Fig. 3B, particularly along
3C, in welds made in a proprietary,
low porosity product (GE-15 produced by General Electric Co., Cleveland).
Gas tungsten-arc welds in CVD
tungsten have unusual heat-affected
zones due to the grain structure of the
base metal 7 . Figure 4 shows the face
and corresponding cross section of
such a gas tungsten-arc butt weld.
Note that the fine grains at the substrate surface have grown due to the
heat of welding. Also evident is the
lack of growth of the large columnar
grains. The columnar grains have gas
bubbles at grain boundaries caused by
fluorine impurities 8 . Consequently, if
the fine grain substrate surface is removed before welding, the weldment
does not contain a metallographically
detectable heat-affected
zone. Of
course, in worked CVD material (such
as extruded or drawn tubing) the
heat-affected zone of the weld has
the normal recrystallized grain structure.
Cracks were found in the columnar
grain boundaries in the HAZ of several welds in CVD tungsten. This cracking, shown in Fig. 5, was caused by
rapid formation and growth of bubbles in the grain boundaries at hightemperatures". At the high temperatures involved in welding, the bubbles
were able to consume much of the
grain boundary area; this, combined
with the stress produced during cooling, pulled the grain boundaries apart
to form a crack. A study of bubble
formation in tungsten and other metal
deposits during heat treatment shows
that bubbles occur in metals deposited
below 0.3 T m (the homologous melting temperature). This observation
suggests that gas bubbles form by
coalescence of entrapped vacancies
and gases during annealing. In the
case of CVD tungsten, the gas is
probably fluorine or a fluoride compound 10 .
Electron Beam Welding — Unalloyed tungsten was electron beam
welded with and without preheating.
The need for preheat varied with the
thickness, length and shape of the
specimen. To ensure a weld free of
cracks, preheating at least to the
DBTT of the base metal is recommended. Electron beam welds in powder metallurgy products also have the
weld porosity mentioned previously.
Gas Tungsten-Arc Braze Welding—
In an effort to establish whether braze
welding could be used to advantage,
we experimented with the gas tungstenarc process for making braze welds
on powder metallurgy tungsten sheet.
The braze welds were made by preplacing the filler metal along the butt
joint before welding. Braze welds were
produced with unalloyed Nb, Ta, Mo,
Re, and W—26% Re as filler metals.
As expected, there was porosity at the
fusion line in metallographic sections
of all joints (Fig. 6) since the base
metals were powder metallurgy products. Welds made with niobium and
molybdenum filler metals cracked.
The hardnesses of welds and braze
welds were compared by means of a
study of bead-on-plate welds made
with unalloyed tungsten and W—26%
Re as filler metals. The gas tungstenarc welds and braze welds were made
manually on unalloyed tungsten powder metallurgy products (the low
porosity, proprietary (GE-15) grade
and a typical commercial grade).
Welds and braze welds in each material were aged at 900, 1200, 1600
and 2000° C for 1, 10, 100 and 1000
hr. The specimens were examined
metallographically, and hardness traverses were taken across the weld, heataffected zone, and base metal both
as-welded and after heat treatment.
Table 2—Typical Analyses of Interstitials in Una lloyed Tungsten After Welding
Interstitial content, ppm
Base m e t a l —
Weld metal
N2
0,
N2
O,
C
F
Type of tungsten
F
Typical powder metallurgy
Proprietary powder metallurgy, low porosity
Chemically vapor deposited
Arc cast
14.7
4
56
<20
2.9
5
41
<20
7.9
9.3
1
1
1
1
120
72
10
<20
<20
14
<2
10.5-23
1
1
1
1
140
52
7
<20
<20
14
C
W E L D I N G RESEARCH S U P P L E M E N T | 421-s
..- 7\77 -
;:,.„,... . 1 , - . - ' , . '
::*;.. .
• _ ,
WmmMtZjwms,''.
Fig. 2—Gas tungsten-arc weld in unalloyed tungsten. A (top)—fracture which occurred while welding without a preheat;
B (bottom)—end view of the fracture. Note the contrasting intergranular failure of weld and heat-affected zone versus the
cleavage type failure of the base metal
Since the materials used in this study
were powder metallurgy products,
varying amounts of porosity were
present in the weld and braze weld
deposits. Again, the joints made with
typical powder metallurgy tungsten
base metal had more porosity than
those made with the low porosity,
proprietary tungsten. The braze welds
made with W—26% Re filler metal
had less porosity than the welds made
with the unalloyed tungsten filler metal.
No effect of time or temperature
was discerned on the hardness of the
welds made with unalloyed tungsten
as filler metal. As welded, the hardness measurements of the weld and
base metals were essentially constant
and did not change after aging.
However, the braze welds made with
W—26% Re filler metal were considerably harder as produced than the
base metal (Fig. 7 ) . Probably the
higher hardness of the W-Re braze
weld deposit was due to solid solution
hardening and/or the presence of ophase finely distributed in the solidified s t r u c t u r e . The tungstenrhenium phase diagram 11 shows that
localized areas of high rhenium content could occur during rapid cooling
and result in the formation of the
hard, brittle o- phase in the highly
segregated substructure. Possibly the ophase was finely dispersed in the
grains or grain boundaries, although
none was large enough to be identified
by either metallographic examination
or X-ray diffraction.
Hardness is plotted as a function of
distance from the braze-weld center
line for different aging temperatures
in Fig. 7A. Note the abrupt change
422-s I S E P T E M B E R
1971
in hardness at the fusion line. With
increasing aging temperature, the
hardness of the braze weld decreased
until, after 100 hr at 1600° C, the
hardness was the same as that of the
unalloyed tungsten base metal. This
trend of decreasing hardness with increasing temperature held true for all
aging times. Increasing time at a constant temperature also caused a similar decrease in hardness, as shown for
an aging temperature of 1200° C in
Fig. 7B.
Joining by Chemical Vapor Deposition—Joining of tungsten by CVD
techniques was investigated as a method for producing welds in various
specimen designs. By use of appropriate fixtures and masks to limit deposition to the desired areas, CVD and
powder metallurgy tungsten sheets
were joined and end closures on tubing were produced. Deposition into a
bevel with an included angle of about
90 deg produced cracking, Fig. 8A,
at the intersections of columnar grains
growing from one face of the bevel
and the substrate (which was etched
away). However, high integrity joints
without cracking or gross buildup of
impurities were obtained, Fig. 8B,
when the joint configuration was
changed by grinding the face of the
base metal to a radius of V 2 in.
tangent to the root of the weld.
Fig. 3—Centogenous gas tungsten-arc
welds in tungsten. A (top)—arc-cast
tungsten, B (middle)—typical powder
metallurgy tungsten, C (bottom)—proprietary, low porosity powder metallurgy
tungsten. Etchant: 25% H20», 25%
NH..OH, 50% H.0 (reduced 50% in reproduction.
To demonstrate a typical application of this process in fabrication of
fuel elements, a few end closures were
made in tungsten tubes. These joints
were leak-tight when tested with a
helium mass spectrometer leak detector.
: • ' • •
-:•:-;..7:
... -A
~,
Fig. 4—Gas tungsten-arc butt weld in chemically vapor deposited tungsten; top view and cross section. Etchant: H20a
NHXIH, H20
V
J
Fig. 5—Cracking and bubble formation in heat-affected zone of weld in chemically
vapor-deposited tungsten. Same general area is shown at increasing magnifications.
Etchant: H202, NH,OH, H20
Mechanical Properties
Bend Tests of Fusion
Welds—
Ductile-to-brittle
transition
curves
were determined for various joints in
unalloyed tungsten. The curves in Fig.
9 shows that the DBTT of two powder
metallurgy base metals was about
150° C. Typically, the DBTT (the
lowest temperature at which a 90 to
105 deg bend could be made) of both
materials increased greatly after welding. The transition temperatures increased about 175° C to a value of
325° C for typical powder metallurgy
tungsten and increased about 235° C
to a value of 385° C for the low
porosity, proprietary material. The
difference in the DBTTs of welded
and unwelded material was attributed
to the large grain size and possible
redistribution of impurities of the
welds and heat-affected zones. The
test results show that the DBTT of
typical powder metallurgy tungsten
welds was lower than that of the proprietary material, even though the latter had less porosity. The higher DBTT
of the weld in the low porosity tungsten may have been due to its slightly
larger grain size, Fig. 3A and 3C.
The results of investigations to determine D B T T s for a number of
joints in unalloyed tungsten are summarized in Table 3. The bend tests
were quite sensitive to changes in
testing procedure. Root bends appeared to be more ductile than face
bends. A properly selected stress relief
after welding appeared to lower the
DBTT substantially. The CVD tung-
Fig. 6—Braze welds on powder metallurgy tungsten sheet. The various fille
metals used are columbium, tantalum, W—26% Re, molybdenum, and rhe
nium. As-polished
WELDING RESEARCH SUPPLEMENT
| 423-s
TYPICAL POWDER METALLURGY
o BASE METAL
-4 WELDS
PROPRIETARY LOW POROSITY POWDER METALLURGY
• BASE METAL
* WELDS
r n'
80
BASE METAL
60
•
/
i
20
200
WFI
ns
300
400
TEMPERATURE (°C1
'. TEMPERATURE- 2 0 0 * C
F i g . 9—Ductile-to-brittle t r a n s i t i o n
curves for powder metallurgy tungsten
base metal and welds
^-AS W LMo
""^r^fe0"
7
1
J JZ
1
.
rn
E&
/
wm
!
— — WELO MET
DISTANCE FROM WELD CENTE!
Fig. 7—Effect of aging on hardness of
powder metallurgy tungsten welded
with W—26<% Re filler metal. A (top)—
effect of varying temperature at constant 100-hr aging time. B (bottom)—
effect of varying time at constant 1200°
C aging temperature. Diamond pyramid
hardness tested at 1-kg load
sten had, as welded, the highest DBTT
(560° C ) ; yet when it was given a
1 hr stress relief of 1000° C after
welding, its DBTT dropped to 350°
C. Stress relief of arc welded powder
metallurgy tungsten for 1 hr at 1800°
C reduced the DBTT of this material
by about 100° C from the value determined for it as-welded. A stress
relief of 1 hr at 1000° C on a joint
made by CVD methods produced the
lowest DBTT (200° C ) . It should be
noted that, while this transition temperature was considerably
lower
than any other transition temperature
determined in this study, the improvement was probably influenced by the
lower strain rate (0.1 vs 0.5 ipm)
used in tests on CVD joints.
Fig. 8—Joints made by chemical vapor
deposition process between powder
metallurgy tungsten sheets. A (top)—
crack in joint at intersections of columnar grains. Bevel was 90 deg included
angle. B (bottom)—joint free of cracks.
Bevel was ground to a radius of y2 in.
tangent to the root of the weld. Etchant:
H202, NH,OH, H20 (reduced 57% in reproduction)
Bend Tests of Braze Welds—Gas
tungsten-arc braze welds made with
Nb, Ta, Mo, Re, and W—26% Re as
filler metals were also bend tested and
the results are summarized in Table 4.
The most ductility (a 90 deg bend
angle at 525° C) was obtained with a
rhenium braze weld. (This amount of
bending is probably borderline since
the specimen was cracked when removed from the testing rig.)
Although the results of this cursory
study indicate that a dissimilar filler
metal may produce joints with
mechanical properties inferior to
those of homogeneous welds in tungsten, some of these filler metals may
be useful in practice.
Results for Tungsten Alloys
General Weldability
Gas
Tungsten-Arc
Welding—In
contrast to unalloyed tungsten, W—
26% Re tubing and sheet were autogenously welded without the need for
preheat. Powder metallurgy W—26%
Re, like the unalloyed powder metallurgy tungsten, also exhibited weld
porosity, Fig. 10A. The absence of
porosity in arc cast W—26% Re, Fig.
10B, again illustrates the influence
of the process history of the base
metal upon weldability.
Tungsten-rhenium-molybdenum material can also be welded without
preheating. However, a high temperature stress relief near the recrystallization temperature is needed before
welding. Without a sufficient stress
relief, severe centerline cracking may
be encountered at the centerline, as
shown in Fig. 11. A stress relief of
1300° C for 1 hr eliminated the problem. Powder products again exhibited
large amounts of porosity.
Electron Beam
Welding—Electron
beam welds in tungsten alloys are
illustrated in Fig. 12. End caps were
welded to several test capsules with a
defocused electron beam. Both the
cap and the capsule shown in Fig.
12A were made of powder metallurgy W—25% Re. Note the gross porosity at the fusion lines. The specimen
shown in Fig. 12B had an arc cast
cap and a powder metallurgy capsule.
No porosity was present near the fu-
Table 3—Ductile-to-Brittle Transition Temperatures of Joints in Una lloyed Tungsten
Joining technique
Gas t u n g s t e n - a r c
Electron-beam
CVD
a
b
T y p e of t u n g s t e n
Type bend
Surface
in t e n s i o n
Joint c o n d i t i o n
Powder m e t a l l u r g y
Longitudinal
Longitudinal
Transverse
Longitudinal
Face b
Root
Face
Face
Powder m e t a l l u r g y
(low porosity)
CVD
CVD
Arc cast
Powder m e t a l l u r g y
CVD
Longitudinal
Root
As w e l d e d
As w e l d e d
As w e l d e d
Stress r e l i e v e d , 1800°C
Stress r e l i e v e d , 2800°C
As w e l d e d
Longitudinal
Longitudinal
Longitudinal
Longitudinal
Longitudinal
Face
Face
Face
Face
Face
As w e l d e d
Stress r e l i e v e d , 1000°C
As w e l d e d
Stress r e l i e v e d , 1000°C
Stress r e l i e v e d , 1000°C
Approximate
DBTT" (°C)
450
325
475
<350
>500
385
560
350
425
235b
200"
Ductile-to-brittle transition temperature, defined as lowest temperature at which specimen bent fully (90 to 105 deg without cracking.
Bend test strain rate changed to 0.1 ipm (all others tested at 0.5 i pm).
424-s | S E P T E M B E R
1971
Table 4—Results of Longitudinal Bend Tests at 525° C of Gas Tungsten-Arc
Braze Welds in Powder Metallurgy Tungsten
Filler metal
Bend angle at
which first crack
occurred (deg)
Niobium
Tantalum
14
53
W-26% Re
Molybdenum
Rhenium
17
44
90
Visual observations of tested specimens
Fractured across entire width
Cracks in weld, heat-affected zone, and base
metal
Cracks in weld and heat-affected zone
Fractured across entire width
Very slight crack in weld
sion line with the arc cast cap. Figure
12(c) shows both an arc cast cap and
capsule of W — 5 % Mo, which contains no porosity at either interface.
This type of evidence is found frequently with both unalloyed tungsten
and tungsten alloys.
Component Fabrication
Another part of our program concerns determining the feasibility of
fabricating test components from
tungsten and tungsten alloys. A
demonstration assembly simulating a
corrosion loop was successfully tungsten arc, braze welded, Fig. 13A.
Nondestructive inspection revealed
the welds to be helium leak-tight and
crack-free. It was constructed from
0.275-in. OD by 0.035-in. wall CVD
tungsten tubing. It was welded manually in a chamber with a controlled
atmosphere of very pure argon. In an
effort to reduce the required heat
input, W—26% Re was chosen as the
filler metal.
In general, problems increase in
making manual welds as the temperature increases (glove deterioration,
welder inconvenience). Also, in building complex components, the lower
heat required for braze welding is a
definite advantage in decreasing weld
stresses and reducing the size of the
heat-affected zone. In the case of a
small thin tube welded to a large
component, a filler metal with a lower
melting point increases the ease and
assurance of making the joint without
melting through the thin tube.
Metallographic e x a m i n a t i o n of
these prototype welds revealed that
they had complete penetration, no
cracks and only a small amount of
fine porosity. Figure 13B shows the
difference in grain growth exhibited
by the two tungsten tubes. In the
bottom tube, the grains grew very
little as compared to the rapid grain
growth in the vertical tube. Since the
porosity shown was along the interface between the braze weld deposit
and the fine grained tungsten tube, a
higher concentration of impurities
may be present in the grain boundaries of the fine grained tube, which
may have slowed the grain growth.
Upon closer examination the ophase of the tungsten-rhenium system
was discovered near the edge of the
braze weld, Fig. 13C. It was positively identified by microprobe analysis and by the use of special etching
techniques (0.5 N NaOH solution preferentially attacks o- phase in tungstenrhenium). Sigma phase, since it is hard
and brittle, is an undesired microconstituent. Microsegregation during solidification of the weld metal probably
produced areas high enough in rhenium content to form the o- phase.
W-25% Re
Fig. 11—Effect of stress relief on welds
in W—25% Re—30% Mo. A (top)—stress
relieved 1 hr at 950° C. B (bottom)—
stress relieved 1 hr at 1300° C
Fig. 10—Autogenous gas tungsten-arc
welds in W—26% Re. A (top)—weld in
powder metallurgy base metal. B (bottom)—weld in arc-cast base metal.
Etchant: H202, NH.OH (reduced 62% in
reproduction)
Conclusions
Tungsten and many of its alloys can
be successfully joined by welding,
braze welding, and chemical vapor
deposition, provided certain techniques
are used. Special machining processes must be employed, the material
must be handled with care, and equipment must be capable of producing
and handling the extreme heat needed
for welding tungsten. For single-pass
welds in unalloyed tungsten, the
workpiece must be heated to at least
the DBTT of the base metal before
welding to avoid transverse cracking.
Neither W—26% Re nor W—25%
Re—30% Mo required this preheat
because the transition temperatures of
the base metals are below room temperature. However, a preheat may be
desirable for large or complex structures, which may require multipass
welds.
Possibly because of solid solution
hardening and the presence of ophase, W—26% Re weld metal is
W-25%Re
W-3%Mo
Fig. 12—Electron beam welds in tungsten alloy capsules. Porosity is located only
at the interface of weld metal and powder metallurgy product. A (left)—powder
metallurgy tungsten cap and tube, B (center)—arc-cast tungsten cap and powder
metallurgy tungsten tube, and C (right)—arc-cast tungsten cap and tube (reduced
25% in reproduction)
WELDING
RESEARCH
SUPPLEMENT
| 425-s
Fig. 13—Simulated corrosion loop braze welded with W—26% Re filler metal. A (left)—completed loop, B (middle)—
cross section of one of the braze-welds; C (right)—higher magnification (X100) of the root of the weld. Note the a
phase along the edge of the root. Etchant: 25% NH4OH, 25% H202, and 50% H»0.
harder than unalloyed tungsten (480
vs 400 dph). Aging for increasing
times and temperatures up to 1600° C
and 1000 hr decreases the hardness of
the W—26% Re weld metal to that of
unalloyed tungsten.
Powder products, whether unalloyed tungsten or tungsten alloys, have
porosity in the weld zone, particularly
along the fusion line. The amount of
porosity depends on the process history of the base metal as well as of its
composition.
As expected, the DBTT of all
grades of tungsten was greatly increased by welding. Stress relief before welding reduced the cracking susceptibility of the welds and heat treatments after welding appeared to improve the ductility of the welds. The
DBTT for unalloyed tungsten welds
ranged from 325 to 560° C, depending on the type of base metal and
testing conditions. Use of a dissimilar
filler metal (braze weld technique)
did not improve the properties of the
resulting joint but appeared to cause
further embrittlement.
Chemical vapor deposition is a
feasible and promising process for
joining tungsten. Of all the joints
studied, those made by CVD methods
followed by a stress relief of 1000° C
had the lowest DBTT (200° C ) .
However, more development must be
done before this process can be applied.
A cknowledgements
The authors gratefully thank J. D.
Hudson for preparing, welding and
testing specimens, G. E. Moore of the
Welding and Brazing Facility, Plant
and Equipment Division, for manually
welding the bead-on-plate specimens.
We thank R. L. Heestand (now at
BMI),
R. G. Donnelly, A. C.
Schaffhauser, R. E. McDonald, W. C.
Robinson (now with Union Carbide,
Greenville, S.C.) and J. I. Federer for
their invaluable assistance in material
procurement and consultation. The
work of the following groups of the
Metals and Ceramics Division is also
appreciated: the Mechanical Properties Group for testing the bend specimens, the Metallography Group for
preparing the photomicrographs and
metallographic samples, and the Reports Office for preparing the manuscript.
References
1. Barth, V. D.. Physical and Mechanical Properties of Tungsten-Base
Alloys,
DMIC-127, pp. 6-10 (March 1960).
2. Lessman, G. G, and Gold, R. E.,
"The Weldability of Tungsten Base Alloys," WELDING JOURNAL, 48(12), Research
Suppl., 528-s to 542-s (1969).
3. "Vapors Create Tungsten Joints,"
Iron Age 19(21). p. 76-77 (May 1963).
4. Heestand, R. L., Federer, J. I., and
Leitten, C. F., Jr., Preparation and Evaluation of Vapor Deposited
Tungsten,
ORNL-3662 (August 1964).
5. Schaffhauser, A. C., "Low-Temperature Ductility and Strength of Thermochemically Deposited Tungsten and Effects
of Heat Treatment," pp. 261-276, Summary
of the 11th Refractory Composites Working Group Meeting, AFML-TR-66-179 (July
1966).
6. Evaluation Test Methods for Refractory Metal Sheet Materials, Materials Advisory Board Refractory Metal Sheet Rolling Panel, MAB176-M (Sept. 6, 1961). Revised.
7. Lundin, C. D., and Farrell, K., "Distribution and Effects of Gas Porosity in
Welds in CVD Tungsten," WELDING JOURNAL,
49(10). Research Suppl., 461-s to 464-s
(1970).
8. Schaffhauser, A. C., and Heestand,
R. L., "Effect of Fluorine Impurities on
Grain Stability of Thermochemically Deposited Tungsten." pp. 204-211, 1966 IEEE
Conference Record of the Thermionic Conversion Specialist Conference, Nov. 3 and
If, 1966, Houston, Texas, Institute of Electrical and Electronics Engineers, New
York,
9. Farrell. K., Houston, J. T., and
Chumley, J. W., "Hot Cracking in Fusion
Welds
in
Tungsten,"
NEW WELDING RESEARCH COUNCIL BULLETINS
"High-Frequency Resistance Welding"
by D. C. Martin
WRC BULLETIN 161: "The Fabrication and Welding of High-Strength
Line-Pipe Steel"
by H. Thomasson
The price of either WRC Bulletin 160 or 161 is $1.50 per copy. Orders for
single copies should be sent to the American Welding Society, 345 East
47th St., New York, N.Y. 10017. Orders for bulk lots, 10 or more copies,
should be sent to the Welding Research Council, 345 East 47th St., New
York, N.Y. 10017.
WRC BULLETIN 160:
426-s I S E P T E M B E R
1971
WELDING
JOURNAL,
49(3), Research Suppl., 132-s to 137-s
(1970).
10. Farrell, K., Federer, J. I., Schaffhauser, A. C, and Robinson, W. C., Jr.,
"Gas Bubble Formation in Metal Deposits," pp. 263-267, Chemical Vapor Deposition 2nd Intern. Conf., ed. by J. M. Blocher, Jr., and J. C. Withers, The Electrochemical Society, New York, 1970.
11. English, J. J., Binary and Ternary
Phase Diagrams of Columbium, Molybdenum, Tantalum and Tungsten, DMIC-152,
p. 92 (April 28, 1961).