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 A N D G. M. S L A U G H T E R

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 ductile-to-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 beam-welded 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.

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

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 heat-affected zone. B e c a u s e of 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 cracking1. Shearing causes edge cracking and we have found that grinding and electrodis-charge 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.

W E L D I N G R E S E A R C H S U P P L E M E N T | 419-s

( 3 LT) CD O l

CD i—I CNJ LT)

o o UD CM

O O

CD CD

CM UD CD

CD

V

m C D

CD

m

CD

CD CD CD o

o S 3

o o

o

CD

i-H

t H i-t CD

CNJ CM CD o g

UD UD CD CD CM O O O , - H

• o 0)

> cp u 0)

l/>

< c 0)

• * ^ IA O l

c 3

H • o 0) > 1

o

< no

<

O

IX.

« z

N

o

^r o

CD CNJ

V

o ,—i

a-i

^T

o

LO

O OJ

V

, 1

r*-CM

CM ^ -

O d «fr CD

q ^ CM r H

m CM

C±,M r H

*~~* i—1

oo r- .

E u

j = u "S o '5. >>

OD

a> a >, r-

& y 2? ai 2 E

5 E

t! 8

is .y B | r - C 3 0.

o m h o

j= ° O <

12 <u *H 33 -3: d ^

M °s o c co ' 3 00 3 CA 4 3

D , CD 05 E

>, c 2 iS 3 (_ rt

. 3 O •— -CO oo o a, P. s -a c 4> a. d 5, „,

Q m 3 <u —

M « g „ <« S & c o o

-.23 _g o c

3 ^ 3 _, 2 2

. S t £ & S J

CA c3 ' 3

i to §

TJ *« -ri c « u oi 3 i l

4 3 O

"3 £*£ . S o ? TD .33 3J 3 s o

.3 3 43 oo 3 <U _ -o -a 2

o - £ CD o > l-l

Srg

o -a

3 o 3 •2 S"° a c « 00 ^ 2

-" "3 0 )

ed <D 3

a

I § c 3 u Q. in

•->

5 CA

— C

0 0 S > 3 3 _ C » o

" " « § 1 ) 4 3 C

•s " »-O °

u t l o 1) "_>

3 E C 43

ti £ -c H C3 a " 4 3

c t s O U <D

• - t > r 43 73

£ 2 ~ « H

c > c U , . 0 ) c

g x E.2

w o o o c o — > -n •3 g 3 C 2 0

"a 2 £ s ««

p o c ""

"3 S.S1

P 3 «->

<U S 3

" S E

o g o

0 0

'g -S _g eg H

2 « ° £ 2

0> —

53 2 C/3 t A

o „

P o z t t ,*J J8 1) 8 g 15 ta

CC * - »

"3 S I - « CO 43 O t"

« ^ b oo

43 " J3

> w T J CD

< " •3

JS 3

•c -^ <1> —

S'S

ca _ f rt

3 > ' E o S

^ E

Si •a S

2 o n - a O 1> " C "30 td

CD "3

-* CD w 43 p 2 O E •£ o

u -^ rt 4 3 P . tfl P . Cu a a

o 0 0 CD C O

^ 5 td •a c c c H 3

4 3 C H

C3 c>

CD

a- c CO . 3

CD E

0 rt

B 4= c

- T 3 • • _ CD CD 4 0 CD t - cfl

<u j3 - a

O CD

" O S ^ H 3

M 3 ^ , C P

•g.8 -s < g

3 CD 03 C CD CD CD

3 ca

"3 _ rt O 2 c >-o £ a u 3 IS p " . H o c 3 2 3 o o 2 ? a-w-i o

cu t -

3 cd u CD

a. £ U

05

O

O

•5

c

0

60 r3

1

43

'5 >, ~~ 3

c*- l

p 0

4 2

p U

Ej 0

CA

CD

4 3

y 2

_, cd

u l _

a> 3: V. •J-.

CD

CD 4 3

4 3 M 3 O

4 3

< ob c 3 0

<a 0

"3 0

CD 4 3 ~ J

M-l

c c g rt M O

O O u-.

•3 1- "

' r t

- 3

2 0 3 — p

c CD

p 3

« u 3 . IS o > >*

• 3 < j CD O X C tD

rt CA P

CD rt

' 3 C a 3 P i CD

O j o

d CD

S*

o

0 0 " "

c 3 CD

c

43 43 O t - o ' J

•3

P o o

rt 0 0

CA CA

rt cs V- c w

CD 5 " J 43 -r-

3 3 CA CA

o O 0 0

C

O C 3 "3

CA O CD O

O B " 4 3 O O 1

, t i S 0 3 3

^ 2 4 = *~^ 2 2P

LA 3 C " O •. CD O i - • tS 3. £ c "9 o 3 3 x i £ a 33

C A CD

R fa 0 0 ^

s °

c l E ' ^

0 e ft g

rt " 3

e- 33 0 2 CD t p

P Q a > o

ob 3 . 3 43 43 —

0 0 c rt CD

• 2 S ^ CA CA " O CD

w

"3 > > " C 43 2

«> a o

00 _ _

O rt - & >-.

3

rt 3

CD > 00 'S

x 5 0 H u CD

^ 43 CD 2 O

E .E °

' ^ CD

C T3

•5 3 S3 ta

X o o , ^ 1 r » . * H

4 3 "

0 eo

c 43

b -a > 3 CD CD Pi N CO T3

43 2 3 -

•£ & -0 3 c o rt CD

> a O 4 *

E S S 5

2 CD Ml P CD

X 3 « £ <3 rt _ '5

u 43 g

rt c j

E 42 o a.

- 3 c i

H ^ c. . CD 2

o -a a > rt tR 0 p

43 C CD

* " Oi

•«? £ £ &-- = 4 3 3 .

. 3 CA L. ZS ?3 U * > -a >* o o _

O o o £ ^ E 4 ^ l i 5 1 ^ E ^ 3 43 B ~ to 12 43 o C O " — 1 c rS <- 33

CD CD

3 E ra CD

CQ

4 = CD _

rt E f

<2 T X

O , CM ' O

> 3

3 I i n to

C 4 3 O 0 0

o a "3 CA O CD CA Q . > O CD CD O . T 3 3 T

•a c

c o u

CD P i

o 3 ' w CA

2 CD . £ 43

• - 53 c CA D , c i to ,

CA a 1 3 33 >, CO ' — 1 0 0

i t "^ E S

H C 1_ H CD

CD

5 CD

CD C u CD

s & >, * « «

^3 43 C u m 0 3 3 b

c 2 rt 4 3

rt

CO _ , X O CD J 3

0 0 3

'.a 2 143 43 • & "

a -a L- CD

CD CD CA C

S CD E

0 0 T3 C CD 3j 2 3 g CD 5

4 3 1 CA

u rt C '5 43 ?- O CD 4 3

3 . 5 S -a 8 3 *'& o 2

0 3 — o - " • "

.3 o 0 0 p

> , rt .3 •3 ^ -S, 4 S t o t -

C0 CD O , + - • * - » C4H

P , • rt T 3 c

• 3 3 O

C

CD . 2 43 .13

O H t o CD w o C

a O «J £ 3 . 2 33 •K 14 CQ T3 . , . 2 C . 3

E a 2 O rt 331 CD & . 4 3

10 S3 43 g

CA

0 0 CA c cu

3 p

1-1 S3 CD 4 3

c w CD

D . T 3 t .

•S O ^ c w

T 3 CD CA

.a 3 ' p rt

o pq

" E

5 a 2 3 F . "ra

£ § o *s 3 o 43 N

75 « O

•6 ^ 3 3

73 "3 O 2

• 2 - - 3 N d w ro M r- —1 H

• s a. id

ca id "* o E 4 3 4 ? 3 3

3 43 0 H 1 E 2 S 3 43

D.-3 CD

/ 0 0

CD

O

u tfi rt

4 I -,3 es .5 a a a c - 4 3 ^

1 1 c

K. E .3! > , CA

J 3 I I O u P. 10 Fs 2

T3 <K "3 cu O

«> » & p 00 to

«3 £ > CA -o 2

• 3 CD H

,9 ^ 3 . 2 , i 5 ^

" r t <J 3 C CD S CD " w „ c i ; 0 0 0 0 * "

§ .£ cO H -a '>

o __ P H CO

2 to CA CD 0 0 H R 3 CD

• ° 43

c w o O -f

C .3 O -O

3 S 3 h

H 4>

• 3 ^r W CD O

CD O S 3 u 43 ^ ~i-

o 2 x £ P. 43 43 ' ^

3 I +

^ T 4^ I

a:

toe I

; | 3 H

—'<3 6 "- p .2

CU

c- 7 Pi g

43

P</-, P 6 O '0 CO. ^

" " CA 3

cu a | c r ca CA

CD CD

CA CD . o

43 E ^ " O 3 00

r c t : « i B •5 " i 2 ^D -33

g is £ ° .s'r

•« 3 o 0

CM 2

°.l CD 3

2 > 3 £

I s tD to "3 js CA CD P 3 .ts

43 i"S £ co £

o &E rt

£ CD

Pi o

^ 2 C

-"3 2 * j o 3

•52 S3 CA & -4

— E o 6 0 CD — '

.£ CA-fj

.5 2 3 O 3 £

73

CU

3 u w

c

3 i

" ° ^ c <o

"rt - 3 43 3

5 E

CD — rt E

CA CA

<£ to

P 1-O CU C50 4 3 u P

3 P i <o " 4 =

T 3 U

E oi . 2 3

i« cd —

43 P U fe CD " *

CD

cu P3 U " T3 U rt J , 43 3 cu

« fl ^ s.a rt 'CA cu cu

4 3 T33

<0 6 Di 3 P i " O 3 CD ' - "

•^3 rt p Q M "a fi

CD CD

OOccr;

" 3 P . CO CD

* E

0 0 cd

,5 E

S ' _ 00 D.

HIS CD . 3 rt 3 ) O - 43 p ^ O O 7 j Ei. ft •"

>^ co 00 £ „ CD 2 CD ' 3 O cc B -o .2, 3

C 3 3

2 "d S 00 ±3

2 o

43 " _s

0 0 * J p 3 o

P. 5

43 X! J-„ * ° 2 o cu 2

c

CD

S CA

2 CD

C*

<

r.

£ _& c

c w cH 0

- 3

O

> C3

•a s rt

D

E rt

0JS

c

•C 2 .8 u

GO _ • —

2 9 E 2 > 43

N S

<S P E es •« se - |

U rt o — — fd ^ j

< 3 u f

t u •

cc 3

s * a 2 2 -3 4 3 cA

• - CD > 43

•3 43 CD U

CD 0 T 3 .a u P.T3

1' S Pi

CD Cp

• 3

03 10 — D.

° Jc l CU

S Si IU - H *- cu "

O cu D . 4 3

CU rt 43 CU ~ 43

c c n r ^

3 C

4 3

> 1 0 ^ H

s - ~ c _

2 o ® <o ta rt * J

C O

CD

< B IU Vi OS

IS ' 3

ta a. 0 0

c 2 P

i? 5 D.2 £»-*• - p

2 2 E

3 % CD O 1 - CO 3 6 0 u rt

rt

o

5 43 P i

, , CA

2 o

420-S i S E P T E M B E R 1 9 7 1

["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 heat-affected 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 metal7. 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 impurities8. 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 high-temperatures". 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 Tm (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 com­pound10.

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 tungsten-arc process for making braze welds on powder metallurgy tungsten sheet. The braze welds were made by pre-placing 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 tungsten-arc 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, heat-affected zone, and base metal both as-welded and after heat treatment.

Table 2—Typical Analyses of Interstitials

Type of tungsten

Typical powder metallurgy Proprietary powder metal­

lurgy, low porosity Chemically vapor deposited Arc cast

F

14.7

7.9 9.3 1

in Una lloyed Tungsten After Welding

Interstitial Weld metal

N2 O, C

4

1 1 1

56

120 72 10

<20

<20 <20

14

content,

F

2.9

<2 10.5-23 1

ppm Base

N2

5

1 1 1

meta l — 0 ,

41

140 52

7

C

<20

<20 <20

14

WELDING RESEARCH SUPPLEMENT | 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 o-phase finely distributed in the so­lidified s t r u c t u r e . The tungsten-rhenium phase diagram11 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 o-phase 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

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.

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.

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.

422-s I S E P T E M B E R 1 9 7 1

: • ' • •

-:•:-;..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 DBTTs 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

^ - A S W L M o

""^r fe0"

!

— — WELO MET

7

. '. TEMPERATURE-

wm

200* C

DISTANCE FROM WELD CENTE!

Fig. 7—Effect of aging on hardness of powder metallurgy tungsten welded with W—26<% Re fil ler 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

80

60

20

•

BASE METAL

1 i

J

r n ' rn / 1 WFI ns E&

/

JZ 2 0 0 3 0 0 4 0 0

TEMPERATURE (°C1

F ig . 9—Ductile-to-brittle t r a n s i t i o n curves for powder metallurgy tungsten base metal and welds

contrast to unalloyed tungsten, W— 26% Re tubing and sheet were autog-enously 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

Jo in ing techn ique

Gas tungsten-arc

Electron-beam CVD

Transition Temperatures of Joints in Una

Type of tungs ten

Powder meta l lurgy

Powder meta l lurgy ( low porosi ty)

CVD CVD Arc cast Powder meta l lu rgy CVD

Type bend

Longi tud ina l Longi tud ina l Transverse Longi tud ina l

Longi tud ina l

Longi tud ina l Longi tud ina l Longi tud ina l Longi tud ina l Long i tud ina l

lloyed Tungsten

Surface in tens ion

Faceb

Root Face Face

Root

Face Face Face Face Face

Joint cond i t ion

As welded As welded As welded Stress re l ieved, 1800°C Stress re l ieved, 2800°C As welded

As welded Stress re l ieved, 1000°C As welded Stress re l ieved, 1000°C Stress re l ieved, 1000°C

Approx imate DBTT" (°C)

450 325 475

<350 >500

385

560 350 425 235b

200"

a Ductile-to-brittle transit ion temperature, defined as lowest temperature at which specimen bent ful ly (90 to 105 deg without cracking. b 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

Niobium Tantalum

W-26% Re Molybdenum Rhenium

Bend angle at which first crack occurred (deg)

14 53

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 con­

cerns 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

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

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 o-phase 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 tungsten-rhenium). Sigma phase, since it is hard and brittle, is an undesired microcon-stituent. Microsegregation during sol­idification of the weld metal probably produced areas high enough in rheni­um content to form the o- phase.

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 o-phase, W—26% Re weld metal is

W-25% Re 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)

W E L D I N G R E S E A R C H S U P P L E M E N T | 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 Mechani­

cal 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 Thermo-chemically 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," 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. Blo-cher, 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).

NEW WELDING RESEARCH COUNCIL BULLETINS

WRC BULLETIN 160:

WRC BULLETIN 161:

"High-Frequency Resistance Welding" by D. C. Martin

"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.

426-s I S E P T E M B E R 1971