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Heat Transfer

in

Plasma-Arc W elding

In comparing plasma and gas tungsten welding

arcs significant differences are found in heat

transfer distribution and efficiency

B Y J . C . M E T C A L F E A N D M . B . C . Q U I G L E Y

s t rans fe r re d to the w o rk p iec e

A co m p a r i so n i s m a d e

kW g a s t u n g s t e n -a r c . B o t h

t e m p e r a t u r e ca n b e

s f e r b y co n ve c t i o n is a n i m

p r o ce ss , b y w h i ch 2 7 t o 3 1 %

17 to

1 9 %

o f the to ta l power

w e l d .

T h u s i n p l a sm a -a r c w e l d i n g , u n l i ke

t u n g s t e n - a r c w e l d i n g , c o n

t h e d o m i

e f f e c t s s o m e w h a t

It is shown tha t in a 10 kW

The hea t f l ow to the wo rkp iece in

tung s ten -a rc we ld ing has been

min ed p rev ious ly (Re f . 1) . Then i t

how n tha t in a 100 A, 16 V gas

s ten -a rc on ly 5% ( less than 100

d t o t h e w o r k p i e c e b y c o n

co n d u c t i o n a n d r a d i a t i o n .

Electricity Generating Board March-

Engineering Laboratories March-

Southampton Hants United

The ma jo r i t y (39%, 630 W) i s due to

the su r face e f fec ts a t the anod e .

The a rc used in p lasma a rc w e l d

i ng (PAW ) i s s ign i f i can t l y d i f fe ren t

f rom a gas tungs ten -a rc . I t i s c o n

s t r i c te d by a sma l l nozz le ( t yp ica l l y 3

m m d iam ) and has a mu ch h ighe r gas

ve loc i t y and tempe ra tu re as shown in

Tab le 1 . The h igh gas ve loc i t i es asso

c ia ted w i th p lasma a rc we ld ing have

t w o i m p o r t a n t co n se q u e n ce s .

T h e m o m e n t u m o f t h e g a s s t r e a m

causes a de f o rm a t io n in the we ld po o l

su r face and th is can be deve loped to

for m a 'key ho le ' in the we l d (F igs. 1

and 2 ) . In th i s mo de o f ope ra t ion a

h o l e is f o r m e d co m p l e t e l y t h r o u g h t h e

base me ta l . As the to rch moves a long

th e w e l d , the me ta l wh ich i s me l ted in

advance o f the keyho le reso l id i f i e s a t

the rea r to fo r m the we ld b ead . A l

t h o u g h " ke yh o l i n g " b e g a n a s a g a s

we ld ing tech n iqu e , i t i s se ld om used in

th is manne r today . Keyho les a re a lso

fo r me d in e lec t ron beam and lase r

we ld ing , a l tho ugh in these cases they

a re p ro duc ed ma in ly by the p re ssu re

o f the evapo ra t ing me ta l and ca l l fo r

p o w e r d e n s i t i e s a b o ve 1 0 G W / m

2

.

The p ropo r t ion o f the to ta l a rc

powe r (V I ) t ran s fe r red by conv ect io n

to the wo rkp iece i s l i ke ly to be much

h ighe r fo r the p lasma we ld ing a rc

than fo r the gas

t u n g s t e n -a r c .

In th i s pape r an exam ina t ion i s

made o f the ene rgy t rans fe r to the

w o r kp i e ce w i t h a p l a sm a w e l d i n g a r c .

T h e r e l a t i ve co n t r i b u t i o n s f r o m r a d i a

t i o n ,

convec t ion and e lec t ron e f fec ts

a t the anode w i l l be compared w i th

t h o se f o r t h e G T A W p r o ce ss . A ve r

aged va lues fo r a rc tem pe ra tu re and

emiss iv i t y have been deduced and

a w a i t e xp e r i m e n t a l ve r i f i ca t i o n .

An assess men t o f the power inpu t

p r o ce sse s w i t h i n a p l a sm a w e l d i n g

Fig. 1 Gas flow impinging on surface of

weld pool before keyhole is produced

I n

Fig. 2 Gas flow through keyhole

Table 1 Comparison of Plasma Arc and Gas Tungsten-Arc Welding

GTAW PAW

Gas velocity, m/s

Arc temperature, K

Power density, MW /m

2

8 0 - 150

8 0 0 0 - 1 5 0 0 0

1 0 - 1 0 0

3 0 0 - 2 0 0 0

1 0 0 0 0 - 2 0 0 0 0

10 0 - 10 0 0 0

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 9 9 - s

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CN

10

1

O

CM

E

E

Table 2 Dimensions and

Process Settings

2000 10000 20 000

Fig. 3 Radiation loss

a rc sho u ld he lp fu tu re inves t iga t ions

in to the e f fect o f var ia t ion o f the

we ld in g pa ra me te rs , such as gas

f low,

cu r ren t , e tc . on the dep th o f pene

t ra t ion o f the w e l d . A l t h o u g h p l a sm a

arc we ld ing i s poss ib le w i th a non -

t rans fe r red a rc , th i s mode (wh ich i s

usua l l y rese rved fo r wo rk ing n o n

conduct ing ma te r ia l s ) w i l l no t be c o n

s ide red he re .

P A W P r o p e r t ie s a n d P o w e r F l o w

T yp i ca l d i m e n s i o n s a n d p o ss i b l e

proc ess se t t ings are show n in Tab le 2 .

The va lu es in Ta b le 2 are bas ed on

e xp e r i m e n t a l e xp e r i e n ce a n d r e p

resen t reasonab le mean va lues . Fo r

the pu rpose o f these ca lcu la t ions i t i s

assumed tha t the a rc tempe ra tu re i s

un i fo rm. In p rac t i ce th is w i l l be fa i r l y

va l id across the core (Refs. 2 , 3 ) but

less so in outer reg ions.

Electron Effects at the Anode( P e l .)

Work Function

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asu rem en ts o r ana ly t i ca l s tud ies

t r i b u t i o n a c r o s s t h i s t y p e o f

ing a rc . In these c i rc um stan ces

oac h i s to f ind an ave rag e

(T

a

) ac ross the a rc and

here . I t is , o f co urs e ,

The tem pe r a tu re may be

u ce d f r o m t h e t o t a l e n t h a l p y f l o w

the a rc ( i.e . the ene rgy t ran spo r ted

m H

= VI - e lec trod e e f fects

- rad ia t ion losses

The losses a t the ca thode ( i .e . tha t

wh ich i s con duc ted aw ay and

The rad ia t ion losses ( rad ia l ) we re

1570 - 2000 = 6230 W

H = 27 .9 MJ/kg

p o n d s t o a

t e m

T

a

= 14200 K.

Radiation from the Arc The

P

ra

= 0.057

e A ( T

a

/ 1 0 0 0 )

4

M W

T

a

i s tem pe r a tu r e , A is the su r

t is the emiss iv i ty o f the

d e n

n of a r c t e m p e r a t u r e a n d

The ma in p rob lem in ca lcu la t ing

om th e a rc is the se lec t ion

l d i n g H a n d b o o k ( R e f . 2 ) s u g

tha t up to 20% and Emmons

% i s taken fo r the a rc con s ide red

2

) is 2

the emis siv i ty is

es ted tha t th i s is the most a pp r o

e but th is w i l l need

Radiation to the Weld A s s u m i n g

the sam e as tha t app ly ing

\.e. e = 0 .012 an d tha t the

V then the

1. To the we ld poo l ass um ing a

a rc

A = 113 m m

2

and tha t the m ean

pe r a tu re o f 14200 K s t i l l app l ie s

T a b l e 3

T e m p e r

a tu re ,

K

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

25000

P r o p e r t ie s o f A r g o n a t 1 A t m o s p h e r e ( f r o m R e f s .

En tha lpy ,

H ,

M J / k g

2.600

3.127

3.651

4.228

4 .965

6.124

8.184

11.83

17.78

26 .12

35.53

43 .83

49 .77

53.71

56 .59

59 .44

63 .12

68.61

76.44

86.65

98 .37

Spec i f i c

hea t ,

C

P

,

k j / k g

K

0.519

0.519

0.540

0.628

0.892

1.511

2.721

4.676

7.242

9.251

9.251

7.158

4.730

3.211

2.733

3.135

4.425

6.572

9.084

11 .218

12.098

Dens i ty ,

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