Heat Transfer and Penetration
Mechanisms w i t h G M A
and Plasma-GMA W e l d i n g
The heat content of transferring metal drops appears to determine
the total cross-sectional area of weld penetration,
while the
impact of the drops on the liquid metal weld pool
determines
the depth of
penetration
BY W. G. ESSERS A N D R. WALTER
ABSTRACT. Heat input to the workpiece during G M A and plasma-GMA
welding can be measured w i t h a simple calorimetric apparatus. A distinction can be made between the heat
supplied by the arc via convection,
radiation and conduction, the heat
generated in the cathodic region and
the heat contained in the transferring
metal drops. For weld penetration in
the workpiece, the heat content of the
transferring metal drops appears to
determine in large measure the total
cross-sectional area of the penetration,
while the impact of the drops on the
liquid weld pool determines the depth
of penetration.
An electromagnetic method is described, by means of which every drop
produced when welding with filler
metal wire can be made to move in a
predefined direction. As a result, the
drops strike the weld pool at the
points desired, i.e., the points at which
penetration is required.
Introduction
In gas-metal-arc (GMA) welding,
the heat w h i c h is transferred to the
workpiece (cathode) is determined by
a number of processes:
1. The phenomena which appear in
the cathode region, such as thermionic
emission and the interaction of positive ions w i t h the cathode surface.
2. The energy transferred from the
arc column by convection, radiation
and conduction.
3. The heat developed in the filler
metal. This heat is transferred to the
workpiece via the molten drops.
The individual part played by each
of the various heat transfer mechanisms has been partly reported in a
recent publication. 1 The present paper
extends the earlier work with a number of new observations. In addition, a
picture which has been obtained of
the penetration
mechanism
with
GMA welding is described. Finally, it is
shown how this knowledge can be
used to produce weld penetration of a
required shape.
Heat Transfer D u r i n g W e l d i n g
In order to measure the amount of
heat transferred to the workpiece by
the arc and the transferring drops, a
simple water-filled calorimeter was
used (Fig. 1). A metal strip which was
used for a bead-on-plate test was
placed in the calorimeter in such a way
that it was almost completely immersed in the water. Only the upper
surface was just above water level. The
traverse speed was chosen such that
no gas bubbles appeared in the water.
A rotating blade ensured that the temperature distribution in the water was
virtually constant.
Water temperature variations were
continuously recorded. The maximum
temperature of the water during all
tests was below 35°C (95°F). Although
Paper presented at the 61st AWS Annual
Meeting held in Los Angeles, California,
during April 13-18, 1980.
W. G. ESSERS is Senior Research Scientist
and R. WALTER is Research Assistant, Philips Research Laboratories, Eindhoven, The
Netherlands.
there was some heat loss from the
surface of the metal strip, the resulting
error appeared to be relatively small,
i.e., smaller than 5% of the heat input
to the workpiece.
The plasma-GMA process2 can be
used either w i t h of w i t h o u t filler metal. Therefore, it is possible with this
process to differentiate between heat
due to the arc and heat due to the
transferring weld metal. It was, for this
reason, used in addition to the usual
GMA process.
Figure 1 illustrates the t w o processes—GMA and plasma-GMA. Figure
1A shows GMA welding w i t h cold
shielding gas; Fig. 1B represents plasma-GMA welding, using an experimental welding torch. In the latter
case the filler metal is surrounded by
thermally-ionized gas, a plasma created by a second arc between a nonconsumable electrode and the workpiece. The plasma has a temperature of
more than 10,000 K:i and is, therefore,
an excellent conductor in which filler
metal melt-off takes place under control.
As both views of Fig. 1 show, the
workpiece consisting of 10 mm (0.39
in.) thick mild steel plate is placed in a
water-filled calorimeter. The dotted
line in Fig. 1 B illustrates a possibility by
which welding with a non-transferred
plasma can be performed. In this case
switch S2 is closed and switch SI is
opened. These arrangements allow the
heat input to the workpiece to be
measured in varying conditions.
The filler metal consisted of mild
steel w i t h a diameter of 1.2 mm (0.05
in.), the shielding gas was argon + 7%
C O , and the plasma orifice was 10 mm
W E L D I N G RESEARCH S U P P L E M E N T I 37-s
10
—i
1
a) o plasma
current
8 _ b ) » plasma
^nsa
| 6
1
1
1
1
1
r
without wire addition and JTO
through the workpiece,
without wire
addition
c ) a plasma-GMA welding
d) x GMA-welding
D
Q.
C
o
0)
100
I
200
300
400
total current in (A)
500
Fig. 2—Measured heat input to the workpiece as a function oi the total current
supplied to the system
Fig. 1—Schematic representations oi GMA
welding (A) and plasma-GMA welding (B)
above a calorimeter: 1—shielding gas;
2—filler metal; 3—workpiece; 4—water-filled
calorimeter; 5—plasma-arc
(0.39 in.) in diameter. The following
tests were carried out:
1. Using the arrangement of Fig. IB,
the heat input to the workpiece was
measured w i t h an arc of which the
current did not flow through the workpiece but was collected by the lower
nozzle. Switch S1 was opened and S2
was closed.. No filler metal wire was
added.
In other respects this non-transferred arc had the same shape as a
transferred arc but only touched the
workpiece. In this case heat transfer
takes place by convection, radiation
and conduction. The workpiece was
not subjected to cathodic heat. The
efficiency of this heat transport mechanism was 23%, defined by the expression:
Heat input
to workpiece
Efficiency
-X 100%
Total power
input to process
The value of the efficiency appears
to be more or less constant over the
whole range of currents used ( ± 3 % ) .
2. When the current through the arc
also passed through the workpiece, so
that this formed the cathode, significantly more heat was absorbed by the
workpiece. (Once again, no filler was
used.) The efficiency in this case was
found to be 54% and this remained
constant over the whole range of currents ( ± 3 % ) .
38-s I FEBRUARY 1981
3. In the next tests a filler metal
wire was fed into the plasma, the wire
being connected to a second power
source as is usual with plasma-GMA
welding. At higher currents even more
heat per ampere supplied to the process was transferred to the workpiece.
The efficiency was found to be 65%
over the whole range of currents
(±3%).
4. Finally, normal GMA welding was
used (Fig. I A ) . This gave the highest
heat input to the workpiece per
ampere supplied to the process. This
also determines the efficiency, w h i c h
was f o u n d to be 71% over the w h o l e
range of currents used ( ± 3 % ) .
Figure 2 shows plots of the measured heat input to the workpiece
against the total current supplied to
the system for test nos. 1 to 4 as
described above. There is a notable
difference between the heat absorbed
by the workpiece in GMA welding
(curve d) and in plasma-GMA welding
(curve c).
In the case of GMA welding more
heat is taken up by the workpiece per
ampere supplied. This can be explained by the fact that in both cases
there is only one cathode—namely, the
workpiece. W i t h G M A welding there
is also only one anode—the filler metal. W i t h plasma-GMA, however, there
are t w o anodes—the filler metal and
the non-consumable plasma anode. In
the latter case, part of the anode heat
is removed by the cooling water of the
non-consumable electrode.
If one compares curves a, b and d in
Fig. 2, one can draw a conclusion as
regards GMA welding. Although the
ratios between the amounts of power
transferred to the workpiece are not
exactly equal over the whole range of
currents, the amount of heat transferred to the workpiece by radiation,
convection and conduction amounts
to about 34% (±3%) of the total heat
input in the case of G M A welding. The
passage of current in the cathode area
delivers about 41% ( ± 3 % ) . Finally, the
metal drops account for about 25%
(±5%) of the total heat transfer to the
workpiece.
The part played by the drops is
further discussed later. The data
quoted above apply naturally only to
the conditions described.
Penetration
Workpiece
of the W e l d
in
the
Heat from the Arc
From the tests described above, it
appears that of all the heat transferred
to the workpiece during welding, the
heat from the arc accounts for the
largest part, about 75%. However, o t h er tests have shown that this power
does not contribute significantly to
weld penetration in the workpiece.
Experiments using a torch as shown in
Fig. 1B and no added filler metal, w i t h
only the plasma arc transferred to the
workpiece, have shown that radiation,
convection and conduction from the
arc together w i t h current passage at
the cathode can have only a very limited influence on the depth penetration.
A 10 mm (0.39 in.) diameter transferred plasma arc was used with an
argon gas flow of 8 liters/min (17 cfh)*
plasma gas and 18 liters argon + 2
*Liters/min
0.472 = cfh.
Table 1 - I n f l u e n c e of Heat Content of M o l t e n W e l d Metal Droplets on W e l d
Penetration 1 * 1
lw""
Depth of
penetration,
mm
M,
g/cmlc'>
HM,
J/cm,di
H„,.
J/g"'
J/cm,n
100
160
190
216
265
312
322
1.4
1.7
2.5
3.4
4.3
4.6
5.5
4.62
5.02
5.16
5.32
5.66
6.01
6.18
60.57
6581
6765
6975
7420
7879
8102
1773
1801
1821
1834
1857
1886
1898
6702
6808
6883
6931
7091
7129
7174
H„
100
200
current in filler wire (A)
300
Fig. 3—The ratio Hl/Hyl as a function ol the
current through the 1.2 mm diameter filler
metal (mild steel)
18
'150 A plasma current. 8 liter argon/min plasma gas. 18 liler argon + 2 liter CO,/min shielding gas, diameter plasma orifice
10 mm, wire extension 33 mm, 3.78 g filler metal per cm weld length, 1.2 mm diameter mild steel filler metal composition:
0.06-0.12% C; 1.3—1.7% Mn; 0.70-0.95% Si; 0.01% Al max.
""[„ is the current through the filler metal.
lrl
Total amount of molten metal per cm weld length.
""Heat necessary to melt M from room temperature at 1536°C (2795°F).
le
'Heat content of the droplets.
<rl
Heat content of the filler metal per cm weld length.
liters C 0 2 / m i n s h i e l d i n g gas. T h r e e
runs w e r e m a d e o v e r a 10 m m (0.39 in.)
t h i c k m i l d steel p l a t e w i t h 180, 240 a n d
300 A p l a s m a c u r r e n t . T h e traverse
s p e e d was 0.24 m / m i n (9 i p m ) . T h e
p e n e t r a t i o n d e p t h s w e r e 0 . 1 , 0.2 a n d
0.3 m m (0.0039, 0.0078, a n d 0.012 i n . ) ,
r e s p e c t i v e l y . T h e s e tests i l l u s t r a t e t h e
small e f f e c t o n t h e p e n e t r a t i o n d e p t h
of t h e c o m b i n a t i o n of r a d i a t i v e , c o n vective and c o n d u c t i v e transfer and
c u r r e n t passage.
O b v i o u s l y these small penetrations
were o b t a i n e d o n l y w i t h the fairly low
arc-current
densities
mentioned.
Higher current densities (smaller d i ameter of t h e plasma orifice) w o u l d
produce increased penetration. H o w ever, c u r r e n t d e n s i t i e s o f t h i s o r d e r o f
m a g n i t u d e w i l l c o n t i n u e t o b e disc u s s e d t h r o u g h o u t this p a p e r . O t h e r
e x p e r i m e n t s h a v e also s h o w n t h a t w i t h
G M A and p l a s m a - G M A w e l d i n g the
arc heat i n f l u e n c e s t h e d e p t h ot p e n e tration only to a limited degree.'
T h e heat s u p p l i e d f r o m t h i s s o u r c e
has a great i n f l u e n c e o n t h e w i d t h o f
the w e l d and o n the contact angle
b e t w e e n t h e w e l d b e a d a n d t h e surface o f t h e w o r k p i e c e . T h e g r e a t e r t h e
a m o u n t of this h e a t , t h e w i d e r t h e
w e l d bead and the better the w e t t i n g in of t h e w e l d m e t a l t o t h e w o r k piece.-
couple. The temperatures s h o w e d a
v a r i a t i o n o f a b o u t 7 0 ° C (126°F) f o r
w e l d i n g c u r r e n t s of 125 t o 235 A
t h r o u g h t h e 1.2 m m (0.047 in.) d i a m e ter m i l d steel f i l l e r m e t a l , u s i n g t h e
p l a s m a - G M A process. T h e v a l u e s o b t a i n e d w e r e 2100 a n d 2 1 7 0 ° C (3812 a n d
393~8°F), r e s p e c t i v e l y .
In o r d e r t o d e t e r m i n e t h e i n f l u e n c e
of t h e heat c o n t e n t o f t h e d r o p s o n
w e l d p e n e t r a t i o n in t h e w o r k p i e c e a
n u m b e r o f tests w e r e c a r r i e d o u t u s i n g
the p l a s m a - G M A process. T h e plasma
c u r r e n t w a s k e p t c o n s t a n t at 150 A.
T h e p l a s m a gas f l o w w a s 8 liters
a r g o n / m i n w i t h 18 liters a r g o n + 2
liters C 0 2 / m i n
s h i e l d i n g gas. T h e
d i a m e t e r o f t h e p l a s m a o r i f i c e was 10
m m (0.39 in.). T h e tests w e r e of t h e
Heat of the Filler Metal
It is k n o w n f r o m t h e l i t e r a t u r e t h a t
the metal drops transferring f r o m the
f i l l e r m e t a l t o t h e w o r k p i e c e are
s t r o n g l y o v e r h e a t e d . It c a n b e r e a s o n a b l y a s s u m e d t h a t this extra heat c o n tributes to the m e l t i n g of the w o r k piece. The values of d r o p t e m p e r a t u r e
f o u n d by A n d o et a/.' a n d J e l m o r i n i e t
a/ 3 are i n g o o d a g r e e m e n t w i t h e a c h
other. A n d o measured the heat c o n tent of the drops by means of a calorim e t e r a n d f r o m this c a l c u l a t e d t h e
temperature. Jelmorini measured the
temperature
of
the
falling
drops
directly by c a t c h i n g t h e m o n a t h e r m o -
Kinetic Energy
0
2
In a d d i t i o n t o h e a t , a n o t h e r f o r m of
energy can be transferred f r o m the
drops to the workpiece: their kinetic
e n e r g y . For e a c h c e n t i m e t e r o f w e l d
l e n g t h , this is g i v e n b y :
tn
ll
(i
CD
*o
>a
b e a d - o n - p l a t e t y p e u s i n g a 1.2 m m
(0.047 in.) d i a m e t e r m i l d steel f i l l e r
m e t a l a n d a 10 m m (0.39 in.) t h i c k m i l d
steel p l a t e . T h e traverse s p e e d w a s
chosen such that the a m o u n t of w e l d
m e t a l per c m w e l d l e n g t h w a s c o n stant.
The d e p t h of penetration and the
c r o s s - s e c t i o n a l area o f t h e w e l d w e r e
m e a s u r e d in each of t h e tests. F r o m
these v a l u e s t h e t o t a l a m o u n t of m o l t e n m e t a l per c m w e l d l e n g t h ( M ) w a s
c a l c u l a t e d . T h e heat c o n t e n t of t h e
drops f r o m the filler metal was d e r i v e d
from the following equation:6
H d r = 0.81 T d r + 92
(1)
w h e r e H d r = heat c o n t e n t o f t h e d r o p s
) / g : Tdr = t e m p e r a t u r e of t h e d r o p s ,
° C , as o b t a i n e d b y J e l m o r i n i et al.
T h e tests w e r e c a r r i e d o u t f o r a w i d e
range o f c u r r e n t s t h r o u g h t h e f i l l e r
m e t a l ( l „ ) . T h e results are g i v e n in
Table 1
W h e n H, is t h e heat c o n t e n t of t h e
f i l l e r m e t a l per c m w e l d l e n g t h a n d H j ,
is t h e h e a t necessary t o h e a t a n d m e l t
t h e mass M f r o m r o o m t e m p e r a t u r e t o
1536°C (2797°F), t h e n t h e r a t i o H , / H „
c a n b e d e t e r m i n e d . T h i s r a t i o is p l o t t e d against t h e c u r r e n t t h r o u g h t h e
f i l l e r m e t a l in Fig. 3. It c a n b e seen t h a t
H , / H M is c l o s e t o 1 o v e r t h e w h o l e
range of c u r r e n t s u s e d , v a r y i n g in f a c t
f r o m 0.89 at 322 A t o 1.11 at 100 A.
W i t h c u r r e n t s l o w e r t h a n 250 A , all
the energy required to melt the w e l d
area m e t a l is c o n t a i n e d in t h e o v e r h e a t e d d r o p s . A b o v e 250 A t h e h e a t
c o n t e n t of t h e d r o p is s l i g h t l y t o o
small.
1
E
f mv"
E
100
200
300
current in filler wire (A)
Fig. 4- -The drop mass (A), the drop frequency (B) and the drop velocity (C) as a function of the current through the filler metal
(mild steel). The polarity and diameter of
the filler metal are indicated, (v in illustration has same meaning as f)
2s
(2)
In w h i c h f is t h e d r o p f r e q u e n c y , m
t h e average mass of t h e d r o p s in k g , v
t h e average v e l o c i t y of t h e d r o p s o n
h i t t i n g t h e w e l d p o o l in m / s e c , a n d s is
t h e traverse speed in c m / s e c . T o
o b t a i n an idea o f t h e v a l u e o f t h i s f o r m
WELDING
R E S E A R C H S U P P L E M E N T I 39-s
of energy, the number, the velocity
and the mass of the transferring metal
drops must be determined. In the present case they were measured during
plasma-GMA welding on mild steel.
For this purpose the metal transfer
was filmed at a rate of 7000 frames per
second, and the film was examined on
a motion analyzer. From the results the
following data could be calculated:
the drop mass (m), the number of
drops reaching the weld pool per second (f) and the velocity (v) of the
drops immediately before impact. The
main variables were then the wire
diameter and the wire polarity, with a
constant traverse speed of 0.41 m /
min.
The plasma current was 150 A and
the wire extension 40 mm (1.57 in.).
For welding w i t h positive polarity on
the wire, 8 liters argon/min plasma gas
was used, and for negative polarity
welding 0.1 liters C O , / m i n was added.
In Fig. 4A the drop mass is plotted as
a function of the current in the filler
metal (l w ). It is obvious that, with 1.6
mm (0.063 in.) diameter filler metal,
larger drops detach than w i t h the same
current and 1.2 mm (0.047 in.) diameter filler metal. The difference between positive and negative polarities
of the filler metal is marked only
below 140 A in the 1.2 mm (0.047 in.)
diameter filler metal, when significantly larger drops are detached w i t h negative polarity welding. At currents
below 110 A drop detachment at the
negative pool becomes irregular, w i t h
very large drops.
Figure 4B shows the relationship
between the drop frequency and the
magnitude of the current, l w . In this
respect there is no difference between
positive and negative polarity when
1.2 mm (0.047 in.) diameter filler metal
is used. The diameter of the filler metal
does have a great influence, however,
at least with welding currents above
130 A. For the same value of l„, a 1.6
mm (0.063 in.) diameter delivers fewer
drops per second.
The velocity of the drops at the
moment of impact w i t h the weld pool
as a function of l„ is plotted in Fig. 4 C
This shows clearly that there is a difference between the three examples:
drops from a 1.2 mm (0.047 in.) diameter filler metal are faster with positive
polarity than with negative polarity,
but drops from the 1.6 mm (0.063 in.)
diameter filler metal are slower, at
least at high values of l„.. The last result
is to be expected, since the current
density, which determines the magnitude of the electromagnetic pinch
force, is greater in the 1.2 mm (0.047
in.) than in the 1.6 mm (0.063 in.)
diameter filler metal (for the same
value of current) while the restraining
40-sl FEBRUARY 1981
0
1000
2000
3000
4000
product of average momentum and drop
frequency . v xp (10"°kgm/sec I
Fig. 5—Depth of penetration of weld bead
as function of v x P: x —1.2 mm mild steel
filler metal, positive polarity; 0—1.2 mm
filler metal, negative polarity;o—1.6 mm
filler metal, positive polarity oi the metal, (v
in illustration has same meaning as i)
force through surface tension is smaller, being proportional to the diameter.
The lower velocity of the drops
detaching from the negative wire can
be explained from the fact that, at least
for part of the time, the point of
contact of the cathode was somewhat
above the detaching drop. As a result
the current did not always pass
through the liquid drop, so that the
pinch force must have been smaller
than w i t h positive polarity.
For the whole range of currents
used, the various diameters of filler
metal and different polarities the
kinetic energy transferred to the workpiece by these drops was less than
1J/cm weld length. Thus in comparison w i t h the heat content of the drops
(Table 1), their kinetic energy can be
neglected. In the f o l l o w i n g sections it
will be shown that the high speed of
the drops is important in another
way.
Momentum of Drops
High-speed cinematography shows
(hat the impact of each drop causes a
marked indentation in the weld pool,
particularly at high currents. At low
currents, i.e., 100 A through a 1.2 mm
(0.047 in.) diameter mild steel filler
metal, the pit fills immediately after
impact and there is an appreciable lag
before the following drop creates a
new pit. At higher currents, i.e., 170 A
and more through the 1.2 mm (0.047
in.) diameter filler metal, the pit no
longer fills before the subsequent
impact. The drops fall constantly into
the same pit so that the heat contained
in the overheated drops is transferred
very efficiently to the bottom of the
weld pool.
This causes the w e l l - k n o w n fingershaped penetration. For this reason we
have calculated the momentum of the
drops when they reach the weld
pool.
In Fig. 5 the depth of penetration of
the weld bead is plotted against the
product of the average m o m e n t u m (P)
of the drops and the drop frequency
(/). It appears that there is a relationship between the value of the "total
impact per second" and the depth of
penetration, irrespective of the history
of the drops, whether they have been
formed from 1.2 or 1.6 mm (0.047 or
0.063 in.) diameter filler metal, or
whether the wire has positive or negative polarity.
When we consider GMA welding
processes in which the transferring
metal follows more-scattered paths we
find that the penetration exhibits other
profiles. W i t h C O , welding, for example, the transfer of metal is much less
focused on one point; in this case the
drops strike the weld pool over a wide
area. As a result, there is no fingershaped penetration.
The same effect can be seen w i t h
plasma-GMA and GMA welding when
very high current densities and short
electrode (filler metal) extensions are
used. No true rotational transfer
occurs, but a small conical arc is created. High-speed cinematography has
shown that the very thin tip of the wire
tends to rotate. In consequence the
drops are less directed to one point
only, and finger-shaped penetration is
absent. This applies even more to
welds made with rotational transfer. In
that case the drops are deposited
along the circumference of a wide
circle having a diameter of 10 mm or
more. The total impact per second at
any point where the drops strike the
weld pool is so small that penetration
is very limited.
Magnetic Effects on the Direction
of M o v e m e n t of the Detaching
Drops
Fig. 6—Schematic representation of arc
deflection with a transverse magnetic field:
1-welding torch; 2—filler metal; 3—arc;
4—workpiece;
5—electromagnet
From
the considerations
given
above it appears that w i t h GMA and
plasma-GMA welding, the penetration
of the weld in the workpiece takes
place mainly at the point where the
drops fall into the weld pool. As a rule
that is also the point at which the arc
strikes the workpiece. However, as will
be seen from the f o l l o w i n g , this is not
0
o
v
,
0
r,
0.5*10 '
magnetic induction
10'
(T )
Fig. 8-Penetration
depth (A) and weld
width at P-1 mm (B) as a function of the
magnetic induction of the pulsed
alternating transverse field. The pulse frequency
was 85 Hz. The current through the 1.2 mm
diameter mild steel filler metal was 230 A
and the plasma current was 100 A. 1 T = 10*
Gs
Fig. 9-Cross-sections
of
bead-on-plate
tests: A—without
a transverse
magnetic
field; B—with a pulsed 85 Hz
altetnating
transverse magnetic field, of 0.4 x 10-' T;
C-with a sinusoidal alternating
transverse
magnetic field (20 Hz). Wire current-230 A,
plasma current-100
A, 1.2 mm
diameter
mild steel filler metal
Fig 7—A—motion analysis of two successive
drops detaching irom a 1.2 mm diameter
filler metal (mild steel), wire current = 200
A, plasma current = 100 A; the alternating
field was pulsed with a frequency of 31 Hz.
B—drops deposited
on a rapidly
moved
plate (1.5 m/min), when oscillating the wire
tip. C— cross-section
of a bead on plate
when the traverse speed is 0.23
m/min;
otherwise all the parameters were the same
as with A and B
a l w a y s necessarily t r u e .
T h e p o i n t of i m p a c t o f t h e d r o p s o n
t h e w o r k p i e c e c a n be i n f l u e n c e d in a
n u m b e r o f w a y s . O n e w h i c h is n o r m a l
w i t h arc w e l d i n g is t o m o v e t h e t o r c h
in t h e r e q u i r e d d i r e c t i o n . If t h i s is d o n e
at a g i v e n f r e q u e n c y a n d s p e e d t h e
result is t y p e s o f w e t t i n g - i n w h i c h
cannot otherwise be o b t a i n e d .
A n o t h e r m e t h o d is t o d e f l e c t t h e arc
by m e a n s o f a transverse m a g n e t i c
f i e l d . This m e t h o d has b e e n d e s c r i b e d
several t i m e s in t h e l i t e r a t u r e , b y D i l t h e y / D i c k 8 a n d A k u l o v e t al", a m o n g
others.
A n a r r a n g e m e n t f o r m a g n e t i c arc
d e f l e c t i o n is s h o w n s c h e m a t i c a l l y in
Fig. 6. T h e f i e l d o f t h e t w o c o i l s are at
r i g h t angles t o t h e arc a n d p a r a l l e l t o
t h e d i r e c t i o n o f w e l d i n g . If an a l t e r n a t i v e m a g n e t i c f i e l d is u s e d , t h e arc w i l l
o s c i l l a t e at r i g h t a n g l e s t o t h e w e l d i n g
d i r e c t i o n at t h e s a m e f r e q u e n c y as t h a t
of t h e f i e l d . T h e d e f l e c t i o n o f t h e arc is
proportional to the strength of the
magnetic field and the current through
t h e arc
W i t h G M A w e l d i n g , a n d also w i t h
p l a s m a - G M A w e l d i n g , t h e use o f s u c h
a m a g n e t i c f i e l d results n o t o n l y in
o s c i l l a t i o n o f t h e arc b u t also causes
Fig. 10—Root run oi a V-groove joint without support at the back of the joint; 185 A through
1.2 mm diameter mild steel filler metal and 150 A plasma current. The pulsed
alternating
transverse magnetic field had a frequency of 46 Hz; the wire speed was pulsed with a
frequency oi 92 Hz
t h e d e t a c h i n g m e t a l d r o p t o m o v e in
t h e s a m e d i r e c t i o n as t h e arc. T h e
l i q u i d d r o p , so l o n g as it is still
a t t a c h e d t o t h e w i r e , carries c u r r e n t
a n d is, t h e r e f o r e , s u b j e c t e d t o a s i m i l a r
f o r c e as t h a t o n t h e arc. In t h e last
p h a s e o f d e t a c h m e n t in p a r t i c u l a r ,
w h e n a l i q u i d n e c k has f o r m e d , t h e
d i r e c t i o n o f t h i s m e t a l p a r t i c l e w i l l also
be d e t e r m i n e d by the magnetic field.
W h e n t h e d r o p d e t a c h e s , it w i l l m o v e
f u r t h e r in t h e d i r e c t i o n g i v e n t o it by
t h e m a g n e t i c f i e l d at t h e m o m e n t of
detachment.
If t h e f r e q u e n c y o f t h e m a g n e t i c
f i e l d is set at h a l f t h a t o f t h e d r o p
frequency, a d r o p w i l l detach each
t i m e t h e e n d o f t h e w i r e reaches its
m a x i m u m d e f l e c t i o n . T h i s is p a r t i c u larly t h e case w i t h a p u l s e d m a g n e t i c
field. The oscillation of the w i r e end
t h e n p r o m o t e s a regular d e t a c h m e n t
o f t h e d r o p s , p r o b a b l y as a result o f t h e
extra a c c e l e r a t i o n i m p o s e d o n t h e
d r o p w i t h respect to t h e w i r e e n d at
each m a x i m u m d e f l e c t i o n of t h e liquid
section
High-speed
cinematography
was
used t o i n v e s t i g a t e this m e t h o d of
drop detachment and the movement
t o w a r d s t h e w e l d p o o l . Figure 7A gives
t h e m o t i o n analysis o f t w o successive
d r o p s . T h e p l a s m a - G M A process w a s
used w i t h a 1.2 m m (0.047 in.) d i a m e t e r
m i l d steel f i l l e r m e t a l . T h e d i s t a n c e of
the drops w h e n falling into the w e l d
p o o l , w a s 9 m m (0.35 in.). As t h e f i l m
shows, the liquid tip of the w i r e moves
t o t h e right a n d to t h e left u n d e r t h e
i n f l u e n c e of t h e p u l s e d a l t e r n a t i n g
m a g n e t i c f i e l d . It has a l r e a d y b e e n
remarked that the drops detach w h e n
t h e w i r e f i l l e r m e t a l e n d is at its m a x i m u m d e f l e c t i o n . This process can be
e n h a n c e d by g i v i n g t h e w i r e m o t i o n
an a c c e l e r a t i n g p u l s e at t h a t m o m e n t .
A c u r r e n t p u l s e is less s u i t a b l e f o r this
purpose since the force o n the w i r e
filler metal tip:
F = /• x B
(3)
w h e r e F = t h e Lorenz force on the
w i r e t i p i n d u c e d b y t h e transverse
WELDING
R E S E A R C H S U P P L E M E N T I 41-s
magnetic field; / = the current density
in the wire tip; B — the induction of
the transverse magnetic field.
F also increases as a result of the
increased current density, so that the
deflection of the filler metal end is
changed. An accelerating pulse on the
wire filler metal speed, however, has a
definite effect on drop detachment:
the drop is shaken off the end of the
wire. By synchronizing the accelerating pulse with the change over of the
magnetic field, each drop is shaken off
the wire at just the right moment.
Figure 7A indicates that the drops
follow a path which is in line with the
alignment of the wire tip at the
moment of detachment. The film
speed was 3000 frames per second. The
position of the drop in the figure was
obtained from every 5th frame of the
film.
If a workpiece in the form of a metal
plate is rapidly moved under such an
oscillating metal shower, a result as
pictured in Fig. 7B is obtained. The
material has no chance to melt c o m pletely together and the drops lie
apart, or in some cases fused together
at the points on which they were
deposited. This is well demonstrated
in Fig. 7B. O n the metal plate in Fig. 7B
there are only three drops fused
together. The speed of the plate under
the torch was 1.5 m / m i n (59 ipm).
The bead on plate shown in Fig. 7C
was made under conditions different
from those of Fig. 7B, in that the
traverse speed was lower (i.e., 0.23
m / m i n or 9 ipm). Otherwise, all the
parameters were the same, including
the dimensions of the plate on which
the weld was made. Figure 7C shows
once again that penetration occurs at
the point where the drops strike the
plate.
The deflection of the wire tip
decreases and increases proportionally
w i t h the strength of the magnetic
field. The effect of variation of the
strength of the magnetic field on the
shape of the weld bead, w i t h constant
traverse speed, is shown in Fig. 8. The
bead-on-plate specimens were all
made w i t h 230 A through a 1.2 (0.047
in.) mm diameter mild steel filler metal
and a plasma current of 100 A. Only
the strength of the magnetic field was
varied. The alternating transverse magnetic field used in these tests was
pulsed w i t h a frequency of 85 Hz.
In Fig. 8A the penetration depths in
the bead-on-plate specimens are plotted against the magnetic induction of
the field. Since with increasing amplitude of deflection of the oscillating
wire tip the drops land further and
further from the center of the w e l d , so
that their heat is dissipated over a
greater area, the penetration steadily
decreases.
In Fig. 8B the w i d t h of penetration,
4 2 - s l FEBRUARY 1981
measured at 1 mm (0.039 in.) above the
deepest point, is plotted against the
magnetic induction. This shows that
the w i d t h can increase by a factor of 3,
w i t h 40% less depth of penetration, as
shown in Fig. 8A.
The cross-sections of a few of these
bead-on-plate weld tests are shown in
Fig. 9. Figure 9A is the cross-section of
a bead-on-plate weld without the
effect of a transverse magnetic field.
The penetration has the well-known
finger shape. The w i d t h of penetration
was 2.5 mm (0.10 in.). Figure 9B shows
the cross-section obtained with a
pulsed alternating magnetic field w i t h
a strength of 0.4 X 1 0 - T = 40 Gauss.
The w i d t h of the penetration is d o u bled to 5 mm (0.20 in.).
If a sinusoidal alternating magnetic
field is used (with a frequency so
related to the drop frequency that the
drops are spread regularly across the
width of the weld), a completely different penetration profile is obtained.
The conditions for the weld illustrated
in Fig. 9C were identical to those of
Fig. 9A, except that a sinusoidal transverse magnetic field was used to
spread the drops. The field alternated
with a frequency of 20 Hz. In contrast
with the finger-shaped penetration of
the bead-on-plate weld as in Fig. 9A, a
very regularly-distributed penetration
was obtained across the whole w i d t h
of the w e l d . This effect can be very
useful in some practical applications.
Finally, Fig. 10 shows a possible
application of a pulsed alternating
magnetic field. W i t h
single-sided
welding of the root run of a V-groove
joint w i t h o u t support at the back of
the joint, the hot metal falls out of the
joint when a spray-arc is used for
welding. In the test described below
our objective was as follows: If we
could succeed in directing all the
drops to the plate edges alongside the
gap, the arc w o u l d be able to pass over
the gap regularly but w i t h o u t any
unfortunate consequences. As has
been demonstrated above, it is the
impact w h i c h determines the depth of
penetration.
Figure 10 shows the cross-section of
such a root run. The current through
the 1.2 mm (0.047 in.) diameter mild
steel filler metal wire was 186 A and
the plasma current was 150 A. Since
the drop frequency with these currents
was 92 Hz the frequency of the transverse magnetic field was set at 46 Hz.
In addition, at the end of each maximum deflection of the wire tip the
wire speed was given a very brief
accelerating pulse, also at a frequency
of 92 Hz. The result was a very regular
weld penetration.
Conclusion
Under the conditions described in
this paper, 34% of the heat transferred
to the workpiece during GMA welding
is supplied from the arc as a result of
convection, radiation and conduction.
The heat generated in the cathode
area contributes 41% while 25% is provided by the overheated metal drops.
The heat of the arc and the passage of
current from the arc through the workpiece has only a slight influence on the
penetration depth when the plasmaGMA process is used in the conditions
described.
In plasma-GMA welding, and also in
GMA welding, it is the heat in the
transferring drops w h i c h determines
the mass of workpiece metal which is
melted. The overheated metal drops
deliver their excess heat to the molten
pool. This effect is more intense w i t h
high currents, since the drops are then
driven deep into the weld pool as a
result of the rapid succession of
impacts. The penetration is a function
of the magnitude of f X P, the total
momentum per second.
Penetration of the weld in the workpiece can be regulated during GMA
and plasma-GMA welding by controlling the direction of movement of the
transferring metal drops. An alternating transverse magnetic field is an
effective measure for this purpose.
W i t h the help of such a field the drops
can be spread over the w h o l e width of
the weld or focused at predetermined
locations. As a result, the shape of the
penetration can be varied from the
normal finger shape to one w h i c h ,
although less deep, is far wider.
References
1. Essers, W.G., and Walter, R., "Some
aspects of the penetration mechanisms in
metal-inert-gas welding," International
Conference on Arc Physics and Weld Pool
Behaviour, London, May 1979.
2. Essers, W.G., "New Process Combines
Plasma with CMA Welding," Welding journal, 55 (5), May 1976, pp. 394-400.
3. Ton, H., "Physical properties of the
plasma-MIG welding arc," /. Phys.D. (Appl.
Physics), 8 (8), 1975, pp. 922-933.
4. Ando, K., and Nishiguchi, K., "Mechanism of formation of pencilpoint-like wire
tip in MIC welding," IIW Doc. 212-156-68.
5. lelmorini, C , Tichelaar, G.W. and van
den Heuvel, G.I.P.M., "Droplet temperature
measurements in arc welding," IIW Doc.
212-411-77.
6. Erokhin, A.A., "Kinetics of the metallurgical processes in arc welding" (in Russian), Moscow, 1964.
7. Dilthey, U., "Beitrag zur Lichtbogensteuerung durch magnetfelder bei mechanisierten
Lichtbogenschweiszverfahren."
"Dissertation," 1972, Technische Hochschule, Aachen.
8. Dick, N.T., "The application of magnetic fields to TIG welding arcs," Welding
Research International, 2 (1), 1972.
9. Akulov, A.I., and Kopaev, B.V., "Magnetic control of the arc during argon MIG
welding," Avt. Svarka, 1972, pp. 39-42.