Visible Light Emissions during Gas Tungsten
Arc Welding and Its Application to Weld
Image Improvement
Spectral windows where external sensors will have the least
radiance disturbance were established, providing for the
possibility of an improved weld pool image
BY E. VV. KIM, C. ALLEMAND AND T. VV. EAGAR
ABSTRACT. An experimental study was
carried out to map the light emissions
from a gas tungsten arc using 24 combinations of weld parameters. Data were
collected with a computer-interfaced
double monochromatic imaging system
which has a rejection ratio of 106. The
intensity scale was calibrated throughout
the wavelength range from 4880 A to
7300 A, using a spectral radiance standard lamp which is traceable to NBS
standards. Lines were identified by element and wavelength with 0.65 A wavelength resolution.
The effect of intensity calibration was
found to be very important in terms of
comparing spectral maps and intensities.
The emissions were found to be dramatically different with different shielding
gases, welding current and base metals.
Spectral windows, where the external
sensors will have the least disturbance
from welding arc emission, were also
determined. An improved image of the
weld pool can be obtained by operating
within these spectral windows. Twodimensional distributions of major elements in an arc plasma are also presented, and the anode spot on the molten
weld surface was observed.
Introduction
Improvement of automated welding
will require development of new sensor
systems. As the welding arc is a harsh
environment, noncontacting sensor devices are preferred, since they do not
disturb the process and do not require
extra equipment in the welding setup. A
number of investigators have developed
methods for viewing the weld pool in
order to control joint tracking and weld
E W. KIM, C ALLEMAND and T. W. EAGAR
are with the Massachusetts Institute of Technology, Cambridge, Mass.
bead width; however, each of these
systems must exclude the arc light in
order to obtain a clear image of the weld
pool (Refs. 1-4). Since the arc is a spectral
radiator, and this spectra is influenced
strongly by the metal vapors emitted
from different base metals, it would be
desirable to map the spectra on a variety
of base plates in order to select the
windows in the spectrum which are relatively free of arc light.
Arc light emissions also have potential
as a welding information source. The
emission changes with weld parameters
and responds well into the kilohertz
range (Ref. 5). Unfortunately, spectral
data on arcs are currently available under
only a few combinations of weld parameters and wavelengths. Furthermore, in
most cases, the intensities from different
investigations and even from the same
investigation are not comparable because of a failure to calibrate the measurement systems (Refs. 5-8). Most of the
work has been done to determine the
feasibility of using the arc light as a sensor
or to investigate the characteristics of arc
emissions. It can be seen from a review
of these papers that few studies have
been devoted to mapping the spectra
and determining windows which are relatively free of arc light.
KEY WORDS
GTAW Light Emissions
Emission Intensity
Intensity Calibration
Monochromatic Image
Image Improvement
Anode Spot Motion
Spectral Data System
Arc Plasma Elements
Arc Current Effects
Shielding Gas Effect
Some researchers have applied spectra
for weld monitors or sensor systems
(Refs. 9-15). In these studies, it was
shown that the emission spectra can be
used to detect changes in the welding
arc, due to such phenomena as reduction
in shielding gas flow rate, loss of flux in
flux cored welding wire or the presence
of hydrogen. Another study incorporated
welding spectra to enhance the image
quality of the weld pool (Ref. 4).
The major purpose of the present
work is to spectrally map light emissions
in the range of 4880 A to 7300 A during
gas tungsten arc welding, and to correlate changes in the spectra to changes in
the welding parameters. The intensities
are calibrated with reference to a standard lamp, the radiance of which is traceable to NBS standards. In this way, all
intensities are measured on the same
scale. Mapping the emission with various
combinations of weld parameters provides comparison of the emission characteristics. Further, this mapping defines
spectrally blind regions (of minimum
spectral intensity). These regions can be a
basis from which to choose windows in
the spectra where external sensors will
have the least disturbance from welding
arc emissions. The spectral distribution of
the major spectral lines were also photographed based on the results of this
spectral mapping.
Experimental W o r k
Equipment
The spectra obtained in this study
were made with the equipment schematically shown in Fig. 1. Direct current
electrode negative (DCEN) polarity was
supplied by a commercial welding power
supply followed by a current regulator,
which gives control over the welding
current to within 1% of the set value.
Light from the arc was reflected by a
WELDING RESEARCH SUPPLEMENT 1369-s
0-
C u r r e n t to
Double
Strip Chart
Filter 25 Hz
Recorder
Voltage
Monochromator
Scan
L o w Pass
Converter
ADC
Control
PDP 11/23
Computer
Tektronix
VT 105
Plotter
Graphic
ADC : A n a l o g u e to D i g i t a l C o n v e r t e r
Fig. 1 — Schematic
arrangement
Lens
of spectral
Disk
Monitor
data analysis
PM
Storage
: Photomultiplier
system
Grating Mirror
Collimator 1
Actual subtractive configuration
Light Source
is shown here as an additive
entrance
Slit
one for clarity purpose.
Rotation of the grating was controlled
by a stepping motor. The number of
stepping pulses and the pulse rates were
also used as data-sampling trigger pulses
for a data collecting computer.
Calculated spectral band pass and
measured values are compared in Table
1. The spectral resolution used is 0.65 A
with a 20% maximum error in determining the peak intensity. The rejection ratio
of the monochromator as a band pass
filter was also measured to be better than
106. The half-band widths at which the
1 0 - 3 peak intensity was transmitted are
tabulated in Table 2.
The intensities were calibrated by
comparison with a lamp whose radiance
is traceable to NBS standards. This lamp
was used as the light source at the
position of the arc source, and output
intensities were taken through the entire
opto-electronic system. The ratio of this
output to the lamp radiance was used as
an intensity calibration factor. This factor
Grating Mirror
Collimating and focusing are accomplished
I
by the same mirror labeled collimator
PM : Photomultiplier
Fig. 2 — Light path
in the
flat mirror and was focused onto the
entrance slit of the imaging doublemonochromator (Ref. 16). The arc was
viewed at 75 deg from the arc column
axis to imitate a realistic position for an
arc sensor. The image comprised the
molten surface, the arc column and the
cathode tip. The image center was
aligned with the center of the slit to
provide a symmetrical image.
The imaging double-monochromator
used in this study has a 500-mm focal
length and a reflection grating of 1180
grooves/mm. A schematic drawing of
this instrument in Fig. 2 shows the light
path through the instrument. A photomultiplier (PM) was attached just after the
exit slit in order to monitor the spectral
intensity. It is also possible to use a beam
splitter to monitor the monochromatic
image and intensity simultaneously. Spectral data were collected at the exit slit
without using the beam splitter in order
to obtain better signal intensity. The
image was used only to check the position of the arc and to take pictures of the
two-dimensional monochromatic light
distribution.
double-monochromator
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100
50
40
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of He arc on
copper
FWHI< a (A) at
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(b) Ne I—Ne neutral line.
0.56
—
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Table 2—Half-Band Width at 1/1000 Peak
Transmission
Table 3—Measured Wavelength Deviation
Z"
Wavelength (A)
Slit Width
(micron)
Single Side Band
1000
750
500
242
14.4
10.3
(A)
Element
From Table
Difference (A)
4917.87
4931.21
5016.30
5105.54
5153.18
5218.25
5292.27
5700.44
5782.19
5875.44
6678.24
7065.39
4918.37
4931.56
5016.61
5105.30
5153.24
5218.20
5292.52
5700.24
5782.13
5875.62
6678.15
7065.12
-0.50
+0.29
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-0.24
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-0.27
Q.
Cu
W
Cu
Cu
Cu
Cu
Cu
Cu
Cu
He
He
He
7.1
3.7
w a s used t o normalize the intensity o v e r
t h e entire w a v e l e n g t h range studied;
h o w e v e r , since b o t h the e n t r a n c e slit size
and t h e s o u r c e position m a y b e c h a n g e d
d u r i n g the m e a s u r e m e n t t o o b t a i n the
best signal c o n d i t i o n f r o m t h e w e l d i n g
arc, t h e calibrated spectral intensities are
o n l y relative. Nevertheless, t h e relative
intensities at any t w o w a v e l e n g t h s are
linearly p r o p o r t i o n a l .
UJ
Measured
II
I
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T y p e AISI 304 stainless steel, Ti-6Al-4V
alloy, 5083 a l u m i n u m alloy a n d c o p p e r
plate w e r e used as base metals. Three
d i f f e r e n t w e l d i n g currents, 100, 150 and
200 A , w e r e u s e d w i t h e l e c t r o d e negative polarity (DCEN). The 3 / 3 2 - i n . (2.4m m ) diameter E W T h 2 e l e c t r o d e was
used w i t h an 8 - m m (0.3-in.) arc length.
T h e arc v o l t a g e w a s 12 V f o r a r g o n
shielding and 22 V f o r helium shielding.
T h e arc w a s r u n o n a specimen plate
b a c k e d by a w a t e r - c o o l e d c o p p e r block.
W e l d i n g grade a r g o n and helium w e r e
used as shielding gases at f l o w rates of 20
c f h (9.4 L/min) f o r Ar a n d 25 c f h (11.8
L / m i n ) f o r H e . Spectra w e r e t a k e n at
each o f these 24 e x p e r i m e n t a l c o m b i n a tions.
Results and Discussion
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a r r o w s . T h e relative intensities in t h e
w a v e l e n g t h tables are s o m e w h a t different f r o m those of t h e e x p e r i m e n t a l
results. By a n d large, s t r o n g t a b u l a t e d
lines m a t c h t h e test results, b u t differences o f t e n o c c u r w h e n t h e intensity
level of a spectral line is l o w . This m a y be
d u e t o t h e interaction effects of each
element in the arc.
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Spectral Map
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e v e n w i t h d i f f e r e n t arc c u r r e n t levels.
These lines w e r e used as a r e f e r e n c e f o r
w a v e l e n g t h calibration b y assigning the
characteristic w a v e n u m b e r t o the c o r r e s p o n d i n g data p o i n t in a data file (Ref.
20). T o check the accuracy in d e t e r m i n i n g
wavelengths, a comparison was made
f o r t h e case of a 200-A helium (He) arc o n
a c o p p e r plate. T h e results are listed in
Table 3, w h i c h s h o w s v e r y g o o d accurac y w i t h i n 0.7 A . M e a s u r e d values w e r e
referenced
to
standard
wavelength
tables (Refs. 1 7 - 1 9 ) .
T h e spectral m a p s f o r the case o f the
entire 24 c o m b i n a t i o n s of w e l d p a r a m e ters are s h o w n in Figs. 3 - 1 0 . The spectra
at w a v e l e n g t h s longer than 6 9 0 0 A usually consist only of s t r o n g a r g o n (Ar) lines,
namely Ar I 6 9 6 5 . 4 3 , 7 0 6 7 . 2 3 , 7147.04
a n d 7270.94. These w e r e o m i t t e d f r o m
the spectral m a p . S o m e lines w h i c h satu-
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Fig. 7-Effect of current on 4880 A-7300 A spectra of He arc on AA 5083
spectra were obtained with different
base plates. This is due to the different
metal vapor species which evaporate
from the weld pool. Since these metal
vapor species may dominate the temperature profile and the electrical properties
of the arc, it can be seen that a change in
the composition of the workpiece can
dramatically change the nature of the
welding arc (Refs. 8, 21-23).
As shown in Figs. 3 and 4, the major
lines from the He arc on a water-cooled
copper plate are Cu I and He I. In the case
of the Ar arc, Ar I and Ar II lines are
dominant. With argon shielding, copper
lines appeared only at the 200-A current.
The greater intensity of the Cu lines in the
He arc is due to the melting of the plate.
This is due to the fact that He gas liberates
more heat than Ar gas. This can be
explained by the fact that the arc voltage
of He shielding is about 10 V higher than
that of Ar shielding, due to the higher
ionization potential of He. This trend was
consistent with each experiment performed.
Figures 5 and 6 show the results on AISI
304 stainless steel. The major metallic
lines observed were Cr I and Fe I. In Figs.
7 and 8, the dominant metallic lines from
the 5083 aluminum alloy experiment are
shown to be Mg I and Al II. Magnesium
lines saturated the upper limit of the data
collecting system. Figures 9 and 10 show
the results of the Ti-6AI-4V alloy experiment. The major lines from this experiment are Ti, Ti I and Al II. Thermodynamic
calculations made by Block-Bolten, Eagar
and Cobine, ef al, are consistent with
these observations (Refs. 24-26).
The data can also be presented in finer
detail, as shown in Figs. 11 and 12. An
atlas of calibrated light emission intensities
during gas tungsten arc welding with 24
combinations of welding parameters has
been produced (Ref. 20).
Effect of Intensity Calibration
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7000.
A comparison between a spectral map
with calibrated intensities and one without calibration is shown in Fig. 13. It is
seen that the uncalibrated intensity ratio
between t w o lines, compared with the
calibrated ratio, can be in error by as
much as 15 times. This strong effect of
calibration on relative intensities explains
some of the difficulty in comparing other
spectra that have been presented in the
literature.
The light transmission characteristics
through the entire optical system
depends upon the optical components
used. The sensitivity and noise of the
electrical data collecting and processing
equipment also affect the characteristics
of the system. It is important to take both
factors into account in an integrated manner, as is done in this work, in order to
obtain consistent results.
Effects of Welding Parameters
It is clear that most of the lines appear
at all current levels on the same material
but with quite different relative intensities. Some of the relative intensity
changes are quantified in Figs. 14, 15 and
16. The intensities from neutral lines
increase with Ar shielding but decrease
after reaching a maximum with He shielding. On the contrary, the intensity of
singly ionized lines in Fig. 16 shows a
consistent decrease. This may be due to a
transition from a singly ionized Fe ion
changing to a secondary ion emitting light
at a wavelength which is not within the
range concerned in this research. Essentially, the plasma may become more
optically thick at this wavelength as the
current is increased (Ref. 27). It is also
seen that the intensity of the tungsten line
from the electrode increases with current
when using He shielding.
It was also found that the background
intensity with Ar shielding at 200 A is
approximately 20 times higher than that
with He shielding. One possible explanation for this higher background intensity is
the higher electrical conductivity and the
lower thermal conductivity of the Ar
plasma compared to the He plasma (Ref.
8). Another possible explanation may be
the free-to-bound electron transition of
the singly ionized Ar atoms, due to their
lower ionization potential compared to
that of He. Although the background is
lower, the peaks are higher with He for
any line which appears in both shielding
gases. This may be due to the higher arc
power of the He gas shielding caused by
the higher total arc potential drop, or it
may be due to the fact that He is optically
thinner than Ar.
The base metal also affects the intensities of some lines. For example, as shown
in Figs. 3, 5, 7 and 9, the intensities of the
same line, 5875.62 He I, differs greatly
according to the composition of the base
metal. This probably results from the
interactions of each vapor or ion species
in the arc plasma, again confirming the
fact that the metal vapor can dominate
the properties of the arc. The property
change of the plasma due to the presence of metal vapors requires a more
rigorous study in the future.
Spectral Window
The spectrally blind regions where
external sensors will have the least disturbance from welding arc emissions were
determined easily in the figures presented in this work. Four spectra from four
different base materials are superimposed in one graph on a coarse wavelength scale in Fig. 17. It was found that
windows on the coarse wavelength scale
are present at 5790-5870 A, 5890-6010
A, and 6440-6630 A for these four base
metals with He and Ar shielding gas.
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and He shielding
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5500.0
6000.0
6500.0
WAVELENGTH CAD
Cr
O
Windows for fine wavelength scales can
also be found in the same manner from
graphs similar to Figs. 11 and 12 (Ref.
20).
LU
Monochromatic Photography
X
CJ
To see the effect of the spectral windows^ photographs were taken with a
6520-A interference band pass filter, as
proposed in the previous section. These
are compared with the photographs taken with neutral density (ND)_ filters. The
interference filter has a 100-A band pass
FWHI. As shown in Figs. 18 and 19, better
images of molten surfaces were obtained
with the narrow band filter than with the
ND filter.
As stated previously, Ar shielding has a
higher continuum emission than He;
hence, the arc light is still present in Fig.
18A. This emission is concentrated in the
vicinity of the tungsten electrode. Figures
20 and 21 are images from a video
monitor screen made with the double
monochromator, using its 14-A band pass
FWHI. These photographs show the distribution of Cr4-Fe and Mn, respectively.
These are heavily concentrated just
above the molten pool. This observation
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cr
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cr
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ui
cr
Fig. 18 - Image of Ar arc on 304 stainless steel. Fig. 19- Image of He arc on 304 stainless steel.
A — 652 ± 5 nm interference filter; B — neutral A — 652 ± 5 nm interference filter; B - neutral
density filter
density filter
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Fig. 20 — Distribution of the Cr+ Fe line on 304 stainless steel at 520.6 ±1.4 nm. A—Ar arc; B — He arc
WELDING RESEARCH SUPPLEMENT 1375-s
Fig. 21 — Distribution of a Mn line on 304 stainless steel at 601.6 ± 1.4 nm. A-Ar
arc; B-He arc
Fig. 22 —Anode spot mode on 304 stainless steel at Cr + Fe 520.6 1.4 nm. A -Spot mode, 150 A, Ar shielding; B-ring mode, 200 A, Ar/2H2
shielding
is in accordance with the results of population density measurements in an arc
plasma (Ref. 28).
Metallic elements are also observed to
be more concentrated at the center of
the molten surface, as seen in Fig. 21.
Figure 22A also shows a spot at the
center. This picture was taken with a
reduced exposure to improve the image
of the spot. This bright spot is believed to
be an anode spot where the current
density and temperature is higher than
other locations on the weld pool surface.
It was found that the shape of the
anode spot changes from a spot to a ring
with increasing current. Figure 22B shows
this. The center ring is believed to be the
locus of the anode spot, and the inverted
cone is the reflected image of the arc
column. The fusion boundary is marked
with a white curve. The transition occurs
at around 150 A. The ring shape is
believed to be related to the depressed
molten anode surface (Ref. 29). At high
376-s I DECEMBER 1987
currents, above 150 A, the anode spot is
thought to move around the ridge of the
depressed pool surface with a nonrandom motion. The video observation
showed a rotating motion of the ring.
This will affect the distribution of the
metallic elements in an arc plasma, and
thus the heat input mode to the workpiece. A ring-shaped heat input model
should be considered, and the electromagnetically driven convection flow in
the weld pool will be changed in comparison with typical mathematical models of
arc welding (Refs. 30-32). The rotating
spot along the ridge may drag the molten
metal and work as an asymmetric driving
force for the weld pool circulation.
Conclusion
Spectral maps for 24 combinations of
weld parameters were made in the
wavelength range from 4880 A to 7300 A
using a calibrated intensity scale with
0.65-A wavelength resolution. Lines can
be identified by species and wavelengths
within this resolution.
The effect of intensity calibration was
found to be very important in comparing
different spectral maps and intensities, as
the uncalibrated ratio between t w o lines
could be in error by as much as 15
times.
The major metallic lines identified were
Cr I and Fe I for AISI 304 stainless steel, Cu
I for copper, Ti, Ti I, and Al II for Ti-6AI-4V
alloy, and Mg I and Al II for 5083 aluminum alloy. The lines from the shielding
gases were always very strong. Argon
gas shielding at 200-A arc current showed
approximately 20 times higher background emission level compared to the
He gas shielding, but the peak intensities
were generally higher with He gas shielding than with the Ar gas shielding.
The intensity changes at different current levels were compared graphically
and numerically. The characteristics of
these changes were found to vary from
element to element and were also found
t o b e a f f e c t e d b y t h e shielding gas.
Spectral w i n d o w s w e r e d e t e r m i n e d
f o r b o t h the Ar a n d He shielding gases f o r
a n u m b e r o f d i f f e r e n t base plate c o m p o sitions. It w a s f o u n d that w i n d o w s are
present at 5 7 9 0 : 5 8 7 0 A , 5 8 9 0 - 6 0 1 0 A ,
a n d 6 4 4 0 - 6 6 3 0 A. By i n c o r p o r a t i n g these
spectral w i n d o w s , a clearer image of a
w e l d p o o l is possible.
By means of m o n o c h r o m a t i c p h o t o g r a p h y , the a n o d e spot w a s o b s e r v e d t o
change its shape f r o m a spot t o a ring
w i t h increasing c u r r e n t . It is b e l i e v e d that
this m a y b e d u e t o m o v e m e n t o f t h e
a n o d e spot a r o u n d the o u t e r ridge of the
depressed w e l d p o o l surface. H e n c e , it is
seen that a n o d e s p o t m o t i o n o n m o l t e n
w e l d pools at high w e l d i n g current
( a b o v e 150 A) is n o n - r a n d o m . This c o u l d
h a v e an i m p o r t a n t e f f e c t b o t h o n the
heat distribution o f non-planar m o l t e n
w e l d pools a n d o n the e l e c t r o m a g n e t i c a l ly d r i v e n c o n v e c t i o n w i t h i n t h e w e l d
pool.
It is r e c o m m e n d e d that this w o r k be
e x t e n d e d t o the case of a m o v i n g arc. A
study o n t h e t e m p o r a l change o f an arc
emission will also help lead t o an u n d e r standing of the w e l d i n g arc plasma.
Acknowledgments
T h e authors appreciate t h e s u p p o r t f o r
this w o r k f r o m t h e O f f i c e o f Naval
Research, u n d e r C o n t r a c t N 0 0 0 1 4 - 8 0 - C 0 3 8 4 , and t h e D e p a r t m e n t o f Energy,
u n d e r C o n t r a c t DE-FG02-85ER13331.
References
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2. Kawahara, M., and Tsuji, H. 1984. A
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3. Linden, C , and Widfeldt, M. 1985. Sensors in automated arc welding summary of
recent work in IIW Commission XII. IIW Doc.
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3) — improvement of image quality by incorporation of spectrum of arc. Transactions of JWRl
10(1):13-18. Welding Research Institute of
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5. Shaw, C. B. 1975. Diagnostic studies of
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6. Peterson, D. W., and Ransom, P. L. 1983.
The calibration and cataloging of spectral emissions from gas metal arc welding of steel from
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7. Key, J. F., Mcllwain, M. E., and Isaacson,
L. 1980. A plasma diagnostics approach to
welding heat source/molten pool interaction.
Sixth International Conference on Gas Discharges and Their Applications — Part 2. Conference Publication No. 189, pp.235, Institute
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8. Dunn, C. ). 1984. Metal vapors in gas
tungsten welding arcs. M.S. thesis, Massachusetts Institute of Technology.
9. Hyatt, R. W., Ullrich, O. A., and Mishler,
H. W . 1982. Spectral monitoring of argon —
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10. Shea, ). E., and Gardner, C. S. 1984.
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84-3603, Report No. 103, Radio Research
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11. Blackmon, D. R., and Hock, V. F. 1984.
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Influence of arc pressure on weld pool geometry. Welding Journal 64(6):163-s to 169-s.
30. Eagar, T. W., and Tsai, N. S. 1983.
Temperature fields produced by traveling distributed heat sources. Welding
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62(12):346-s to 355-s.
31. Goldak, )., Chakravarti, A., and Bibby,
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Weldability of Steels— Fourth Edition
By Prof. R. D. Stout
This w e l l - k n o w n b o o k on t h e w e l d i n g m e t a l l u r g y of steels has been extensively revised by Prof. S t o u t ,
Dean E m e r i t u s of t h e G r a d u a t e S c h o o l , Lehigh University. T h e fully i n d e x e d , 4 5 0 - p a g e , c l o t h - b o u n d
v o l u m e , w h i c h c o n t a i n s n u m e r o u s i l l u s t r a t i o n s a n d t a b l e s , w a s p u b l i s h e d in A p r i l 1 9 8 7 .
T h e p r i c e o f Weldability
of Steels—Fourth
Edition
is $ 4 0 . 0 0 p e r c o p y , p l u s $ 5 . 0 0 f o r p o s t a g e a n d
h a n d l i n g . O r d e r s s h o u l d b e s e n t w i t h p a y m e n t t o t h e W e l d i n g R e s e a r c h C o u n c i l , S u i t e 1 3 0 1 , 3 4 5 E. 4 7 t h
St., N e w York, NY 1 0 0 1 7 .
W E L D I N G RESEARCH S U P P L E M E N T 1377-s