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Effects of Shielding Gas on Gas Metal Arc Weld ing A luminum
Contrary to the conclusion of previous investigators, argon may be the optimum shielding gas for the gas metal arc welding of aluminum
BY W . R. REICHELT, J. W . E V A N C H O A N D M . G. H O Y
ABSTRACT. A r g o n , h e l i u m , a n d m i x tures o f these gases are used fo r sh i e l d ing in gas meta l -a rc w e l d i n g o f a l u m i n u m . Cons ide rab le w o r k has been d o n e p rev ious ly by o t h e r invest iga tors to d e f i n e t h e ef fects of these gases o n p e n e t r a t i o n , arc s tab i l i t y a n d po ros i t y . M o s t of th is w o r k , h o w e v e r , was perf o r m e d by m a i n t a i n i n g an " o p t i m u m " arc gap. C o n s e q u e n t l y , for var ious c o m p o s i t i o n s a n d f l o w rates s tud ies , w e l d parameters w e r e also va r ied .
To fu r t he r unde rs tand the ef fects o f sh ie ld ing gas o n gas meta l -a rc w e l d i n g a l u m i n u m , a s tudy was c o n d u c t e d w h e r e b y al l w e l d set t ings w e r e preset at cons tan t va lues and ef fects o f sh ie ld ing gas c o m p o s i t i o n a n d f l o w rate o n arc gap , vo l t age , and cu r ren t , in a d d i t i o n t o p e n e t r a t i o n , arc s tab i l i t y , and po ros i t y w e r e eva lua ted . Sh ie ld ing gas c o m p o s i t i o n and f l o w rate b o t h a f fec ted d e p t h o f p e n e t r a t i o n . M a x i m u m pene t ra t i on was o b t a i n e d w i t h pu re argon at 150 c f h , whe reas m i n i m u m d e p t h o f p e n e t r a t i o n was assoc ia ted w i t h a 25% a r g o n / 7 5 % h e l i u m m ix tu re at 200 c f h .
Effects o f sh ie ld ing gas o n d e p t h of pene t ra t i on co r re la ted d i r ec t l y w i t h ef fects o f s h i e l d i n g gas o n wa t t age . Gas c o m p o s i t i o n s h igh in a rgon also p r o d u c e d best arc s tab i l i t y , a n d t h e m i n i m u m a m o u n t of m i c r o - p o r o s i t y . Results of th is w o r k suggest t ha t , c o n trary to t he c o n c l u s i o n s o f p rev ious invest igators , a rgon my be the o p t i m u m sh ie ld ing gas for gas meta l -a rc w e l d i n g o f a l u m i n u m .
Paper presented under sponsorship of the Aluminum Alloys Committee of the Welding Research Council at the AWS 60th Annual Meeting in Detroit, Michigan, during April 8-12, 1979.
W. R. REICHELT, J. W. EVANCHO and M. G. HOY are with the Joining Division of Alcoa Laboratories, Alcoa Center, Pennsylvania.
I n t r o d u c t i o n
Sh ie ld ing gas, as an i m p o r t a n t va r i ab le in gas meta l -a rc w e l d i n g o f a l u m i n u m , serves t h ree p r ima ry f u n c t i o n s : It p rov ides a p lasma fo r c o m m u t a t i o n o f cu r ren t ; it p ro tec ts t h e w e l d p o o l f r o m react ing w i t h t he air e n v i r o n m e n t ; and w h e n w e l d i n g us ing d i rec t cu r ren t reverse po la r i t y (DCRP) , it can p r o v i d e the c l ean ing ac t i on w h i c h par t ia l l y removes a l u m i n u m o x i d e f r o m the a l u m i n u m p la te .
The phys ica l p roper t i es a n d character ist ics o f b o t h argon and h e l i u m , t h e t w o p r i n c i p l e s h i e l d i n g gases used f o r w e l d i n g a l u m i n u m , are c o m p a r e d in Tab le 1 . The i o n i z a t i o n p o t e n t i a l o f argon is m u c h l o w e r t h a n that of h e l i u m ; i o n i z a t i o n p o t e n t i a l is t h e vo l tage requ i r ed to m o v e an e lec t ron f r o m an a t o m , t he reby t u r n i n g t he a t o m i n to an i o n . C o m p a r i s o n o f i o n i za t ion po ten t i a l s gives an i n d i c a t i o n as
t o t he re lat ive ease w i t h w h i c h the sh ie ld ing gas f o rms the p lasma; t he h igher t he i o n i z a t i o n p o t e n t i a l , t h e more d i f f i cu l t it is t o i n i t i a te an arc. H igh i o n i z a t i o n po ten t i a l s can also result in p o o r arc s tab i l i t y ; h e l i u m , t he re fo re , w e l d s w i t h a less s tab le arc.
Therma l c o n d u c t i v i t y is i m p o r t a n t , because a gas w i t h g o o d t h e r m a l c o n d u c t i v i t y w i l l he lp t o c o n d u c t heat i n to t h e w o r k p i e c e . Deg ree of t h e r m a l c o n d u c t i v i t y has been r e p o r t e d to af fect t he shape of t h e w e l d bead and the c o n d i t i o n o f t he meta l next t o t he w e l d . H e l i u m ' s h i gh t h e r m a l c o n d u c t i v i t y re lat ive t o tha t of a rgon is o n e fac to r w h y w e l d s m a d e w i t h h e l i u m show a b roader w e l d p u d d l e . Dens i t y o f h e l i u m is o n e - t e n t h tha t o f a rgon . Because o f th is d i f f e rence in dens i t y , h igher f l o w rates are genera l l y requ i r ed w h e n w e l d i n g w i t h h e l i u m . A rgon possesses exce l l en t c l ean ing
Table 1—Properties of Shielding Gas
Ionization potential Arc init iat ion Arc stability
Thermal conductivi ty (cal/sq. cm/cm/ °C /s )
Density (relative to air) Cleaning action
Argon
15.8 eV Good Good
0.406 x 10
1.38 Good
Helium
24.6 eV Poor Poor
3.32 X 10-
0.137 Poor
Table 2—Advantages/Disadvantages of Hel ium and Argon
Advantages
Helium • Higher arc voltage and assumed greater heat input (deep penetrat ion and high welding speeds)
• Broad weld root width Argon • Good arc init iat ion and stability
• More effective shielding • Low cost • Good cleaning
Disadvantages
• Poor cleaning • Poor arc init iat ion and stability • Cost • Requires greater f low rates • Narrow weld root width
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 I 147 -s
Fig. 1—Grooved 5083 plate. Groove dimensions: VA in. (6.3 mm) deep and Yi in. (12.7 mm) wide
Fig. 2—Commercially available equipment utilized during weld evaluation. Shown are the welding torch, arc viewer, track wire filler metal feed controller
action. Helium on the other hand provides no cleaning.
Because of the differences in physical properties and characteristics of helium and argon, each shielding gas has its advantages and disadvantages-Table 2. A higher arc voltage associated wi th helium results in a greater heat input and supposedly a deeper penetration. This greater heat input also permits welding at higher speeds. The main advantage associated wi th the high heat input, however, is the broad weld root width obtained when welding wi th helium.
Although the greater heat input is a major advantage for hel ium, this gas also has several disadvantages. Helium does not assist in cleaning the oxide from aluminum plate, and its high ionization potential results in difficult arc initiation and poor arc stability. A greater f low rate is also required wi th helium because of its lower density. Probably the greatest disadvantage of this shielding gas is related to its cost. Helium continues to be more costly than argon.
Argon has many advantages over helium. Because of its low ionization potential, it demonstrates good arc initiation characteristics and excellent arc stability. Argon is also effective in assisting cleaning of the oxide from aluminum plate. Because of its higher density, argon provides a more effective shield from the environment at lower f low rates than are required for helium; also, most importantly, argon is cheaper than helium. The main disadvantage attributed to argon is the narrow weld root width that results when using this shielding gas.
Many of these statements about argon and helium as shielding gases have resulted from qualitative evaluations over several years. However, only limited quantitative data on the effects of shielding gas composit ion or f low rate are available in the literature. As a
result, several "rules of t humb" have evolved as to the benefits of the various shielding gases or the mode of using the various shielding gases w i th out substantial theoretical verification. In this regard, most references on arc welding indicate that shielding gas flow rate should be selected by reducing f low rate until an unstable arc is observed and then increasing the f low rate slightly for a safety margin. Another statement is that helium provides greater penetration.
Previous in-house process development work has suggested that shielding gas f low rates may be just as significant as shielding gas composition in affecting weld penetration and weld cross-section geometry. To quantify the effects of shielding gas composition and f low rate, a program investigating both argon and helium was conducted.
Procedure
Effects of shielding gas composit ion and f low rate on weld penetration and weld cross-section geometry were determined by automatic GMA welding in the flat position. Shielding gas compositions ranging from pure argon to pure helium, including various mixtures, were evaluated at flow rates ranging from 50 cfh (23.5 l iters/min) to 300 cfh (141 liters/min). The program was conducted in two phases as described below.
Phase l-No Arc Gap Control
Initial weld parameters were determined using a shielding gas composed of 50 cfh (23.5 liters/min) helium and 50 cfh (23.5 liters/min) argon. Wire filler metal feed speed, current and travel speed were adjusted to produce a smooth welding arc and a sound weld. The weld settings determined by an experienced welder's observations fol low: voltage-35 V; current-404 A;
travel speed—12 ipm (5 mm/s). These parameters resulted in an arc
length of 0.148 in. (4 mm). Subsequent welds were made maintaining these machine settings as shielding gas composition and f low rate were varied. Initially, three welded samples were made for each condit ion of shielding gas composit ion and f low rate. However, uniform results permitted the number of samples to be reduced to one per shielding gas variable.
Phase II—Arc Gap Control
The procedure for Phase II was similar to that of Phase I. In this phase, however, a constant arc gap of 0.150 in. (4 mm) was maintained first by adjusting current. The procedure was then repeated and arc gap was adjusted by voltage (filler metal feed).
Accurate shielding gas flow rates were maintained through the use of thermal mass meters. Voltage and current were recorded using a Brush recorder, and arc gap was observed by means of an arc viewer mounted on the travel carriage.
All welds were made on a grooved 5083-0 plate, 1 X 9 X 1 2 in. (25.4 x 228 X 305 mm), using 3/32 in. (2.4 mm) diameter 5183 electrode. The groove was VA in. (6.3 mm) deep and Vi in. (12.7 mm) wide—Fig. 1. Shielding gas was commercial welding grade argon and helium.
All welds were cross-sectioned at the midpoint perpendicular to the weld. The cross-section was polished and etched; and weld penetration, width at varying depths,* and crown height were measured. In Phase II, welders' comments concerning arc characteristics were also documented.
*Weld width measured approximately VA in. (6.4 mm) from bottom of weld
148-sl M A Y 1980
Equipment
Commercially available equipment was utilized for this investigation—Fig. 2. This equipment included a water-cooled welding torch wi th a 3/4 in. (19 mm) gas cup, a wire filler metal feed controller, a Gulco KAT track, and a drooping volt-ampere characteristics power supply.
Results
Phase I—No Arc Gap Control
When arc gap was allowed to vary, both shielding gas composit ion and f low rate significantly affected weld penetration—Fig. 3. For pure argon, maximum penetrations were associated wi th 150 cfh (70.5 liters/min) f low rates. Flow rates greater than or less than 150 cfh (70.5 l iters/min) resulted in lesser penetrations. M in i mum penetrations were associated wi th shielding gas compositions between 60 and 80% helium at approximately 150 cfh (70.5 l iters/min) to 200 cfh (94 liters/min) f low rates. Increasing or decreasing the f low rate or increasing or decreasing the percentage of helium in the shielding gas resulted in increased penetration. In no case, however, was penetration as great as the level obtained wi th 150 cfh (70.5 liters/min) pure argon.
Shielding gas f low rate had little effect on weld root w id th . However, shielding gas composit ion significantly affected weld root width. Root widths less than 0.250 in. (6.3 mm) were produced wi th pure argon and increased to 0.4 in. (10 mm) with compositions
NO ARC GAP CONTROL
100
ARGON - CFH
Fig. 3—Effect of shielding gas on weld penetration
containing 70% helium—Fig. 4. Effects of shielding gas on weld
cross-sectional area were complex and are shown in Fig. 5. For shielding gas compositions rich in argon, f low rate had a significant effect on cross-sectional area. Maximum cross-sectional areas of 1 square inch (645 mm2) was obtained at f low rates between 100 cfh (47 liters/min) and 150 cfh (70.5 liters/min) for compositions con
taining 50% or more argon. As the compositions became richer in helium, f low rate had less effect; however, variations in composit ion between 70 and 90% helium did affect cross-sectional area.
These results are further illustrated in Fig. 6, which shows weld cross-section profiles as affected by shielding gas composition and f low rate. Of particular interest is the effect of f low
90% He 80% 60%
50%
20%
10%
150
E 100
s => Zl III
50
9 0 %
/ /
He 80%
Jj
/
70%
/
. 8 5 0 ^ - -
60%
NO ARC GAP CONTROL
\ . 850 SQ. IN.
/ \A.900 \
^V.950 V A A
\ \ j . O O o \ \ Y \
j.ooo \ J>«r\ \ \ 20
50 100 ARGON - CFH
150
Fig. 4—Effect of shielding gas on weld root width
50 100 ARGON - CFH
Fig. 5—Effect of shielding gas on weld cross-sectional area
150
W E L D I N G RESEARCH SUPPLEME NT I 149-s
200 NO ARC GAP CONTROL
U
uJ I
0 0
50
100
ARGON-CFH
150 200
Fig. 6—Effect of shielding gas on weld cross-section geometry
rate on increasing the weld root width when pure argon is used. Weld root profile for 150 cfh (70.5 l iters/min) argon is similar to profiles using an argon-helium gas mixture. A flow rate of 150 cfh (70.5 liters/min) pure argon provides adequate weld width and deep penetration and is a significant
improvement over the narrow weld spike typically associated wi th pure argon. Wide weld widths generally associated wi th pure hel ium or compositions high in helium are most evident at the top of the weld. However, this wide weld bead provides no significant benefit.
Phase II—Arc Gap Control
Arc Cap Controlled by Current. When arc gap was adjusted by current to 0.150 in. (4 mm), penetration was primarily affected by shielding gas f low rate; shielding gas composit ion had essentially no effect—Fig. 7.
80% 60% 90% He 80%
X O 100
50
r \.350 IN. I I / ! 1
I / /
\i //yy^
) \ .350 IN.
Jy^/
/ \ ARC GAP - 0.150"
/ # \ / / J \ ADJUSTMENT BY
/ ^ y \CURRENT
\ / y
JS \ ^^250 I N ^ \
.200 IN. _ \ — * \ "
30%
20%
50 100 ARGON - CFH
50 100 ARGON - CFH
Fig. 7—Effect of shielding gas on weld penetration Fig. 8—Effect of shielding gas on weld root width
150-sl M A Y 1980
Increasing the gas f low rate beyond 100 cfh (47 liters/min) decreased weld penetration.
Both shielding gas flow rate and composit ion affected weld root width—Fig. 8. Mimimum weld root widths were associated wi th pure argon shielding at 50 cfh (23.5 liters/ min). Maximum weld root widths were associated wi th pure helium at f low rates exceeding 100 cfh (47 liters/ min).
Flow rate had a significant effect on weld cross-sectional area for shielding gas compositions rich in argon—Fig. 9. Increasing flow rate decreased weld cross-sectional area. On the other hand, shielding gas compositions rich in helium were not affected by f low rate, although changes in helium concentration did affect the weld cross-sectional area. Maximum cross-sectional areas of 1 square inch (645 mm'2) were obtained wi th pure helium.
The results are further illustrated in Fig. 10. The spike typically associated wi th welds made using pure argon shielding gas are evident in this figure. It is also obvious that the larger cross-sectional area associated wi th welds made using pure helium shielding gas results primarily from a broadening of the top of the weld (weld surface bead). Contrary to what would be expected, the weld root width is not significantly affected either by shielding gas composit ion or f low rate.
Effects of shielding gas composit ion
I50
90% He 80% 70% 60%
E _ i UJ
x
50%
40%
30%
20%
10%
50 100
ARGON - CFH Fig. 9—Effect of shielding gas on weld cross-sectional area
150
and f low rate on arc characteristics are summarized in Fig. 11. A smooth welding arc was obtained wi th a wide range
of shielding gas compositions at f low rates between 100 cfh (47 liters/min) and 125 cfh (58 l i ters/min). Increasing
X
I00
_ j
jjj 50
0
ARC GAP =0.150 in.
ADJUSTMENT BY
CURRENT
0 50 100 150
£L
o _ l UJ >
X o DC < UJ CO
z UJ
0. o _ l UJ
> UJ Q X o DC < UJ CO UJ
or
o. o
X o DC UJ CO UJ
DC
Q .
o
> UJ
o -̂ X DC
< UJ CO
a. o _ J UJ
> UJ
o >*. x o DC < UJ CO UJ DC ARGON-CFH
Fig. 10—Effect of shielding gas on weld cross-section geometry
WELDING RESEARCH SUPPLEMENT I 151-s
100
50
FUZZY \ ARC \
~ VIOLENT / ARC /
I
SMOOTH ARC
V
UNDULATING PUDDLE
I
VIOLENT ARC -
UNDULATING PUDDLE
V I
90% He 80%
Fig. 11-Effect of shielding adjustment
100 150 ARGON -CFH
gas on arc characteristics—current
Fig. 12— Effect of shielding gas on weld penetration SO 100
ARGON - CFH
f low rates beyond 125 cfh (58 liters/ min) resulted in a rough, undulating arc. Violent arc characteristics were associated when pure helium was employed.
Arc Cap Adjusted by Wire Feed. Contrary to the results obtained when arc gap was adjusted by current, when arc gap was adjusted by wire feed, shielding gas f low rate did not affect weld penetration significantly. Penetration, however, was significantly affected by shielding gas composit ion — Fig. 12. Maximum penetrations were obtained for pure argon and decreased
as percentage of helium was increased.
Weld root width was affected primarily by shielding gas composit ion. However, shielding gas flow rate slightly affected root w id th , especially at f low rates exceeding 150 cfh (70.5 l iters/min). Min imum weld root widths were associated wi th pure argon, and maximum weld root widths were associated wi th pure helium, as shown in Fig. 13.
Both shielding gas flow rate and composit ion affected weld cross-sectional area slightly. Maximum cross-
sectional areas are associated wi th pure argon at f low rates less than 100 cfh (47 liters/min)—Fig. 14. These results are dramatically illustrated in Fig. 15. The tremendous benefit of pure argon on weld penetration is obvious from Fig. 15. Of particular interest is the tendency towards a wider weld root width for pure argon at 100 cfh (47 liters/min) than is normally associated wi th pure argon shielding. Weld root w id th increased considerably when pure helium was employed; however, penetration decreased significantly. Benefits of ad-
150
100
SO
,_
9 0 %
450 IN
'y
He
\
Y\400 IN.
w .500 IN.\
4
i.
/ /
\ 1
/ '
8 0 %
. /
/ \ /
.350 IN. /
y^y~-— ^ T "
7 0 %
/SoO IN
6 0 %
ARC GAP - 0.150" ADJUSTMENT BY / WIRE FEED y
/ \ ^y / V \^^/
V ^ " x < 2 5 0 IN.
\ -200 IN. -«^-
J \
90% He
<-> 1 0 0
s 3
50
I \ / / / \ 1 1/
\ i\/y
/
\i
/ \ /
/ \ ARC GAP - 0.150 V / \ ADJUSTMENT BY N ^ WIRE FEED
r \ N , > v ^ / * N s > ^ 6 0 0 SQ. IN. <f
\ \ .650 SQ. I N ^ " > v \
*5*^S^\ ** ^*< y' ^\ \^sv6 0 0 SQ" IN*\
\s ^yy^ \ \ ~ S < - - " ' ' ' ' .650 SQ. I N . \ yc~~~~
,-.700 SQ. I N ! ^ \ _ \X—V*
\ i X \ A
50%
30%
50 100
ARGON - CFH
Fig. 13-Effect of shielding gas on weld root width
50 100
ARGON - CFH 150
Fig. 14—Effect of shielding gas on weld cross-sectional area
152-sl M A Y 1980
150
X y !00
I 5 0
0
ARC GAP • 0.150 in. ADJUSTMENT BY
WIRE FEED
0 50 100
ARGON-CFH 150
Fig. 15—Effect of shielding gas on weld cross-section geometry
100
I o
D _j UJ X
50
0
fig. 1 ad jus
SLUDGE I
FUZZY I ARC
-
/ /
6-Effect yment
i i
SLIGHT \ \ UNDULATING J PUDDLE j
/ ^ \ / \ ROUGH ARC \
HEAVY SLUDGE \
50
of shielding Z
i i
SMOOTH ARC
100
ARGON
as on
(CFH)
VIOLENT ARC • UNDULATING
PUDDLE
SLIGHT UNDULATING
PUDDLE
i
\ \ I
150
arc characteristics-wire
"
feed
z
z o
< DC
III z 111
Fig. 17—Effect of welding current on weld penetration
y
•
-200 - 220 O • A -=£ 200 © a / \ - •- ?2o mm A
ARC GAP CONTROL
O CURRENT • 50% CURRENT - 50% WIRE FEED
A WIRE FEED
ding helium to the shielding gas when adjustments of arc gap are made by wire feed are not evident from these data.
The effects of shielding gas on arc characteristics are summarized in Fig. 16. As was the case when arc gap was adjusted by wire filler metal feed, a wide range of shielding gas composi-
300
tions and flow rates resulted in a smooth welding arc. Increasing flow rates above 150 cfh (70.5 l iters/min) resulted in an undulating puddle, and pure helium created a fuzzy arc which produced shallow penetration.
4 0 0 500
C U R R E N T ( A M P E R E S )
Discussion
It is evident from this investigation that both shielding gas composit ion and flow rate significantly affect weld cross-section geometry. These effects,
a. O _ i UJ >
O DC < LU (/) LU DC
0. o —i UJ
>
x o DC < LLi
z UJ
C L
O _ i UJ
> UJ
a x o DC < UJ
z UJ
a. o _ l UJ > UJ a x o DC < UJ
en UJ DC
Z UJ
a. O _! LU > LU
a x o DC < UJ CO LU DC
WELDING RESEARCH SUPPLEMENT I 153-s
1 1 1
ARC GAP = 0.150*
0 CURRENT ADJUSTMENT
O W I R e F E E D ADJUSTMENT
"
• o •
ft °o%§*
% i U i i
I
• o
I
•
I
•
I
-
-
-
60 70 JOULES x 103
.600
.200
I I
"•
8? o 0
I I
I
o°° o
i
i i ARC GAP - 0.150"
m CURRENT ADJUSTMENT
O WIRE FEED ADJUSTMENT
• •
I I
40 50 80 90
Fig. 18—Effect of heat input and method of arc adjustment on weld cross-sectional area
60 70
JOULES X 103
Fig. 19— Effect of heat input and method of arc adjustment on penetration
.6
J5
A
3
2
I
-
o
-
-
I
I
o
CD O
O
• • •
i
J? ?: o
I
• •
9
I
1
• o
1
1 1
ARC GAP = 0.150"
CURRENT ADJUSTMENT
WIRE FEED ADJUSTMENT
• •
I I
-
-
-
-
-
-
I
\cA* ^
i
i
C\ rP \ b^Jgn-
i
o • A 0
\
^ C p
I
ARGON
100%
— 50%
25%
^
I
HELIUM
-100% 50%
75%
-
60 70
JOULES x 103
Fig. 20—Effect of heat input and method of arc adjustment on weld root width
WIDTH (IN)
Fig. 21—Effect of shielding gas composition on weld penetration/ weld root width ratio
however, are not consistent w i th those previously reported in the literature. Depending on how arc gap was adjusted, much deeper penetration could be obtained when pure argon was employed. More important, however, is the effect of f low rate. Flow rate not only affects weld penetration but weld root width and, by employing proper controls, a desirable weld cross-section geometry can be obtained when using pure argon.
Reasons for the effects of shielding gas composit ion and f low rate on weld cross-section geometry are not obvious. Contrary to what had been reported previously, greater penetration did not result from a higher arc voltage associated wi th pure helium shielding. Penetration, however, was affected by the welding parameters, particularly current (Fig. 17), regardless of voltage (wire feed speed) employed. The differences in weld cross-
section geometry then are attributed to the effects of changing shielding gas composition and f low rate on welding current. This is further illustrated in Figs. 18,19 and 20.
The effect of heat input on weld cross-sectional area is indicated in Fig. 18. Regardless of shielding gas composition and method of arc gap adjustment, weld cross-sectional area increased as expected wi th increasing heat input. The method of arc gap adjustment did, however, affect the weld cross-section geometry. As indicated in Fig. 19, increasing heat input increased weld penetration when adjustment in arc gap was made by wire feed adjustments. Current adjustments, however, resulted in a constant weld penetration even though total heat input was affected substantially. The opposite effect is noted for weld root width—Fig. 20. Increasing heat input by wire filler metal feed adjust
ment decreased weld root width as heat input increased. Conversely, increasing heat input increased weld root width when arc gap was maintained by current adjustment.
True improvements in weld bead geometry are associated wi th increasing weld root width whi le maintaining a constant penetration or increasing penetration. Such improvement has normally been attributed to the use of helium mixtures in shielding gas. Figure 21 shows the ratio of weld penetration to weld root width as a function of shielding gas composit ion. Greater weld root widths for a given penetration were obtained when pure helium was employed; however, greater depths of penetration were obtainable with pure argon. Most interesting, however, is that additions of helium to shielding gas up to a level of 75% helium did not significantly affect ratio of weld penetration to weld root
154-sl M A Y 1980
width.
Conclusion
Results of this work indicate that selection of shielding gas in gas metal-arc welding is an important criteria which affects not only the arc characteristics but the weld geometry. Although much concern has previously been expressed regarding the nar
row weld root widths resulting from argon shielding, proper selection of shielding gas f low rate and method of arc adjustment can optimize weld root width when pure argon shielding is employed and produce satisfactory weld geometries. Consequently, the fol lowing conclusions are warranted:
1. Shielding gas composit ion and f low rate can affect weld penetration and cross-sectional area.
2. The manner by which shielding gas composition and flow rate affected weld penetration and cross-sectional area is dependent on method of arc gap adjustment.
3. Maximum penetrations are produced with pure argon or argon-rich shielding gas.
4. Desirable weld cross-sectional geometry can be obtained wi th pure argon shielding provided f low rate is optimized.
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WRC Bulletin 256 January 1980
Review of Data Relevant to the Design of Tubular Joints for Use in Fixed Offshore Platforms
by E. C. Rodabaugh
The program leading to this report was funded by 13 organizations over a two-year period. The objective was to establish and/or validate design methods for tubular joints used in fixed offshore platforms. The report is divided into four self contained chapters (1) Static Strength (2) Stresses (3) Fatigue (4) Displacements, wherein detailed cross comparisons of various types of test data and design theories are reviewed.
Publication of this report was sponsored by the Subcommittee on Welded Tubular Structures of the Structural Steel Committee of the Welding Research Council.
The price of WRC Bulletin 256 is $13.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St., Room 801, New York, NY 10017.
WRC Bulletin 257 February 1980
Analysis of the Ultrasonic Examinations of PVRC Weld Specimens 155, 202 and 203 by Standard and Two-Point Coincidence Methods
by R. A. Buchanan, prepared for publication by 0. F. Hedden
This report describes two methods of analysis of ultrasonic examination data obtained in a 13-team round-robin examination of three intentionally flawed weldments. The objective of the examinations is to determine the accuracy of independently detecting, locating and sizing the weld flaws, using a fixed procedure.
Computer programs to facility comparison of flaw locations with the ultrasonic data, for each of the specimens and both of the methods, are appended to the report.
Publication of this report was sponsored by the Subcommittee on Nondestructive Examination of Materials for Pressure Components of Pressure Vessel Research Committee of the Welding Research Council.
The price of WRC Bulletin 257 is $11.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St., Room 801, New York, NY 10017.
W E L D I N G RESEARCH SUPPLEMENT I 155-s