Anode/Cathode Geometry and Shielding Gas Interrelationships in GTAW

Electrode tip geometry and groove geometry must be compatible to ensure arc stability

BY J. F. KEY

ABSTRACT. A comprehensive set of experiments to determine specific effects of tungsten electrode tip geometry and shielding gas composi­tions (consisting of pure argon, pure helium, five mixtures of argon and helium, and one mixture of argon and hydrogen) on penetration was con­ducted. Performance of various com­binations of t ip geometry and shield­ing gas in three common weld groove geometries was assessed.

No opt imum electrode t ip geometry or shielding gas composit ion was found for all GTAW conditions. Pene­tration increased wi th electrode vertex angle when welding on flat plate. Helium additions generally increased penetration.

Electrode tip geometry and groove geometry must be compatible to ensure arc stability. Conclusions are based on heat transfer models that are heavily dependent on tungsten elec­trode vertex angle and shielding gas thermophysical properties.

Introduction

Early work by Savage et a/.1 and Chihoski2-' on penetration effects of tungsten electrode (cathode) tip geometry in autogenous gas tungsten arc welding (GTAW) was put in per­spective by Spiller and MacGregor4

who differentiated between partial penetration welds on thick plate and full penetration welds on thin plate. A study limited to Alloy 600 has been presented more recently by Glickstein et a/.5 Various heat transfer models6"15

have been presented to rationalize these results. These works were gener­ally concerned with arcs shielded by inert, pure gases, usually argon.

Since many GTAW procedures call for shielding gas mixtures of argon and

helium or argon and hydrogen as well as edge preparations whose geometry varies for the joint, the interrelation­ships between shielding gas composi­t ion and electrode geometry (both anode and cathode) need to be assessed. This task was initiated to provide this assessment, and, as such, is part of a larger program to fully characterize fusion welds from heat source through unaffected base mate­rial.

Experimental Procedure

All materials used were from the same heat of 12.7 mm (0.5 in.) thick AISI Type 304 stainless steel plate. For anode geometry studies, standard 75 deg V grooves, 40 deg U grooves [4.76 mm (0.188 in.) root radius], and 10 deg narrow grooves [2.5 mm (0.10 in.) root face extension] were machined into the plate.

A solid-state, series-regulated 150 A GTAW power supply was used as the current source. A precision positioner driven by a stepping motor was ut i­lized to move the specimen under a fixed torch. Two-percent thoriated tungsten electrodes, 2.38 mm (0.094 in.) in diameter, and custom, premixed shielding gases were used. Argon-helium compositions were 10, 25, 50, 75, and 90 vol-% hel ium, with the balance argon. A 95 vol-% argon-5 vol-% hydrogen mixture also was eval­uated. Ultra-high purity argon and helium were used as pure, unmixed shielding gases.

Paper presented at the 61st AWS Annual Meeting held in Los Angeles, California, during April 13-18, 1980.

I. F. KEY is Scientist, Materials Engineering Branch, Fuels and Materials Division, EG&G Idaho, Inc., Idaho Falls, Idaho.

Partial penetration (=±30% of plate thickness) single spot welds and con­stant current bead welds were made on flat plates and in grooves to encom­pass a wide range of welding methods. The spot welds represented overlap­ping spot welds produced using high current-short duration current pulsa­tions. Spot welds were 2.0 s in dura­tion, and bead-on-plate welds were made at 3.0 mm/s (7.09 ipm). An arc gap of 1.00 mm (0.039 in.) was used with pure argon but had to be increased as helium was added. Pure helium required a gap of 1.50 mm (0.059 in.).

High-speed cinematography was used to analyze arc dynamics and arc interactions wi th the groove. A 16-mm rotating prism camera was used to f i lm all welds wi th the exception of the bead-in-groove welds (in which groove walls obscured a lateral view of the arc). A camera speed of 500 frames/s was adequate for the pur­poses of this study. Neutral density filters were used to attenuate arc intensity. Timing light marks on fi lm (Eastman Kodak MS 2256 color instru­mentation film) edges allowed quan­tification of temporal events. (See Rey­nolds and Key16 for more details.) Films were viewed using a motion analysis projector.

Table 1 gives the experimental matrix of weld types produced as a function of electrode tip geometry and shielding gas composit ion. All welds were sectioned and prepared for met­allographic examination. Macrophoto-graphs were taken of each section. Fusion zone depth, w id th , and area were measured on the flat plate welds. Only the fusion zone area was mea­sured on groove welds since fiducial reference marks needed for depth measurements were destroyed during welding.

364-s I DECEMBER 1980

Results

Figure 1 shows arc shape and result­ing fusion zone profile as a function of electrode tip geometry for stationary spot-on-plate welds. Note that the arc changes from a "be l l " shape at small vertex angles to a more constricted "ba l l " shape at large vertex angles. Also, the arc is uniformly more con­

stricted for the larger truncation diam­eter, 0.500 mm (0.020 in.).

Macrophotographs of the fusion zone show a corresponding shape change. As the arc becomes more con­stricted, the fusion zone becomes nar­rower and much deeper. These fusion zone dimensions are reasonably con­stant at vertex angles over 90 deg. Figure 2 shows a graphical plot of

these results and corresponding bead-on-plate welds (only the depth /wid th ratio need be plotted since it reflects trends in depth, w id th , and area); the correlation coefficient, r2, only applies to the 15-90 deg portion of the plot. In subsequent plots, r2 wi l l apply to the entire plot.

Figure 3 shows the effect of helium and hydrogen additions to argon

Table 1—Experimental Matrix

Vertex angle and truncation

diameter, deg/mm

Shielding gas composition, vol-%

100 Ar 90Ar/10He 75 Ar/25 He 50 Ar/50 He 25 Ar/75 He 10 Ar/90 He 100 He 95 Ar/5 H2

15/0.125 /0.500

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/0.500

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180

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SOP, BOP SIG, BIG

SOP, BOP SIG, BIG

SOP, BOP SIG, BIG

SOP, BOP

SOP, BOP SIG, BIG

SOP, BOP SIG, BIG

SOP, BOP SIG, BIG

SOP, BOP

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SOP, BOP SIG, BIG

SOP, BOP SIG, BIG

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SOP, BOP SIG, BIG

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a 'Spot-on-plate {2.0 s). " "Bead-on-p late (3.0 m m / s ) . l c lSpot- in-groove {75 V-groove, 40 U-groove, 10 narrow groove), 2.0 s. " "Bead- in-groove (75 V-groove, 40 U-groove, 10 narrow groove), 3.0 mm/s .

Vertex angle

0.125 *fe *^ +. -A. «4fc- ^ 0.500

0.125

WHmyuuuiH WUMHMMI IHUHUHIIilHiB

0.500 mm

w Truncation

Fig. 1-Arc shape and fusion zone profile as a function of electrode tip geometry in a pure argon shield (150 A, 2.0 s spot-on-plate)

W E L D I N G RESEARCH SUPPLEMENT I 365-s

shielding gas on fusion zone profile in spot-on-plate welds. Figure 4A shows graphical results of depth /w id th as a function of helium content for spot-on-plate welds. As expected, helium additions cause a large increase in penetration (273%) when using an electrode with a sharp tip geometry such as a 30 deg vertex angle w i th 0.125 mm (0.005 in.) diameter trunca­t ion. A significant increase (148%) is obtained for a moderate 60 deg tip. However, electrodes wi th blunt 90 or 180 deg vertex angles produced only slightly improved penetration (22 and

8%, respectively) when helium was added.

For these particular welding condi­tions, helium has little benefit if a blunt t ip geometry is used. The poor correlation coefficient for the 180 deg plot and, to a lesser extent, the 90 deg plot is attributed to arc instability. Arcs were generally less stable for blunt electrodes than sharp ones regardless of shielding gas composit ion. Arcs from a 180 deg tip geometry are very unstable unless the weld is made at the electrode's maximum current l imit to ensure uniform emission from the

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SOP: Spot-on-plate weld (2.0 s) BOP: Bead-on-plate weld (3.0 mm/s) Current: 150 A The correlation coefficient (r2) applies only to the 15-90 degree portion of the plot

I I I I 30 60 90 120

Vertex angle (degree)

150 180

Fig. 2—Fusion zone depth/width as a function of electtode tip geometry in a pure argon shield (150 A, 2.0 s spot-on-plate and 3.0 mm/s bead-on-plate)

entire tip. Effects of helium and hydrogen

additions to argon were sought for bead-on-plate welds. These welds were made at a fairly rapid 3.0 mm/s (7.09 ipm) rate. Figure 4B shows results which differ somewhat from corre­sponding spot-on-plate welds. All electrode tip geometries performed similarly to the 90 deg electrode in spot-on-plate welds, where the pene­tration is somewhat greater, but the influence of helium and the amount of scatter, reflected in r2, is similar. How­ever, Larson" reports results from bead-on-plate welds made at slower travel speeds ~1.0 mm/s (2.36 ipm) that are similar to spot-on-plate results in Figs. 3 and 4A. Obviously, welding speed effect on molten pool geometry is responsible for these differences. Figure 5 shows variations in dep th / width due to electrode vertex angle in a 95 Ar-5H2 shield for both spot-on-plate and bead-on-plate welds. Al­though the depth /w id th is uniformly greater at all vertex angles, the trends are the same as for a pure argon shield—Fig. 2.

Figure 6 shows the fusion zone area as a function of helium content and electrode tip geometry for spot-in-groove welds. Figure 6A is for a 75 deg V groove, Fig. 6B is for a 40 deg U groove, and Fig. 6C is for a 10 deg narrow groove. Although helium has a moderate to significant effect on fusion zone area in both the V-groove and U-groove, it generally has less effect in the narrow groove. In general, the sharper electrode t ip geometries produce larger fusion zone areas.

Fig. 3—Fusion zone profile as a function of electrode tip geometry and helium or hydrogen content of shielding gas (150 A, 2.0 s spot-on-plate).

366-s I DECEMBER 1980

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Figure 7 shows c o r r e s p o n d i n g b e a d -i n -g roove w e l d s , w h i c h i nd i ca te that h e l i u m add i t i ons are m u c h m o r e e f fec t i ve fo r inc reas ing f us i on z o n e area in b e a d - i n - g r o o v e t h a n fo r spot -i n -g roove we lds . A l t h o u g h e l ec t r ode t ip g e o m e t r y has l i t t l e e f fec t o n fus ion z o n e area in pu re a rgon , t h e sharp geomet r i es are c lear ly super io r in h igh h e l i u m - c o n t e n t sh ie l d i ng gases. Figure

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8 shows the d e p e n d e n c e of t h e f us i on zone area o n e lec t rode ver tex ang le in a 95 A r - 5 H , sh ie ld fo r b o t h s p o t - i n -g roove and b e a d - i n - g r o o v e w e l d s .

A l i m i t e d n u m b e r o f expe r imen ts w e r e repeated to p r o v i d e an i n d i c a t i o n o f t he stat ist ical va l i d i t y o f results. For s p o t - o n - p l a t e expe r imen ts , errors in d e p t h / w i d t h w e r e less t h a n 10% for " s h a r p , " e.g., 30 deg e l ec t r ode t i p

0 .80

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0.20 -

I I I 9 5 A r - 5 H 2

• Spot -on-p la te

0 Bead-on-p la te

30 60 90 120 150 180 210 240 Vertex angle (degree)

Fig. 5—Fusion zone depth/width as a function of electrode tip geometry in a 95 argon-5 hydrogen volume petcent shielding gas for both spot-on-plate (150 A, 2.0 s) and bead-on-plate (150 A, 3.0 mm/s) welds.

geomet r i es , and up to 25% for " b l u n t , " e.g., 90 deg e l ec t r ode t i p geomet r i es . Decreased arc s tab i l i ty fo r b l u n t e lec­t rodes may a c c o u n t f o r t h e increased va r ia t i on . For b e a d - o n - p l a t e exper i ­ments , errors in d e p t h / w i d t h fe l l w i t h ­in a 10% var ia t i on fo r all e l ec t r ode t i p geomet r ies .

Discuss ion

S p o t - o n - p l a t e and b e a d - o n - p l a t e re­sults f r o m th is i nves t i ga t i on and arc t e m p e r a t u r e measurements 1 8 for t he same c o m b i n a t i o n s o f e l ec t r ode t i p g e o m e t r y and sh ie ld ing gas c o m p o s i ­t i o n suppor t heat t ransfer m o d e l s tha t d e p e n d , in par t , o n e l ec t r ode t ip g e o m e t r y and t h e r m o p h y s i c a l p rope r ­t ies of t he sh ie l d i ng gas. Shaw 6 charac­ter izes an arc p r o d u c e d by an e lec­t r o d e w i t h a smal l ver tex ang le as a l ine heat source tha t is p o o r l y d i s t r i b u t e d . Tempera tu res w o u l d be h igh a long the axis bu t m u c h l owe r e l sewhere . C o n ­versely, an e lec t rode w i t h a large ver­tex ang le p roduces a d i s t r i b u t e d heat source w h i c h is m u c h m o r e e f f i c ien t in p r o d u c i n g d e e p p e n e t r a t i o n w e l d s . L u d w i g 8 f o u n d tha t h igh t h e r m a l c o n ­d u c t i v i t y gases such as h e l i u m a n d hy d r ogen (at cer ta in tempera tu res ) p r o d u c e a u n i f o r m l y d i s t r i b u t e d heat source regardless of t he e l ec t r ode t i p

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 367 -s

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geometry. The high thermal conduc­tivity and specific heat of helium and hydrogen (compared to argon) also aid in transferring more heat to the anode.

Results were compared to previous­ly published data-Table 2.'•'•s Trends in data were similar to Savage et al.' and Spiller et al.* w i th the fo l lowing exceptions: a maximum in fusion zone depth and a minimum in width occurred for electrode vertex angles in

the 75 to 90 deg range. Savage's1 data showed a uniform increase in depth and decrease in width up to 180 degrees. Spiller's4 data showed the opposite trend, i.e., a slight decrease in depth with increasing vertex angles, but a similar trend to Savage's1 data in width. It should be noted that Savage used carbon steel and Spiller used Type 321 stainless steel.

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368-sl DECEMBER 1980

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WELDING RESEARCH SUPPLEMENT I 369-s

ly smal l t r u n c a t i o n , p r o d u c e the mos t stable arcs, espec ia l ly in V g roove and n a r r o w g roove we lds . This may ac­c o u n t , in par t , for the i r equa l or super ior p e r f o r m a n c e c o m p a r e d to b l u n t geomet r i es . A l t h o u g h U grooves accept most e l ec t r ode t i p geome t r i es , t hey are t o o large fo r e c o n o m i c a l m e c h a n i z e d w e l d i n g .

The g roove g e o m e t r y has an a d d i ­t i ona l e f fect o n heat t ransfer to t he anode . Since g roove w a l l s c o n f i n e t h e arc and l im i t rad ia t ion and c o n v e c t i o n losses to t he e n v i r o n m e n t , e l ec t r ode t i p g e o m e t r y has less e f fec t on p e n e ­t ra t i on . This is espec ia l ly t rue in t h e case of n a r r o w - g r o o v e j o i n t s w i t h nearly ver t ica l wa l ls . The o v e r r i d i n g cons ide ra t i on appears t o be arc s tab i l i ­ty w h i c h ind ica tes use of a m o d e r a t e l y sharp e l ec t r ode t i p g e o m e t r y (30-60 deg ver tex ang le) .

Conclusions

Results i nd i ca te t he f o l l o w i n g c o n ­c lus ions on a n o d e / c a t h o d e g e o m e t r y and sh ie ld ing gas i n te r re la t i onsh ips in a u t o g e n o u s , p a r t i a l - p e n e t r a t i o n G T A W :

1. Pene t ra t ion increases as t he tungs ten e lec t rode t i p is m a d e m o r e b l un t (15-90 deg) fo r b o t h s p o t - o n -p la te a n d b e a d - o n - p l a t e w e l d s .

2. O p t i m u m e lec t rode t i p g e o m e t r y is gove rned by arc s tab i l i t y w h e n w e l d ­ing in a g roove fo r b o t h s p o t - o n - p l a t e and b e a d - o n - p l a t e w e l d s . Path l eng th t o g r o u n d must be c o n s i d e r e d . Sharp t i p geomet r ies (30-60 deg) are pre­fe r red .

3. H e l i u m and h y d r o g e n have a benef i c ia l e f fec t on p e n e t r a t i o n for b o t h s p o t - o n - p l a t e and b e a d - o n - p l a t e we lds . The e f fec t is espec ia l ly s t rong in h e l i u m mix tu res fo r sharp e l ec t r ode t i p geomet r ies , A specia l case arises fo r s p o t - o n - p l a t e w e l d s in w h i c h b l u n t e l ec t rode t i p geomet r i es are q u i t e e f fec t i ve in pu re a rgon .

4. H e l i u m also has a bene f i c ia l ef fect o n p e n e t r a t i o n w h e n w e l d i n g in a g roove fo r b o t h spot and bead we lds .

5. Data favor heat t ransfer mode l s tha t d e p e n d , in part, o n e l e c t r o d e t i p g e o m e t r y and t h e r m o p h y s i c a l p roper ­t ies of t h e sh ie ld ing gas. These p a r a m ­eters af fect b o t h d i s t r i b u t i o n a n d t ransfer of heat to the a n o d e w i t h resu l t i ng s t rong ef fects on fus ion z o n e p ro f i l e .

6. N o o p t i m u m c o m b i n a t i o n of e lec t rode t i p g e o m e t r y or sh ie l d i ng gas c o m p o s i t i o n was f o u n d fo r al l G T A W c o n d i t i o n s .

Acknowledgments

A p p r e c i a t i o n is e x t e n d e d to L. D.

Fig. 9—Influence of electrode tip geometry and groove geometry on path length to ground: A—75 deg V groove; B—40 deg U groove; C—10 deg narrow groove

Reynolds for his c o n t r i b u t i o n s in e q u i p m e n t des ign and f ab r i ca t i on and fo r p e r f o r m i n g h i gh -speed c i n e m a t o g ­raphy and w e l d i n g .

This w o r k was s u p p o r t e d by t h e U. S. D e p a r t m e n t o f Energy, Assistant Sec­retary for Research and D e v e l o p m e n t , O f f i ce of Basic Energy Sciences, unde r D O E Con t rac t No . DE-AC07 -76ID01570.

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