wj_1971_11_s467 tig electrode material vs arc carachteristics

7/29/2019 Wj_1971_11_s467 Tig Electrode Material vs Arc Carachteristics

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E f f e c t of A n o d e C o m p o s i t i o n o n T u n g s t e n

A r c C h a r a c t e r i s t i c sR e l a t i v e l y m i n o r c h a n g e s in a n o d e c o m p o s i t i o n

c a u s e s i g n i f i c a n t c h a n g e s in t h e a r c p o t e n t i a l -

a r c c u r r e n t c h a r a c t e r i s t i c s of i n e r t g a s

s h ie ld e d t u n g s t e n a r c s .

B Y T H E O D O R E F. C H A S E , J R . A N D W A R R E N F . S A V A G E

A B S T R A C T . T he effect of alloy com positio non the propens i ty for hot c racking hasrece ived cons iderable s tudy . However ,l i t t le effort has been expended in investigating the influence of alloy compositionon the fus ion zone d imensions and weld

T . F . CHA SE a nd W. F . SA VA GE a r e w i t hR ensse laer Polytechnic Ins t i tu te , T roy,New York.

penet ra t ion . T he welding arc has beeninves t iga ted and analyzed by many inves t i ga tors . Unfor tunate ly many of the repor tss tudying the arc have depended upon orhave repor ted fac ts concerning arc phenomena which may not apply to the ac tua lwelding process. I t is interesting to notetha t few repor ts have been publ ished whichconsider the behavior of an ac tua l weldingarc . For example , quant i ta t ive descr ip t ions

of the arc ar rived a t by ca lor imetr ic m ean s ,s tudies of h igh-current shor t dura t ion arcsand low-current long-arc-gap carbon arcsmay not be appl icable to welding arcs .These var ia t ions in exper imenta l techniqueshave a lso produced disagreement amonginves t iga tors regarding the ro le the anodeplays in a welding arc.

T his inves t iga t ion cons is t s of a s tudy ofthe effect of anode composition on the arc

'r—~"

A l

SI *• A l SJ ' M N

AN *T I

F i g . 1—Cross-sections of fusion w elds in Inconel 600 with s elected eleme nt addit io ns

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 467-s



Table 1— i

Al loyinge lemen t

Fe %CrNiSPSi

MnTi

Al

CCu

Composition of 1

f

1

9.6314.874.50.0050.0010.007

0.0560.02

0.03

0.0030.016

4

9.4714.873.80.0040.0010.14

0.0420.02

0.03

0.0040.012

:usion Zone in Weld Specimens Shown In

5

10.114.674.10.0130.0010.07

0.9000.02

0.03

0.0020.012

— Ident i f ica t ion6

10.614.973.60.0040.0020.08

0.0740.22

0.03

0.0020.014

7

9.5214.873.60.0040.0020.06

0.0450.02

0.30

0T0020.013

number17

9.7914.675.30.0040.00070.29

1.500.01

0.03

0.0020.01

19

9.9214.874.50.0040.00050.23

0.0500.01

0.21

NTAT

0.01

Fig. 1

20

10.314.874.30.0050.00090.07

1.300.30

0.03

N.A.0.01

22

9.2314.575.00.0040.0010.07

0.0600.35

(j\33

0.0030.014

Table 2—Summary of Dimensions ofWelds and Element Additions Involved

Depth Cross-of pene- sectional

Iden t i f i - t ra t ion area E lementca t io n cm sq cm added

potential-arc current characteristic curve.Precautions were taken to assure thatequivalent experimental procedures werefollowed for all runs and typical weldingparameters were used. To minimize thecomplex interaction effects one would have

in investigating a multicomponent alloysuch as stainless steel, single elementadditions were made to high purity nickel.These minor element additions were foundto influence an argon-shielded tungstenarc considerably.

T he arc potential at a given current andarc length was altered by changing theanode composition. The slope of the arcpotential-arc current characteristic curvewas also found to be influenced by theelement addition. A lthough no simplecorrelation was found between the observedarc characteristics and the physical properties of the minor alloying additions, theobserved effects are believed to be ofsignificant practical impo rtance. A secondpaper, currently in preparation will describethe results of a study of the voltage distribution in a GT A arc which was performedas part of the investigation of the effectof anode composition on tungsten arccharacteristics.

Introduction

Weldability as defined by A WS is"the capacity of a metal or combinationof metals to be welded under fabrication conditions into a specific suitablydesigned structure and to performsatisfactorily in the intended service."

Weldability of materials has been thesubject of innum erable investigationssince Nikolas Von B ernardos andS tanislov Olczewski received theirpatent in the field of arc welding in1885. During the past decade greatstrides have been made in relatingweldability to the chemical compositionof the weldment and base material

Hot cracking propensity is onecriterion often chosen as a means ofevaluating weldability. It is generallyagreed that hot cracking occurs abovethe solidus temperature of the lowestmelting phase present. 2 T he effect ofalloy composition on the propensityfor hot cracking has received considerable study. 3 ' 4i 5 However, the in

fluence of alloy composition on thefusion zone dimensions and weldpenetra tion should also be included inany list of considerations for judging amaterial's weldability. Unfortunately,little effort has been expended in in

vestigating the effects of material composition on fusion zone dimensions.

Observations have been reported byOyler

6et a l , Goodwin 4 , and Ludwig7

concerning differences in fusion zonedimensions as a result of minor composition variations. T hese investigations involved complex alloys but theconsensus of the authors was thatminor composit ional var ia t ions markedly influenced the melting efficiencyof the gas tungsten arc (GT A ) process.

Since the present investigation alsoinvolves the GT A process, a brief

review of earlier welding arc investigations is warranted.

Welding Arc Physics

One of the first reviews on the weldingarc was by S praragen an d Lengyel8 anda more recent one was prepared byJackson.1 Jackson sta tes, "A weldingarc consists of a sustained electricaldischarge through a high temperatureconducting plasma, producing sufficientthermal energy so as to be useful forthe joining of metal by fusion. "

T he welding arc has been analyzed

by many investigators . Unfor tunate lymany of the reports studying the welding arc have depended upon or havereported facts concerning arc phenomena which may not apply to the actualwelding process. It is interesting to notethat few reports have been publishedwhich consider the behavior of anactual welding arc. For example, quantitative descriptions of the arc arrivedat by calorimetric m e a n s

9 , 1 0' "•12 studies

of high-current short duration arcs13

and low-current long-arc-gap carbonarcs

14 may not be applicable to weldingarcs .

Goldman and White15 concluded intheir investigation of six anode materials (aluminum, silver, cobalt, copper,

1456

717

19

20

22

0.3790.4750.4670.635

0.3440.537

0.635

0.546

0.291

0.4450.6160.5520.758

0.4580.575

0 .633

0.685

0.302

S i

MrT i

AlS iM r

SiAlMr

T iT i

Al

N o n e

( 0 . 1 4%)i ( 0 . 9 %)

( 0 . 22%)

( 0 . 30 %)( 0 . 29 %)

i ( 1 . 5 0 %)( 0 . 23%)( 0 . 21 %)

i ( 1 . 30 %)( 0 . 30 %)

( 0 . 35 %)

( 0 . 33%)

tungsten and zinc) that there was noappreciable effect of anode composition on the arc potential-arc current

characteristic curve. However, theseinvestigators used anodes so efficientlycooled as to remain solid. Skolnik andJones

16 have reported that "in the inertgas arc the anode drop is zero ornegligibly small." However, they conducted their investigation at arc currents less than 100 am p. In a dditio nMorr is and Gore1 7 have stated that"the anode plays no part in the arcmechanism except to serve as thepositive electrode and there is noreason why the arc mechanism shouldvary with a change in the anode mater ia l ." Morr is and Gore a lso reportedthat "the melting rate of the workpieceis,'for a given current, dependent onlyupon the thickness of the metal and itsthermal conductivity, while the arcdelivers the same power to the workregardless of the work metal."

A nother investigator , R abkin, 18 statesthat the anode voltage drop does notdepend on the material of which theelectrodes are made or on the composition of the gases in the arc column,and is a constant quantity equal to 2.5± 0.2 volts. However, recent investigat ions by Goodwin 4 and Ludwig7 con

clude that the welding arc is sensitiveto the anode material. Ludwig suggested that an energy balance existsat the anode and he postulates themechanisms by which heat is bothsupplied to the anode and transferredfrom the anode to its surroundings.Ludwig suggests that the variations infusion zone dimensions which he observed for different heats of the samealloy are due to changes in the anodespot size and that this change is relatedto the amount of easily ionized components present as impurit ies . S ubsequently, Savage et al19 have postula tedthat the formation of oxides on thesurface of a weld puddle may alter themagnitude of the anode drop. T his in

468-s | N O V E M B E R 1 9 7 1



turn could modify the fusion zonedimensions.

Goodwin4

and Dickinson 20observed

considerable variations in fusion zonedimensions which appeared to beat t r ibutable to the concentrat ion ofminor elements in Inconel 600. T heseinvestigators studied 64 different alloycompositions within the compositionlimits for Inconel 600. A dditions ofsix minor elements were investigatedusing a full factorial experimental des ign . Photomacrographs of the crosssections of typical welds made underidentical conditions in 9 of the 64alloys constituting Goodwin's six factorial experiments are shown in Fig. 1.T able 1 summarizes the chemicalcompositions of the nine specimenswhich are identified by the numbersdirectly above the weld fusion zones inFig . 1 . T he underl ined composi t ionsrepresent the intentional additions tothe basic ternary, whose compositioncorresponds to that of Specimen 1.

T able 2 summarizes the depth ofpenetration and cross-sectional areas ofthe weld fusion zones shown in Fig. 1.Note that the depth of penetrat ion

varies by a factor of ' =2.18 (com

pare Specimens 19 and 22) while the

cross-sectional area varies by a factor of

' = 2 .51 (compare S pecimens 6

and 22). Of the listed additions, onlyaluminum and a combinat ion of t i tanium and aluminum decreased thepenetrat ion , while only the combinat ion

of t i tanium and aluminum decreasedthe cross-sectional area. A ll otheraddit ions summarized in T able II increased both the penetration and thecross-sectional area of the fusion zonesabove the corresponding values for thebasic ternary (Specimen 1).

A s a result of these and othe r complex interaction effects noted by Good

win and the obvious disagreementamong invest igators regarding the ro lethe anode plays in the arc voltage distribution, it was decided to initiate a

comprehensive study of the effect ofanode composi t ion on the characteristics of a gas shielded tungsten weldingarc .

High purity nickel was chosen as thebasic component of the anode in theinterest of simplicity and the effect ofaddit ions of control led amounts ofselected alloying elements on the arccharacteristics was studied. T he following report summarizes the objectives ofthe investigation, the materials and procedures used and the results obtained.T he effort was actually divided into twophases : T he effect of anod e com position on the arc potential-arc currentcharacteristics, while a second paper nowin preparation will be concerned withthe influence of anode composition onthe potential distribution in the arc.

O b j e c t i v e

T he objective of this part of the investigation was to study the effect of

anode composi t ion on the arc potent ial-arc current characteristics associatedwith an inert gas shielded tungsten arcwelding process.

Material and Apparatus

High purity nickel bar stock i}/i by2 in.) was selected as the base metal forthis investigation to avoid the complexinteractions one might encounter with amult i -component m aterial . T he chemical analysis of this bar stock is summarized in Ta ble 3. The as-receivedmaterial h ad been annealed for 3 hr at1350F subsequent to cold-rolling to

gage.T he composition of the fusion z one

of autogenous GT A spot welds wasmodified by introducing inserts ofselected elements in a J-fg "i - diameterby }•{& in. deep hole drilled in 2 by 2 byYi in specimens of the high purity barstock. A plug cut from J/fg in . nickelwire (composi t ion summarized in Tab le3) was then pressed in place to seal th eaddit ions from the ambient atmosphere .B y posi t ioning the GT A torch directlyabove the insert, the encapsulated alloy

addition was melted and mixed with thehigh purity base material during thearc on-time, thus creating a moltenanode of modified composition.

T he elements added to the basematerial in this fashion were selectedaccording to their availability in pureform and their physical and chemicalproperties . A ll elements chosen foraddition were of 99.9 % purity orbetter. Wherever possible, materialsavailable in bulk form were employedas additions to minimize the effect ofsurface contaminants characteristic ofpowders (which have an inherently highsurface-to-volume ratio).

T able 4 lists the elements chosen asadditions and the form in which theywere added, together with those physicalproperties

21'

22considered to be perti

nent. T he work functions listed inTable 4 were obtained from Fomenko 'shandbook which was judged the m ostrecent and authoritative reference. Ofthe several methods for determining

the work function of an element,determinations based upon thermionicmeasurements were judged most pe rt i nent to this investigation and valuesobtained by this method are thus citedwherever available. It is of interest tonote, howe ver, that significant differences in the reported values of the workfunctions of some elements were encountered in searching the literature.

Sample Preparation

Figure 2 shows in schematic form the

details of the specimens used in this

portion of the investigation. Elevenspecimens were prepared, each con

taining a 50 milligram insert of one of

the eleven addition s listed in T able 4

(including one specimen with only Ni

wire added as a control specimen).

Whether in bulk or powder form, the

insert was compacted in the predrilled

hole using a unit pressure of the order

of 100,000 psi . A N i-wire plug was then

inserted and peened flush with the sur

face to seal the co mpa cted insert in

place.

T a b l e 3—Chemical C o m p o s i t i o n s

Base

C

S

M n

Fe

C u

N i

Nickel-\

C

S

M n

Fe

C u

SiT i

N i

metal

0.03 %

0.005

0.25

0.04

0 .01

b a l a n c e

vire plugs

0.03 %

0.005

0.24

0.07

0 .01

0.363.06

b a l a n c e

T a b l e 4—Physical Proper t ies of Anod

E l e m e n t

Al

Ca

Y

Cr

Sn

M n

N i - w i r e

C u

Fe

Si

S b

A r g o n

F o r m

B u l k

P o w d e r

B u l k

P o w d e r

B u l k

P o w d e r

B u l k

B u l k

P o w d e r

P o w d e r

B u l k

C h a r a c t e r i

(99 .999%)

e Compom

a t . w t .

26.97

40.08

88.92

52 .01

118.70

54.93

58.69

63.54

55.85

28.06

121.76

sties of Sh

39.94

ents

<t> ( e v )

4.25

2.80

3.30

4.58

4.38

3.83

4.58

4.40

4 .31

4.80

4.08

i e l d i n g G a s

I.P.

( v o l t s )

5.96

6.09

6.58

6.74

7.30

7 .41

7 .61

7.68

7.83

8.12

8.50

15.68

B.P. (°C)

2457

1487

2927

2482

2270

2097

2732

2595

3000

2355

1380

- 1 8 5 . 7

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 | 469-s



Apparatus

A gas-tight chamber (8 in . long by6 in. wide by 4 in. high) was constructedof lucite with two 8 by 4 in. plateglasswindows serving as the front and rearwalls of the chamber in order that highspeed motion pictures could be obtainedof the arc . A water-fil led U-tub emanometer was employed to measurechamber pressure during each run . Inaddition, a dewpoint monitor installedwithin the chamber was employed toinsure that the shielding gas was sufficiently free of moisture to preventgross oxidation of the molten pool . A nexhaust valve connected to the chamberwas adjusted to maintain the chamberpressure at a constant level.

Electrical Equipment. A ll welds weremade with straight polarity dc using a600 amp saturable reactor type powersupply in a constant current mode.Electrodes were centerless ground 0.125-in . d iameter, 2%-thoriated-tungsten ,dressed to a conical tip with a 90 deg

apex angle. Direct-inking recorderswere used to monitor and record the arcpotent ial and arc current . T he currentand vol tage measurements were madewith an accuracy of ± 3-4% by calibrating the recorders against a suitablelaboratory s tandard .

Exper imenta l Procedure

Pre-Run Procedure

Each specimen received equivalentpre-run preparat ion, including debur-ring, degreasing and fixturing. Precaut ions were taken to assure thatequivalent experimental procedures werefollowed for all runs. T hese prec aution sincluded the establishment of equivalent chamber pressure, arc gap spacing,

1/16" DIA HOLE- 1 / 16" DEEP

ELEMENT^ADDITION

0 . 06 0 N i WIRE

FUSIONZONE

- 9 9 . 5 8 % Ni BASE METAL

Fig. 2—Schematic representation ofsample preparation

470-s | N O V E M B E R 1 9 7 1

dew point , c inematographic condit ions ,recorder cal ibrat ion , durat ion of cham

ber purge and equipment warm-uptime.

Welding Procedure

T he welding conditions used for thisinvestigation are listed in T able 5.Prior to making the arc potent ial-arccurrent data runs a mixing run wasmade on each specimen to insure that

melting and mixing of the insert materialwas complete. High-speed movies weretaken during th is run . A rc potent ial-arccurrent characteris t ic data were obtained by recording the arc voltage as afunct ion of arc current at 25 ampintervals. T hese data were fed into acomp uter to generate leas t-squares analysis plots of arc voltage vs arc currentover a range of arc currents extendingfrom 200-350 amp for each specimen.

Resul ts and Discuss ion

Effect of Additions onArc Characteristics

Figures 3 to 13 present the arc-potential, arc-current characteristics obtained for the eleven different a no decompositions studied. In all cases, thesolid circles represent the actual dataand the height of the vertical bar througheach point corresponds to the 95%confidence limits for the computergenerated least squares fit (heavy solidline). In every instance the data fallwithin the 95% confidence limits for astraight line described by the equation:

E = £,oo - RP(I - 200) (1)where : E = arc potent ial at a currentlevel I (volts); / = arc cu rrent (am p);£200 = arc potent ial at 200 amp; Rp =dynamic resistance of the arc.

Figure 3 shows the arc potential as afunction of the arc current for the purenickel anode containing a nickel wireinsert used as a control specimen. T his

14

o

<

UJ1-o

otc<

II

IO

• ARC PC T E N T I A L-CURF

CHA RACT ERI ST I CS FOR 9<

EN T

) .58%Ni

2 5 0 3 0 0AR C C U R R E N T ( A M P E R E S )

Fig. 3—Arc potentia l-arc current characterist ic curve for 99.58% Ni

arc characteristic is reproduced in Figs.4 to 13 as a dashed line to permitcomparison of each of the other ninearc characteris t ics with that of the control sample. Inspection of these figuresreveals that the 50 mg additions eachhad a distinctive influence on the arccharacteris t ic . For example, note thecurve for an aluminum addit ion shownin Fig. 4 . A lthou gh th e slope is nearlythe same, the arc potential is shifted

upward by nearly 0.5 volt over theent ire range from 200 to 350 amp. Onthe o ther hand, the 50 mg addit ion ofcalcium caused a marked change in theslope of the arc characteristic as may beseen by inspecting Fig. 5.

T he s lope of the arc potent ial , arccurrent characteristic corresponds toRp, the dynamic resistance term inEqua t ion 1 . T he dynamic res istance isdefined as the change in arc potentialper unit change in arc current and iscalculated as follows from the data inFigs . 3 to 13:

AE ^ E3IQ — £200

A /=

350 - 200(2 )

where : AE = change in arc p otentia lfor a g iven change, A / , in arc curre nt ;£350 = arc potent ial at 350 amp; E i00 =arc potent ial at 200 am p; Rp = dynamicresistance in ohms.

T he reciprocal of the dynamic resistance, AI/AE = 1/Rp, to borrow aterm from vacuum tube technology,may appropriately be termed the " tran s-conductance" of the arc, p ., and wouldhave units of reciprocal ohms, or mhos,if calculated as follows from the datain Figs . 3 to 13:

M =A / 350 - 200

AE £350 — £21(3 )

T able 6 summarizes the values of

£350, £200, RP, and p . obtained from Figs .

2 5 0 3 0 0A R C C U R R E N T ( A M P E R E S )

Fig. 4—Arc potential-arc current characteristic curve for 99.58% Ni + 50mg Al



_ARC P O T E N T I A L - C U R R E N TC H A R A C T E R I S T I C S

FOR 9 9 . 5 8 N i + 5 0 m g Co- F O R 9 9 . 5 8 N i

2 5 0 3 0 0A R C C U R R E N T ( A M P E R E S )

Fig. 5—Arc potentia l-arc current characteristic curve for 99.58% Ni + 50mg Ca

3 to 13. T he values listed should be

compared with the corresponding valuefor the Ni-wire control specimen todeduce the effect of the individualadditives. Note that in no case is thevalue of £2oo with an added elementless than that for the control specimen.S ilicon exerts the greatest influence(+1.09 vol ts) , fo l lowed in descendingorder by ant imony, copper, manganeseand aluminum (both + 0.65 volts), tin,y t t rium, iron (+ 0 . 32 vo l ts ) . T he l as ttwo elements , calcium and chromium,have no significant influence on £>oo-

A t a current of 350 am p, s i l icon,

aluminum, ant imony, i ron, copper,and tin additions increased the value of£350, while calcium, yttrium, chromiumand manganese decreased £350 belowthat of the control sample .

T he dynamic res is tance values , tabulated in milliohms (1 0

_ sohms) for all

ten elemental additions are less than the

Table 5—GTA Welding ConditionsUsed to Obtain Arc Potential-Arc Current Characteristic Curves

200 250 3 0 0 3 5 0A R C C U R R E N T ( A M P E R E S )

Fig. 6—Arc potentia l-arc current characteristic curve for 99.58% Ni + 50mg Y

value of 20.0 milliohms observed for the

control sample, with calcium having thegreatest effect (R p = 13.9 milliohms)and iron the least (R p = 19.4 milli

ohms). T he percentage change in Rp

(r ight-hand colum n in T able 6) rangesfrom — 30.5% for calcium to — 3.0%

for Fe. T he values of the tran scon-ductance, M, in mhos show the opposi tetrend, being the reciprocals of the Rp

values . Note that p varies from a lowof 50 mhos for the control sample to ahigh of 71.94 mhos for calcium.

Pract ica l S ign i f icance

T he practica l significance of theseobservations can best be demonstratedby the following hypothetical examples.Firs t , suppose that a GT A process wereselected to weld commercially purenickel using a power supply operatingin the constant current mode togetherwith a welding head designed to maintain a constant arc vol tage by automatically adjusting the arc length.A ssume that a welding curre nt of 300am p and a welding voltage of 12.8 v

_ARC P O T E N T I A L - C U R R E N T

C H A R A C T E R I S T I C SFOR 99.58 %Ni + 5 0 m g C rFOR 9 9 . 5 8 % Ni

200 250 300 350ARC CURRENT (AMPERES)

Fig. 7—Arc potentia l-arc current characteristic curve for 99.58% Ni + 50mg Cr

was found to provide the desired weld

geometry in the pure nickel materialused in obtaining the data shown inFig. 3 .

Now suppose that a heat of nickel isobtained with an aluminum contentcorresponding to that of the specimenused in obtaining the data shown inFig. 4 . Note that for an arc length of5^2 in., Pig- 4 indicates that this materialwould exhibit an arc voltage of 13.3 vat 300 am p. T hus the automa tic headwould decrease the arc length until thepreset voltage of 12.8 v was establishedwith the 300 amp welding current .

Obviously such a reduction in arclength would decrease the width of theweld pool and increase the depth ofpene tration. T herefore, the weld in theheat contain ing aluminum would benarrower and deeper than that in thepure nickel used in establishing theprocedure, in spite of the fact that theprocedure was followed rigorously.

On the other hand, suppose that theprocedure was established for a constant arc length of ^32 in . with an arccurrent of 300 amp. Under these condi-

W e l d i n g c u r r e n t

W e l d i n g v o l t a g eA r c g a p

( m e a s u r e d w i t h

c o l d e l e c t r o d e )

S t i c k - o u t ( c o l l e t t ot i p dis tance)

W e l d i n g s p e e dS h i e l d i n g g a s f l o w

C h a m b e r p r e s s u r e

p r i o r t o r u n

d u r i n g r u n

C h a m b e r d e w -

p o i n t

E l e c t r o d e

200 to 350 (25 a m p

i n c r e m e n t s )

( d e p e n d e n t v a r i a b l e )

0.156 ± 0.002 in.

0.438 ± 0.010 in.

0 ( G T A s p o t we l d )30 c fh a rg on

( 9 9 . 9 9 9 % p u r i t y )

3 0 m m H20

35 m m H20

- 6 4 F t o - 6 0 F

Ya in . d iam, 2%t h o r i a t e d t u n g

s t e n , g r o u n d t oc o n i c a l t i p w i t h 9 0

d e g a p e x a n g l e

„

T a b l e 6—Summary o f A r c P o t e n t i a l - A r c Cu r r e n t Ch a r a c t e r i s t i c s

E l e m e n t

a d d e d t o

n i c k e l a n o d e

AlCa

YCr

Sn

M n

N i - w i r e

C u

Fe

Si

S b

* See text fo r

A r c

at 200A EM0

( v o l t s )

11.4610.87

11.2710.86

11.405

11.46

10.81

11.53

11.13

11.90

11.58

d e f i n i t i o n .

v o l t a g e

at 350A E350

( v o l t s )

14.2612.95

13.5713.59

13.97

13.66

13.81

13.99

14.04

14.39

14.19

D y n a m i c

r e s i s t a n c e *

R„( IO"

3o h m s )

18.713.9

15.318.2

17 .1

14.7

20.0

16.4

19.4

16.0

17.4

T r a n s -

c o n d u c t a n c e *

( m h o s )

53.4871.94

65.3654.95

58.48

68.03

50 .

60.98

51.46

62.5

57.47

P e r c e n t a g e

c h a n g e i n

RP

(%)

- 6.5- 3 0 . 5

- 2 3 . 5- 9.0

- 1 4 . 5

- 2 7 . 0

C o n t r o l

- 1 8 . 0

- 3,0

- 2 0 . 0

- 1 3 . 0

W E L D I N G R E S E A R C H S U P P L E M E N T \ 471-s



250 300ARC CURRENT (AMPERES)

Fig. 8—Arc potentia l-arc current characteristic curve for 99.58% Ni + 50mg Sn


Fig. 9—Arc potentia l-arc current characterist ic curve for 99.58% Ni + 50mg Mn

/ARC POTENTIAL-CURRENT

CHARACTERISTICS —-FOR 99.58 %Ni + 50mg Cu-FOR 99.58 %Ni


Fig. 10—Arc potent ia l -a rc curren t characte rist ic curve for 99.58% Ni + 50 mgCu

t ions , the welding voltage of 13.3 v

would be 0.5 v higher for the heatconta ining a luminum than the 12.8 vcharacteristic of the pure nickel heatused in establishing the procedure.While this is a difference of only 4% inthe total arc voltage, the influence ofthis change on the weld geometrywould depend on the region of the arcin which the most significant proportion of the change occurred. Forexample, if the 0.5 v ch ange was confined to the anode drop region and increased the anode drop from say 3.5 vto 4.0 v, the variation would correspond

to a 12.5% change in anode drop, andthus in all probability would cause asignificant increase in melting efficiency.

From the above discussion, it isevident .that a knowledge of the effect ofanode composit ion on the voltage-current characteristic alone is notsufficient to predict the influence ofano de com position on the weld geometry . R ather , one must a lso determinethe effect of the anode composition onthe voltage distribution as well. T hus, amore complete analysis of the overallinfluence of anode composition mustbe deferred pending publication of the

report on the effect of alloying additions on the voltage distribution in anarc . None-the-less, the significance ofthe data shown in Figs. 2 to 4 should beobvious to the practicing weldingengineer.

Poss ib le In f luence o fAdd i t i ves

T he physical properties of the alloy

ing addit ions summarized in Table 2were selected on the basis of their

possible influence on the characteristics

of a GT A welding arc . Unfor tunate ly,the observed variations in arc charac

teristics do not seem to bear a straight

forward relationship to any of the prop

erties listed. T hus , it is only possible toreview the probable role of each of theproperties listed and indicate what additional data not currently available maybe necessary to explain the observedchanges in arc characteristics.

T he ionization potential, I .P., of anelement in an arc should influence thearc characteristic since metals in generalhave a much lower ionization potentialthan the shielding gases used in welding.Morr is and Gore 17 have shown that thecharacteristic curves associated w ith fivedifferent shielding gases possess dif

ferent slopes and that the curves areshifted to higher voltages according tothe ionization potential of the shieldinggas. Mantel23 suggests that metal vapororiginating from either of the electrodesincreases the conductivity of the arc,because of the lower ionization potentials of the metal vapors. However, theconcentration of the added elements isonly of the order of 1 % in the weld pooland it seems unlikely that this couldcause a large enough change in the ionization potentia l of the plasma. Furthermo re, inspection of T able 6 reveals thatthe variations cannot be correlated with

the ionization potential of the additive.A ccording to the Langevin equation,

the mobility of a charged particle isinversely proportional to the mass of theparticle. T hus the atomic weight of themetal whose vapor enters the arc shouldalso influence the conductivity of theplasma as a result of the influence ofmass on the mobility of the metal ions.However, no correlation with atomicweight appears in the data in Ta ble 6.

T he electronic work function, 4> , ofthe element addition might also be expected to influence the volt-ampere

characteristic of an arc . Specifically, analloy addition which lowers the workfunction of the anode should increase

the number of secondary e lectrons con

tributed to the negatively charged sheathadjacent to the node. Finally, the boiling points of the elemental additionsshould be considered, since the ease withwhich an element can leave the anodeand enter the plasma as metal vaporshould influence the arc characteristic .However, the boiling point of the element itself is not an adequate criterionon which to base the escaping tendencyof an atom from a molten binary alloy.T his follows since the boiling poin t ofbinary systems depends upon the bonding preferences of the elements involved.

In general, a preference toward dissimilar bonds would tend to increase theboiling point, whereas a preference forlike bonds would tend to decrease theboil ing point . Unfor tunate ly, in adequatedata on bonding energies prevents quantitative treatment of this problem atthis time.

A nother c ircumstance which couldinfluence the escape of the element fromthe anode to the plasma is the involvement of the element in a chemicalreaction at the surface of the weldpuddle . A strong tendency for someelements to react with contaminants in

the welding atmosphere could fix theelement in the form of a stable reactionproduct on the surface of the moltenweld poo l. T o com plicate matters further, stable reaction products floatingon the surface of the molten pool havebeen observed to influence GT A arcsconsiderably. S imilar effects were reported by Wilkinson and Milner24 andby S avage, et al.19 Such effects may wellinfluence the heat transfer from the arcto the work in some complex fashion.

T here is also the possibility that t hesurface tension of the molten metal may

be changed by the presence of the addedelement. T he natu re of this effect w ouldin turn depend upon whether the ele-

472-s | N O V E M B E R 1 9 7 1



/AR C P O T E N T I A L - C U R R E N T

C H A R A C T E R I S T I C SF O R 9 9 . 5 8 % N i + 5 0 m g Fe

-F0R99.58%Ni

2 5 0 300

AR C C U R R E N T ( A M P E R E S )

Fig. 11—Arc potential-arc current character is t ic for 99.58% Ni + 50 mg

/_ A R C P O T E N T I A L - C U R R E N T

C H A R A C T E R I S T I C SFO R 9 9 . 5 8 % N i + 5 0 m g SiFO R 9 9 . 5 8 % Ni

2 5 0 30 0

A R C C U R R E N T ( A M P E R E S )

Fig. 12—Arc potential-arc current character is t ic for 99.58% Ni + 50 mg

2 5 0 300

A RC C U R R E N T ( A M P E R E S )

Fig. 13—Arc potential-arc current character is t ic for 99.58o/o Ni + 50 mg

ment remained in solution or becameassociated with reaction products uponthe surface. Since the arc exerts a forceon the molten anode, a change in th e

effective arc gap can occur if a changein the surface tension of the liquidalters th e con tou r of the surface of th e

weld pool. T his ar c force has been investigated.

25-

26- •' However, nei ther the

influence of the anode composi t ion u ponthe magnitude of arc force, nor upon the

surface contour of the weld pool ha s

been investigated quantitatively.

High speed movies taken during thisphase of th e study revealed some dif

ferences in the plasma configurationamong the eleven trials . However, thesedifferences were not significant enoughto make a quantitative analysis possible.B u t some of the movies did show dif

ferences inpuddle contour believed to

result from differences in surface tension.In addition, various unidentifiable re

action products were observed on the

surface of the molten pool .T hus al though analysis of both the

data obtained from the characteristiccurves as presented inT a b l e 6and the

high speed motion pictures of the plasma

revealed that the arc is indeed sensitiveto the anode composi t ion , the mechanism or mechanisms by which the individual elements influence the arc is not

clear. All efforts torelate the data ob

tained from the characteristic curves to

the physical properties of the alloyingadditions mentioned above failed.Therefore, it must be concluded that the

observed influence of minor alloyingadditions on the characteristics of a

welding arc may involve complex interactions not anticipated in the designof this experiment. It is entirely possiblethat the mechanisms are too complex

to be resolved until more accuratevalues for properties listed and for other s not considered become available.

Conclusions1. The arc in aGTA process is sensi

tive to the anode composi t ion .2. The arc potent ial at a given current

and arc length is altered by slightchanges in the composi t ion of the anode.

3. The slope of the arc potential-arccurrent characteristic curve is changedby minor additions of alloying elementto the anode .

4. No satisfactory model could be

formulated to explain observed influence of various elements studied on characteristics ofGTA process.

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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 | 473-s

wj_1971_11_s467 tig electrode material vs arc carachteristics

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