in Proc. of the Int. Conf. on Welding Research in the 1 98O1sl Osaka University, Osaka, Japan, 1 13, 1980.

Oxygen arid Nitrogen Contamination During Submerged Arc Welding of Titanium

T. W. Eaqar*

The oxygen content of titanium submerged arc weld metal i s primarily dependent upon the pur i ty and composition of the s ta r t ing f lux while nitrogen contamination of the deposit i s shown to r e su l t from adsorption of atmospheric gases on the hot electrode and baseplate p r io r t o me1 t inq of the flux. As reported previously by other invest igators , chloride additions improve the operating charac te r i s t i c s of the f luor ide f luxes, b u t i t i s shown here t ha t the oxygen content of the weld metal may be affected adversely. The reprocessing and reuse of the fused slaq was a lso investigated as a means of reducing the costs of submerged a rc welding of titanium. In general i t i s found t ha t the cost of titanium submerged arc welding does not compare favorably with the cost of a l t e rna t e processes such as qas metal arc welding of titanium. The advantages and disadvantages of f lux shielded weldinq of t i tanium are out1 ined.

I . Introduction

Welding of heavy section titanium s t r u c t u ~ e s in the open atmosphere i s current ly performed by e i t he r the'gas tungsten arc o r the gas metal a rc processes; however, these processes present numerous d i f f i c u l t i e s , primary of which i s prevention of oxygen and nitrogen contamination of the weld metal. In order t o prevent such contamination, copious amounts of argon or helium gas, combined with elaborate sh ie lds , a re used. These arrangements a r e capable of reducing the to ta l contamination level t o an increase of l e ss than 100 ppm oxygen and 20 ppm nitrogen; however, the shie lds a re cumbersome and the i n i t i a l j o in t preparation must be closely control led t o provide adequate cleanliness. If any e r ro rs are made during the process, the system i s unforgiving; and the contaminated weld, i f found, can only be corrected by removal and rewelding. All of the above requirements lead to increased fabr icat ion costs f o r titanium s t ructures .

Although f i r s t developed twenty-five years ago by Gurevich ( I ) , the process of submerged arc welding (SAW) of titanium has not been used widely outside of the Soviet Union. This i s due primarily t o the poor operating charac te r i s t i c s and bead qual i ty of welds produced by other investigators (2-5). In pa r t i cu la r , two s tudies ( 4 , 5 ) indicate t ha t both the oxygen and the nitrogen content of the weld deposit might be too high t o produce jo ints of adequate f rac tu re toughness f o r many applications. Nonetheless, i t has been found t ha t welding of titanium in the presence of a metal halide f lux e . g . , CaF2) has the advantage of providing chemical control of the oxygen content of the weld metal ( â ‚ ¬ 1 ~ 7 Such control would make the welding process more forgiving of operator e r ro r and hence might be expected t o simplify and reduce the costs of titanium s t ruc tu ra l fabr ica t ion. The present study was undertaken t o determine the source of oxygen and nitrogen contam- at ion of the titanium SAW weld pool and t o develop techniques f o r control of the process,

* Associate Professor, Department of Materials Science and Engineering, Massachusetts I n s t i t u t e of Technology, Cambridge, Massachusetts, 02139, U.S.A.

I I. Experimental

Bead on pla te welds were made on 19 mm thick Ti-6A1-4V extra low in te r - s t i t i a l grade material. Electrodes of s imi lar composition of diameters 1.6 mm, 2.4 mm and 3.2 mm were used; however, i t was found t ha t the l a rges t diameter electrode produced the most consistent r esu l t s . The oxygen and nitrogen contents of the base pla te were 0.12% and 0.012% respectively. In the e lect rodes , the values were 0.08% oxygen and 0.008% nitrogen.

Although a number of d i f fe ren t welding conditions were t es ted , the majority of the welds were produced with DC electrode posi t ive , 36 vo l t s , 300 amperes, 15 mm electrode extension a t 30 cm/min travel speed. Several welds were made with external argon shieldinq in order t o reduce atmospheric contamination and hence t o measure the contamination produced by the f lux alone; however, most welds were made without external shielding.

Reagent grade chemicals were used t o produce the f luxes , most of which were melted under vacuum or i n e r t gas. llonetheless, several fused fluxes were found t o be contaminated with several percent oxygen due t o adsorption of water vapor on the s t a r t i ng material. I t was found tha t high puri ty f lux could only be obtained from optical qual i ty c rys ta l s which had been fused in an atmosphere of HF o r HC1. The oxygen content of such f lux i s believed t o be l ess than 50 ppm.

After melting, a l l f luxes were crushed and screened t o 60 to 160pm powder. Care was taken t o avoid contamination of the crushed powder with the moisture in the atmosphere.

After welding, cores were machined from the center of the weld bead. These samples were analyzed by i n e r t qas fusion f o r oxygen and by the Kjeldahl method f o r nitrogen. The oxygen content of the fused fluxes was determined from x-ray powder d i f f rac t ion i n t ens i t i e s .

III., Results and Discussion

Over one hundred weld beads were produced f o r t h i s study; only a few of which a r e presented in the following tables. Inspection of Table I reveals much useful information concerning the chemical propert ies of several titanium fluxes. Weld 1 , produced with optical qual i ty c rys ta l s has an oxygen content of 1000 ppm which does not d i f f e r from the base metal/electrode analysis; however, t h i s weld i s contaminated with 500 ppm nitrogen. I t i s in teres t ing t o note t ha t although-atmospheric contamination i s present , as evidenced by the elevated nitrogen in the weld deposit, the oxygen content of the weld bead i s not increased. This r e su l t has been reproduced with optical qua1 i t y CaF2 fluxes more than a dozen times. If the oxygen content of the f lux i s raised t o 1.2% CaO as in Weld 2 , the oxygen content of the weld metal i s increased t o approximately 2800 ppm even i n the presence of atmospheric shielding, cf. Weld 3. I t has been found in a l l welds tha t the oxygen content of the deposit made with CaF2-CaO fluxes i s d i r ec t l y proportional t o the amount of CaO impurity in the flux. For example, x-ray analysis of the slag from Weld 6 revealed t ha t 1.6% CaO was present. Use of argon shieldinu reduces the oxi- dation of the f r e e calcium in the f l ux , as seen by comparing Helds 7 and 8 ; thereby reducing the oxygen content of the metal deposit. The oxygen level of the deposits made with pure CaF2 appears t o be independent of the amount of atmospheric contamination. The atmospheric contamination may be estimated from the nitrogen content of the weld deposits.

The above finding t ha t the oxygen content of titanium submerged arc weld deposits i s controlled sole ly by the puri ty of the s t a r t i ng f lux i s consistent with the report of Gurevich tha t the CaO content of CaF2 fluxes must be main- tained below 0.5% (8) . For many applicat ions, even t h i s level may be too high t o maintain suf f i c ien t ly low oxygen in the weld metal.

The r e su l t s of Welds 2 through 5 indicate t ha t nitrogen contamination of the weld deposit r esu l t s from adsorption of atmospheric gases on regions of the hot electrode and base pla te which are not protected by the flux. I t wil l be noted t ha t argon shielding, which d i lu tes the nitrogen in the surrounding atmosphere, always reduces nitrogen con tami nat ion. Longer electrode exten- sions always increase the ni troqen content. Anodizing the t i tanium electrode and baseplate have been found t o reduce the level of nitrogen contamination by 50 t o 100 ppm, while not measurably increasing the oxygen content. Vaporiza- t ion of vo l a t i l e halides such as KC1, NaF, or NaCl provide some shielding from atmospheric gases, thereby reducing the amount of nitrogen contamination, cf . Welds 9 through 15. The most e f fec t ive method f o r controll ing nitrogen con- tamination i s external argon shielding.

In many of the e a r l i e s t repor ts , Gurevich noted t ha t chlorides must be added t o titanium SAW fluxes in order t o improve the operating characteris- t i c s . A number of such f luoride-chloride fluxes have been studied and have been found t o provide greater weld penetration than pure CaF2 fluxes. None- the less , Welds 9 through 17 indicate that the addition of chlorides and in one case, the addition of NaF, tends t o increase the oxygen content of the weld deposit. Weld 16 suggests t ha t the increase in oxygen may be related more t o reduced ac t i v i t y of the calcium cation in the other blended fluxes than t o the addition of the chloride anion. Based upon a Temkin regular sol - ution model, the addition of 20 mole percent NaCl or KC1 t o CaF2 would reduce the a c t i v i t y of CaF2 by 40 t o 50% ( 9 ) . This reduced CaF2 ac t i v i t y could ex- plain the increased oxygen content of welds 9-15 and 17. Gurevich a lso found t ha t the addition of a cation with a very high oxyqen a f f i n i t y such as lantha- num may reduce the oxygen content of the weld deposit t o 700 ppm (10).

One obvious d i f f i cu l t y with the use of optical pur i ty CaF2 as a welding f lux i s the expense; however, since t h i s f lux i s e ssen t ia l ly neutra l , i t may be possible t o reuse the fused slag thereby reducing the cost of the flux. The r e su l t s of an experiment examining the reuse of the titanium slag i s shown in Table 11. I t i s readily noted t ha t both the oxygen and nitrogen content of the fluxes increase with increasing use. In one case, the f lux was dried in an HCI-Clz furnace a t 500° in order t o remove the CaO which remained a f t e r the fourth reuse. I t i s seen t ha t the drying did reduce the amount of oxygen by a fac to r of twoà If the f lux i s dried a f t e r each use, i t i s possible t ha t i t would maintain i t s low oxygen potential f o r many uses.

One advantage of reusing f lux , in addition t o improved economy, i s t ha t the contamination of the f lux by small amounts of titanium, as measured by x-ray fluorescence, improves the operating charac te r i s t i c s of the f lux without the addition of hygroscopic chloride s a l t s . The reused f luor ide f lux produced a more s tab le a r c , a be t t e r weld bead contact angle and a smoother surface than did the unused CaF2.

An economic comparison of SAW versus GMAW of titanium has been performed. A t the measured f lux consumption r a t e of 4.5 kg of f lux per kg of electrode deposited, and the present cos t of optical qua l i ty CaF2 of $18 per k g , SAW i s approximately s i x ty percent more expensive than GMAW. I f multiple reuse of of the f lux could reduce the cost t o $2.25 per kg of f lux , the two processes

would be roughly equal in cos t (11).

I t should be a l s o be noted t h a t the submerged a rc weld deposi ts produced i n t h i s study are general ly i n f e r i o r in shape and surface appearance t o those cu r ren t ly produced by GMAW. A parametric study of the welding process para- meters provided no s i g n i f i c a n t improvement in t h i s a rea ; however, the improve- ments in bead shape with addi t ion of chlorides t o the f lux or with f l u x reuse suggest t h a t f u r t h e r improvements a r e possible. There i s l i t t l e doubt t h a t Soviet i nves t iga to r s may be capable of producing technica l ly useful submerged a r c welds in t i tanium, but i t i s not c l e a r t h a t the process i s economical compared t o the a1 t e rna t ives .

In conclusion, the use of ha l ide f luxes when welding t i tanium shows promise of providing means of improving the a r c behavior, of cont ro l l ing the weld bead shape and penetrat ion and of con t ro l l ing the oxygen content of the deposi t . On the negative s i d e , t he f luxes a re required t o be of high pur i ty , which makes them expensive; they are often hygroscopic and in many cases they produce noxious fumes. The advantages of f lux shielded welding of t i tanium wi l l only be r ea l i zed i f these disadvantages can be overcome.

Acknowledgements

The author wishes t o express appreciat ion t o the many people who have contr ibuted t o t h i s work. These include C. S. Chai, J . J s G u l l o t t i , - G. Hunter, Dr. G . B. Kenney, M . Ring, B o A. Russell and Professor D. R. Sadoway. The support of the Office of Navel Research i s a l s o appreciated.

REFERENCES

( 1 ) S. M. Gurevich and S. V . Mischenki , A v t , Svarka, No. 5 , 1956, p.1. ( 2 ) B. Cl iche, Report No. RAM-00-107, Canadair L t d . Montreal, Canada, 1964. ( 3 ) H a F. Petsch, Final Report, Contract No. N00140-68-C-0148, General

Dynamics Corporation, Groton, CT, 1968. ( 4 ) D. C. Hill and C. L. Choi, Neld J . , 55 No. 6 , 1976, p. 152-s. ( 5 ) S. Marya and F. LeMaitre, 5emes ~our%es d'Etude d u t i t a n e e t de ses

A1 l i a g e s , Nantes, France, November, 1978. ( 6 ) S. M, Gurevich e t a l . , A v t . Svarka, No. 4 , 1964, pp..93-94. ( 7 ) C. S. Chai , J . J . Gu l lo t t i and T. W. Eagar, Technical Report No. 2 ,

Contract N00014-77-C-0569, Mass. Ins t . of Technology, Cambridge, MA, 30 Sept. 1978.

( 8 ) 5. M. Gurevich and L . K. Bosak, A v t . Svarka, No. 11, 1964, pp. 47-50. ( 9 ) F. D. Richardson, Physo Chem. of Me1 t s in Metal1 urgy, Academic Press,

New York, 1974, ~ o l 1 . (10) S. M. Gurevich e t a l . , A v t . Svarka, No. 4 , 1964, pp. 93-94. (11 ) G. Hunter e t a1 ., Technical Report No. 3, Contract N00014-77-C-0569,

Mass. Ins t . of Technology, Cambridge, MA, 30 Sept. 1979.

Tab1 e I . Effec ts of t i tanium f l u x composition and welding process parameters on the oxygen and ni trogen content of the weld depos i t .

Weld Flux Oxygen Nitrogen Weld Bead No. Composition Percent Percent Appearance Remarks

Fused Fluxes

1 Optical CaF2

2 CaF2-1.2% CaO :

3. CaF2-1.2% CaO

4 CaF2-1.2% CaO

5 CaF2-1.2% CaO

6 CaF2-20% Ca

7 CaF2-2% Ca

8 CaF2-2% Ca

Blended Fluxes*

9 CaF2-1% NaF

10 CaFz-5% NaCl

11 CaF2-10% NaCl

12 CaF2-20% NaCI

13 CaF2-5% KC1

14 CaF2-10% KC1

15 CaF2-20% KC1

16 CaF2-10% CaCl2

17 CaF2-10% SrC12-3% LiCl

good

poor

f a i r

poor

f a i r

cracked

poor

poor

very poor

poor

poor

very poor

poor

poor

poor

poor

very poor

Argon shielded

19 mm e lec t rode extensi on

Anodized e lec t rode and basepl a t e

- - Argon shielded

Suggested by reference (5 )

* A1 1 blended f luxes were made with optical q u a l i t y CaF2 and KC1 c rys ta l s . The CaC12, SrCl2 and LiCl were dried in an HCL-C12 atmosphere a t 500°C

Table 11. Oxygen and nitrogen content of titanium submerged arc welds made with reused optical qua l i ty CaF;? flux.

No. of f lux Oxygen N i t r o g e n Reuses Percent Percent Remarks

New 0.10 0.06 - -

0.09 Flux dried in liC1-C12 furnace a f t e r fourth reuse.