Effect of Hydrogen in an Argon GTAW Shielding Gas: Arc Characteristics and Bead Morphology Hydrogen content in the shielding gas may have as strong an influence on joint penetration as welding current BY M. ONSOIEN, R. PETERS, D.L. OLSON AND S. LIU ABSTRACT. The influence of hydrogen idation, undercutting, porosity, slag in- rent-carrying capacity of the arc at a additions to an argon shielding gas on the clusions, fusion, and joint penetration given voltage, or arc length (Refs. 5, 6). heat input and weld bead morphology (Refs. 1-4). The actual voltage to main- An addition of hydrogen to argon or was investigated using the gas tungsten tain a stable high-pressure arc is para- helium gives an arc a wider temperature arc welding process. Variations in weld metrically determined by the arc length distribution, a larger heat input and a bead size and shape with hydrogen ad- and welding current level, and is influ- slightly reducing atmosphere (Reg. 2, 7). ditions were related to changes in the enced by the conductivity of the arc. The higher heat input can partially be re- ability of the arc to generate heat and not Charge carrier concentration and the lated to the dissociation of diatomic hy- to generate perturbations in the weld mean free path between electron-parti- drogen in the arc to form atomic hydro- pool caused by Marangoni fluid flow. cle interactions are the two major factors gen, which then recombines to the that determine arc conductivity. A gas molecular form in the cooler regions of the Introduction with low-ionization potential will easily arc and at the surface of the workpiece ionize to form a plasma. As a result, the (Ref. 7). Ionization of atomic hydrogen Shielding gases have as a primary role "large" number of electrical charge carri- contributes further to the increase in heat the protection of the weld and its sur- ers in the arc increases the electrical con- input. Similarly, the presence of hydrogen roundings from atmospheric oxygen and ductivity. The ionization potential of an also affects the current-carrying capacity nitrogen. At an adequate flow rate, they inert gas is an indirect measure of the cur- of argon or helium. The reducing atmos- displace these two gases from the weld phere will lead to a cleaner weld bead sur- zone. The ability of a shielding gas to per- face and, in multiple pass welds, reduces form the protective role will depend on the tendency for oxide/slag buildup. In gas its chemical composition, which has a tungsten arc welding, hydrogen also significant influence on arc initiation, arc serves to prevent oxidation of the non- stability, plasma diameter, and plasma KEY WORDS consumable electrode (Refs. 2-4). temperature (radial temperature distribu- Addition of hydrogen to the shielding tion and peak temperature). The physical Hydrogen Effect gas has also been reported to increase the and chemical nature of the shielding gas Argon Shielding Gas surface tension of liquid stainless steel because of a decrease in surface oxygen also affects other arc and weld properties GTAW concentration (Ref. 8). Surface tension such as metal transfer, spatter, surface ox- Arc Characteristics and its gradients can significantly affect Bead Morphology weld bead size and morphology. A clean Heat Input M. ONSOIEN, D.L. OLSON and S. LIU are liquid metal surface promotes conditions Arc Resistance with the Center for Welding and Joining Re- where Marangoni fluid flow (surface ten- Joint Penetration search, Colorado School of Mines, Golden, sion driven flow) and the dominant Colo. R. PETERS is with Delft Technological Heat Gener. Process mechanism in the overall weld pool me- University, Delft, The Netherlands. Welding Potential chanics, which results in a broader and Paper presented at the AWS 75th Annual shallower weld bead (Refs. 9-11). If the Meeting, held April 10-14, 1994, in Philadel- reducing action of hydrogen is sufficient phia, Pa. to remove surface oxygen, then hydro- 10-s I JANUARY 1995 gen additions should promote Marangoni fluid flow and form shallow 17 " I " I " I " I " I " I " I " and wide weld beads. Since hydrogen IS additions may affect many physical prop- 1S erties of the plasma, this investigation is performed to determine the hydrogen in- J teraction or interactions that control the ."" V reported variations in heat input and |,, bead morphology. II Experimental Procedure 11(10 ' 80, . 1~, . 120, . 140, . |Eo, . 180, . 200, . 220 EO 100 120 140 180 180 200 220 o 0 vol.pct. H 2 w@d]ng Cu~nt (~1 Tests were designed and performed to Weldlne Cummt (A) • 1 vol.pct. H 2 v 2 vol.pct. H 2 evaluate the influence of hydrogen addi- • 3 voi.pct. H2 tions to argon shielding gas on the arc a 4- vol.pct. H 2 characteristics and bead morphology in 17 • , • , . , . , - , . , . , . Arc LJreU~ - 8.5 mm A~-IIm . • . ; 18 gas tungsten arc welding of Type 304 18 stainless steel. The shielding gases used 15 were pure argon and argon with 1, 2, 3 14 and 4 vol-% hydrogen. A constant-cur- 13 rent power source was used with a direct 13 current electrode negative polarity to |,, 12 make bead-on-plate welds. The diameter 11 of the 2% thoriated tungsten electrode 10 10 was 3.2 mm (1/8 in.), and the electrode , . , . , . • , . i . i , , . , • , . J . 80 1~ 120 140 1110 180 200 22O 980 ' 80/ , IO0' • 120' • 140' ' 1(10 i80 2OO 220 tip had a vertex angle of 30 deg. The tip (A) Wmkllnql cummt (A) Wlldlng Cu~.t of the electrode was truncated with a flat top of 1.3-mm diameter (0.05-in.). The base material used was a Type Fig. 1 -- Arc characteristics for GTA welds with five different argon-hydrogen shielding gas mix- 304 stainless steel, and its chemical com- tures and arc lengths of: A -- 2.5 ram; B -- 5 mm; C -- 6.5 ram; D -- 8 mm. position is shown in Table 1. The test plates measured 12.7 x 76 x 152 mm (0.5 x 3 x 6 in.). The plate thickness satisfies Table 1--Chemical Composition of the 304 Stainless Steel Base Plate the usually accepted conditions of three- Alloying Elements (wt-%) dimensional heat transfer (-0.5 in.) Bead-on-plate welds were made C Si Mn Ni Cr P S Cu Co Mo V using systematic variations in welding 0.03 0.44 1.54 9.3 18.9 0.025 0.003 0.10 0.11 0.01 0.01 parameters. The current settings were 80, 100, 130, 160 and 200 A. The arc lengths were 2.5, 5.0, 6.5 and 8.0 mm (0.1, 0.2, 0.25 and 0.3 in.). The shielding gas flow rate was fixed at 11.8 L/min (5.6 ftg/h), and the welding travel speed for all the welds was 2.1 mm/s (5 in./min). The travel speed was maintained constant to welding voltage to 18 I I I I ensure similar effect of travel speed on maintain the welding bead shape in each specimen. arc. 2 vol.pct. H 2 The weld specimens were sectioned Hydrogen addi- 16 and prepared according to standard met- tions also increased [] a ,'",/: allographic techniques and macroetched the welding voltage, • . • ,~r /// using Villela's reagent to reveal the bead confirming the influ- 14 size and shape, that is, area, joint pene- ence of dissociation • • - . ~" - tration and width• ionization of the di- atomic gas. With in- 12 Results and Discussion creasing arc length, the welding potential "1o 10 The arc characteristic curves for vari- increases, and the in- ous arc lengths and hydrogen contents in fluence of the hydro- S • IOOA the shielding gas are shown in Fig.lA to gen content becomes / V 130A 1D. For a constant arc length, welding even more apparent. • 160A voltage was observed to increase with in- It is known that in- [] 200 A creasing welding current. This change is creasing the arc 6 I , I , t I 2 4 6 8 1o expected because of the arc temperature length increases the 0 increase with increasing welding cur- arc resistance, and it Arc Length (ram) rent. A higher arc temperature generally appears that further indicates a more chaotic charge carrier increases in arc resis- Fig. 2 -- Extrapolation of welding voltage to obtain zero arc length motion path, which requires higher tance occurs with in- potentials. WELDING RESEARCH SUPPLEMENT I 11-s for both dissociation of 7 .... , .... J .... 4 .... [ .... ; .... , .,.. ,. H 2 and ionization of : ~. tooA // ¢~l I=mA o / ,L ~" I atomic hydrogen. The addition of hy- i ,~v"~1,,'~" /" / drogen to argon requires i higher applied potential I 3 to maintain a stable arc, and increasing hydro- gen content further in- creases the welding po- "~2;; .... f1 4 i • Io l 4 • | IO tential. If the welding arc is modeled as a series circuit, it is possible to isolate the portion of the potential drop that is un- C,j~ * 200 A related with the arc phe- ¶ cunw,t = Iio A / / nomenon by extrapolat- *~- // • o 0 vol.pct. H2 I1" ,J" • 1 vol.pct. H 2 ing the data of measured v 2 vol.pct. H 2 /~/ / voltage to zero arc 4 /. ~11 o • 3 volpct H 2 It / length -- Fig. 2. The o 4 vol.pct. H2 I ! nonarc portion includes I /,~¢ O1" the tungsten electrode, = ;s /.. ~" the work and other con- ductive paths of the , .... | SO 1 * I welding circuit. By sub- A,c ~ (ram) tracting the nonarc po- tential from the welding Fig. 3 -- Effect of arc length on arc voltage for GTA welds made with five argon-hydrogen shielding gas mixtures potential (voltage) data, and welding current levels of: A -- 80 A; B -- 100 A; C -- 130 A; D --180 A; E -- 200 A.