Numerical Simulation of Gas Tungsten Arc Welding in Different Gaseous Atmospheres. Tashiro, Shinichi; Tanaka, Manabu; Ushio, Masao

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1 Title Autho(s) Numeical Simulation of Gas Tungsten Ac Welding in Diffeent Gaseous Atmosphees Tashio, Shinichi; Tanaka, Manabu; Ushio, Masao Citation Tansactions of JWRI. 3() P.1-P.5 Issue Date 5-1 Text Vesion publishe URL DOI ights

2 Tansactions of JWRI, Vol.3, (5), No. Numeical Simulation of Gas Tungsten Ac Welding in Diffeent Gaseous Atmosphees TASHIRO Shinichi *, TANAKA Manabu ** and USHIO Masao *** Abstact In ode to claify the fomative mechanism of penetation geomety at the anode in an ac melting pocess using gas tungsten ac plasma, a numeical model is useful to undestand quantitative values fo the balances of mass, enegy and foce in the melting phenomena. In the pesent pape, the whole egion of an ac melting pocess, namely, tungsten cathode, ac plasma and anode is teated in a unified numeical model to take into account the close inteaction between the ac plasma and the liquid anode. Calculations ae made fo the time dependent development of the penetation at the anode fo the ac plasma in diffeent gaseous atmosphee at a cuent of 15 A. The anode penetation geomety as a function of time is pedicted in the ac melting pocess in agon and also helium. KEY WORDS: (Numeical model), (Ac plasma), (Anode), (Melting), (Welding), (Fluid flow), (Agon), (Helium). 1. Intoduction Ac plasmas have been applied in the industial pocessing of mateials, fo example in suface teatment, ac funaces, the pocessing of mineals and in ac melting. Pobably the most impotant application in ac melting is ac welding. Fo the undestanding and also the contol of such pocesses, it is impotant to develop a modeling capability, pefeably to be able to pedict popeties of the total ac pocessing system. Fo example, in ac welding, one of the most impotant paametes is the geomety of the weld penetation, as this geomety detemines the thickness and thus the stength of the weld. The pincipal contol paamete in welding is the ac cuent. Howeve, the penetation geomety depends not only on the total enegy input to the metal but also on the enegy flux distibution at the metal suface and the balance of whole foces in the welding phenomena [1-], as shown in Figue 1. This pape pesents a methodology fo pedicting the geomety of the molten liquid poduced by the ac. The whole egion of gas tungsten ac welding consisting of a tungsten cathode, an ac plasma and a molten anode, Received on Novembe 7, 5 * Designated eseache ** Associate pofesso *** Pofesso emeitus namely, a weld pool is teated in a unified numeical model to take into account the close inteaction between the ac plasma and the weld pool. The basic model and pocedue ae the same as ou pevious papes used [5, ], but ae extended to employ diffeent shielding gases. We give pedictions of the time dependent two-dimensional distibutions of tempeatue and velocity in the whole egion of gas tungsten ac welding pocess in agon and also in helium.. An Ac-Electode Model The tungsten cathode, ac plasma and anode ae descibed elative to a cylindical coodinate, assuming otational symmety aound the ac axis. The calculation domain is shown in Figue. The flow is assumed to be lamina, and the ac plasma is assumed to be in the local themodynamic equilibium (LTE). Futhemoe, the anode suface is assumed to be flat and unpetubed by the ac pessue. The diamete of the tungsten cathode is 3. mm with a degees conical tip. The anode is stainless steel SUS3 and its tempeatue coefficient of suface tension above the melting point is assumed as Tansactions of JWRI is published by Joining and Welding Reseach Institute, Osaka Univesity, Ibaaki, Osaka 57-7, Japan 1

3 Numeical Simulation of Gas Tungsten Ac Welding in Diffeent Gaseous Atmosphees shown in Figue 3 [7]. The govening equations, bounday conditions and numeical method have been detailed in ou pevious papes [5, ], so that only the most petinent points ae explained hee. The mass continuity equation (1) is t 1 (v ) (v ) the adial momentum consevation equation () is v t 1 P ( v ) ( v v ) j B 1 v v v v ( ) ( ) the axial momentum consevation equation (3) is v t P 1 ( vv ) ( v ) jb v 1 v v ( ) ( ) g the enegy consevation equation () is h 1 1 h h ( vh) ( vh) ( ) ( ) t c c j E j E U the cuent continuity equation (5) is 1 ( j ) ( j ) the Ohm's law () is V V j ; j and Maxwell's equation (7) is 1 (B ) j. Each physical symbol in the above equations indicates the same meaning as in geneal use. At the anode suface, BE in Fig., thee ae two souces of adial momentum as shown in Fig. 1. The fist is the dag foce, namely, the shea stess which is applied by the cathode jet on the suface of the weld pool, and the second is the suface tension gadient foce, namely, the Maangoni foce. The dag foce is aleady eflected in equation () fo the adial momentum consevation. The viscosity makes the dag foce at the anode suface. Theefoe, the Maangoni foce would need to be included in the adial momentum consevation at points on the anode suface, BE. In most cases, the diffeence in suface tension aises fom the tempeatue vaiation at the weld pool sufaces [], and then the Maangoni foce can be expessed by p p v T T whee is the suface tension of the weld pool. Theefoe, the additional tem fo equation () at the anode suface F A is Anode () F A T. (9) T The diffeential equations (1) to (7) ae solved iteatively by the SIMPLEC numeical pocedue [9] fo the whole egion of the stationay gas tungsten ac welding pocess as shown in Fig.. Mass balance Dag of cathode jet Ac plasma Suface tension Cathode jet Tungsten Cathode Ohmic heating Ohmic heating Conduction Themionic emission of electon Radiation Ohmic j B heating Buoyancy Enegy balance Radiation & Conduction Conduction, and Neutaliation of ion Conduction, and Electon absoption Radiation & Conduction Conduction Anode Fig. 1 Schematic illustation of enegy and mass balance in the ac melting/welding pocess. -3 A B 1 C 15 Tungsten cathode Ac plasma Anode Fig. Schematic illustation of calculated domain. F E D

4 Tansactions of JWRI, Vol.3, (5), No. Suface tension (N/m) T N mk Tempeatue (K) Fig. 3 Assumption of suface tension of molten SUS3 [7]. 3. Results and Discussion 3.1 Agon shielding gas Figue shows distibutions of tempeatue and fluid flow velocity of the weld fo the individual diving foces in agon gas tungsten ac. These calculations wee made in the steady state unde the conditions of a welding cuent 15 A and an ac gap 5 mm. Each calculation was caied out by taking into account the diving foce fo the dag of the cathode jet in equation (), the buoyancy in equation (3), the electomagnetic in equations () and (3), and the Maangoni in equation (9). Each maximum velocity fo dag of cathode jet, buoyancy, electomagnetic and Maangoni foce is 7 cm s -1, 1. cm s -1,.9 cm s - 1 and 1 cm s -1, espectively. Fig. suggests that the calculated convective flow in the weld pool is dominated by the dag foce of the cathode jet and the Maangoni foce as compaed with the othe two diving foces in agon ac. Figue 5 epesents the two-dimensional distibutions of tempeatue and of fluid flow velocity in the whole egion of the stationay gas tungsten ac weld fo a 15 A welding cuent at seconds afte ac ignition. The maximum calculated velocity of the cathode jet in Fig. 5 eaches 1 m s -1. This cathode jet changes its diection in font of the anode suface, and then its adial component of fluid flow dags the suface of the weld pool. The outwad fluid flow is also caused by a nomal negative tempeatue coefficient of suface tension as shown in Fig. 3. Fig. 5 epesents a numeical esult coesponding to stong outwad fluid flow with a wide and shallow weld penetation which is caused by both outwad foces of the dag foce and the Maangoni foce. This weld penetation geomety is a typical one in the agon gas tungsten ac welding pocess. The maximum calculated velocity in the weld pool fo all consideed cases eaches 5 cm s -1 at s afte ac ignition. 175 K 15 K 1 K 1 (a) Dag 175 K Max. 3 cm/s Dag Max. cm/s 15 K K 1 K Electomagnetic 1 (c) Electomagnetic K 175 K 15 K 1 K Buoyancy K (b) Buoyancy 15 K 1 K Max. 3 cm/s Max. 3 cm/s Maangoni 1 (d) Suface tension gadient Fig. Tempeatues and fluid flow velocities in the welds fo individual diving foces in agon ac K K 15 K K 5 K 3 K 35 K 1 K A, 15 A = s 1 K, Inteval K 17 K 5 K SUS 3 (LS) A, 15 A = s Max. 15 m / s Max. 3 cm / s SUS 3 (LS) 1 Fig. 5 Two-dimensional distibutions of tempeatue and of fluid flow velocity at seconds afte ac ignition fo a 15 A in agon gas tungsten ac weld on SUS3. 3. Agon shielding gas Figue shows the adius distibutions of cuent density at the suface of a wate cooled coppe anode fo 15 A of ac cuent, in the cases of agon and helium. These calculations wee made in the steady state. The cuent density in the helium ac is much moe intense than that in the agon ac, which suggests that the electomagnetic foce in the weld pool becomes a majo foce in the helium gas tungsten ac. The electical conductivity fo helium plasma is significantly smalle than that of agon plasma fo T < 1, K due to the 3

5 Numeical Simulation of Gas Tungsten Ac Welding in Diffeent Gaseous Atmosphees highe ioniation potential of He [1]. Fo example, the electical conductivity at, K is about only 1 A/Vm fo helium plasma but about 1, A/Vm fo agon plasma [1]. It can be deduced fom this low electical conductivity that constiction of the helium ac close to the anode suface occus. Figue 7 shows the same distibutions as in Fig. fo the individual diving foces but fo the helium gas tungsten ac. Each maximum velocity fo dag, buoyancy, electomagnetic and Maangoni foce is 11 cm s -1, cm s -1, 5 cm s -1 and cm s -1, espectively. Fig. 7 suggests that the calculated convective flow in the weld pool is dominated by the electomagnetic foce and the Maangoni foce in the helium ac. Figues epesent the two-dimensional distibutions of tempeatue and of fluid flow velocity in the whole egion of the helium gas tungsten ac welding fo 15 A of welding cuent, seconds afte ac ignition. The weld penetation inceases with the elapse of time. It is found, in Fig., that thee ae two e-ciculatoy flows in the weld pool, namely, an inwad fluid flow close to the cente at the inne of the weld pool and an outwad fluid flow at the suface of the weld pool. The inwad fluid flows is caused by the electomagnetic foce. The outwad fluid flow is pincipally caused by the Maangoni foce with a nomal negative tempeatue coefficient of suface tension. Both diving foces dominate the e-ciculatoy flows in the weld pool, and then a special geomety of weld penetation appeas as shown in Fig., which is obviously changed in compaison with that in agon ac. Max. 11 cm/s K 15 K 1 K 175 K Dag 175 K Max. cm/s 15 K 1 K K Buoyancy 1 1 (a) Dag (b) Buoyancy K 175 K Max. 5 cm/s 15 K 1 K Electomagnetic 1 (c) Electomagnetic (d) Suface tension gadient K 175 K 15 K Max. cm/s 1 K Maangoni 1 Fig. 7 Tempeatues and fluid flow velocities in the welds fo individual diving foces in helium ac. Cuent density at anode suface (A/cm ) A He Ac cuent: 15 A Ac gap: 5 mm Anode: Wate-cooled Cu 1 Radius (mm) Fig. Two-dimensional distibutions of tempeatue and of fluid flow velocity at seconds afte ac ignition fo a 15 A in a helium gas tungsten ac welding of SUS3. A gas flow Fig. Radius distibutions of cuent density at the suface of a wate cooled coppe anode fo agon ac and helium ac fo a 15 A in ac cuent. Tungsten cathode ( degees, 3. mm) 5 mm Ac Anode mateials (SUS3, 5 mm, 1 mmt) Wate in Coppe wall Wate out Fig. 9 Schematic illustation of the appaatus fo stationay gas tungsten ac weld on SUS3.

6 Tansactions of JWRI, Vol.3, (5), No. 3.3 Pedictions with expeimental esults Stationay gas tungsten ac welding was pefomed fo compaison of the weld penetative pofiles with calculated esults fo agon ac and helium ac. The expeimental setup is shown in Figue 9. The anodes wee disks, 5 mm in diamete and 1 mm in thickness, of SUS3, mounted into a wate-cooled coppe plate. The expeiments wee made fo s of acing time allow compaison of expeiment with theoy, so that conditions wee then simila to the bounday condition of calculations, to be compaable to the esults of calculations. Figues 1 and 11 show pedicted weld penetative pofiles afte seconds, compaed with expeimental esults, in the case of agon and helium, espectively. We conside that the ageement between the theoetical pedictions and the expeimental weld pofiles of Figs. 1 and 11 is vey good. The deep penetation at the cente of the weld, which would be caused by the electomagnetic foce, in the helium ac was expeimentally obseved and was simila to the theoetical pediction, wheeas its deep penetation at the cente was not found in pediction of expeiment in the agon ac. 1 15A, Low Sulfu Expeiment = s Calculation 1 Fig. 1 Compaison of expeimental and theoetical weld pofiles fom 15 A afte s in an agon ac. 1 15A, Low Sulfu Expeiment = s Calculation 1 Fig. 11 Compaison of expeimental and theoetical weld pofiles fom 15 A afte s in helium ac. Conclusions The conclusions in the pesent pape ae summaied as follows. (1) The whole egion of the stationay gas tungsten ac welding pocess, namely, tungsten cathode, ac plasma and anode was teated in a unified numeical model to take into account the close inteaction between the ac plasma and the liquid anode. () Calculations wee made fo the time dependent development of the penetation of the anode fo the ac plasma in diffeent gaseous atmosphee, namely, in agon and also in helium. (3) The calculated maximum velocities of the weld pool fo each foce, namely, dag of cathode jet, buoyancy, electomagnetic and Maangoni foce wee 7, 1.,.9 and 1 cm s -1, espectively, in agon gas tungsten ac welding. () The calculated maximum velocities of the weld pool fo each foce, namely, dag of cathode jet, buoyancy, electomagnetic and Maangoni foce wee 11,, 5 and cm s -1, espectively, in helium gas tungsten ac welding. (5) The above numeical calculations showed that the calculated convective flow in the weld pool was pincipally dominated by the Maangoni foce independently of the gaseous atmosphee. Howeve, anothe dominant foce was the dag foce of the cathode jet in the agon ac and the electomagnetic foce in the helium ac. () It was concluded that a balance of these diving foces could change the diection of e-ciculatoy flow in the molten anode and damatically vaied the weld penetation geomety. Refeences [1] Oepe G.M., et al.; Welding J., 193; : p37s-31s. [] Choo R.T.C., et al.; Welding J., 199; 71: p77s-93s. [3] Matsunawa A.; Poc. 3d Int. Conf. Tends in Welding Res., Gatlinbug, Tennessee, USA, 199; 3: p3-1. [] Goodai M., et al.; J. Phys. D: Appl. Phys., 199; 31: p [5] Tanaka M., et al.; Metall. Tans. A, ; 33A: p3-5. [] Tanaka M., et al.; Plasma Chem. & Plasma Pocess., 3; 3: p55-. [7] Zachaia T., et al.; Metall. Tans. B, 199; 1B: p-3. [] David S.A., et al.; MRS Bulletin, 199; 19 (1): p9-35. [9] Patanke S.V.; Numeical Heat Tansfe and Fluid Flow, Hemisphee Publishing Copoation, 19. [1] Boulos M.I., et al.,; Themal Plasmas, Plenum Pess,