Efflux Plasma Charge Sensor for Weld Joint Penetration in Keyhole Plasma Arc Welding

Transcription

1 Efflux Plasma Charg Snsor for Wld Joint Pntration in Kyhol Plasma Arc Wlding S. B. Zhang and Y. M. Zhang Wlding sarch Laboratory Cntr for obotics and Manufacturing Systms and Dpartmnt of Elctrical Enginring Collg of Enginring Unirsity of Kntucky Lxington, KY 456 Abstract. An fflux plasma charg snsor is proposd to monitor wld joint pntration in kyhol plasma arc wlding. For conninc of application, th snsor is intgratd as part of th fixtur. Basd on xprimntal data, a linar modl is fit to stimat th joint pntration from th output of th snsor. To undrstand th principl of th proposd snsor, th physical procss associatd with plasma jt and spac charg is analyzd. Exprimnts ha also bn conductd to xamin th potntial of th proposd snsor in monitoring burn-through and kyhol collaps. Introduction obustnss, conomy, and accuracy ar among th major critria to aluat snsors for automatd wlding systms. Du to its robustnss with rspct to th harsh nironmnt during arc wlding and its grat conomy, through-th-arc snsing has found applications in arc lngth control and sam tracking (fs. 1-6). It has also bn usd to analyz th oscillation of th wld pool for monitoring wld joint pntration in gas tungstn arc wlding (GTAW) (f. 7) du to th rlationship btwn th oscillation of th wld pool and its pntration. Kyhol plasma arc wlding (PAW) is on of th primary arc wlding procsss for prcision joining of critical mtal componnts (fs. 8-9). To nsur wld quality, th pntration must b monitord and controlld. It is known that th wld pool is subjct to th impact of th high spd

2 plasma jt in kyhol PAW and that no studis ha disclosd rlationships, if any, btwn pool oscillation and wld pntration. Hnc, although utilization of th arc has grat adantags in trms of robustnss and conomy, highly dsird by wlding industry, through-th-arc mthods ha not found applications in pntration control for kyhol PAW. If an arc basd robust, conomical pntration snsor can b dlopd for kyhol PAW, its potntial in application could b quit positi. During kyhol PAW, th plasma jt pntrats through th workpic, forming a funnl-shap caity rfrrd to as kyhol, as shown in Fig 1 (f. 1-11). To dtrmin possibl corrlation btwn pntration and plasma arc, charactristics associatd with th kyhol should b takn adantag of. In an arly study, th light of th plasma fflux from th kyhol was masurd for maintaining th kyhol (f. 1). Anothr xampl inold dtcting spcific spctral lins of mtal apor for kping th kyhol through which th apor of bas mtal scaps (f. 13). cntly, th authors studid th possibility of using th diamtr of th kyhol to monitor pntration (f. 14). Exprimntal rsults showd that, aftr th kyhol is stablishd, incrasing/dcrasing th wlding currnt/wlding spd incrass joint pntration until burn-through. Similarly, dcrasing/incrasing th wlding currnt/wlding spd dcrass th joint pntration until th kyhol collapss. Howr, xprimnts also rald that th diamtr of th kyhol dos not chang with th hat input which altrs th joint pntration (f. 14) aftr th kyhol is stablishd. This implis that th xistnc of or th diamtr of th kyhol dos not proid sufficint information for monitoring joint pntration. This study aims at dloping a pntration snsor, rfrrd to as fflux plasma charg snsor (EPCS), for kyhol PAW basd on th rsidual of th nrgy of th plasma jt xiting from th kyhol. This ida is undrstandabl bcaus, for a gin matrial of bas mtal, th strongr th

3 rsidual nrgy is, th mor nrgy th bottom of th workpic should absorb from th plasma jt; thus th widr th back-sid width of th wld pool should b. For possibl practical applications, th snsor has bn intgratd as a part of th fixtur. It is xpctd that th rsults of this study would proid foundations to dlop a cost ffcti, robust, and accurat snsor for joint pntration in kyhol PAW. Exprimntal Apparatus and Procdur Kyhol mod is usd in most PAW applications. Chill clamping and gas backup shown in Fig. (f.11) proid an undrsid shild which is ncssary for mtals such as stainlss stl, titanium. For wld joints which rquir or allow gas backup, th proposd snsor can b applid with a minor modification of th backing bar. Fig. 3 shows th proposd snsor built into th backing bar by using an insulator, a capacitor, and a rsistor. Th insulator is usd to prnt th oltag, causd by th spac charg of th plasma jt, from bing short-circuitd so that it can stablish a oltag drop across th rsistor. Th oltag drop srs as th output of th snsor. Th capacitor,.1 µ F in this study, is usd to form a filtr with th rsistor. A computr data acquisition with high input impdanc of masurmnt circuit and softwar program was usd to sampl th output oltag of th snsor. Bcaus th snsor masurs th spac charg gnratd by th fflux plasma jt, it is rfrrd to an fflux plasma charg snsor. In th proposd mthod, th backup bar can b ithr moing or fixd. In this study, a fixd dsign as shown in Fig. 4 was usd. Th distanc btwn th workpic and th dtction plat, rfrrd to as masurmnt hight h, was 1 inch (5.4 mm). Pur argon was usd as shilding gas for both th top-sid and th back-sid. To idntify th modl of th snsor, th rsistanc of th output rsistor of th snsor,, was adjustabl. Th output signal of th snsor was rcordd by a data acquisition

4 systm. Th back-sid width of th wld was masurd off-lin aftr xprimnt using a ision snsor dlopd arlir (f. 15). Bad-on-plat wlds wr mad on stainlss stl (34) plats using a constant currnt (CC) inrtr. Th currnt rangd from 1 A to A. Th dimnsions of th workpic wr mm( in.). Wlding currnt and wlding spd wr changd in ordr to chang th joint pntration and th procss mod (kyhol or non-kyhol). Othr wlding paramtrs shown in Tabl 1 wr constant during xprimnts. Tabl 1 Constant Wlding Paramtrs Orific diamtr 1.57 mm (.6 in.) Flow rat of plasma gas 1.4 L/min (3 ft 3 /h ) Flow rat of shilding gas 11.5 L/min (5 ft 3 /h ) Flow rat of backing gas 9. L/min ( ft 3 /h ) Stand-off 6 mm (.4 in.) sults and Discussion Pntration Prdiction Exprimnts ha bn conductd by using arious wlding currnts and spds to chang th joint pntration. Figs. 5 and 6 plot th xprimntal data. It can b sn that no mattr th caus of pntration chang, whthr arying th wlding currnt or th wlding spd, th output of th snsor gis us a ry good ida of th back-sid width of th wld pool. To furthr rify th corrlation btwn joint pntration and snsor output, th xprimntal data in Figs. 5 and 6 ar plottd as {, wb } pair in Fig. 7. It can b sn that if th snsor output is usd to prdict th back-sid width of th wld pool basd on th following quation: w = ( V 65. V) (1) b

5 whr and w b ar masurd by olt and mm rspctily, th maximum rror is approximatly ±5. mm. Such an accuracy should b sufficint for wld pntration control in most applications. Procss Diagnosis In addition to th joint pntration, th proposd snsor may also monitor th collaps of th kyhol. Fig. 8 shows an xprimnt in which kyhol collaps occurrd. During th initial priod, th output from th snsor was zro. Th wld pntration corrsponding to this priod is partial. Whn th currnt incrasd to 7A from 65 A, th snsor output ros to a significant ll which indicatd th stablishmnt of th kyhol. Howr, th back-sid width of th wld pool thn dcrasd gradually du to procss disturbancs. At approximatly t = s, th output of th snsor suddnly oscillatd btwn zro and V. Th kyhol oscillatd btwn collapsing and opning. Whn th currnt was incrasd to 75 A, th output of th snsor bcam stabl and so did th kyhol. At t = 4 s, th currnt was dcrasd to 65 A and th kyhol closd. Th rsultant output of th snsor was zro. It can b sn that, whn th kyhol closs, no plasma jt xits from th kyhol. Th output of th snsor is zro. Hnc, th snsor output proids information to monitor th xistnc of th kyhol. Similarly, if burn-through occurs, a strong plasma jt will xit from th workpic. Th rsultant output of th snsor will b much highr than in a normal kyhol procss. In th xprimnt shown in Fig. 9, th wlding spd was dcrasd from.7 mm/s to.8 mm/s at t = 35 s. Bfor th chang, th output of th snsor was approximatly 3.5 V. Aftr th chang, th output of th snsor suddnly incrasd to abo 8 V. Burn-through occurrd immdiatly. Hnc, th output of th snsor proids information to monitor th burn-through.

6 Snsor Fundamntals According to th plasma physics, in an ionizd plasma, th man thrmal locitis of th lctrons and th man positily-chargd ions ar dtrmind by thir tmpraturs as wll as th Boltzmann s constant (f. 16): V 8kT = πm 1/ () V i 8kTi = πmi 1/ (3) whr k is Boltzmann s constant, V, T, V i, T i, m th locity, tmpratur, and mass of th lctron, and m i th locity, tmpratur, and mass of th ion. Th ratio btwn V and V i is thrfor 1/ 1/ V / Vi = ( mi / m ) ( T / Ti ) (4) Bcaus th lightst spci of th positi ions is or tims hair than th lctron (f. 17) and th tmpraturs of th lctrons and ions ar approximatly th sam (as th tmpratur of th plasma), th abo locity ratio V / V ( m / m ) 1/ = 44 (5) i i This implis that in th fflux plasma, th man locity of th lctrons is at last 4 tims highr than that of th ions. Basd on th rsults in (f. 17), th diffrnc btwn th locitis of th ngatily-chargd lctrons and th positily-chargd ions rsults in a spac charg bing stablishd around th plasma until th opposing fild du to this spac charg rtards approaching lctrons sufficintly for a charg balanc to occur. Howr, such a charg balanc would nr occur if th stram of th (nw) fflux plasma is maintaind. Hnc, as long as th kyhol xists such that th continuous stram of th fflux plasma is kpt, th fflux plasma causd spac charg maintains a potntial btwn

7 workpic and dtction plat. As drid in (fs. 17, 18), such potntial can b dtrmind by th following quation:.5ktp (6) whr is th charg of th lctron, and T p is th tmpratur of th fflux plasma. In th quialnt circuit of th snsor shown in Fig. 1, a oltag sourc and an intral rsistor ar usd to approximat th fflux plasma. Th oltag drop on th output rsistor, dnotd as, is masurd as th output of th snsor. (Not that th potntial of th dtction plat is ngati in rlation to th workpic ground.) Disrgarding th capacitor which is usd to filtr th nois, th snsor s modl can b writtn as: = + (7) In Eq. (7), is xactly known, is th masurmnt, and and ar unknown. Th following qualitati analysis will show that can b usd as th critical paramtr for dtrmining joint pntration. Th ssnc of th proposd mthod lis in a possibl rlationship btwn joint pntration, masurd by th back-sid width of th wld pool, and th rsidual nrgy of th plasma jt, i.. th nrgy of th fflux plasma. In fact, for kyhol PAW, th width of th wld pool aris slightly along th thicknss dirction of th workpic. This suggsts that th radial-axis hat transfr from th plasma jt, rathr than th hat transfr along th thicknss dirction, is dominant in th workpic. This implis that th width of th wld pool in a particular layr during kyhol PAW may b mainly dtrmind by th hat absorbd in this layr from th plasma jt. On th othr hand, th hat transfr rat, thus th hat absorption, in gnral incrass whn th local tmpratur gradint btwn th

8 hating sourc and th objct bing hatd incrass. Whn th tmpratur of th objct bing hatd is insignificant in comparison with th tmpratur of th hating sourc, th local tmpratur gradint is dtrmind by th tmpratur of th hating sourc. Hnc, th rsidual nrgy of th fflux plasma controls th local tmpratur gradint, thus th hat absorption and th width of th wld pool, on th bottom layr of th workpic. Such rsidual nrgy may b usd to dtrmin th back-sid width of th wld pool: whr w = f Q ) (8) b ( r Q r dnots th rsidual thrmal nrgy of th plasma jt. Furthr, th thrmal nrgy of th fflux plasma can b dtrmind by its tmpratur (f. 19): Q kt p (9) On th othr hand, th tmpratur dtrmins th oltag of th spac charg quation (6). Hnc, w b = g( ) (1) That is, th back-sid width of th wld pool can b dtrmind by th oltag of th spac charg of th fflux plasma. W should thrfor dtrmin basd on th masurmnt to monitor th joint pntration. Eq. (7) gis + = (11) It can b sn that in ordr to calculat basd on Eq. (11), must b known. Th lctrical rsistiity of ncountrs btwn chargd particls (f. ) can b xprssd as: 3 ln Λ η =.53 1 ohm/cm (1) 6 3 / T p

9 whr ln Λ is a constant dtrmind by th lctron dnsity of th plasma and th plasma tmpratur. Bcaus is dtrmind by both th masurmnt hight h and th lctrical rsistiity η, aris with th tmpratur of th plasma. To larn mor about, th authors ha conductd xprimnts to dtrmin it. Suppos two masurmnts, and (1) (), ar aailabl. If thy ar causd by th sam undr th sam, but two diffrnt ( and (1) () ), w will ha = = + (1) (1) + () () (1) () (13) It is idnt that th two unknown paramtrs and can b xactly dtrmind from quation systm (13). Hnc, if w can accuratly control th wlding paramtrs and conditions to prform kyhol plasma arc wlding, w will b abl to rpat th tim-arying procsss of and : ( 1), (),..., ( N) and ( 1), (),..., ( N). Whn th tim-arying procsss of and ar rpatd, w can us diffrnt ( and (1) () ); thus th rspcti masurmnts will b (1) (1) (1) () () () (1), (),..., ( N) and (1), (),..., ( N) : () i = () i = () i + () i + ( 1) ( ) ( 1) ( ) ( 1) ( ) () i () i ( i= 1,,..., N) (14) Hnc, ach pair of ( i), ( i)}( i = 1,,..., ) can b dtrmind from quation systm (14). { N Basd on th abo principl, two diffrnt s, 1 K and 35 K, wr usd to conduct xprimnts using th xact th sam conditions and squnc of wlding paramtrs. Th xprimntal data ar

10 shown in Fig. 11. Fig. 1 plots th rsultant 1), (),..., ( ) and 1), (),..., ( ). It can ( N b sn that significantly aris with wlding conditions and paramtrs. ( N Th ariation of complicats th dtrmination of from th masurmnt. Fig. 1 shows 1kΩ 8kΩ. If w choos a ry larg, * + (15) whr = 45kΩ is an stimat of. In this study, = 36 kω, th maximum rror is * ± 35 / 36 = ± 1%. Hnc, = 36 kω can nsur that b dtrmind basd on with an accptabl accuracy. Bcaus of this mapping rlationship btwn and, it is rasonabl to dtrmin th wld pntration basd on. Conclusions Efflux plasma charg snsor tchnology for joint pntration has bn dlopd for th kyhol plasma arc wlding procss. To undrstand th proposd snsor, fforts ha bn mad to analyz th snsing fundamntals. Exprimntal rsults showd that joint pntration can b monitord by th proposd snsor with ±5. mm accuracy. Th proposd snsor may also b usd to monitor th xistnc of th kyhol and th burn-through. Acknowldgmnt Th authors thank Dr. Arthur B. Nuns, Jr. for his aluabl suggstions. frncs 1. Cook, G. E., "obotic arc wlding: rsarch in snsory fdback control," IEEE Transactions on Industrial Elctronics, IE-3(3): Cook, G. E. and Wlls, A. Michal, "Sidwall-matching adapti control systm for wlding," U. S. Patnt # 4,477,713.

11 3. ichardson,. W "obotic wld joint tracking systms thory and implmntation mthods," Wlding Journal 65 (11): Hunright, J "obotic arc wlding undr adapti control- a sury of currnt tchnology," Wlding Journal 65(11): Sugitani, Y. and Mao, W "Automatic simultanous control of bad hight and back bad shap using an arc snsor in on-sidd wlding with a backing plat," Wlding sarch Abroad, 4(1): Ohshima, K., tc., "Controlling of torch attitud and sam tracking using nuro arc snsor," IECON Procdings of th 1995 IEEE 1 st Intrnational Confrnc on Industrial Elctronics, Control, and Instrumntation, Part, pp , Los Alamitos, CA. 7. Xiao, Y. H. and dn Oudn, G., "Wld pool oscillation during GTA wlding of mild stl," Wlding Journal, 7(8): 48s-434s. 8. Martikaimn, J. K. and Moiso, T. J. I., "Instigation of th ffct of wlding paramtrs on wld quality of plasma arc kyhol wlding of structural stls," Wlding Journal, 7(7): 39s- 34s. 9. Nuns, A. C., Jr. and Baylss, E. O., "Variabl polarity plasma arc wlding on th spac shuttl xtrnal tank," Wlding Journal, 63(9): ussll, G. K. and ubinsky, B., "Thr-dimnsional simulation of th plasma arc wlding procss," Intrnational Journal of Hat and Mass Transfr, 36(13): O Brin,. L., Wlding Handbook, Vol., pp Amrican Wlding Socity, Miami, FL. 1. Mtcalf, J. C. and Quigly, M. B. C., "Kyhol stability in plasma arc wlding," Wlding Journal, 54 (11): 41s-44s. 13. Martinz, L. F., Marqus,. E., Mcclur, J. C. and Nuns, A. C., Jr., "Front sid kyhol dtction in aluminum alloys," Wlding Journal, 7(5): Zhang, Y. M. and Zhang, S. B., "Obsration of kyhol during plasma arc wlding," Wlding Journal, 78(): 53s-58s. 15. Zhang, Y. M. and Koacic, "al-tim snsing of sag gomtry during GTA wlding," ASME Journal of Manufacturing Scinc and Enginring, 119(): Locht Holtgrn, W. 1968, Plasma Diagnostics, pp73-739, North Holland Publishing Company, Nw York. 17. Li, L., Brookfild, D. J., and Stn, W. M., "Plasma charg snsor for in-procss, noncontact monitoring of th lasr wlding procss," Masurmnt Scinc & Tchnology, 7(4): Fink, B.., Kapadia, B. D., and Dowdn, J. M., 199. "A fundamntal plasma basd modl for nrgy transfr in lasr matrial procssing," Journal of Physics, Part D: Applid Physics, Vol. 3, pp Howr, S., Plasma Physics and Magntofluid-Mchanics, pp McGraw-Hill Book Company, Inc., Nw York.. Spitzr, L., Jr., 196. Physics of Fully Ionizd Gas, pp Intrscinc Publishrs, Nw York.

12 Fig. 1 Kyhol in plasma arc wlding.

13 Fig. Typical backing bar for kyhol plasma arc wlding [11]. Fig. 3 Intgratd snsor and fixtur.

14 Fig. 4 Schmatic of clamping apparatus.

15 EPCS output signal () and wlding currnt I (A) A 14 EPCS signal Wlding currnt I 75A Tim (s) (a) 34 65A 54 4 w b Back-sid width w b (mm) Tim (s) (b) (c) Fig. 5 Pntration prdiction xprimnt 1. Tral spd:.5mm/s. (a) Snsor output and wlding currnt. (b) Back-sid width of wld pool. (c) Wld back-sid photo.

16 EPCS output signal ()and wlding currnt I (A) mm/s EPCS signal Wlding currnt I 7A 75A 14 Tim (s) (a) 34.9mm/s Back-sid width w b (mm) 3 1 w b Tim (s) (b) (c) Fig. 6 Pntration prdiction xprimnt. (a) Snsor output and wlding paramtrs. (b) Back-sid width of wld pool. (c) Wld back-sid photo.

17 5 Back-sid width w b (mm) EPCS output signal () Fig. 7 Prdiction of pntration.

18 EPCS output signal () and wlding currnt I (A) EPCS signal No kyhol Unstabl kyhol Wlding currnt I 65A 7A 75A 65A Tim (s) (a) 4 6 w b Back-sid width w b (mm) Tim (s) (b) Fig. 8 Kyhol collaps monitoring. Tral spd:.9mm/s. (a) Snsor output and wlding currnt. (b) Back-sid width of wld pool. (c) Wld back-sid photo. (c)

19 EPCS output signal () and wlding currnt I (A) Burn-through Tral spd.7mm/s Tral spd.8mm/s EPCS signal 7A 65A Wlding curnt I Tim (s) Fig. 9 Burn-through monitoring. 75A 4 6

20 Fig. 1 Snsor quialnt circuit. EPCS output signal () and wlding curnt I (A) ( : 35k) ( : 1k) 65A 7A 75A 65A Tim (s) Fig. 11 Snsor output with diffrnt load rsistors.

21 Sourc oltag () and wlding currnt I (A) A A Tim (s) A 36 4 (a) Sourc oltag. Sourc rsistanc (k) and wlding currnt I (A) A A Tim (s) A 36 4 (b) Sourc rsistanc. Fig. 1 Idntifid spac charg oltag and sourc rsistanc.