Research on laser weld penetrationmonitoring

Research on laser weld penetration monitoringw ith laser induced plas ma signals*D uan A iqin,H u Lunji and W ang Yajun段爱琴,胡伦骥,王亚军**A bstract In t h is pap er,laser induce d p l as m a si gnals w ere anal y zed dur i ng keyhole w el d i ng t hrough t hree m et hod s.A ccording t o t he results,the relativit y bet w een op tical and acoustic si gnals of p las ma is sho w n when w elds are in full p enetrati on,or partial penetra tion an d non p enetration.K ey word s laser w e l d i ng,plas m a,opti ca l si gna,l acoustic signa l0 IntroductionIt is w ell known that laser i n duced p las m a is a so rt of co mm only i m portant physical pheno m ena acco m pan ied w it h K eyho le duri n g laser deep penetrati o n w elding.It plays an i m portan t role i n for m i n g ofK eyho le,ener gy and m aterials transfer during laser w e l d i n g.So it can be rea l ized that laser we l d penetration are diagnosed and con tro lled by plas m a signals[1-4].M easuring p las m a signals fr o m back ofw eld m ay be a m ost si m ple m ethod o f m on itori n g w e l d penetration i n ti m e.But i n m any cases,the m easure m ents can on l y be taken fro m w e l d face.The m ain princi p les of m onitori n g w e l d penetrati o n w ith plas m a signals are due to different m oving states o f plum e bet w een full penetrati o n and non penetration(or partial penetration).It is kno w n that laser induced plas m a co m es fro m ion ization o fm etal vapo rs and sh ieldi n g gas duri n g laserw e l d i n g.Plas m a has optica,l acoustic and char ged c haracteris tics,and has so m et h i n g w ith w elding para m eters,m aterial co m positi o ns,shielding gas and quality ofw e l d.By the spectr ograph ic ana lysis of plas m a,laser i n duced plas m a consisted ofm any d ifferent waveleng th.The br i g h t n ess o f spectru m is different for d ifferent m aterial co m positi o ns,shie l d i n g gas or o t h er w e l d i n g para m eters.The acoustic e m issi o ns are a co m p lex process,such as ther m al expansion,so li d ificati o n,vaporization,keyhole osc illation,p l a s m a particle ejection and gas flo w.The spectr um o f acoustic e m i s sion duri n g laser w elding can ex tend to MH z frequenc ies,but useful infor m ati o n can be obta i n ed fro m spectra recorded over a m uch s m a ller range (!100k H z).Th i s is particularly the tr ue under the con d itions of keyho le w elding w hen vaporization and p las m a for m ation are closely re lated to i n stabilities and m ass m o tion i n t h e m e lt surr ound i n g the keyhole.The frequency o f osc illations characterizing t h ese processes are typically! 10k H z,so acoustic spectra i n t h e range of100H z-20 MH z w ill be i n dicati v e o f keyho le stab ility and w elding cond iti o ns[3].1 Experi m ental apparat us and procedureI n th is paper,three m et h ods w ere used to study the p l u m e over m o lten pool f o r know i n g optic and acousti c characters of plas m a.M easuring re lative i n tensity o f p las m a signals(optic and acoustic si g na ls)by a spec i a l p las m a m easure m ent apparatus w as one of the m ethods.B e sides,h i g h speed ca m era had been used to study shapes o f p las m a,and at sa m e ti m e,optic e m issions o f plas m a w ere ana l y zed by spectrograph ic m easure m en.t Reducing laser* **Sponsored by De fence Fund.Duan A i q i n,H u L unji andW ang Y a j un,N ationa lK ey L abo rato ry o fH i gh D ensity Beam P ro cessi ng T echno logy,B eiji ng A eronauti ca lM anu f ac t ur i ng T echno l ogy R esearch Insti tute,Be iji ng,100024. E m a i:l hb104@ba m tr.i com (D uan A i q i n) D uan A i q i n and Hu Lun j,i D epart m ent ofM a teria l s Sc ience and Eng i neering,H uaz hong U n i versity o f Sc i ence and T echnology,W uhan,410073.po w er and changing focus positi o n conti n ually w ere used to produce d ifferent penetration w e l d s so that re lativ ity bet w een w elds penetrati o n and p las m a si g na ls can be stud ied .The experi m en tal setup is sho wn i n F i g .1.A 2k W fast ax ia l fl o w C O 2laser w as used to m ake buttw elds .T ri al m ateri a l w as sta i n l e ss steel 1C r18N i 9T ,i and its size w as 150mm ∀60mm ∀2mm.F ig .1 Sche m atic d iagra m of exper i m en t syste mA spec i a l plas m a m easuring apparatusw as used .The sa mp li n g frequenc ies w as 50kH z for one sensor .I n the experi m en,t Phanton V4h i g h speed ca m era w as used .Its speed w as 1000fra m es per second and reso luti o n w as 512∀512.2 M easure m ent and ana l y sis2.1 Optical and acoustic e m ission of plas m aThe i m ages can g ive m any infor m ati o n about p las m a .F i g .2sho w s the plas m a pictures of four kinds of m ateria ls i n l a ser keyho le w el d i n g .It is c lear thatm aterial co m posi ti o ns affect the shape and intensity o f p las m a seriousl y ,so m aterial m ust be considered w hen plas m a si g na ls w ere used .Fig .3sho w s a series o f i m ages of plas m a in laser keyho le w e l d i n g by high speed ca m era .Fro m the fi g ure ,it is clear that plas m a has v isible fluctuation and uncertain period icity .Optical si g na ls analysis de m onstrated th is characters a lso .F i g .4recorded a process of p las m a usi n g the special plas m a m easuring apparatus w hen laser po w er reducedconti n ua lly .F i g .4a sho w s t h e variati o n o f sa m pling position vs optical signal i n tensity ,and F i g .4b show s acousti c signal i n tensity at t h e sa m e positi o n .Fro m F i g .4,it is shown that the plas m a signals have v i s ible fluct u ation and uncerta i n shape at sa m e ti m e ,but a lso have so m e trends .Fig .2 Shap es of p las m a at differen t m ater i al co mpositionF ig .3 Shapes of p l as ma(A series of p las m a record ed by 1000fra m e per second )Thr ough above research,w e kno w that only the tran sient i n tensity of laser i n duced p las m a signals can t be used to sho w stability o f w elding and w e l d penetration .Bu t their trends have obv i o usly re lativity w it h w eld pene tration .So so m e data processing m ethods w ere used to study th is relativ ity .2.2 R elativity bet w een plas m a signals and w eld pen etrationI n this study ,so m e data pr ocessi n g m ethods w ere used to deal w it h t h e or i g i n al da ta .Through analysis o f m any sa m pli n g data ,such as average relative intensityF ig.4 M easured p las m a si gn als at a w el d whenpo w er reduced cont i nually processi n g,FFT processing and calculation of ener gy spectra,so m e resu lts cou ld be found.F irs,t t h ere w ere no spec ial frequent to be found both optica l and acoustic si g nals.N ex,t energy spectra and average relati v e intensity had si m ilar trends,but energy spectra had m ore fluctua s,t though optical and acoustic si g na ls had d iffer ent variety r u le fo r sa m e w e l d,they had sa m e trendsw hen w eld penetration w as var i e d.Fig.5sho w s the variati o n of optical signal intensity (acoustic signal i n tensity)at different positi o n of the w e l d after the data sho w n i n Fig.4w ere processed.Fro m Fig.5,it is clear that si g na ls can be d i v i d ed i n to four ran ges,and these range have obv iousl y relativ ity w ith w e l d penetrati o n.Table1sho w s all i n for m ation.Fro m F i g.5and Tab le1,so m e resu lts sho w fo llo w ing:First range(R1):I n th i s range,acoustic si g na ls in tensity are lo w er,average intensity is on ly166and the h i g hest is less than500,and fl u ctuati o ns are s m al.l Bu t optica l signals have the h i g hest i n tensity and s m all fluctua tions.In this range,the w e l d is full penetra ti o n and has good appearance of back.Tab l e1 Con trast between opt i cal and acoustic si gnals during d ifferen t w eld range R ange R1R2R3R4P ositi on1-218218-367367-441441-500 L aser powe r1300-950W950-716W716-600W0W e l d stateFull pene trati on;G ood appearance o f back o f we l dPartia l penetra tion;W eld w i dth i s reduceN on penetrati on W eldi ng stopA ve rage i ntensity ofop tica l s i gna ls12818306936A ve rage i ntensity ofacoustic si gna l s1666398030Second range(R2):In th i s range,acoustic si g na ls i n tensity i n crease suddenly and have i n tense fl u ctuati o ns, average i n tensity is639and i n so m e positi o n signals are higher than1700.A t the sa m e ti m e,optica l signals have very obv iousl y fl u ctuati o ns.R elative to parts of we l d are partial penetrati o n and w eld w i d th beco m e narro w.It can be consi d ered t h at keyho le is i n un stab ility.Third range(R3):In this range,a ll signals obv i o us ly reduce till very l o w leve.l Average i n tensity o f acousti c signals is80and optica l is69.This process w as in heat conduct w eldi n g.Fourth range(R4):w e l d i n g stop,all signals co m e fro m background.F ig.5 Variation of we l d position vs op tica l si gna l inten sityand acoustic si gna l inten sity3 D iscussi o nA ccor d i n g to above results,w e can diagnose and con tro lw eld penetrati o n as fo llo w s:Suppose that m aterial co m position and sh i e l d i n g gas are constan,t so w hen w e l d s are i n full penetrati o n|#S t|=|S t-S0|!#S0(1)|#I t|=|I t-I0|!#I0(2) W hen w elds are in partial penetration or non penetrati o n:|#S t|=|S t-S0|>#S0(3)|#I t|=|I t-I0|>#I0(4) W here,S t,I t:acoustic and optica l signals intensity at a position t;S0,I0:the i n itialization of acoustic and optical si g na ls intensity;#S0,#I0:the i n itialization o facoustic and optica l signal fl u ctuation.Though S0,I0,#S0,#I0are re lativ ity w ith w elding para m eter,for a w e l d i n g process,it can be defi n ed w hen m ateria l co m positi o n and w eld i n g para m eters are fi x ed. Therefo re,by t h e resu lts obta i n ed fro m above ana l y sis, deter m ined w e l d s penetration can be m onitored in ti m e.4 C oncl u sionsThe m a j o r conc l u si o ns obtained fro m this study are as fo ll o w s:(1)Laser induced plas m a si g na ls have c l o se re lativi ty w ith m ater i a l co m positi o ns and w eld i n g para m eters i nC O2laser w elding.(2)The acoustic signals i n tensity are d ifferent at full penetrati o n w elds,partial penetrati o n and non pene tration w elds.A t full penetrati o n w e l d,acoustic signals i n tensity are very lo w er,but obviousl y i n crease at partial penetrati o n.W hen m aterial and other w e l d i n g para m eters are unc hanged,w e l d penetrati o n are unchanged,only w eld po w er have littl e influence w ith acoustic signal i n ten sity.(3)The optical signal intensity have close relativ ity w ith w eld penetration.A t fu ll penetration w elds,the opti ca l signal intensity are stab le,w hile the optical signals are fluctuation at parti a l or non penetrati o n.(4)It is a available m ethod using both optical and a coustic signals for m on ito ri n g w eld penetration i n keyhole w elding.R eferences[1] Isam a M iya m oto.Spati a l and te m po ra l character istics o f l aseri nduced plas m a i n CO2laser w eldi ng.P ro ceedings LAM P 92,N agaoka,June,1992.[2] Luo H ong,H u Lunj,i H u X i yuan,e t a.l L ase r keyho le effec ton opti ca l and acoustic signa l s dur i ng l ase r w e l d i ng.Ch i na M echan i ca l Eng i neering,1998,9(11):84-86(i n Chinese) [3] D uley W W.L aser w eldi ng.N e w Y ork:John W il ey&SonsInc,1998.[4] Su Y andong.Spectrographic ana l y si s of laser i nduced plas m ai n process o f high powe r l ase r we l d i ng.China W eldi ng,1999,8(2):121-126。