CORRELATION BETWEEN MELT POOL TEMPERATURE AND CLAD FORMATION IN PULSED AND CONTINUOUS WAVE ND:YAG LASER CLADDING OF STELLITE 6

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1 Proceedings of the st Pcific Interntionl Conference on Appliction of sers nd Optics 4 <Edited y iln Brndt nd Erol Hrvey> CORREATION BETWEEN ET POO TEPERATURE AN CA FORATION IN PUSE AN CONTINUOUS WAVE N:YAG ASER CAING OF STEITE 6 Shoujin Sun, Yvonne urndet, iln Brndt Industril ser Applictions ortory, Industril Reserch Institute Swinurne Swinurne University of Technology, Hwthorn, Vic, 3, Austrli Astrct The melt pool temperture in pulsed lser cldding of stellite 6 ws mesured with two-colour pyrometer. The pulse pek temperture ( T p ), durtions when the melt pool temperture is ove the melting points of stellite 6 powder ( t ) nd sustrte ( t ) in ech pulse hve een clculted nd the effects of lser operting prmeters (pulse energy, pulse frequency nd spot overlp) on these vlues hve een exmined. It is found tht T p only depends on the pulse energy nd determines the melt pool size ( ), wheres the rtios of oth t nd t to the pulse length increse with pulse energy, pulse frequency ( f ) nd spot overlp. The predictions of cld height, the totl thickness of cld lyer nd dilution y T p, t nd t re given for oth pulsed nd continuous wve (CW) lser cldding of stellite 6. Comprison etween the experimentl dt nd prediction hs een mde. Thick cld lyer with low level of dilution requires higher vlue of the product of t f, longer em interction time ( τ ) nd lrge. Introduction ser cldding is surfcing technique used to pply metl coting onto metl sustrte to improve its wer, oxidtion, erosion nd corrosion resistnces [-3]. Its dvntges include low het input, low distortion of the workpiece nd finer dendrite structure of the cld lyer compring with the conventionl techniques such s PTA nd TIG [4]. In some cses, it is required to produce thick cld lyer with minimum dilution to explore the mximum properties of the cld lloy such s the ppliction of colt-sed stellite lloy for improving the lifetime of low-pressure (P) stem turine lde [5, 6]. Therefore, the prediction nd control of oth cld height nd dilution re demnded to meet the industril requirement. There re mny reports on the continuous wve (CW) lser cldding [-9], where the control of cld height nd dilution is chieved simply y operting t n pproprited level of lser power nd chnging the scn rte nd powder mss flow rte. However, there is little reserch pulished on the pulsed lser cldding, which is n effective wy to reduce further the het input into the sustrte nd refine the lyer microstructure. It is however more difficult to control the cld height nd dilution of the pulsed lser cldding ecuse of the dditionl vriles, such s pulse frequency, pulse durtion nd pulse energy []. This pper presents generl model sed on the mesurement of melt pool temperture to predict the cld height nd dilution for oth CW nd pulsed lser cldding. The model nlysis hs een verified y the experimentl results. Experimentl procedures Single-trck cldding experiments involving oth CW nd pulsed lser clddings of stellite 6 on the AISI 4 grde stinless steel hve een crried out. The chemicl composition (wt.%) of stellite 6 powder (prticle size rnge 53 to 8µm) ws.57 C, 8.7 Cr,. Si, 3.9 W,.4 Fe,.6 Ni,.3 n with the lnce Co. The chemicl composition (wt.%) of steel sustrte (thickness mm) ws.4 C 4.3 Cr.7 Si,.45 n nd the lnce of Fe. The CW lser cldding ws crried out t lser power of, 4, 6, 8 nd W respectively. The scn rte ws set t 9, nd 5mm/min respectively t ech level of lser power. The pulsed lser cldding ws crried out t the pulse frequency of 4 nd 6Hz respectively, the pulse length (the durtion when the lser power is switched on) ws fixed t 8ms for oth pulse frequencies. At ech pulse frequency, the pulse energy ws set t 5, 8 nd 4J/pulse respectively. The scn rtes were selected to represent the spot overlps of 89 nd 83%. etils on the pulsed lser cldding cn e found in []. The lser spot size ws 3.5mm nd the powder mss flow rte ws set t low level of.7g/min for oth CW nd pulsed lser clddings in order to exmine the effect of lser operting prmeters on the cld height nd dilution. uring cldding, the melt pool temperture ws mesured y two-color pyrometer which ws set up coxilly with the lser em. The spot size of pyrometer ws mm, nd the smpling rte ws khz.

2 Cld height Anlysis A typicl temperture profile of the melt pool within pulse is shown in Figure. The temperture increses to the pek ( T p ) efore the lser power is switched off s p following which it rpidly cools down. T m nd T m re the melting points of the sustrte nd powder respectively. t nd t re the durtions when the melt pool temperture is ove the melting points of powder nd sustrte respectively. Temperture ( o C) Figure. A typicl melt pool temperture profile of pulsed lser cldding t pulse frequency of 4Hz. t points reflect the smpling rte of khz. It is ssumed tht the powder injected into the melt pool is melted nd deposited onto the surfce of sustrte only during the time the melt pool temperture is ove the melting point of powder (i.e., within t ). Therefore, the nominl cld height produced in one pulse is the volume of powder deposited on the sustrte in one pulse divided y the cld re, i.e.: α t m& h = ρ where, α is powder efficiency (the percentge of delivered powder melted in the melt pool), m& is powder mss flow rte nd ρ is the density of the lloy powder. Since the cld lyer is the overlp of numer of pulses, nd the numer of overlpped pulses is f, v Therefore, the single-trck cld height is: h H = v t f α m& = ρ t Time (ms) f t T p () τ s T m p T m () where, is the melt pool dimeter nd τ = is v the lser em interction time, is the lser spot size nd v is the scn rte. Thickness of cld nd dilution The cld thickness is the thickness of overll melted mterils including the powder nd sustrte. The cld thickness in one pulse is dependent on the verge temperture nd the effective em interction time s follows: s n ( T T ) ( f t τ) C T = K (3) i m + where, K, n nd C re the constnts relted to the therml properties of the sustrte nd lloy powder, T i is the verge melt pool temperture within durtion t in pulse. The dilution is cused y the melting of sustrte during cldding. It is clculted y: T H H = % = % (4) T T Appliction of model nlysis Continuous wve lser cldding The effects of lser power nd scn rte on the cld height nd totl thickness of single-trck cld lyer re shown in Figure. It cn e seen tht the cld height increses slightly with lser power, wheres the totl thickness of cld lyer increses drmticlly with the lser power. Both the cld height nd totl thickness of cld lyer increse with decresing scn rte. Cld height (mm) mm/min ser power t workpiece (W)

3 Totl tickness of cld (mm) mm/min elt pool temperture ( o C) mm/min ser power t workpiece (W) Figure. Effects of lser power nd scn rte on () cld height nd () totl thickness of cld lyer for continuous wve lser cldding The effects of lser power nd scn rte on the melt pool temperture re shown in Figure 3. At the sme powder mss flow rte, the melt pool temperture increses linerly with incresing lser power regrdless of the scn rte. The melt pool size ws mesured from the trnsverse cross-section imges of the single-trck cld. The reltionship etween the melt pool size nd melt pool temperture is shown in Figure 3. Within the lser spot size nd nozzle dimeter, the melt pool size increses linerly with the melt pool temperture. In the cse of continuous wve lser, f t = nd f t = therefore equtions () nd (3) cn e simplified s: α m& H = τ ρ s n ( T T ) τ C (5) T = K (6) i m + The comprisons etween the mesured nd clculted cld height nd totl thickness of cld lyer re shown in Figures 4 () nd (). The clcultions were mde using equtions (5) nd (6) for α = 5%, = 3.5mm, m & =.7g / min,. 3 o.3 nd K =.35mm/ C / sec 3 ρ = 8.4g / cm, n =, C =.3mm. The vlues of the constnts K, n nd C re determined empiriclly. The good greements etween the mesured nd clculted cld height nd totl thickness of cld lyer indicte tht the modeling nlysis cn predict the cld formtion of CW lser cldding. elt pool size (mm) ser power t workpiece (W) mm/min elt pool temperture ( o C) Figure 3. Reltionships etween () lser power nd melt pool temperture, () melt pool temperture nd melt pool size. esured cld height (mm) mm/min Clculted cld height (mm)

4 esured totl thickness of cld (mm) mm/min Clculted totl thickness of cld (mm) Figure 4. Correltion etween the clculted nd mesured () cld height nd () totl thickness of cld lyer for CW cldding. Pulsed lser cldding The experimentl results of single-trck pulsed lser cldding of stellite 6 re shown in Tle. Both the cld height ( H ) nd totl thickness of cld lyer ( T ) increse with pulse energy ( E ), pulse frequency ( f ) nd spot overlp ( SO ). The effects of these prmeters on the cld height re more severe thn on the totl thickness of cld lyer. Tle Results of single-trck pulsed lser cldding E f SO H T t t (J/pulse) (Hz) (%) (µm) (µm) (ms) (ms) The verges of t nd t clculted from ll the pulses for the single-trck cldding re lso listed in Tle. espite the sme pulse length (8 ms), the durtions t nd t in ech pulse depend on the pulse energy, spot overlp nd pulse frequency. The durtions increse with pulse energy, pulse frequency nd spot overlp ecuse of the higher effect of the previous pulse, ut the effect of pulse energy is more significnt. Regrdless of pulse frequency nd spot overlp, the pek temperture increses linerly with pek lser power s shown in Figure 5. This is similr to continuous wve lser cldding, where the melt pool size is strictly dependent on the melt pool pek temperture (Figure 5). The melt pool size increses linerly with melt pool pek temperture nd reches its mximum vlue, i.e. the lser spot size (3.5 mm) roughly t the melt pool temperture of 95 o C. Pulse pek temperture ( o C) elt pool size (mm) Hz, 89% overlp 4 Hz, 83% overlp 6 Hz, 83% overlp Pulse pek power (W) Hz, 89% overlp 4 Hz, 83% overlp 6 Hz, 83% overlp elt pool pek temperture ( o C) Figure 5. Reltionships etween () pulse pek power nd pulse pek temperture nd () pulse pek temperture nd melt pool size. Compring Figures 3 nd 5, it cn e found tht the reltionship etween the melt pool size (in mm) nd melt pool temperture (in o C) cn e roughly descried y: =.36 Tp 3.5 when Tp 95 C (7) = 3.5 when Tp > 95 C (8) The correltion etween the mesured nd clculted cld height nd totl thickness of cld lyer for the pulsed lser cldding is shown in Figure 6. The clcultions were mde using equtions () nd (3) t the sme vlues of the constnts s those for the CW lser cldding. It is cler tht the model cn lso predict the cld height nd totl thickness of cld lyer for the pulsed lser cldding. o o

5 esured cld height (mm) Hz, 89% overlp 4 Hz, 83% overlp 6 Hz, 83% overlp the effects of the product of t f nd t f s shown in equtions () nd (3), ut the difference of totl thickness is more significnt. Since the product of t f increses with the melt pool temperture for the pulsed lser cldding, the difference of cld height etween PC nd CWC is smller with incresing melt pool temperture. The vrition of dilutions with melt pool temperture shows totlly different trend for PC nd CWC s shown in Figure 7. The dilution of CWC increses with melt pool temperture ut tht of PC decreses with incresing melt pool temperture which is ttriuted to the fster increse of cld height with melt pool temperture. esured cld thickness (mm) Clculted cld height (mm) Hz, 89% overlp 4 Hz, 83% overlp 6 Hz, 83% overlp Clculted cld thickness (mm) Figure 6. Comprison of the mesured nd clculted () cld height nd () totl thickness of cld lyer for the single-trck pulsed lser cldding. Comprison etween CW nd pulsed lser cldding There re mny wys to compre the CW nd pulsed lser cldding, such s sed on the sme lser power t the workpiece, or the sme pek power. In this pper, comprisons hve een mde sed on the (pulse pek) melt pool temperture s shown in Figure 7 nd on the verge lser power s shown in Figure 8. The solid lines re drwn sed on the prediction from equtions () nd (3) for pulsed lser cldding (PC) nd equtions (5) nd (6) for CW lser cldding (CWC). The symols re otined from experimentl mesurement. In this experiment, the powder mss flow rte ws.7g/min, the scn rte ws 9mm/min, nd the pulse frequency for PC ws set t 4Hz. Both the cld height nd totl thickness of cld lyer of pulsed lser cldding re smller thn those of CW lser cldding t the sme melt pool temperture ecuse of Cld height or thickness (mm) ilution (%) PC cld height PC cld thickness CWC cld height CWC cld thickness elt pool temperture ( o C) PC dilution CWC dilution elt pool temperture ( o C) Figure 7. Comprison of () cld height nd totl thickness of cld lyer nd () dilution etween pulsed nd CW lser cldding on the melt pool temperture. One dvntge of pulsed lser cldding is tht it cn e done t very low verge lser power (Figure 8) t which good onding cld lyer cn not e produced y CW lser. The rtio of cld height etween PC nd CWC

6 t sme lser power is smller thn the product of t f due to the lrger melt pool size of pulsed lser cldding. The dilution of pulsed lser cldding is lower t higher lser power wheres tht of CW lser cldding is lower t lower lser power. For pulsed lser cldding, the durtions when the melt pool temperture is ove the melting points of stellite 6 powder ( t ) nd sustrte ( t ) in ech pulse ply importnt roles for cld height nd totl thickness in ddition to the pulse pek temperture of the melt pool. Cld height or thickness (mm) PC cld height PC cld thickness CWC cld height CWC cld thickness ilution of CW lser cldding increses with melt pool temperture (or lser power) wheres the dilution of pulsed lser cldding decreses with melt pool temperture (or lser power) t the investigted rnge of lser power. The higher vlue of the product of t f, longer em interction time ( τ ) nd lrge rtio of melt pool size ginst lser spot size ( ) re required to produce thick cld lyer with low level of dilution for pulsed lser cldding. References ilution (%) Averge lser power t workpiece (W) PC dilution CWC dilution Averge lser power t workpiece (W) Figure 8. Comprison of () cld height nd totl thickness of cld lyer nd () dilution etween pulsed nd CW lser cldding s function of the lser power t workpiece. Conclusion The model developed sed on the melt pool temperture mesurement in the present study cn predict the stellite 6 cld formtion nd chrcteristics in oth pulsed nd continuous wve lser cldding. In the cse of CW lser cldding, the cld height increses slightly ut the totl thickness of cld lyer increses drmticlly with melt pool temperture. Therefore, the melt pool temperture must e strictly controlled to control the dilution. [] Colco R., Crvlho T. nd Vilr R. (994) ser cldding of stellite 6 on steel sustrte, High Temp. Chem. Processes 3, -9. [] So H., Chen C. T. nd Chen Y. A. (996) Wer ehviors of lser-cld stellite lloy 6, Wer 9, [3] e Hosson J. T.. nd e ol vn Otterloo. (997) Surfce engineering with lsers: ppliction to Co sed mterils, Surfce Engineering 3, [4] Kthuri, Y. P. () Some spects of lser surfce cldding in the turine industry, Surfce nd Cotings Technology 3, [5] Peters T. nd Jhnen W.. Stem turine leding edge repir y stellite lser cldding, Proceedings of EPRI, ST7. [6] Shepelev., edres B., Kpln W.., Bmerger. nd Weisheit A. () ser cldding of turine lde, Surfce nd Cotings Technology 5, [7] Frenk A., Vndyoussefi., Wgniere J.-., Zryd A. nd Kurz W. (997) Anlysis of the lser-cldding process for stellite on steel, etllurgicl nd terils Trnsctions 8B, [8] Steen W.., Weersinghe V.. nd onson P. (986) Some spects of formtion of lser cld trcks, in Proceedings of SPIE Vol. 65: High power lser nd their industril pplictions, [9] Weersinghe V.. nd Steen W.. (987) ser cldding with lown powder, etl construction 9, [] Sun S., urndet Y. nd Brndt. (3) Pulsed lser cldding of stellite 6 on stinless steel, in Proceedings of WTIA Surfce Engineering Conference, Sydney, Austrli.