JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 Twn Gas Jet-asssted Pulsed Green Laser Scrbng of Sapphre Substrate X.Z. Xe, Z.Q. Luo, X. We, F.L. Zhang, W. Hu and Q.L. Ren School of Eletromechancal Engneerng, Guangdong Unversty of Technology, Guangzhou, Guangdong 5006,Chna E-mal: xaozhuxe@gdut.edu.cn Sapphre s an mportant materal for LED substrates. Because of ts unque physcal property, sapphre s strongly resstant to wet and dry chemcal etchng. Laser-based scrbng technques have a good prospect on sapphre scrbng applcaton. Durng the laser scrbng, the assstng gas sgnfcantly affects the scrbng depth, wdth and qualty. A pulsed laser wth a wavelength of 53 nm s used to scrbe the sapphre substrate by twn assstng gas mpngng both n co-axs and off-axs. Ths process s smulated by a computatonal flud dynamcs code, FLUENT. Durng the smulaton, nlet pressure of co-axal nozzle and off-axal nozzle are vared at dfferent ntersecton angle. By analyzng the statc pressure and flow velocty at dfferent nlet pressure and ntersecton angle, optmal laser processng parameters are obtaned to mprove the groove qualty and reduce the groove wdth. Experments are carred out to testfy the smulaton result. The result of both the smulaton and experment show that when laser pulse energy s 150μJ wth co-axal nlet pressure bar, offaxal nlet pressure ~3 bar and ntersecton angle 70, a better groove qualty and an optmal groove sze can be obtaned. DOI:.961/lmn.16.0.0006 Keywords: assstng gas, laser scrbng, sapphre, flow feld, FLUENT 1. Introducton Sapphre has been wdely used n ndustry, natonal defense and scentfc research. And t s an mportant materal for LED substrates and optcal wndows for ts unque physcal property and strong capacty to resst wet and dry chemcal etchng [1]. Snce the sapphre s hard and fragle, t s dffcult to fabrcate by tradtonal mechancal machnng. Laserbased scrbng, beng a non-contact process s a sutable and effcent method for precson machnng of sapphre wthout the problems of mechancal damage and tool wear. Sapphre scrbng has a great mpact on the yelds of the component and pacagng effcency. Durng the laser scrbng, redeposton layer around the scrbng groove ncreases the groove wdth. That s why reducng and elmnatng the fused deposton s the ey to mprove the groove qualty and ncrease components yelds. The assstng gas has been wdely used for removng the meltng materal n laser metal cuttng and drllng. Effcency and qualty are closely related to gas pressure, nozzle geometry and stand-off dstance. By adustng these parameters, the generaton of normal standoff shoc whch reduces the total pressure loss can be avoded []. W O Nell [3] ndcates that n laser metal cuttng, the angle between the laser beam and the off-axal gas et determnes the maxmum groove depth & wdth and the generaton of a sutable clocwse rotatng vortex may ad the removal of molten materal. Sezer [4] nvestgates the nfluence of angle n twn gas et-asssted laser drllng and analyzes the flow feld at dfferent angles. Ma [5] studes the phenomena of shoc wave nduced by a supersonc mpngng et emanatng from a straght nozzle onto a substrate wth varyng nclned angles by numercal smulaton and experment. He ponts out that f the total pressure s hgh, a Mach shoc ds s formed and the gas pressure drops dramatcally beyond the shoc ds. Melhem [6] calculates the sn frcton along the erf surface for four average et veloctes at the nozzle ext and two erf wall wedge angles. It ndcates that the sn fcton decreases along the erf surface. B Trumala Rao [7] ponts out that f we want to acheve the mnmum melt flm thcness on the cut surface, laser cuttng should be performed at optmum cuttng velocty and maxmum gas pressure so that lamnar gas flow s sustaned n the entre erf depth. Jn-chen Hsu [8] demonstrates that the use of the ntermttent gas ets can effectvely ncrease the materal removal rate and reduce the consumpton of assstng gas. As a result, n laser metal processng the man factors are standoff dstance, ntersecton angle and nlet pressure. Especally, nlet pressure s the most mportant factor because an excessve nlet pressure nduces the generaton of normal standoff shoc whch wll deterorate the processng qualty. On the other hand, other researchers promote the melt remove ablty of assstng gas by mprovng supersonc nozzle structure. Guo [9, ] maes an 170
, JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 analyss of relatonshp between nozzle structure and dynamcs characterstc of flow feld. Qu [11] establshes the turbulence model of subsonc convergent nozzle, analyzes the nfluence law of gas dynamcs performance about et flow affected by the nozzle structure, constructs a three-dmensonal symmetry model of laser cuttng mpngng flow and studes the nfluence on the gas dynamcs performance by laser cuttng parameters n erf. Due to the dffculty of processng the supersonc nozzle, modfyng the shape and parameters nsde the nozzle to mprove the functon of gas dynamcs n the process of laser cuttng costs too much. Therefore, the man method to optmze the dynamc performance of the laser cuttng process s to examne the nfluence of dfferent asssted gas parameter on the gas flow feld. Assstng gas s not only used n metal materal laser processng, but also used n other materals. Sulaman [1] nvestgates that hgh assstng gas pressure results n slghtly large erf wdth but small out-of-flatness and erf wdth ratos n Kevlar lamnate laser cuttng. Elhad S [13] uses an assstng gas n lowerng treatment temperatures and changng nterfacal and bul chemstry to lmt capllarydrven flow n laser slca ablatng. In ths paper we smulate the assstng gas flow by computaton flud dynamcs software FLUENT based on the Naver-Stoes equaton of compressble gas, so as to nvestgate the nfluence of dfferent processng parameters on the flow feld of assstng gas. Combng wth experments we study how nlet pressure and ntersecton angle of co-axal and offaxal et nozzle affect the depth and wdth of the groove, so as to obtan optmum laser processng parameters to scrbe a deep and narrow groove wth good qualty.. Theoretcal modelng.1 Basc equaton of turbulent flow Durng the laser scrbng, the nteracton of assstng gas and worpece surface can be smulated by symmetrcal et mpact model, whch s based on steady compressble Reynolds average Naver-Stoes equaton [14]. Reynolds average Naver-Stoes equaton: Conservaton of mass: ( ) 0 x u (1) Where ρ s the gas densty and u s the gas velocty n the drecton. Conservaton of momentum p ( u) ( uu ) x x u ef x x u x u 3 x x ' ( u u ' ) () Where p s the gas pressure, μ ef s the effectve vscosty and u s the gas velocty n the drecton. Turbulence model: RNG -ε s adopted because a bg gap occurs by usng standard -ε equaton n smulatng the mpngement of assstng gas [15, 16]. Where s the turbulent netc energy and ε s the dsspaton rate of the turbulent netc energy n the RNG -ε turbulence model. The values of and ε can be obtaned from Esq.(3) and (4). D D x D D x ef f G Y x ef f C x 1 M G C * (3) Where α and α ε are the effectve prandtl numbers of and ε respectvely. Here both α ε and α are 383. G s the the generaton term of turbulent netc energy, ' ' u G u u (5) x G Accordng to Bousssnesq hypothess, S Where C, S S S 1 u u S x x Y M s the correcton term of compressble gas. Y M M (4) (6) (7) Where M, RT, 3 C 0 * 1 C C 3 1 Whereη=S/ε, S s the tensor module of tensor rate, C 1ε =1.4, C ε =1.68, C μ =845, η 0 =4.38, β=1. Consderng vortex flow n the average flow, RNG -ε equatons have corrected the turbulent vscosty. By addctng a correcton term n the ε equaton, stan rate of the manstream appears. Therefore, the generaton term n RNG -ε equatons not only connects wth the flow condton but also s the functon of spatal coordnates n the smulaton case. RNG -ε equatons s wdely used n calculatng parameter of nozzle et flow because t s better dealng wth hgh stran rate flow whch has a 171
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 large curvature degree steam lne. Tang the accuracy of smulaton nto account, the RNG -ε equatons are selected to buld turbulence model.. Boundary condton and meshng In the smulaton of the supersonc assstng ets mpngng on the sapphre substrate, the computaton doman s selected as the regon between the nozzle and the substrate shown n Fg.1. There s a co-axal et nozzle and an off-axal et nozzle n the numercal model, where center lne of co-axal nozzle s perpendcular to the surface of worpece and the ntersecton angle between co-axal nozzle and offaxal nozzle (θ) s varyng from 5 to 85. Stand-off dstances of both co-axal nozzle and off-nozzle are selected as 5 mm because the pressure mpngng on the worpece surface s n the second hgh pressure area at ths dstance [17]. Ext dameter of co-axal nozzle s mm and ext form of off-axal nozzle s rectangle wth sze of mm 1mm. Compressble ar s used as the assstng gas. P e1 and P e are the total gauge pressures of the co-axal et and off-axal et, whch s varyng from 1 bar to 4 bar. Pa s the ambent gauge pressure whch s set to zero. The boundary condton of the worpece-sapphre substrate s set to wall. Relatonshp between dfferent processng parameters and removal rate of melt s studed by varyng the P e1, P e and the ntersecton angle (θ) of the nozzles. The computatonal doman shown n Fg. s 75.0 37.5 5 mm 3. Grd selecton s an mportant factor to ensure the soluton accuracy n most numercal smulaton methods. Hence fnte volume s meshed wth unstructured mesh. The sze of fnte volume s 0.1 mm and the amount of fnte volumes s 177744. In order to present the complex wave front of the et, fnte volumes should be remeshed by adaptve grd reconstructon method durng the calculaton process [18]. 3. Numercal results 3.1 Flow feld of co-axal et Fg.3 shows the contour of statc pressure at dfferent nlet pressure n the symmetry plane. The flow feld of assstng gas et s dvded nto free et area, mpngement area and wall et area. When P e1 s 1 bar, there s hardly any shoc n the flow feld. Gas velocty s subsonc at the nozzle ext and statc pressure n the wall et area s almost 1 bar. There s not any pressure loss durng the gas mpngng on the substrate. When P e1 s bar, radcal gas expanson appears n the ext of co-axal nozzle obvously. Statc pressure at nozzle ext drops below the standard atmospherc pressure. Due to the pressure of expanson area s less than the pressure n the free boundary, expanson wave wll be reflected whch develops compresson wave. Under ths crcumstance, the compresson wave s oblque shoc whch wll reduce the statc pressure n the wall et area slghtly. When nlet pressure s 3 bar, the flow behavor s smlar to the one n bar, but the poston of oblque shoc s closer to the substrate. When nlet pressure s 4 bar, the oblque shoc dsappears and a strong normal standoff shoc s generated, whch maes the flow velocty drop acutely and leads to a serous loss of energy and momentum. Fg.4 also shows the statc pressure of wall et area s only.3 bar when nlet pressure s 4 bar. As a result a hgh pressure nlet not only reduces statc pressure but also generates vortex whch fnally decreases the absorpton rate of laser and does harm to the melt materals removal. l l Fg.1 Model of twn gas et-asssted laser scrbng. Fg. Grd structure of computaton doman n the cross secton vew (X-Y plane) and axonometrc vew 17
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16.0 1.8 1.6 Free et area Impngement area Wall et area Statc pressure( atm) 1.4 1. 0.8 0.6 0.4 0. 0 40 60 80 0 nterscton angle (Degree) Fg.3 Oblque shoc Normal shoc Contour of statc pressure at dfferent nlet pressure 1 bar, bar, 3 bar and 4 bar. Dynamc pressure( atm) 1. 0.8 0.6 0.4 0. 0 40 60 80 0 Off-axs et angle ( degree) Statc Pressure (bar) Statc Pressure (bar) 0.8 0.6 0.4 0. 3.0.5.0 1.5 0.5-0.5 - -5 0 5 Poston(mm) - -5 0 5 Poston(mm) Statc Pressure (bar) Statc Pressure (bar).0 1.5 0.5-0.5 - -5 0 5.5.0 1.5 0.5-0.5 Poston(mm) - -5 0 5 Poston(mm) Fg.4 Statc pressure n the groove when nlet pressure s 1 bar, bar, 3 bar and 4 bar 3. Flow feld of off-axal et Fg.5 shows the statc pressure and dynamc pressure at laser rradate regon at dfferent θ when co-axal nlet pressure s zero and off-axal nlet pressure s bar. Analyzng the effect of off-axal et, we suppose that the horzontal drecton of off axs et s the same wth the laser scannng drecton. When θ s less than 30, statc pressure n wall et area s approxmate bar. When θ s larger than 30, statc pressure decreases wth the ncreasng θ. When ntersecton angle ascends to 85, the statc pressure gets the mnmal value of 0.4 bar. It s dffcult for mpngng et to remove the redeposted debrs for the drag force s too low. On another hand, when ntersecton angle s less than 30, dynamc pressure s about 0. bar. When θ s larger than 30, dynamc pressure ncreases as the ntersecton angle grows. In other words, flow velocty and mass flow rate of assstng gas n the wall et area ncrease wth the ncreasng ntersecton angle. Fg.5 Statc pressure and dynamc pressure at dfferent θ when Pe1=0 bar and Pe= bar In order to mprove the groove qualty, t s necessary to ensure that mass flow rate s hgh enough to remove the melt materal. That s to say, θ should be larger than 70, whch can ensure a hgh mass flow rate and reduce the loss of gas et momentum n laser scrbng. For another, a hgh ntersecton angle reduces the drag force to remove the melt materal from the groove, whch leads to a poorer cut qualty. As a result, a too hgh or low ntersecton angle s not favorable for laser scrbng. In order to enhance drag force and mass flow rate of assstng gas, ntersecton angle between co-axal and off-axal nozzle should be set to 70. Fg.6 Contour of statc pressure when off-axal nlet pressure s 1 bar, bar, 3 bar and 4 bar wth θ of 70 173
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 Statc Pressure (bar) 1.6 1.4 1. 0.8 0.6 0.4 0. 1.5.0.5 3.0 3.5 4.0 Co-axal nlet Pressure (bar) Fg.7 Statc pressure at dfferent off-axal nlet pressure when Pe1 s 0 bar and θ s 70 Fg.6 & Fg.7 show the contour of statc pressure and the statc pressure at dfferent off-axal nlet presure when co-axal nlet pressure s zero and θ s 70. There s no normal standoff shoc appears wth the ncreasng off-axal nlet pressure (P e ). Obvously the statc pressure ncreases wth the ncreasng nlet pressure. However, statc pressure rses gently when P e s hgher than 3 bar. Because the type of off-axal nozzle s nonaxsymmetrcal dfferent from co-axal nozzle wth convergent shape, t s dffcult to generate the normal standoff shoc. Therefore flat fan nozzle s better than the convergent nozzle n laser scrbng. But t s stll dffcult to ncrease statc pressure when P e s more than 3 bar, because the shoc reflecton effect of ar weaens the netc energy of the assstng gas whch maes a consderable total pressure loss. Therefore the deal off-axal nlet pressure s about 3 bar. 3.3 Flow feld of twn assstng gas Fg.8 shows the velocty vector of flow feld at dfferent θ when both co-axal and off-axal nlet pressure are bar. When θ s smaller than 45, coaxal gas et and off-axal gas et crash and reflect above the laser rradate regon, whch causes a hgh pressure area n the wall et area where gas velocty and mass flow rate s extremely low. As the ntersecton angle θ ncreases, stagnaton area dsappears and the flow velocty and mass flow rate ncrease. As a result t s easer to remove the melt materal from the groove at ths crcumstance. When θ s hgher than 70, off-axal et gas colldes wth the reflected gas et of co-axal et, whch decreases the total pressure acutely. Therefore excessve hgh ntersecton angle causes a great total pressure loss, whch not only reduces the drag force of assstng gas but also reduces the mass flow rate n the wall et area. Therefore, the optmum ntersecton angle s around 70. Fg.9 and Fg. show the velocty vector and statc pressure of the flow feld at dfferent nlet pressure combnaton respectvely. When P e s smaller than 3 bar, the collson and reflecton of the two assstng gas s slght, whch can obtan an deal mass flow rate n laser scrbng. When P e s 4 bar, a strong collson and reflecton occur and mass flow rate decrease. It means that a great loss of energy and momentum wll occur f P e exceeds 3 bar. That s why the statc pressure s hgher than other nlet pressure combnatons when P e1 = bar and P e =3 bar. In the same way, excessve co-axal nlet pressure blocs the flow of off-axal et and reduces the total pressure n the wall et area. To prevent the furous collson of the two mpngng et, co-axal nlet pressure should be reduced approprately. Therefore the optmum nlet pressure combnaton s the one co-axal nlet pressure of bar and off-axal nlet pressure of 3 bar. Fg.8 Velocty vector of flow feld at dfferent θ 45, 70 and 85 when nlet pressure Pe1=Pe= bar. (e) Fg.9 Velocty vector of flow feld at dfferent nlet pressure combnaton Pe1= bar,pe=bar; Pe1= bar, Pe=3 bar;pe1= bar, Pe=4 bar; Pe1=3 bar, Pe=3 bar;(e) Pe1=4 bar, pe=4 bar. 174
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 Statc Pressure (bar).5.0 1.5 0.5-0.5 P e1 = bar, P e = bar P e1 = bar, P e =3 bar P e1 = bar, P e =4 bar - -5 0 5 Poston(mm) Fg. Statc pressure on the centerlne of co-axal nozzle at dfferent nlet pressure combnaton Pe1=bar, Pe= bar;pe1= bar, Pe=3 bar; Pe1= bar,pe=4 bar. 4. Experments 4.1 Expermental faclty A 53 nm DPSS (Dode Pumped Sold State) laser (Lghtwave, Seres G) at a pulse duraton of 45 ns s used as a lght source of laser processng system to verfy the smulaton results. Its spot dameter can be adusted at the range of ~μm. In order to protect the focusng lens and remove the melt materal, co-axal and off-axal assstng gases are used wth nlet pressure ranged from 0 to 4 bar. The structures of the nozzles are the same wth the one n the smulaton. Horzontal drecton of the off axs et s the same wth the laser scannng drecton. The translatons n the x/y/z drectons and the rotaton n the x y plane are controlled by the computer generated sgnals. The pulse repetton frequency used n ths experment s fxed at 1 Hz. The substrate s the sngle sde polshng sngle crystal sapphre substrate wth dmenson of nch 43μm(dameter thcness). The groove wdth and depth are measured by optcal mcroscope (Zxss AX) and laser scannng confocal mcroscope (Olympus OLS4000). The front surface and secton of the groove are observed by laser scannng confocal mcroscope and envronmental scannng electron mcroscopy (FEI Quanta 400 FEG). 4. Influence of co-axal et Fg.11-1 shows the relatonshp between coaxal nlet pressure and the groove depth & wdth at dfferent laser pulse energy and scannng velocty. We can now that hgher P e1 leads to the shallower groove when the laser pulse energy s 150 μj and the scannng velocty s 1mm/s. However, when the scannng velocty s hgh (>), the dfference s small n groove depth at dfferent nlet pressure. The reason s that when the scannng velocty s 1mm/s, the assstng gas cools the surface of substrate, whch reduces the absorpton of laser and decreases groove depth when the scannng velocty s 1mm/s. When the pulse energy s hgh (650μJ), the dfference n groove depth s mnor wth the ncreasng the nlet pressure at dfferent scannng velocty. The reason s that laser energy absorpton s much hgher than the one assstng gas carred off. As a result, there s lttle dfference at dfferent nlet co-axal pressure. When laser pulse energy s low (150μJ), groove wdth ncreases wth the ncreasng co-axal nlet pressure. Ths phenomenon wll become more obvous when scannng velocty s lower (1mm/s). Then more meltng materals deposts and adheres to the groove edge whch enlarges the groove wdth. When laser pulse energy s hgh (650μJ) and scannng velocty s low (), groove wdth also ncreases wth the ncreasng nlet co-axal pressure. However, when laser pulse energy s hgh (650μJ) and scannng velocty s hgh ( ), groove wdth shows a trend from declne to rse and reaches the mnmum at an nlet pressure of 3 bar. It matches the smulaton result because when nlet pressure s about 3 bar, the statc pressure n the wall et area get the maxmal value, whch can offer the largest drag force to carry out the melt materal from the groove. When the nlet pressure s larger than 3 bar, normal standoff shoc wave wll be generated. It results n the scattered assstng gas whch wll reduce the statc pressure and the ablty to remove the melt. In bref, smlar to the result of chapter 3.1, the optmal coaxal nlet pressure s n the range of -3 bar. Groove depth (μm ) 50 40 30 0 4 Groove wdth (μm ) 18 16 14 1 Pulse energy: 150 J 0 1 3 4 Co-axal nlet pressure (bar) Pulse energy: 150 J 0 1 3 4 Co-axal nlet pressure (bar) Fg.11 Groove wdth and groove depth at dfferent coaxal nlet pressure when pulse energy s 150 μj 175
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 Groove depth (m) 0 90 80 70 60 50 40 30 Pulse energy: 650 J groove can be observed n Fg.14. When θ s 70, the groove edge s flatter than other condtons. In concluson, the best ntersecton angle between coaxs and off-axs s around 70. 0 0 1 3 4 Co-axal nlet pressure (bar) 40 38 Groove wdth (μm ) 36 34 3 30 8 6 4 Pulse energy: 650 J 0 1 3 4 Co-axal nlet pressure (bar) Fg.1 Groove wdth and groove depth at dfferent co-axal nlet pressure when pulse energy s 650 μj Fg.14 Front surfaces of the groove at dfferent θ wthout any assstng gas, θ=45, θ=70 and 85 when pulse energy s 150μJ,scannng velocty of 8mm/s, Pe1=0 bar and Pe= bar 4.3 Influence of off-axal et Groove wdth (m) 18 16 14 1 Pulse energy: 150 J Ar pressure: bar 0 40 60 80 0 Interscton angle (Degree) Fg.13 Groove wdth at dfferent θ when pulse energy s 150 μj, co-axal nlet pressure of 0 bar, off-axal nlet pressure of bar and θ of 70 Fg.13 shows the relatonshp between the ntersecton angle and the groove wdth. If θ s smaller than 45 wth P e1 =0 bar & P e = bar. When θ s hgher than 45, groove wdth decreases wth the ncreasng ntersecton angle. Whle θ reaches 70, the groove wdth reaches the mnmum. The reason of ths phenomenon s that dynamc pressure n wall et area s less than 0.3 bar when θ s smaller than 45. When ntersecton angle s hgher than 45 dynamc pressure rses rapdly. Ths greatly mproves the mass flow rate and enhances the melt removal capablty. The reason why groove wdth does not decrease anymore when ntersecton angle s hgher than 70 s that an exorbtant angle leads to a low drag force, whch wll hnder the melt removal. Front surfaces of Groove wdth (m) 18 16 14 1 Pulse energy: 150 J 0 1 3 4 Off-axal nlet pressure(bar) Fg.15 Groove wdth wth dfferent off-axal nlet pressure at dfferent scannng velocty when pulse energy s 150 μj Fg.16 Front surfaces of groove at dfferent off-axal nlet pressure 0 bar, 1 bar, bar, 3 bar when pulse energy s 150μJ and scannng velocty s 8mm/s 176
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 Fg.15 shows the groove wdth as a functon of off-axal nlet pressure. The groove wdth decreases wth the ncreasng off-axal nlet pressure and saturates when off-axal nlet pressure s hgher than 3 bar. Ths s because f nlet pressure s lower than 3 bar, statc pressure ncreases wth the ncreasng nlet pressure. However statc pressure does not ncrease further when nlet pressure s hgher than 3 bar. The optmal off-axal nlet pressure should be ~3 bar whch s also verfed from the front surfaces of the groove, shown n Fg.16. In order to acqure a hgh qualty groove n pulsed green laser sapphre scrbng, t s necessary to use assstng gas et both n co-axal nozzle and off-axal nozzle. 4.4 Influence of twn assstng gas Groove wdth (m) 18 16 14 1 8 Pulse energy: 150J Wthout gas Co-axal gas Off-axal gas Co-axal+Off-axal gas 0 4 6 8 1 Scannng velocty (mm/s) Fg.17 Groove wdth at. dfferent scannng velocty and dfferent assstng gas condton when nlet pressure s bar Fg.17 shows the groove wdth wth dfferent scannng velocty under dfferent assstng gas condtons. It s clear that n most cases a smaller groove wdth can be obtaned when usng the offaxal assstng gas and twn assstng gas. Fg18 shows the front surfaces of groove under the assstng gas condtons of wthout assstng gas, co-axal assstng gas only, off-axal assstng gas only and twn assstng gas respectvely. The melt materal can t be effectvely removed from the groove and adheres on the edge whch magnfes the groove wdth f no assstng gas s used. When usng co-axal assstng gas, the groove wdth s slghtly smaller than the one not usng any assstng gas, but the groove edge s stll rough. The groove wdth reduces and the groove qualty mproves when usng off-axal assstng gas. When usng twn assstng gas both n co-axs and off-axs, an optmum groove qualty wll be obtaned, whch has a flat groove edge wthout adherng a lot of redeposton layer. Although drag force to carry out the melt materal s larger enough when only usng co-axal assstng gas, the mass flow rate s stll too small to remove the melt materal snce the velocty of the et s stll small n the laser rradate regon. Wthout enough mass flow rates the strong pressure wll compact the melt materal and mae t dffcult to elmnate. By addng off-axal assstng gas whch accelerates the gas velocty n the wall et area, an deal assstng gas et wth hgh drag force and hgh mass flow rate wll remove the melt materal from the groove effcently. Fg.18 Front surfaces of groove at dfferent condton of assstng gas wthout assstng gas, co-axal assstng gas only, off-axal assstng gas only and twn assstng gas when pulse energy s 150 μj, scannng velocty of 8mm/s and Pe1=Pe= bar. 5. Conclusons The laser scrbng capacty or dross-removal capablty s determned by the drag force and the mass flow rate of the assstng gas. The melt materal can be carred out from the groove effcently under a hgh level et drag force and mass flow rate. The coaxal gas et provdes a hgh drag force whle the offaxal gas et mproves the mass flow rate n laser sapphre scrbng. To optmze the groove qualty t s necessary to use the twn assstng gas both n co-axs and off- axs. When usng the co-axal assstng gas et n laser sapphre scrbng, the co-axal nlet pressure should not be set to a hgh value because hgher nlet pressure (>3 bar) of convergent nozzle results n the generaton of normal stand-off shoc. It wll reduce the statc pressure n laser rradate regon and dsrupt the gas flow drastcally. Therefore, the optmum coaxal nlet pressure s bar n order to maxmze the drag force of assstng gas and avod the generaton of normal stand-off shoc. Only usng the co-axal assstng gas can not remove the meltng materal effcently because the mass flow rate s stll too small n ths way. Usng off-axal assstng gas can accelerate the gas velocty n laser rradate regon and remove the redeposted debrs and avod the redeposted debrs wth suffcent gas flow wth suffcent mass flow rate. The deal ntersecton angle (θ) and nlet pressure of off-axal assstng gas s 70 and ~3 bar respectvely. Usng twn assstng gas both n co-axs and offaxs can provde a hgh drag force and mass flow rate whch mproves the groove qualty and optmzes the groove sze n pulsed green laser sapphre scrbng. 177
JLMN-Journal of Laser Mcro/Nanoengneerng Vol. 11, No., 16 When co-axal nlet pressure s bar, off-axal nlet pressure of ~3 bar, ntersecton angle (θ) of 70 and laser pulse energy of 150 μj, a good groove qualty and sze wll be obtaned. Acnowledgments Fnancal assstance for ths wor s granted by the Natonal Nature Scence Foundaton of Chna (No.51575114 & 5175096), Guangdong Natural Scence Foundaton (No.S130014070) and Ordnary Unversty Characterstcs Innovaton Proect of Guangdong Provnce (Natural Scence, No.14KTSCX059). References [1] Elena R. Dobrovnsaya, Leond A. Lytvynov, and Valeran Pshch: Sapphre: Materal, Manufacturng, Applcatons, (Sprnger, New Yor, 09) p.33. [] K. Chen, Y. Lawrence Yao, and V. Mod: J. Manuf. Sc. E.-T. ASME., 1, (00) 49. [3] W. O'Nell, M. Voglsanger, A. Elboughey, and W.M. Steen: P. I. Mech. Eng. B-J. Eng., 15, (01) 51. [4] K.H. Sezer, L. L, and S. Legh: Int. J. Mach. Tool Manu., 49, (09) 116. [5] C.C. Ma, and J. Ln: Opt. Laser Technol., 34, (0) 479. [6] O.A. Melhem, B.S. Ylbas, and S.Z. Shua: Opt. Laser Eng., 49, (11) 384. [7] B. Trumala Rao and A.K. Nath: Sadhana- ACAD. P. Eng. S., 7, (0) 569. [8] H.J. Chen, W.Y. Ln, and Y.J. Chang: Int. J. Adv. Manuf. Tech., 79, (15) 449. [9] S.G. Guo, J. Hu, and L. Luo: Appl. Laser, 7, (07) 403. [] J. Hu, S.G. Guo, L. Luo, Z.Q. Yao: Int. J. Adv. Manuf. Tech., 39, (08) 716. [11] M.Y. Qu and J. Hu: Chn. J. Lasers, 5, (09) 196. [1] F. Al-Sulaman, B.S. Ylbas, M. Ahsan and C. Karates: Int. J. Adv. Manuf. Tech., 45, (09) 6. [13] S. Elhad, M.J. Matthews, G.M. Guss, and I.L. Bass: Appl. Phys. B-Lasers O., 113, (13) 307. [14] V. Yahot and S.A. Orszag: S. Sc. Comput., 1, (1986) 1. [15] V. Yahot and S.A. Orszag: Phys. Fluds, A4, (199) 15. [16] V. Yahot and S.A. Orszag: J. Sc. Comput., 1, (1991) 39. [17] A.D. Brandt and G.S. Settles: J. Laser Appl., 3 (1997) 69. [18] J. Xu and C. Zhao: Int. J. Heat. Mass. Tran., 50, (07) 434. (Receved: May 4, 15, Accepted: Aprl 6, 16) 178