oblika pulznih laserskih sledi the shape of pulsed laser
TRANSCRIPT
UDK 535-374
Oblika pulznih laserskih sledi The Shape of Pulsed Laser Traces
GORAZD RAKOVEC - JANEZ MOŽINA
Za določitev različnih vrst sledi, ki nastanejo pri označevanju s pulznimi laserskimi izvori sta vpeljana dva nova parametra, relativna gostota laserskih pulzov G in relativna gostota kraterjev Gk r . Na tej podlagi je izpeljan model superpozicije kraterjev, s katerim je poleg laserskega označevanja mogoče na nov način opisati tudi druge pulzne laserske obdelovalne procese. Opravljene meritve premera, globine in hrapavosti laserskih sledi potrjujejo uporabnost modela. Z modelom, ki privzema posamezni krater kot glavno procesno enoto, se odpirajo nove možnosti za nadzor in optimizacijo pulznih laserskih obdelovalnih procesov.
Two new parameters, relative density of laser pulses G and relative crater density Gkrt have been introduced in order to define various kinds of laser traces obtained in pulsed laser marking. In addition to the description of laser marking, the model of crater superposition which was derived on this basis also enables one to describe other pulsed laser manufacturing processes. The measurements of diameter, depth and roughness of laser traces were performed to confirm the applicability of the proposed model. Assuming a single laser crater as a fundamental process unit, new possibilities have been opened for monitoring and optimizing pulsed laser processes.
0. UVOD
Laserske sledi je mogoče definirati kot različne trajne spremembe, ki se pojavijo v materialu po prehodu laserskega žarka. Lahko jih delimo na sledi z odvzemanjem materiala in brez njega. Za sledi brez odvzemanja materiala so značilne predvsem kemične in mikrostrukturne spremembe materiala, ki so podlaga za toplotno obdelavo in nekatere vrste označevanja. Pri večjih intenzitetah vpadle laserske svetlobe pa se naštetim spremembam pridružijo še geometrijske spremembe površine materiala, ki nastajajo zaradi lokalnega taljenja in izparevanja materiala. Tovrstne spremembe so podlaga številnim laserskim obdelovalnim procesom, kakršni so vrtanje, rezanje, graviranje ipd. Najpogostejša je delitev na pulzne in zvezne laserske procese. V tem prispevku se omejujemo na pulzne laserske procese, pri čemer pa izbrani način omogoča tudi opis zveznih procesov.
Laserski obdelovalni procesi so v literaturi izčrpno obdelani 111, 121, 131. Pri tem pa je zanimivo, da je o geometrijskih lastnostih laserskih sledi najti le malo podatkov. S tem prispevkom posegamo na to področje in želimo definirati geometrijske lastnosti laserskih sledi in njihove povezave z obdelovalnimi parametri. Takšen način naj bi v naslednji fazi omogočil novo klasifikacijo laserskih obdelovalnih pocesov in odprl tudi nove možnosti za njihovo krmiljenje 141, 151.
0. INTRODUCTION
Laser traces can be defined as various permanent changes occuring in a material after the passage of a laser beam. A distinction can be made between those with and those without the ablation of material. Chemical and microstructu- ral changes in the material, which form the basis for heat treatment and some types of marking, are characteristic of traces without material ablation. With high peak intensities of incoming laser light, these changes are coupled with geometrical changes in the material surface as a consequence of localized melting and vaporization. Such changes are the basis for numerous laser manufacturing processes, such as drilling, cutting and engraving. Most frequently, a distinction is made between pulsed and continuous laser processes. This paper is limited to pulsed laser processes; however, the chosen approach also makes it possible to describe continuous processes.
Laser manufacturing processes are described in detail elsewhere 111, 121, 131. Nevertheless, it is interesting that there is only a little information on the geometrical properties of laser traces. Our paper deals with this area and its objective is to define the elementary geometrical properties of laser traces and their correlations to manufacturing parameters. In the next phase, this approach should make a new classification of laser manufacturing processes feasible, as well as opening new possibilities for controlling them 141. 151.
1. OZNAČBE 1. SYMBOLS
As m razdalja med dvema zaporednima As m distance between two successivekraterjema, craters,
Dk m premer kraterja, Dk m diameter of crater,J energija laserskega pulza, E, J energy of laser pulse,J povprečna energija sledi, J mean energy of trace,
f s~1 frekvenca pulzov, f s-1 pulse frequency,G relativna gostota laserskih pulzov, G relative density of laser pulses,G0 statična relativna gostota laserskih Go static relative density of laser
pulzov, pulses,Gk globina kraterja, Gk depth of crater,CN m globina sledi, CN m depth of trace,Ckr relativna gostota kraterjev, ?kr relative density of craters,A W /m 2 povprečna intenziteta laserskega A W /m 2 mean intensity of laser pulse,
pulza,Am W /m 2 povprečna intenziteta sledi, Am W /m 2 mean intensity of trace,N število laserskih pulzov, N number of laser pulses.Pt W povprečna moč laserskega pulza, P, W mean power of laser pulse,
W povprečna moč sledi, r n W mean power of trace,m srednje odstopanje profila, R a m standard deviation of profile,
Rm m največja višina odsekov neravnin, R m m max. height of sectional unevennessRt m največja višina neravnin, Rt m maximum height of unevenness,t s čas, t s time,At s čas trajanja laserskega pulza, At s duration of laser pulse,to s čas med dvema zaporednima pulzo to s time between two successive
ma, pulses,A t0 s časovna perioda vlaka laserskih At0 s time period of train of laser pulses,
pulzov,S m2 prerez laserskega žarka, S m2 laser beam cross-section,Sk m2 površina kraterja, Sk m2 surface area of crater,V k m višina kraterja, V k m height of crater,V m /s relativna hitrost med obdelovancem v m /s relative speed between workpiece
in žarkom, and beam,Ax m širina laserskega žarka v smeri x , Ax m width of laser beam along the x-axis,Ax0 m presledek med dvema zaporednima Ax0 m shift between two successive
pulzoma v smeri x, pulses along the x-axis,VK m /s relativna hitrost v smeri x. VX m /s relative speed along the x-axis.
1.1 Kratice 1.1 AcronymsLP laserski pulz, LP laser pulseTLP vlak laserskih pulzov. TLP train of laser pulses
2. KRATER2.1 Laserski pulz
Laserski žarek v splošnem ponazarjamo s prostorsko in časovno porazdeljeno intenziteto I(x, y, t), kjer sta x in y radialni koordinati. Z integracijo intenzitete po prerezu laserskega žarka S dobimo trenutno moč:
2. CRATER2.1 Laser pulse
A laser pulse is generally described by spatially and temporally distributed intensity / (.V, y, t), where .v and y are radial coordinates. By integrating the intensity over the laser beam cross-section S, we obtain power at a given time:
Pit) = JJ/C v , y, t)dy d.vS
( 2. 1).
Za opis laserskih procesov v mnogih primerih zadošča poznavanje energije posameznega pulza:
In many cases, for a description of the laser processes, it is sufficient to know the energy of a single pulse:
Eì = J j J / ( at, y, t)dy d.v <it = J P(t)dt ( 2 .2. )
A t x A t
in frekvence pulzov f. Povprečna moč laserskega žarka P je v tem primeru enaka:
P =
V praksi se pogostno uporablja časovno neodvisna povprečna moč laserskega pulza Pv določena z enačbo:
and the pulse frequency f. The mean power of a laser beam P is in this case equal to:
Ey f (2.3).
Frequently applied in practice is the time-in- dependent mean power of the laser pulse P ,, which is defined as:
A tAt (2.4).
Analogno lahko določimo povprečno intenziteto /,, neodvisno od časa:
The mean intensity /i independent of time as:
is analogously defined
E,SA t (2.5).
Pri pulznih procesih sta intenziteta oziroma moč sestavljena iz zaporedja med seboj časovno ločenih bliskov — pulzov. Zaporednje večjega števila pulzov imenujemo vlak laserskih pulzov (TLP).
2.2 Relativna gostota laserskih pulzov — C
Kot parameter laserskih obdelovalnih procesov bomo definirali relativno gostoto laserskih pulzov (7(61. Pri gibanju v smeri A' je gostota Gx definirana z enačbo:
AyG X = AX A A'n
In pulsed processes, the intensity or the power is composed of a sequence of flashes - pulses separated by time. A sequence of a larger number of pulses is called a train of laser pulses (TLP).
2.2 Relative density of laser pulses — G
As a parameter of laser manufacturing processes, the relative density of laser pulses G (61 will be defined. When moving along the x-axis, density Gx is defined by the equation:
Aa-vxAf0
(2 .6 ) .
Če obdelovanec miruje glede na žarek (vx= 0), je treba uporabiti drugačno definicijo relativne gostote pulzov. V tem primeru definiramo statično gostoto laserskih pulzov G0 kot število laserskih pulzov N, ki se popolnoma prekrivajo:
beam ( vx = 0 )If the workpiece is stationary relative to the
a different definition of relative density of pulses should be applied. In this case, the static density of laser pulses G0 is defined as the number of laser pulses N which overlap completely:
Gn = N (2.7).
Gostota G vlaka laserskih pulzov je vedno manjša od statične gostote G0, ker vlak laserskih pulzov pri gibanju zavzame večjo dimenzijo v smeri radialne hitrosti.
The density C of a train of laser pulses is always lower than the static density G0, since the train of laser pulses has a greater magnitude in the direction of the radial velocity.
2.3 Oblika k ra te rja
Sled, ki nastane z vpadom posameznega laserskega pulza na površino materiala, imenujemo krater. Oblika kraterja je v splošnem odvisna od parametrov laserskega pulza in lastnosti obsevanega materiala (71. Za poenostavljen opis oblike kraterja je primerno izbrati njegov premer Dk in globino Gk (sl. 1). Premer in globino je mogoče pripisati za procese z odvzemanjem (sl. la) in brez odvzemanja materiala (sl. Ib).
2.3 Shape of c ra ter
A single incoming laser pulse leaves a trace, called a crater, on the surface of the material. The shape of a crater generally depends on the parameters of the laser pulse and the properties of the irradiated material 171. For a simplified description of the crater shape, it is logical to choose a diameter Dk and depth Gk (fig. 1). Diameter and depth can he applied to processes with (fig. la) and without ablation of material (fig. lb).
Slika 1a) krater pulz nega procesa z odvzemanjem materiala,b) krater pulznega procesa brez odvzemanja materiala
D k — premera kraterja, D k h — premer kraterja brez odvzemanja materiala. Ck - globine kraterja, Gkb ~ globina kraterja brez odvzemanja materiala. Vk - višina kraterja
Figure 1a) crater of a pulsed process with ablation of material,
b) crater of a pulsed process without ablation of material D k — diameter of crater, Z)kb — diameter of crater without ablation of material, Gk — depth of crater,
Ckb - depth of crater whithout ablation of material. V’k - height of crater
V splošnem so laserski procesi sestavljeni iz množice interakcij laserskih pulzov z materialom. Potemtakem so tudi vse laserske sledi oziroma obdelave sestavljene iz kraterjev. Le v posebnih primerih je laserski obdelovalni proces sestavljen iz enega samega pulza in njemu pripadajočega kraterja. Krater kot sled posameznega laserskega pulza je zato mogoče izbrati za osnovno enoto laserske obdelave. Za opis laserskih obdelovalnih procesov, ki jih sestavlja vlak laserskih pulzov, lahko uporabimo poenostavljen model, v katerem se celotna laserska sled poraja kot skupek ali su- perpozicija posameznih kraterjev.
3. SUPERPOZICIJA KRATERJEV3.1 Relativna gostota kraterjev — Gkr
V praksi se srečujemo z različnimi laserskimi sledmi 181. Od takšnih, pri katerih se posamezni kraterji po večkrat delno ali popolnoma prekrivajo, do takšnih, pri katerih so posamezni kraterji medsebojno povsem razmaknjeni. Vse te sledi lahko enotno kolikostno opišemo tako, da vpeljemo relativno gostoto kraterjev Gkr kot količnik med premerom kraterja Dk in relativnim premikom As obdelovanca glede na laserski žarek v času med dvema zaporednima laserskima pulzoma:
Relativna gostota kraterjev ima velik pomen za opis laserskih obdelovalnih procesov. Z njo je mogoče na povsem nov način ne samo karakterizirati različne vrste laserskih sledi, ampak na povsem nov način klasificirati pulzne laserske obdelovalne procese. V posebnem primeru, ko je relativna hitrost enaka nič, kar pomeni, da laserski pulzi vpadajo na isto mesto, definiramo statično relativno gostoto kraterjev, ki je enaka številu vpadlih pulzov:
Laser processes usually consist of Interactions between laser pulses and material. Thus all laser traces or manufacturing processes consist of craters. Only in special cases is a laser manufacturing process composed of a single pulse and its appertaining crater. A crater as a trace of a single laser pulse can therefore be chosen as a fundamental unit of laser manufacturing processes. For a description of laser manufacturing processes, consisting of a train of laser pulses, a simplified model can be used in which the entire laser trace occurs as a group or superposition of individual craters.
3. SUPERPOSITION OF CRATERS3.1 Relative density of craters — Gj„.
In practice, one finds various laser traces 181,i.e., those where individual craters often partially or completely overlap, and those with craters completely separated. All these traces can be unified in a quantitative description if the relative density of craters Ckr is introduced as a quotient of the crater diameter Dk and the motion As of the workpiece relative to the laser beam in the period of time between two successive laser pulses:
(3.1).
The relative density of craters is of great importance for the description of laser manufacturing processes. It enables one not only to characterize various types of laser traces but also to approach the classification of all pulsed-laser manufacturing processes in a new way. In a special case, when the relative velocity vanishes, which means that the laser pulses fall on the same spot, one can define the relative static density of craters, which equals the number of incoming pulses:
7kr n 1As Uk v
' k r o N (3.2).
Tako kakor gostota pulzov, je tudi gostota kraterjev enakega vlaka laserskih pulzov vedno manjša od statične gostote kraterjev. Relativna gostota kraterjev je manjša od relativne gostote pulzov, ker je premer kraterja manjši od premera laserskega pulza, statični gostoti kraterjev in laserskih pulzov pa sta enaki.
Pri obravnavi laserske sledi na materialu nas zanima, koliko energije v povprečju pride na enoto površine materiala <Sk. V ta namen uporabimo relativno gostoto laserskih pulzov G in zapišemo:
The crater density of a train of laser pulses is, like the density of pulses, always lower than the static density of craters. The relative density of craters is lower than the relative density of pulses, since the diameter of the crater is smaller than the diameter of the laser pulses, whereas the static densities of the craters and the laser pulses are equal.
When considering laser traces in the material, we are interested in the mean quantity of energy per unit of material surface area Sk. For this purpose, we use a relative density of laser pulses G and write:
(3.3),En = GE\
kjer je i?N energija laserske sledi. Analogne enačbe veljajo tudi za PN in /N. S spreminjanjem G torej spreminjamo količino energije na površini sledi laserskega žarka in s tem določamo lastnosti in vrsto sledi. Z gostoto G preslikamo energijske razmere procesa pri enem — enotskem pulzu v energijske razmere celotne laserske sledi. Vse sledi laserskega žarka pa lahko enotno razvrstimo z relativno gostoto kraterjev.
Za procese odvzemanja materiala lahko Gkr prikažemo kot povprečno globino sledi, izraženo v številu globin enega kraterja. Vrednosti Gkr so pri procesu odvzemanja teoretično ponazorjene z idealiziranim modelom naslikah 3 in 4. Teoretični model je močno idealiziran zato, da smo lahko vsaj približno nakazali superpozicijo kraterjev. Le ta je v splošnem zapletena, tako da je analitični način vsaj za prakso neuporaben. Prikazane so sledi vlaka pravokotnih pulzov (sl. 2) po procesu odvzemanja materiala, z idealno pravokotnim laserskim pulzom; z veliko globinsko ostrino, z idealnim procesom uparjanja homogenega materiala in s pravokotnim profilom intenzitete.
P
R ............. .................. ...
where EN is energy of the laser trace. Analogous equations are also applied to PN and /N. Therefore, by modifying G, we modify the quantity of energy on the surface of a laser beam trace, thus determining the properties and type of trace. Energy conditions of a process at one unit pulse are applied with density G to the energy conditions of the entire laser trace. With the assistance of the relative density of craters, we were able to classify all traces of the laser beam in a unified manner.
In processes with ablation of material, Gkr can be presented as the mean depth of trace, expressed in terms of the number of depths of crater. Gkr values are, in ablative processes, theoretically illustrated by an idealized model in fig. 3, 4. The theoretical model is greatly idealized, so that we could, at least approximately, indicate the superposition of craters, which is generally complicated, thus preventing the application of a practical analytical approach. Figure 2 shows a train of rectangular pulses with an ideally rectangular laser pulse; with a great depth sharpness, with an ideal vaporization process of homogeneous material and with a rectangular profile of intensity.
* j g
i l w bA l i A l i „ ___aJL
SI. 2. Vlak pravokotnih laserskih pulzov. Fig. 2. Train of rectangular laser pulses.
0 < Gkr < 1 — pri teh vrednostih se posamezni kraterji ne prekrivajo. Imamo pikčasto označevanje z odvzemanjem materiala. Globina sledi se giblje od 0 do 1 globine kraterja. Sled je pikčasta. Maksimalna globina sledi je enaka globini enega kraterja. Površina vzdolžnega prereza sledi ima obliko stopničastega profila (sl. 4a).
Gkr = 0,5 — razdalje med robovi kraterjev (pik) so enake premeru enega kraterja.
0 < Gkr < 1 — at these values, single craters do not overlap. There is dotted marking with ablation of material. Depth of trace varies from 0 to 1 of the depth of crater. The trace is dotted. The maximum doptli of trace equals the depth of one crater. The surface of a longitudinal cross-section of trace has the shape of a graded profile (fig 4a).
Gkr = 0.5 — distances between the edge craters (dots) equal the diameter of one crater.
Sl. 3. Statična relativna gostota kraterjev: , ? ~ _Fig. 3. Static relative density of craters: 0 ’ 0
SI. 4. Relativna gostota kraterjev vlaka pravokotnih laserskih pulzov:a) 0 < Gkr = 1. b) Ckr = 1. c) 1 < Gkr < 2. d) Gkr = 2
Fig. 4. Relative density of craters of a rectangular pulsed laser train:a) 0 < Gkr = 1. b) Ckr = 1. O 1 < C kr < 2. d) Ckr = 2
£ kr = 0,33 — razdalje med robovi kraterjev so enake dvema premeroma kraterja.
Gkr = 1 — sled postaja zvezna. Globina sredine sledi je enaka globini enega kraterja in ima največjo mogočo ravnost na tej globini. Debelina sledi ni enakomerna (sl. 4.b).
1 < Gkr < 2 — zvezno označevanje s stopničastim vzdolžnim profilom. Od tu naprej je sled zvezna. Debelina sledi je enakomernejša. Globina sledi se giblje od 1 do 2 v stopničastem profilu (sl. 4c).
Gkr = 2 —zvezno označevanje z gladkim vzdolžnim profilom. Globina sledi je enaka približno dvema globinama kraterja in ima največjo mogočo ravnost na tej globini. Debelina sledi je enakomerna (sl. 4d), kakovost sledi je v tem primeru najboljša.
2 < Gkr < 3 — globina sredine sledi se giblje od 2 do 3 v stopničastem profilu. Debelina sledi je enakomerna.
£ kr = 0.33 — distances between the edges of craters equal two diameters of one crater.
Ckr = 1 — The trace is becoming continuous. The depth in the middle of the trace is equal to the depth of one crater and has the greatest possible evenness at this depth. The thickness of the trace is not even (fig. 4b).
1 < Gkr < 2 — Continuous marking with a graded longitudinal profile. From here onwards, the trace is continuous. The thickness of the trace is more even. The depth of the trace varies from 1 to 2 in the graded profile (fig. 4c).
Gkr — Continuous marking with a smooth longitudinal profile. The depth in the middle of the trace equals appproximately two depths of a crater and has the greatest evenness possible at this depth. The thickness of trace is even (fig. 4d). In this case, the quality of the trace is the best.
2 < Gkr < 3 — The depth in the middle of the trace varies from 2 to 3 in a graded profile. The thickness of the trace is even.
Gkr = 3 — globina sledi je približno enaka trem globinam kraterja in ima največjo ravnost na tej globini; debelina sledi je enakomerna.
Na podlagi nadaljnjega analognega opisovanja sledi lahko izvedemo klasifikacijo laserskih sledi z relativno gostoto kraterjev, npr.:
0 < Gkr S 1 — pikčasto označevanje,1 < Gkr ^ 2 — nezvezno označevanje,2 < Gkr S 4 — zvezno označevanje,4 < Gkr ž 6 — graviranje,6 < Gkr — globoko graviranje ali rezanje ob-
delovanca.Največja globina sledi v tem modelu je:
Gkr = 3 — the depth of the trace equals approximately three depths of a crater and has the greatest evenness at this depth. The thickness of the trace is even.
On the basis of further analogous descriptions of the trace, one can classify laser traces with the assistance of the relative density of craters,for example:0 < Gkr š 1 — dotted marking,K C k r S2 — discontinuous marking,2 < G k r š 4 — continuous marking,4 < Gkr š 6 — engraving,6 < Gkr — deep engraving or cutting of a
workpiece.The maximum depth of trace in this model is:
C N “ G o G k (3.4).
Poleg vrste obdelav je s številom Gkr mogoče določiti ali sklepati o: obliki prereza sledi, hrapavosti sledi, globini sledi, premeru kraterja, referenčni hitrosti laserskega obdelovalnega procesa, adaptivni hitrosti pri adaptivnem krmiljenju 141, 191, razporeditvi energije po sledi itn.
3.2 Poimenovanje laserskih sledi z Ckr
terjev je mogoče dobiti več značilnih razporeditev kraterjev oziroma laserskih sledi na površini. Različne sledi, ki imajo skupni imenovalec v relativni gostoti kraterjev Gkr, smo razmejili in njihove značilnosti združili pod različnimi imeni 161. Imena smo skušali določiti tako, da je vsaka vrsta sledi s čimmanj besedami čimbolj enopomensko določena in hkrati tudi opisana.
Za pulzne procese definiramo naslednja imena:— sled vlaka LP (sl. 4, 5): sprememba ma
teriala zaradi obsevanja z vlakom LP,— krater (sl. 3a): sled enega LP; G0 = 1,— luknja (sl. 3b): prekrita sled vlaka LP,
C0 > i,— pikčasta sled (sl. 4a): neprekrita sled vlaka
LP; 0 < G <1,— verižna sled (sl. 4b—d): delno prekrita sled
vlaka LP; Gkr ^ 1.
4. PREIZKUSI
Veljavnost zamisli relativne gostote kraterjev smo skušali potrditi tudi s preizkusi. Opravljene so bile predvsem meritve verižnih sledi, izdelanih pri izbranih parametrih f, At in Ev Vsi preizkusi so bili izvedeni v enakem zaporedju:
In addition to a variety of manufacturing processes, the number Gkr enables us to define or infer: the shape of the trace cross-section, the roughness of the trace, the depth of the trace, the diameter of the crater, referential speed of the laser manufacturing process, adapted speed at adaptive control [4], 191, distribution of energy along the trace, etc.
3.2 Denotine of laser traces by £7kr
With the help of the described model of crater superposition, it is possible to acquire several typical allocations of craters or laser traces on the surface. Different traces which have a common denominator in the relative density of craters Gkr were separated and their properties were combined under various names 161. We tried to choose names which have the smallest number of words, only one meaning and which at the same time are also descriptive.
For pulsed processes, the following names are defined:
— trace of an LP train (analogous to figs. 4, 5): change in the material due to irradiation with an LP train,
— crater (fig. 3a): trace of one LP; G0 = 1,— hole (fig. 3b): an overlapped trace of an
LP train; G0 > 1,— dotted trace (fig. 4a): not an overlapped
trace of an LP train; 0 < Gkr < 1,— chain trace (fig. 4b-d): partially over
lapped trace of an LP train; Gkr è 1.
4. EXPERIMENTSWe also tried to confirm the concept of relati
ve crater density experimentally. In the first place, measurements of chain traces occuring at selected parameters f, At and E were carried out. All experiments were conducted in the same sequence:
— obdelava preizkušancev,— izvedba merilnih postopkov in— analiza rezultatov.Za merjenje je bila izbrana verižna sled. Kot
preizkušanec je bila uporabljena finozrnata jeklena pločevina (zrnat cementit v feritni osnovi) z 0,2- odstotnim deležem ogljika, ki se odlikuje tudi z veliko absorptivnostjo (okrog 35 %) svetlobe laserskega izvora Nd-YAG.
Izdelava laserskih sledi je potekala na laserskem sistemu Nd-YAG, ki daje laserske pulze z naslednjimi karakteristikami: At = 0,23—25 ms, fmaks = 4,5—40 Hz in P, = 2,8—28 J. Z opisanim sistemom so bile narejene sledi v industrijskem okolju. Verižne sledi so bile izdelane pri konstantni frekvenci laserskega izvora ob spreminjanju hitrosti koordinatne mize.
Parametre sledi smo merili z napravo za merjenje hrapavosti. Fotografije so bile posnete na vrstičnem elektronskem mikroskopu.
4.1 Dimenzije verižnih sledi
Za meritve hrapavosti verižne sledi je bi[a z vlakom laserskih pulzov TLP(Af = 0,5 ms, Pi = = 3000 W, f = 60 Hz) izdelana verižna sled v merilnem območju merilnika hrapavosti. Izmerki so prikazani na sliki 5, kjer je potrjeno, da je pri manjših hitrostih hrapavost verižne sledi manjša.
Druga serija preizkusov je bila izvedena s parametri TLP (0,13 ms, 3850 W, 3Hz). Pri teh parametrih laserskega žarka je znašal povprečni premer kraterja 300 pm, globina pa okrog 600 pm. Moč je bila za 22 odstotkov večja kakor v prvi seriji. Kraterji so skoraj neodvisni drug od drugega. Z večanjem gostote kraterjev se veča tudi globina verižne sledi, ki v končni fazi prebije 1,3 mm debelo pločevino (sl. 6). Značilni primeri tako nastalih sledi so prikazani na fotografijah, ki smo jih posneli z elektronskim mikroskopom.
— processing of samples,— performance of measurement procedures,— analysis of results.A chain trace was chosen as the object of
measurement. For the sample, we used a fine- -granulated steel plate (granulated cementite in ferrite base) with 0.2 percent of carbon, which is notable for its high absorbtion (around 35%) of the Nd-YAG laser light.
Traces were made with Nd-YAG system, which renders laser pulses with the following properties: At = 0.23—25 ms, /Vnax = 4.5—40 Hz and P, = 2.8—28 J. With the described system, traces were made in an industrial environment. Chain traces were made with constant frequency of the laser source while changing the speed of the coordinate table.
The parameters of the trace were determined with a device for roughness measurements. Photographs were taken with a scanning electron microscope.
4.1 Dimensions of chain tracesFor the measurements of the roughness
of a chain trace, a chain trace was made with a train of laser pulses TLP (At = 0.5 ms, P = = 3000 W, f = 60 Hz) in a measurement range of the roughness meter. The results of the measurements are shown in Fig. 5, where it is confirmed that at lower speeds, the roughness of chain trace decreases.
The second series of experiments was carried out with parameters TLP (0.13 ms, 3850 W, 3 Hz). With these parameters, the mean diameter of a crater was 300 pm, while the mean depth was around 600 pm. In comparison with the first series, the power was 22% higher. Craters are almost independent of each other. An increase in the relative density of craters causes an increase in the depth of chain trace which, in the final phase, pierces through a 1.3 mm thick steel plate (fig. 6). Typical examples of traces formed in this way are presented in photographs taken with an electron microscope.
SI. 5. Hrapavost vzdolžnega profila verižne sledi v odvisnosti od hitrosti.+ - P t : * - f i m ; ° - Ra
Fig. 5. Roughness of longitudinal profile of chain trace dependent on speed.
Sl. 6. Globina verižne sledi v odvisnosti od relativne gostote kraterjev. Fig. 6. Depth of chain trace dependent on relative density of craters.
Naslikah 7 do 11 je z vpadne strani prikazano večanje gostote kraterjev konstantnih parametrov žarka ob spreminjanju hitrosti. Pikčasta sled na sliki 7 (Gkr = 0,88) preide v verižno na sliki 8. (Gkr = 0,97), kjer se kraterji le dotikajo, nato pa se na sliki 9 začnejo prekrivati. Profil sledi je valovit po premeru in po globini. Na sliki 10 (Gkr = 1,73) postane premer vidne sledi enakomeren in za 6 odstotkov manjši od premera pik pikčaste sledi, nato pa ostane konstanten ob hkratnem večanju globine. Kakovost robov verižne sledi je odvisna od učinkovitosti izmetavanja taline.
In figures 7 to 11, the increasing crater density for constant beam parameters while modifying the speed is presented from the incoming side. The dotted trace in fig. 7 (Gkr = 0.88) transforms into a chain one in fig. 8 (Gkr = 0.97), where the craters only just touch, and then they start overlapping in fig. 9. The profile of the trace is wavy with regard to diameter and depth. In fig. 10 (Gkr= = 1.73), the diameter of the visible trace becomes even and 6% smaller than the diameter of dots in the dotted trace, then it remains constant for a simultaneously increasing depth. The edge quality of a chain trace depends on the efficiency of the ejected molten material.
SI. 7. Posnetek (39021 pikčaste sledi pri relativni gostoti kraterjev Gkr = 0,8S. Fig. 7. Photograph (3902) of dot trace at relative density of craters Gkr = O.SS.
SI. 8. Posnetek (3903) verižne sledi pri relativni gostoti kraterjev Gkr = 0.97. Fig. 8. Photograph (3903) of chain trace at relative density of craters Gkr = 0.97.
4Ì0S 13KW K i 80 M i
SL 9. Posnetek (3905) verižne sledi pri relativni gostoti kraterjev Ckr = Fig. 9. Photograph (3905) of chain trace at relative density of craters Gkr
SI. 10. Posnetek (3906) verižne sledi pri relativni gostoti kraterjev Gkr = Fig. 10. Photograph (3906) of chain trace at relative density of craters Gkr
SI. 11. Posnetek (4106) verižne sledi pri relativni gostoti kraterjev Gkr = Fig. 11. Photograph (4106) of chain trace at relative density of craters Gkr
SI. 12. Posnetek (4001) prereza verižne sledi s slike 11.Fig. 12. Photograph (4001) of the profile of chain trace from fig. 11.
1,39.= 1.39.
1,73.= 1.73.
4,62.= 4.62.
Z večanjem globine se pojavi zmanjšanje izmetavanja taline, ki v nekaterih delih zalije prerezano sled. Globina narašča z večanjem Gkr prav do preboja pločevine, ki je prikazan na slikah 11 in 12 (Gkr = 4,62). Kakovost robov je tu največja. Naraščanje globine prikazuje tudi diagram na sliki 6. Odstopa le ena vrednost globine prereza, ki je verjetno dobljena na zalitem delu, preostale dimenzije pa nakazujejo linearno zvezo med Gkr in globino verižne sledi. Globina pikčaste sledi 600 pm je dvakrat manjša od teoretične vrednosti, kar je mogoče razložiti s tem, da gre za Gaussov profil žarka. Ugotovljeno je tudi naraščanje globine sledi z večanjem relativne gostote kraterjev Gkr.
5. SKLEP
Relativna gostota kraterjev Gkr kot razmerje treh laserskih parametrov (hitrosti, frekvence pulzov in premera kraterja) pomeni učinkovit parameter za določanje vrste, globine, premera in hrapavosti pulznih laserskih sledi. Izhaja iz relativne gostote pulzov G, ki določa razporeditev energije po laserski sledi in s tem postaja temelj za klasifikacijo laserskih obdelav. Z gostoto G je mogoče hitro preslikati energijske razmere enega laserskega pulza v proces nastanka celotne sledi. Gostota kraterjev Gkr omogoča razmejitev in s tem poimenovanje vseh vrst pulznih laserskih sledi. Za osnovno enoto laserskih pulznih obdelav smo uporabili krater, ki sestavlja vse druge sledi: pikčasto in verižno sled ter pulzno luknjo. Pri tem je izdelana poenotena definicija kraterja za pulzne laserske procese z odvzemanjem in brez odvzemanja materiala. Ta omogoča tudi poenoteno obravnavo drugih vrst laserskih sledi z relativno gostoto kraterjev.
Opravljene so bile meritve premera, globine in hrapavosti laserske verižne sledi. Z njimi je bila potrjena uporabnost vpeljane relativne gostote kraterjev. Potrjeno je naraščanje hrapavosti verižne sledi z večanjem hitrosti med laserskim žarkom in površino materiala ter naraščanje globine verižne sledi z večanjem relativne gostote kraterjev, pri čemer se premer sledi bistveno ne spremeni.
Opisane raziskave nakazujejo nadaljnje raziskave v smeri določanja dimenzij posameznih kraterjev kot osnovnih enot laserskih obdelovalnih procesov. Takšno orodje omogoča oblikovanje datoteke tehnoloških parametrov, s katerimi je mogoče na podlagi relativne gostote kraterjev optimirati parametre pulznih procesov z metodami adaptivnega krmiljenja.
With an increasing depth, there is a decrease in ejected molten material, which in some parts flushes the trace which has been cut. The depth increases with the Gkr until it pierces through the steel plate as shown in figs. 11 and 12 (Gkr = 4.62). Under these circumstances, the edge quality is the best. An increase in depth is shown in the graph in fig. 6. Only one value of cross-section depth is not consistent. It is probably acquired in the flushed part, since other dimensions indicate a linear relationship between Gkpand the depth of a chain trace. The depth of dotted trace 600 pm is half the theoretical value, which can be explained by the Gaussian profile of the beam. An increasing trace depth was discovered in the case of an increasing relative density of craters Gkr.
5. CONCLUSIONS
The relative density of craters Gkr as a proportion of three laser parameters (speed, pulse frequency and diameter of crater) is useful for defining the type, depth, diameter and roughness of pulsed laser traces. The relative density of craters is derived from the relative density of pulses G, which determines the distribution of energy along the laser trace, thus becoming the basis for the classification of laser manufacturing processes. With a density G, it is possible to convey the energy conditions of single laser pulse to the formation of the entire trace. The relative density of craters Gkr enables us to distinguish and denote all types of pulsed laser traces. As an elementary unit of pulsed-laser manufacturing processes, we used a crater which shows all other types of trace: dotted, chain trace and pulse hole. In this way, a unified definition of a crater for pulsed-laser processes with and without ablation of material was formulated. This enables a unified treatment of the remaining types of laser traces with the aid of the relative density of craters.
Measurements of diameter, depth and roughness of a laser chain trace were taken. They confirmed the applicability of the crater relative density. The increasing roughness of a chain trace is verified where we have an increasing speed between the laser beam and the material surface, while the increasing depth of a chain of trace is verified where we have an increasing relative density of craters, whereby the diameter of the trace is not significantly changed.
The described research calls for further work towards defining the dimensions of single craters as fundamental units of laser manufacturing processes. With this kind of tool, one can form files of technological parameters with which it is possible to optimize, on the basis of the relative density of craters, the parameters of pulsed processes with methods of adaptive control.
6. LITERATURA 6. REFERENCES
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Naslov avtorjev: mag. Gorazd Rakovec, dipl. inž. Authors' Address: Mag. Gorazd Rakovec. Dipl. Ing.prof. dr. Janez Možina, dipl. inž. Fakulteta za strojništvo Univerze v Ljubljani Aškerčeva 6.Ljubljana. Slovenija
Prof. Dr. Janez Možina, Dipl. Ing Faculty of Mechanical Engineering University of Ljubljana Aškerčeva 6 Ljubljana, Slovenia
Prejet0: 11.6.1993Received:
Sprejeto: g g igg3Accepted: