Strojniški vestnik - Journal of Mechanical Engineering 52(2006)10, 645-652
UDK - UDC 621.9.048:004.42
Kratki znanstveni prispevek - Short scientific paper (1.03)
Nekatere možnosti določitve rezalnih podatkov pri laserskem
odrezovanju
Some Possibilities for Determining Cutting Data when using Laser Cutting
Miroslav Radovanovič (University of Niš, Serbia)
Tehnološki problemi, s katerimi se srečujemo na področju uporabe strojev za lasersko odrezovanje, so posledica nezadostnega poznavanja laserske tehnike in pomanjkanja dovolj zanesljivih podatkov iz prakse in poznavanja parametrov, ki vplivajo na sam postopek odrezovanja. Pomemben parameter, ki ga moramo določiti in vključiti v programsko opremo številskega krmiljenja, je rezalna hitrost. Različni avtorji analizirajo postopek lasreskega odrezovanja in podajajo matematične modele, pri katerih je lasersko odrezovanje modelirano z rezalno hitrostjo, kot ključnim parametrom odrezovanja. Številčno vrednost rezalne hitrosti lahko določimo iz priporočil proizvajalca, z določenimi izkušnjami ali z uporabo enačb iz literature. V tem prispevku so predstavljene nekatere možnosti za določitev parametrov laserskega odrezovanja in rezalne hitrosti, na podlagi stopnje odrezovanja, ločilne energije in rabe specifične moči. © 2006 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: rezanje lasersko, hitrosti rezanja, parametri rezanja, podatki o rezanju)
The technological problems faced in the field of the application of laser-cutting machines lie in insufficient knowledge of the laser technique and the absence of both sufficiently reliable practical data and knowledge about the parameters affecting the work process itself. A significant parameter that is necessary to determine and to enter in an NC-program is the cutting speed. Various authors analyze the laser-cutting process and give mathematical models where laser cutting is modeled by using the cutting speed as the key cutting parameter. The numerical value of the cutting speed can be determined from the recommendations of manufacturers, a particular experience or with the aid of equations from the literature. This paper presents some possibilities for determining laser-cutting data and cutting speed based on cutting rate, severance energy, and the specific power consumption. © 2006 Journal of Mechanical Engineering. All rights reserved. (Keywords: laser cutting, cutting speed, cutting parameters, data analysis)
0 INTRODUCTION
Laser technology is a very young technology, which has not even reached the age of 40 years. For a long time it has been a development searching for industrial applications. For most engineering applications, the laser can be regarded as a device for producing a finely controllable energy beam, which, in contact with a material, generates considerable heat. The heat energy is supplied by the laser beam. The laser beam permits tool-free machining with active heat energy. The energy of the light contained in the laser radiation is absorbed by the workpiece and transformed into thermal energy. The laser beam is becoming a very important engineering tool for cutting, welding and heat treatment.
Laser cutting (especially of mild steel) is a well-known and attractive process for cutting thin sheets. It is one of the most important applications for industrial lasers.
Laser cutting is a thermal, non-contact, mechanized process capable of cutting most materials with a high degree of precision and accuracy. Laser cutting is the process of melting or vaporizing material in a very small, well-defined area. The processes of heating, melting, and evaporation are produced by the laser beam, affecting a workpiece’s surface. The desired cut is obtained by moving the laser beam along a given contour. The laser beam is a cutting tool that is able to cut all materials. A laser beam, focused into a very small spot of 0.1 to 0.2 mm in diameter, concentrates thousands of watts. The
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power density for cutting steels is typically 105 to 106 MW/m2. There is no other way to concentrate so much energy into such a small spot. The high power density of the focused laser beam in the spot melts or evaporates material in a fraction of a second. Since our desire is to remove the evaporated and molten material from the affected zone as soon as possible, the laser cutting is performed with a coaxial current of an assisting gas - gas-assisted laser cutting, Fig. 1. When cutting metals, gas-assisted laser cutting is normally applied. In gas-assisted laser cutting, the gas is usually introduced coaxially with the focused laser beam into the cutting area. The gas cools the cut area, thus reducing the heat-affected zone, and also removing the molten dross from the cut.
There are three different methods of cutting with a laser: .    sublimation cutting, .    fusion cutting, .    flame cutting.
In laser-sublimation cutting, the material is removed by evaporating the metal. This requires light densities obtainable by appropriate adjustment of the laser radiation and focusing, so that material removal occurs exclusively by means of evaporation. The metal vapour emitted is blown out of the cut by the cutting-gas jet. Generally, either argon or nitrogen is used in order to avoid oxidation of the cut edge.
Cutting speed
Assisting gas
Molten material Dross
Kerf width       |   Cut roughness Heat affected zone
Since almost no melting occurs, smooth cuts can be obtained.
Laser fusion cutting requires less power than laser sublimation cutting as the material is only melted and then blown out of the cut with an inert-gas jet. With laser flame cutting, instead of an inert gas, pure oxygen is used. The material is heated beyond its ignition temperature until a highly exothermic reaction occurs, which supplies the energy for the cutting process. The liquid material is burned using the oxygen and blown downwards out of the cut by part of the assisting-gas jet. Laser-flame cutting and laser-fusion cutting are the predominant methods in the precision cutting of metals.
1 MELTING SPEED, EVAPORATING SPEED AND CUTTING SPEED
If the intensity of the laser-beam radiation is such that the temperature of the workpiece material for a small time attains the temperature of the melt or the evaporation then a small volume of material can be excluded from the cutting zone. After the temperature of melting or evaporation is achieved in the material a frontier layer is formed between the liquid and the hard phase or the gas phase and the liquid/hard phase. With a melt or evaporation of the layer of material thickness Ds the frontier layer moves in depth, while the laser beam makes a hole, i.e., perforates the material of the workpiece with a thickness s. Fig. 2 shows a model for determining the melting speed, the evaporating speed and the cutting speed.
Fig. 1. Gas assisting laser cutting
Fig. 2. Model for the determination of the cutting speed
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Strojniški vestnik - Journal of Mechanical Engineering 52(2006)10, 645-652
The melting speed can be estimated by employing a simple energy-balance arrangement. The principle is based on the fact that the material locally reaches the melt temperature because of the focused radiation.
The energy required to convert a mass m of a material in a cubic element (cylinder with base Af and height s) at an initial temperature T0 into a melt can be written with the equation:
Em=m[CS(Tm-T0) + Lm
(1),
where E (J) is the energy required to melt the material, m (kg) is the mass of material, C (J/kgK) is the solid specific heat capacity, T (K) is the melting temperature, T0 (K) is the ambient temperature, and L (J/kg) is the latent heat of melting. We then use the following equation:
m=rV = rAfs=rAfvmt = r  4  vmt         (2),
where r (kg/m3) is the mass density, V (m3) is the cubic element (cylinder with base A and height s), A (m2) is the area of the laser spot, s (m) is the material thickness, vm (m/s1) is the melting speed, t (s) is the time, and df (m) is the focal spot diameter. We can put C =C, where C (J/kgK) is the specific heat capacity.
Thus, the equation of the energy balance is given by:
pd2
vmtr[C(Tm-T0)+Lm
(3).
The power required to melt the material can
P = aP, where P
m
be determined by the equation
(W) is the power required to melt a material, a is the
absorptive index, and P (W) is the laser power.
The absorption varies with the temperature for the metal surface and the CO2 wavelength. The absorptive index at ambient temperature for steel is 0.1, at the melting temperature it is 0.55, and at the vaporization temperature it is 0.9 ([1] and [2]).
Since Em /t = Pm= aP we can determine the melting speed in the form:
4aP
 rr              (4).
pd 2 fr[C(Tm-T0) + Lm]
The focused laser beam bores into the surface of the material with a velocity v , directed into the material. Using Equation (4) we m can determine the cutting speed, providing that it does not include the
effect of the assisting gas. If the focused laser beam has a diameter df and is being scanned at a velocity v, Figure 2, then the beam will traverse a distance equal to its diameter in a time t = s/vm. During this time a cubic element of material V will be removed, and we have t = s/vm = df /v.
The equation for determining the cutting speed is:
(5).
The theoretical equation for the cutting speed, in the event that the material is removed by melting, based on a balance of energy, is made with:
4a P
(6),
pdfsr[C(Tm-T0) + Lm\
where v (m/s) is the cutting speed.
From Equation (6) we can see that the cutting speed is linearly proportional to the laser power and inversely proportional to the sheet thickness. The cutting speed is small for materials with a high melting point and for materials with a high latent heat of melting. The cutting speed is also small for a large focal-spot diameter.
Based on Equation (6) we can determine the necessary energy for melting the material per unit area of the cut as:
.
c(7),
= P = pdfr[C(Tm-T0) + L nm    vs                 4a
where E (MJ/m2) is the necessary energy for melting the material per unit area of the cut.
The necessary energy for melting the material per unit area of the cut is a constant value for the concrete material of the workpiece and the system for focusing the laser beam.
For mild steel r=7860 kg/m3, C=620 J/kgK, Tm =1515°C=1788K, T0=20°C=293K and L m=247-103 J/kg. For a focal-spot diameter d=0.2 mm and the absorptivity index at the melting temperature a=0.55 we have Equation (6) in the form:
v = 0.379
and Equation (7) in the form
E
nm
vs
2.63
(8)
(9).
Equations (8) and (9) include v(mm/s), P(W), s(mm) and Enm (MJ/m2).
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The evaporating speed can be estimated using the equal-energy method. The energy required to convert a mass m of a material cubic element at an initial temperature T0 into a vapor can be written with the equation:
Ev=m[CS(Tm -T0) + CL(Tv-Tm) + Lm +Lv]   (10),
where E (J) is the energy required to evaporate the material, CL (J/kgK) is the liquid specific heat capacity T (K) is the vaporization temperature, and L (J/kg) v is the latent heat of vaporization.
Usually, L >>L and T>>T0 and we assume we can put C*CL=C. Thus, the equation of energy balance (10) is given by:
Ev = m(CTv + Lv)
(11).
pd
2
pdf Since m=r       vvt , where vv (m/s) is the
evaporating speed, we have: pd
E =   4f vvtr(CTv+Lv)
(12).
The power required to evaporate the material can be determined by the equation Pv =aP , where Pv (W) is the power required to evaporate the material.
Since Ev /t = Pv =aP we can determine the evaporation speed in the form:
4a P vv=2                                     (13).
pdf r(CTv + Lv)
A focused laser beam boring into the surface of a material with a velocity vv directed into the material.
Using Equation (13) we can determine the cutting speed, providing that the effect of the assisting gas is not included.
If the focused laser beam has a diameter df and is being scanned at a velocity v, Figure 2, then the beam will traverse a distance equal to its diameter in a time t = s/vv. During this time a cubic element of material V will be removed, and we have t = s/vv = df /v.
The equation for determining the cutting speed is:
vv s
(14).
The theoretical equation for the cutting speed, in the event that the material is removed by
vaporization, based on the balance of energy is carried out with:
4a P
p df sr (CTv + Lv
(15),
where v (m/s) is the cutting speed.
Equations (6) and (15) are theoretical, and do not include the influence of the assist gas, especially if it is oxygen. With laser oxygen cutting the oxidation reaction generates heat, which accelerates the cutting process. It contributes approximately 40% of the energy required for the cutting.
The necessary energy for the vaporization of material per unit area of cut is:
vs
pdf r(CTv + Lv)
4a
const.
(16),
where E   (MJ/m2) is the necessary energy for the vaporization of material per unit area of cut.
For mild steel Tv=3160 K and Lv=6.8-106 J/kg. For the focal spot diameter d=0.2 mm and the absorptivity index at the vaporization temperature a=0.9 we have Equation (15) in the form:
P v = 0.083 s
And Equation (16) in the form:
E
nv
12.08
(17).
(18).
nv     vs
In Equations (17) and (18) we have v(m/s), P(W), s(mm) and Env (MJ/m2).
We can determine the severance energy (the energy necessary for cutting the material per unit area of cut) as:
P vs
(19),
where Es (MJ/m2) is the severance energy.
The value of the severance energy is essentially a quantitative measure of the difficulty of cutting: the larger the value, the more difficult a material is to cut. It is evident that the severance energy Es is between the energy necessary for melting the material Enm and the energy necessary for the vaporization of the material Env:
Enm < Es < En
(20).
The theoretical range of values for the severance energy, Est , for mild steel is:
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2.63 < Est < 12.08
(21),
where Est (MJ/m2) is the theoretical value of the severance energy.
2 DETERMINATION OF THE CUTTING DATA
The process of laser cutting can be described by the following characteristics: cutting rate, severance energy, and specific power consumption.
The cutting rate is defined by the equation:
R = v-s
(22),
where R is the cutting rate, v is the cutting speed, and s is the material thickness.
The severance energy is defined by the equation:
P
(23),
where E is the severance energy, P is the laser power, v is the cutting speed, and s is the material thickness.
The specific power consumption is defined by the equation:
P Ps =Q                         (24),
Table 1. Cutting data for laser oxygen cutting
where P is the specific power consumption, P is the laser power, and Q is the material removal rate.
Tables 1 and 2 contain the numerical values of the cutting rate, the severance energy, and the specific power consumption for the laser oxygen cutting and laser nitrogen cutting of mild steel. The values of the parameters are variable and change with the lens, the focal point stand-off, the nozzle diameter, the nozzle stand-off, and the assisting-gas pressure. All the parameters are the middle values, calculated on the basis of cutting data from manufacturers ([10] and [11]).
Between the cutting rate, R, and the severance energy, E, we have a dependence in the form:
R-E = P                               (25),
where P is the laser power.
Between the cutting rate, R, and the specific power consumption, P, we have a dependence in the form:
R-P
(26),
where P is the laser power, sr is the cut width.
Between the severance energy, E, and the specific power consumption, Ps, we have a dependence in the form:
Characteristics Cutting rate R(m2/s)
Severance energy E(MJ/m2)
Specific power
consumption Ps(GJ/m3)
Material: mild steel; assisting gas: oxygen
1000 W
(12*16)10-5
Laser power
2000 W
(20^25)10-5            23^33)10-5
6*8                     8*10                    9*13
3000 W
21*33                   23*42                   25*54
Table 2. Cutting data for laser nitrogen cutting
Characteristics
Cutting rate R(m2/s)
Severance energy E(MJ/m2)
Specific power consumption Ps(GJ/m3)
1000 W
(8.5^12)10-5           (17^20)10-5            (17^25)10-5
Material: mild steel; assisting gas: nitrogen
Laser power
2000 W
8.5*12                    10*12                    12*18
54*72                    60*72                    60*90
3000 W

Nekatere možnosti določitve rezalnih podatkov - Some Possibilities for Determining Cutting Data
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Strojniški vestnik - Journal of Mechanical Engineering 52(2006)10, 645-652
E =sr
Ps
(27),
where sr is the cut width.
With these characteristics we can determine the principal parameters of the laser-cutting process.
2.1 Determining the cutting data using the cutting rate
On the basis of the cutting rate, R, we can estimate: .   cutting speed, v:
v =
.
material removal rate, Q:
(28),
(29),
Q = v ¦ s ¦ sr = R- sr .   time to make the cut, tg:
tg==~Q                               (30),
Example 1: Laser, oxygen cutting; laser, 1000W; material of workpiece, mild steel; material thickness, 4mm; and length of cut, 1m.
For the laser of 1000W, from Table 1 we have a middle value of the cutting rate R=14105m2/s. For the cut width we take s=0.25mm, for the volume of removed material, V=1.0cm3, for the cutting speed, v=0.035m/s, for the material removal rate, Q=3.5 . 108m3/s, and for the cutting time, tg=28.57s.
2.2  Determining the cutting speed with the severance energy
On the basis of the severance energy, E, according to Equation (23), we can estimate the cutting speed v as:
E-s
Example 2: Laser, oxygen cutting; laser, 1000W; material of workpiece, mild steel; and material thickness, 4mm.
For a laser of 1000W, from Table 1 we have a middle value of the severance energy E=7MJ/m2. Cutting speed, v=0.035m/s
2.3 Determination of the cutting speed by specific power consumption
On the basis of the specific power consumption, P, according Equations (24) and (29),
we can estimate the cutting speed v as:
P -s-s
(32).
Example 3: Laser, oxygen cutting; laser, 1000W; material of workpiece, mild steel, and material thickness, 4mm.
For a laser of 1000W, from Table 1 we have a middle value of the specific power consumption of Ps=27MJ/m3. For the cut width we take sr=0.25mm. Cutting speed, v=0.037m/s
2.4 Determination of the cut width by cutting rate and specific power consumption
On the basis of the cutting rate, R, and the specific power consumption, Ps, we can estimate the cut width sr as:
sr=
P
RP
(33).
Example 4: Laser, oxygen cutting; laser, 1000W; and material of workpiece, mild steel.
For a laser of 1000W, from Table 1 we have
. middle values of the cutting rate R=14 10-5m2/s, and
a specific power consumption Ps=27GJ/m3.
Cut width, sr=0.26mm
2.5 Determination of the cut width by the severance energy and specific power consumption
On the basis of the severance energy, E, and the specific power consumption, Ps, according to Equations (25) and (26) we can estimate the cut width sr as:
E
(34).
Example 5: Laser, oxygen cutting; laser, 1000W; and material of workpiece, mild steel.
For a laser of 1000W, from Table 1, we have middle values of the severance energy E=7MJ/m2, and specific power consumption Ps=27GJ/m3. Cut width, sr=0.26mm
3 DETERMINATION OF THE CUTTING SPEED BASED ON EXPERIMENTS
The experiments were performed on a CO2 laser-cutting machine with computer numerical control. The technical characteristics of the CO2 laser
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are: radiation wavelength, 10.6mm; zone of the continual laser power control, 0.2 to 1.3kW; continual work regime; beam divergence less than 4 mrad; mode TEM00; and circular polarization. The optimal laser power is 0.8kW. The focusing system lens is 28mm in diameter and a focal distance of 125mm. The nozzle con opening is 1.6 mm. The material used for the examination is steel Ust 13 (DIN). The work process is carried out using the oxygen process with 99.8% purity.
The experimental equation for the cutting speed is obtained with the aid of a factorial plan of experiments and with ([5] and [8]):
v = 0.379 P s      9                    (35),
where v (m/s) is the cutting speed, P (kW) is the laser power, h (mm) is the dross height, and s (mm) is the material thickness.
Equation (38) can be rearranged to give:
PP0. = 2.64
(36).
The experimental range of values of the severance energy is:
PP0. 2.64
vs
(37),
where Ese (MJ/m2) is the experimental value of the severance energy.
In Equations (36) and (37) we have P/v (kJ/m) and P/vs (MJ/m2).
The tentatively derived Equations (36) and (37) model the interrelation between the cutting speed, the laser power, the material thickness, and the dross height for the laser oxygen-cutting of mild steel.
120 110 100 90 80 70 60 50 40 30 20 10
0           1           2           3           4           5           6           7           8           9          10
Material thickness s (mm)
Fig. 3. P/v vs. s
The experimental range of values of the severance energy for mild steel is:
2 MJ/m2 < Ese < 13 MJ/m2            (38).
The value Ese includes the influence of the reactive gas on the laser-cutting process.
Fig. 3 and Fig. 4 show P/v and P/(vs) against s for various dross heights, molten and evaporated material. From Figure 3 we can see that the relationship between the laser power/cutting speed P/v and the material thickness s is linear. The field of cutting is between the line of melting and the line of vaporization. In this case dependence is a function of the dross height. From Figure 4 we can see that the relationship between the laser power/(cutting speed and material thickness) P/(vs) and the material thickness s is approximately constant. This means that the severance energy Ese, or P/(vs), has a constant value for the concrete material of the workpiece. The required severance energy, Ese, is between the energy for melting and the energy for the vaporization of the material.
4 CONCLUSION
The technological problems related to the application of the laser-cutting process are an insufficient knowledge of the application of the laser technique as well as an absence of sufficiently reliable practical data and knowledge about the parameters influencing the work process itself. High levels of contour precision at a precisely defined cutting speed have an important effect on the quality of the cut. Therefore, in order to contribute to practical data, this paper gives the results of both the theoretical and the experimental research,
26 24 22 20 18 16
hs=0.05 mm hs=0.1 mm hs=0.2 mm hs=0.5 mm Melt Vapor
14 12 10 8 6 4 2 0
+ Vapor ~" hs=0.05 mm
, hs=0.1 mm
k hs=0.2 mm
hs=0.5 mm
Melt
0           1           2           3           4           5           6           7           8           910
Material thickness s (mm)
Fig. 4. P/(vs) vs. s
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referring to the determination of the principal parameters by laser cutting. The cutting data of the laser cutting can be described by the cutting rate, the severance energy, and the specific power consumption. The theoretical range of values for the severance energy is 2.63 MJ/m2 < Est < 12.08 MJ/m2, and the experimental range of values is 2 MJ/m2 < Ese < 13 MJ/m2. The value of Ese is essentially a quantitative measure of the cutting difficulty: the
larger the value, the more difficult a material is to cut. The value Ese includes the influence of the reactive gas on the laser-cutting process. For a determination of the cutting data, as useful initial data it is useful to take the mean values from Table 1 and 2 for the laser cutting of mild steel. Using these values an operator of a laser cutting machine can rapidly estimate the kind of cutting speed for a given thickness of material and the optimal laser power.
5 REFERENCES
[I]    Wilson J., J. Hawkes (1988) Lasers, principles and applications, Prentice Hall International Series in Optoelectronics
[2]    Grum, J. (1997) Laser surface treatment of tool steels, International Conference on Industrial Tools
ICIT’97, Maribor, Slovenia, pp. 273-278 [3]    Kramar, D., M. Junkar (2003) The development of a software tool for the selection of contour-cutting
processes, Journal of Mechanical Engineering 49 (2003) 6, pp. 346-365 [4]    Kramar, D., M. Junkar (2000) Laser and abrasive waterjet cutting economics, International Sheet Metal
Review, 2, pp. 38-41 [5]    Radovanovič, M. (2002) Determination of laser cutting speed based on energy balance, Tribologia, Vol.
33, No. 6, 2002, Warszawa, Polska, pp. 1561-1570 [6]    Radovanovič, M. (2004) A comparison of mathematical models for severance energy by CO2 laser cutting
of mild steel, Academic Journal of Manufacturing Engineering, Vol.2, No. 1, 2004, Timisoara, Romania,
pp. 27-30 [7]    Radovanovič, M. (2003) Determining of severance energy by CO2 laser cutting, Annals of The University
of Craiova, Seria Mecanica, No. 1, 2003, Craiova, Romania, pp. 181-190 [8]    Radovanovič, M. (2002) Mathematical modeling the feed rate by laser cutting and experimental verification,
advanced technology of plasticity, Vol.2, Edited by M. Kiuchi, H. Nishimura, J. Yanagimoto, Japan
Society for Technology of Plasticity, Jokohama, Japan, pp. 1817-1822 [9]    Black, I. (1999) A comparison of severance energies for reactive CO2 laser cutting of mild steel, The
International Journal of Advanced Manufacturing Technology, 15, Springer-Verlag, pp. 832-834 [10] Esab Cutting Systems (2002) Laser cutting machine Alpharex with trumpf laser TLF, cutting parameter for
vertical cutting, May 2002
[II]  Rofin-Sinar Laser (2000a) Rofin DC 0XX CO2 slab laser, adjustment of cutting parameters, March 2000 [12] Rofin-Sinar Laser (2000b) Introduction to industrial laser materials processing
Author’s Address: Prof. Dr Miroslav Radovanovič University of Niš Faculty of Mechanical Engineering Aleksandra Medvedeva 14 18000 Niš, Serbia mirado@masfak.ni.ac.yu
Prejeto: Received:
14.3.2006
Sprejeto: Accepted:
22.6.2006
Odprto za diskusijo: 1 leto Open for discussion: 1 year
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