Laser cutting

Joachim Berkmanns, Cleveland USAMark Faerber, Unterschleiheim Germany

FACTS ABOUT Laser technology

2

1. Introduction 3

2. The laser cutting process 4

2.1 Types of cutting processes 4

2.2 Lasers for laser cutting 5

3. Parameters in laser cutting 5

3.1 Pulsed or continuous wave (cw) laser operations 5

3.2 Laser power and intensity 6

3.3 Focal length of the lens 7

3.4 Laser beam mode 8

3.5 Wavelength of the laser beam 8

3.6 Focal position relative to the workpiece 9

3.7 Nozzle size and standoff distance 10

3.8 Gas type and gas pressure 11

4. Pressure and volume requirements for different materials 12

4.1 Mild steel and low-alloy steel 12

4.2 Stainless steels and other high-alloy steels 15

4.3 Aluminium and aluminium alloys 16

4.4 Titanium 18

4.5 Nickel alloys 18

4.6 Copper alloys 18

4.7 Non-metals 19

5. Literature 19

Table of contents

3

1. Introduction

Over the past decade, laser cutting hasdeveloped into state-of-the-art technology.It is estimated that more than 25,000 cut-ting systems are used for the high-powercutting of metals and non-metals world-wide. When including e.g. low powerapplications, such as plastics cutting andpaper cutting, the numbers are even higher.

Impressive examples of modern laser cut-ting applications are:

Cutting of hydro-formed parts andtubes

High speed cutting of thin-sheet metal

Cutting of thick section-material

Developing lasers with higher outputpowers without sacrificing beam qualityhas been one important goal in the past.Other efforts focused on improving thedrive technology of the motion system andenhancing material handling around thecutting table.

Predictions are that laser cutting based onimproved cutting speeds, little tool wearand unlimited flexibility will further replacecompeting technologies. There are marketsurveys suggesting that the number of flatbed laser cutter installations will doubleover the next ten years. In addition, lasermanufacturers will address new marketssuch as cutting tubes and pipes.

The gases used to generate the laser beamand expel the molten metal out of the cutkerf are important consumables duringlaser operations. They can prolong the life-time of the optical component, increasethe cutting speed and improve the cuttingquality. All the above contribute to moreprofitable laser operation. It is thereforethe objective of this brochure to familiarisepotential users of laser cutting systemswith the technology and to provide guide-lines for proper use of the cutting gases.Many of the results presented in this bro-chure were obtained in projects carried outin the application lab of the Linde Group orin projects that were initiated and spon-sored by the Linde Group.

2.1 Types of cutting processes

An almost parallel laser beam, which isusually invisible, is generated in the lasersource and directed to the cutting head bymirrors, where it is concentrated (focused)by a lens to a small spot, cf. Fig. 1. Depend-ing on the process, the spot is placed on the surface of the workpiece or on thematerial to be cut (Fig. 1, see also Factsabout: Laser Basics).

The intense light beam quickly heats upthe workpiece and melts the material. Theassist gas (also called: cutting gas) isapplied to protect and cool the focusinglens and to remove the molten metal fromthe cut kerf at the same time. There are two cutting processes, depending onthe type of assist gas used:

When cutting with oxygen, the materialis burned and vaporised after being heat-ed up to ignition temperature by thelaser beam. The reaction between theoxygen and the metal actually createsadditional energy in the form of heat,which supports the cutting process.These exothermic reactions are the rea-son why oxygen enables penetration ofthick and reflective materials when it isused as a cutting gas.

When cutting with non-reactive (inert)gases such as nitrogen or argon, thematerial is melted solely by the laserpower and blown out of the cut kerf bythe kinetic energy of the gas jet. As non-reactive gases do not react with the mol-ten metal, and no additional heat is

generated, the laser power required isusually much higher than in oxygen cut-ting of the same thickness. Cutting withnonreactive gases is often referred to asclean cutting or high-pressure cutting.

Sublimation cutting is another cutting pro-cess. In sublimation cutting, the solidmaterial is converted into vapor withoutpassing through a liquid phase. Gases areused to support the process and removevapor.

In cold cutting, the energy of the laserbeam breaks the chemical bonds of thematerial to be cut, thereby producing pow-dery residues. Laser beam energy andchemical bond energy must match, andcutting gas is often not needed.

2. The laser cutting process

Laser

Laser beam

Mirror Mirror

Cutting gas

Focusing lens

Gas nozzle

Workpiece

Fig. 1: Principles of a laser cutting system (schematic)

4

2.2 Lasers for laser cutting

Many lasers can be used for laser cutting,provided their beam can be focused on a small spot with sufficient intensity tomelt the material and their specific wave-length is absorbed in the material. Today,CO2 gas lasers, Nd:YAG solid-state lasersand Excimer gas lasers are those mostcommonly used in the field of materialsprocessing.

Recently introduced Diode lasers are con-sequently not suitable for laser cuttingwith respect to intensity in the focus beingnot sufficient yet. The latest member of thelaser family, the fiber laser, in contrast,seems to provide a beam quality, laserpower, and beam intensity that opens abroad perspective in laser cutting applica-tions.

3.1 Pulsed or continuous wave (cw) laseroperations

The highest cutting speeds can be ob-tained at high power levels in cw-modeoperation. Continuous wave (cw) meansthat the laser power output is constant,without interruption over time. At highspeeds, the laser power is used almost en-tirely to melt or vaporise the material on the cut front and there is relatively little heat conduction into the base materi-al. However, some of the heat is conductedinto the base metal when the cutting direc-tion is reversed or when cutting around asharp corner. This reduces the feed rateand causes the workpiece to heat up, to theeffect that cutting quality deteriorates.

When cutting filigree structures or piercing holes into thicker materials, it canbe especially difficult to achieve acceptable

cutting qualities with a high power cwlaser. Pulsed processing can produce bettercuts under such circumstances. High peakpower in the short pulses ensures efficientheating with an effective removal of hotmaterial from the kerf while low averagepower keeps the workpiece cool.

The cutting speeds obtainable in pulsedcutting are much lower than with con-tinuous wave (cw) laser beams. Averagepower normally has to be reduced to somehundred watts in order to achieve a signifi-cant increase in cutting quality by puls-ing. This often results in cutting speedsthat are only 10 % of those obtainable in the cw mode. When cutting metallicmaterials, the peak power generally mustbe within a range of 1 to 10 kilowatts, andeach pulse must be long enough to melt alayer of the cutting front, which is typically13 milliseconds.

3. Parameters in laser cutting

5

6

3.2 Laser power and intensity

Lasers are usually described in terms ofpower, e.g. 1,000 watts or 6 kW. Laserpower is the total energy emitted in theform of laser light per second. The intensi-ty of a laser beam is equal to its power di-vided by the area over which the power isconcentrated.

For example, focusing a 1,000 watt laserbeam over a diameter of 0.1 mm (0.004 in)will result in a power density of approx.125,000 watts per mm2.

The high intensity causes the material toheat up rapidly so that little time is avail-able for heat to dissipate into the surround-ing material. This produces high cuttingrates and an excellent quality of cut.

A lasers intensity also determines thethickness that can be cut. The thicker thematerial to be cut, the higher the intensityneeded. Higher intensities can be reachedby increasing laser power or by using afocusing lens with a shorter focal length.However, focusing the beam on to a smallerspot also reduces the depth of focus and istherefore unsuitable for cutting thickmaterials.

High intensity can be achieved both in pulsed and continuous beams. Accordingly,either the peak pulse power in pulsed cut-ting or the average power in continuouscutting determines the penetration.

Cutting speed is determined by the aver-age power level. The higher the averagepower, the higher the cutting speed (Fig. 2).However, high-power lasers do not auto-matically deliver high intensity beams.The lens used to focus the beam is veryimportant in the context of cutting speed.

Fig. 2: Typical cutting rates with approx. 3 kW of laser power

Laser power Intensity =

Irradiated area

0 4 8 12 16 20

Sheet thickness (mm/in)

Mild steel Stainless steel Aluminium

0 0.2 0.4 0.6 0.8

4

6

8

10

2

Cutting speed

(m/min)

Cutting speed (ft/min)

10

0

20

30

7

Fig. 3: Focusing a laser beam

Focusing lens

Depth of focus

Short focal distance Long focal distance

3.3 Focal length of the lens

The focal length of the lens defines the shape of the focused laser beam. Theminimum spot size (df) is a function ofwavelength (), beam mode (K), the dia-meter of the unfocused beam (D) at thelens and the focal length of the lens (f)according to:

A small spot diameter is favoured by:

Short focal length (f)

Good mode = even intensity distribution (close to Gaussian, K = 1/M2 = 1) (K)

Short wavelength of the laser beam ()

A large beam diameter at the lens (D)

The depth of focus Zf, which defines the level of tolerance for variation of the distance from the lens to the workpiece aswell as the thickness that can be cut,depends on the same parameters. In gene-ral, a small spot size goes along with ashort depth of focus.

This means that a lens with a short focallength produces a small spot size and ashort depth of focus, generally resulting inhigh speed and good cutting quality of thinsheet metal. However, careful control ofthe distance between the lens and theworkpiece (lens working distance) isnecessary. When thicker materials are cut,the depth of focus must be adapted to thematerials thickness by selecting a longerfocal length (Fig. 3). As the longer focallength also results in greater focal spotpower, it must be increased in order to maintain intensity and cutting speed.

4 f 1df = x x

D K

Laser beam Laser beam

Spot size

df

df

Zf

f

f

Spot size

Focusing lens

Zf

df

D D

3.4 Laser beam mode

A laser beams mode refers to the distribution of energy through its crosssection (Fig. 4, see also Facts about: LaserBasics). The mode affects the cutting pro-cess, because:

It affects the size of the focused spotand the intensity of the focused beamas a result

It affects intensity distribution in thebeam and focus and, as a result, cutquality

A good mode is therefore essential in lasercutting. The best mode is Gaussian inten-sity distribution, which is concomitantwith optimum focusing properties. TheGaussian mode is often termed TEM00(Transverse Electromagnetic Mode of thezero order).

3.5 Wavelength of the laser beam

The wavelength of the laser beam affectsits absorption in the metal to be cut (Fig. 5). The absorption rate of CO2 laserradiation in cold steel, for example, isapprox.15 %, to the effect that 85 % of theradiation is reflected. The absorption rateof Nd:YAG radiation, on the other hand, isapprox. 25 %. The high initial reflectivityof many metals can be overcome by bothNd:YAG and CO2 lasers, provided theintensity of the focused beam is sufficientlyhigh. The absorption rate increases re-markably as the temperature of the material rises. To give another example,glass cannot be cut with laser light of visible or near infrared (Nd:YAG laser)wavelength, as it is transmitted throughthe glass without any energy absorption.

While some highly reflective materials (e.g. aluminium or copper) might absorb

8

Fig. 5: Absorption as a function of wavelength for different materials

Mode printLaser beam

Laser beam

Intensity distribution TEM00 mode

Intensity distribution TEM01 mode

Iron

Steel

Nd:YAG: 1,06 m CO2: 10,6 mAbsorption rate (%)

Aluminium

0

10

15

20

25

30

5

1 10

Wavelength (m)

Fig. 4: Intensity distributionacross the laser beam

Mode print

Silver

Copper

some wavelengths better than others, aspecific type of laser is generally more suitable for a specific application thanothers. Suitability usually depends onother laser parameters, such as peakpower, pulse length and focus ability ratherthan wavelength characteristics. Therefore,if for example, Nd:YAG lasers are claimedto cut with better quality or more precisionthan CO2 lasers, this is true only whencomparing pulsed Nd:YAG lasers with cw CO2 lasers.

3.6 Focal position relative to the workpiece

The small spot size obtained by focusingthe laser beam provides high intensitiesfor material treatment. Above and belowthe focus the intensity in the beam drops.The depth of focus expresses how quicklythe beam becomes wider and the intensitydrops. A shorter focal length results in asmaller spot size and a smaller depth offocus.

Generally, the focal point must be positioned accurately with reference to thesurface of the workpiece, and the positionmust be kept constant during processing.

The beam waist or focus (the point wherethe beam diameter is smallest) should either be located on the surface of the

workpiece (when using oxygen as a cuttinggas, Fig. 6) or up to 75 % of the materialthickness of the workpiece. The sensitivityof focusing is less in high-power lasersthan in lower-power ones. The sensitivityto focal positioning is also greater in somematerials than in others.

However, focal position is a parameter thatmust be controlled in order to ensure opti-mum cutting performance and:

Variations in material and thicknessmay require alterations of focus

Variations in the laser beam shape orthe mode and changes in the tempera-ture of the cooling water and contami-nation on the lens may change the focalposition

9

Focal position: + 3.0 mm

Focal position too high

Focal position: 2.5 mm

Focal position too low

Fig. 6: An illustration of the effect offocal position on cutting performance

Focal position: 0.0 mm

(on the surface)

Focal position correct

10

3.7 Nozzle size and standoff distance

Gas assistance is essential in laser cutting.Therefore, nozzle geometry and standoffdistance are important. Nozzle design andflow dynamics through the nozzle differsubstantially from other thermal cuttingprocesses (Fig. 7). This is mainly due to thecompactness and diameter of the nozzle,which is always larger than the kerf pro-duced below it. As a result, only a portionof the gas jet formed by the nozzle pene-trates the kerf.

Nozzle standoff distances depend on thedesign of the nozzle. The standoff distancefor standard nozzles used in laser cutting

applications should be smaller than thediameter of the nozzle, as turbulence andpressure variations may appear in the cutting gas jet with larger distances (Fig. 8).At short nozzle standoff distances, the kerfitself acts as a nozzle and the geometry ofthe nozzle tip is not so critical. If largenozzle standoff distances are required,great care must be taken in designing thenozzle tip, especially when nozzle pres-sure exceeds 23 bar (3045 psi).

Typical nozzle diameters are in the rangeof 0.83 mm (0.030.12 in), so that thenozzle standoff distance should be in therange of 0.51.5 mm (0.020.06 in) forbest cutting results.

Fig. 8: Effect of nozzle parameterson effective cutting gas pressure

Fig. 7: Nozzle geometry definitions and standoff distance

Laser beam Pressure at workpiece,

bar

1

2

3

Size of the nozzle =

diameter of the orifice, dNozzle

standoff

distance

WorkpieceCutting width

Nozzle tip

0 1 2 3

Standoff distance

Nozzle diameter

psi

10

20

30

0

11

3.8 Gas type and gas pressure

The cutting gas used is crucial to the cut-ting result. Oxygen generally yields goodcutting performance in carbon steels andlow alloyed steels. However, oxygen reactswith the base metal, and the cut edge iscovered with an oxide layer. These are thereasons why high-alloy steels are being cut with nitrogen more and more oftenwhenever sufficient laser power is available.

Nitrogen is also being used increasingly for parts that are subsequently powder-coated. Any oxides on the cut edge decreasethe bond between the coating and thematerial and may therefore result in corrosion problems1).

The change from oxygen to nitrogen as acutting gas requires additional changes inthe set-up of the machine. The followingrules of thumb apply to cutting pressure:

Recent developments in the laser industryhave led to cutting lasers with outputpowers of up to 6 kW. As a result, moreand more businesses are changing over tonitrogen for cutting stainless steels andother high-alloy materials. As there are noexothermic reactions supporting the laserprocess, cutting gas pressure increaseswith increasing material thickness.

As the focusing lens is an integral part ofthe gas chamber, its strength limits gaspressure, which may not exceed 12 bar(175 psi) in older machines, depending onlens material and dimensions. Recently,however, thicker lenses allowing con-siderably higher cutting gas pressure of upto 20 bar have become available. This maybe advantageous in the cutting of stainlesssteel and aluminium.

The following section deals with commonmaterials and their specific cutting para-meters.

General rule for cutting mild steelwith oxygen:

The thicker the material, thelower the pressure Maximum pressure at approx. 6 bar (90 psi)

General rule for cutting stainlesssteel with nitrogen:

The thicker the material, the higher the pressure Minimum pressure at approx. 8 bar (120 psi)

12

4.1 Mild steel and low-alloy steel

Oxygen-cutting with CO2 lasers is the mostcommon method for cutting flat stockmild steels and low-alloy steels. Hollowprofiles and tubular structures as used in,e.g., the car industry are often cut withNd:YAG lasers manipulated by robots.

Today, plates with a thickness of up to 20 mm (3/4 in) can be cut easily with laserpowers of up to 3 kW. The focal point ispositioned close to the upper surface of theworkpiece. Table 1 contains a guideline forthe most important parameters, Fig. 10surveys the gas consumption. The cuttingtables in this brochure should only beregarded as guidelines. Deviations mayoccur, depending on type of laser, beamdelivery system, nozzle arrangements andfocus conditions. This applies particularlyto the cutting of profiles. Here, cuttingspeed is often much lower than indicated

in the cutting tables, because the motionsystem (i.e. robot) cannot follow the contours quickly enough.

A lens with a focal length of 127 mm (5 in)may be used when cutting sheets of platematerial of up to 6 mm (1/4 in). A 190 mm(7.5 in) lens should be used for materialthicknesses over 6 mm (1/4 in).

Laser power of at least 3 kW or higher isrequired when cutting plates with thick-nesses of up to 1225 mm (0.51.0 in).Cutting oxygen pressure is generallyunder 1 bar (14.7 psi), and the nozzle diameter is larger, 23 mm (0.080.12 in).The focus point may be situated 13 mm(0.040.12 in) above the upper surface ofthe workpiece.

Purity of oxygen forms an important para-meter when cutting mild steels and low-alloy steels2). The cutting speed can be

increased by using higher purity oxygen(99.9 % 99.95 %) rather than standardoxygen purity (= 99.5 %) (Fig. 9).

Cutting speed can be increased by1030 %, depending on material thicknessand types of steel, although the produc-tivity gain depends largely on the nature ofthe parts being produced. The positiveeffect is greater when cutting larger partswith simple geometry than when cuttingcomplicated shapes, where cutting speedis limited by the capacity of the mechanicalsteering system rather than the cuttingprocess itself.

When oxygen-cutting mild steels and low-alloy steels, a thin oxide layer is formed onthe cut surface. In most cases, this doesnot cause any problems. However, it maylead to difficulties if the cut parts are to bepainted or powder coated, in which casepaint adhesion and, consequently, corro-

4. Pressure and volume requirementsfor different materials

Table 1: Parameters for laser cutting of mild steels with laser cuttingoxygen including pressure and volume requirements

Laser

power

W

500

800

1,000

1,000

1,000

1,500

1,500

2,000

4,000

Material

thickness

mm (in)

0.5 (.02)

1.0 (.04)

2.0 (.08)

4.0 (.16)

6.0 (1/4)

8.0 (.32)

12.0 (1/2)

18.0 (3/4)

25.0 (1.0)

Nozzle

standoff

mm (in)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.30.8 (.01 .03)

0.30.8 (.01 .03)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

Nozzle

diameter

mm (in)

0.60.8 (.03 .04)

0.60.8 (.03 .04)

0.61.2 (.03 .05)

0.61.2 (.03 .05)

1.01.5 (.04 .06)

1.21.5 (.05 .06)

1.21.5 (.05 .06)

1.21.5 (.05 .06)

1.52.0 (.06 .08)

Oxygen

pressure

bar (psi)

3.56.0 (5090)

3.56.0 (5075)

2.54.0 (3560)

2.04.0 (3060)

1.53.0 (2045)

1.52.5 (2035)

1.02.0 (1530)

0.51.0 (715)

0.50.7 (710)

Gas

volume

m3/h (scf/h)

2.0 (70)

1.8 (64)

3.0 (105)

2.7 (95)

3.2 (115)

3.0 (105)

2.4 (85)

1.4 (50)

1.3 (46)

Cutting

speed

m/min (in/m)

15.0 (600)

11.0 (440)

7.0 (280)

4.0 (160)

2.5 (100)

2.0 (80)

1.0 (40)

0.5 (20)

0.5 (20)

Note: Oxygen pressure should be decreased with increasing material thickness in

order to avoid self-burning and poor cut quality. See page 19(*)

sion protection may be insufficient. Here,high-pressure nitrogen-cutting can be usedas an alternative to obtain oxide-free cuts.When cutting thick materials, however, thecutting speed is reduced. Modern highpower lasers allow for the same cuttingspeeds and sometimes even higher cuttingspeeds when cutting thin material (thick-ness less than 0.08 in).

The surface condition of mild and low-alloy steels is important in laser cutting.Oxygen-cutting of sheets with rust, forexample, may cause dross and notches.Likewise, painted surfaces can also causeproblems. This applies particularly tosheets coated with zinc primers and ironoxide shop primers. When oxygen-cutting,problems only occur when the paint layerfaces the gas nozzle. The cuts then oftenexhibit dross and notches.

These problems do not occur when thepaint layer is on the lower side of the sheet.The problems with painted sheets can beavoided by using high-pressure nitrogen-cutting, although the cutting speed will, ofcourse, be lower.

Zinc coated mild steels, galvanised or hotdipped, cause considerable problemswhen cutting with oxygen. There is alwaysdross formation and the cut surfaces maybe quite rough. High-pressure nitrogen-cutting is therefore used in industry to cutzinc-coated steels. The cutting quality isacceptable and free of adhering dross.

13

Fig. 9: Effect of oxygen purity on the cutting speed for 1 mm (0.04 in) mild steel using 800 Wlaser power and cutting oxygen pressure of 3 bar (44 psi)

Fig. 10: Consumption of oxygen depending on the gaspressure in the nozzle and the size of the nozzle, d

99.0 99.5 100

Oxygen purity (%)

1 mm Mild steel

11

10

9

8

7

6

5

20

30

Cutting speed

(m/min) (ft/min)

Consumption

(m3/h)

0 2 4 6 8 10

Pressure (bar/psi)

45

40

30

20

10

d = 2.5 mm/0.1 in

d = 2.0 mm/0.08 in

d = 1.6 mm/0.06 ind = 1.4 mm/0.05 ind = 1.2 mm/0.04 ind = 1.0 mm/0.03 in

250 50 75 100 125 150

1,400

350

700

1,050

0

Consumption

(ft3/h)

d = 3.0 mm/0.12 in

1414

Table 3: Parameters for laser cutting of stainless steel with laser cutting nitrogen including pressure and volume requirements

Material

thickness

mm (in)

1.0 (.04)

2.0 (.08)

4.0 (.16)

6.0 (1/4)

9.0 (3/8)

12.0 (1/2)

Laser

power

W

1,500

1,500

3,000

3,000

4,000

4,000

Nozzle

standoff

mm (in)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.30.8 (.01 .03)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

Nozzle

diameter

mm (in)

1.21.5 (.05 .06)

1.21.5 (.05 .06)

2.02.5 (.08 .10)

2.53.0 (.10 .12)

2.53.0 (.10 .12)

2.53.0 (.10 .12)

Nitrogen

pressure

bar (psi)

6.0 (90)

9.0 (135)

13.0 (195)

14.0 (210)

16.0 (240)

18.0 (260)

Material

thickness

mm (in)

0.5 (.02)

1.0 (.04)

2.0 (.08)

4.0 (.16)

6.0 (1/4)

9.0 (3/8)

Laser

power

W

1,000

1,000

1,000

1,500

1,500

1,500

Nozzle

standoff

mm (in)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.50.8 (.02 .03)

0.50.8 (.02 .03)

Nozzle

diameter

mm (in)

0.61.2 (.03 .05)

0.61.2 (.03 .05)

0.61.2 (.03 .05)

0.81.5 (.03 .06)

1.01.5 (.04 .06)

1.21.5 (.05 .06)

Oxygen

pressure

bar (psi)

4.06.0 (6090)

4.06.0 (6090)

4.06.0 (6090)

4.05.0 (6090)

3.55.0 (5075)

3.54.0 (5060)

Table 2: Parameters for laser cutting of stainless steel with laser cutting oxygen including pressure and volume requirements

Note: Oxygen pressure and volume are higher compared to cutting of mild steel. See page 19(*)

Gas

volume

m3/h (scf/h)

5.0 (175)

5.0 (175)

5.0 (175)

7.0 (250)

7.0 (250)

5.5 (195)

Cutting

speed

m/min (in/m)

15.0 (600)

11.0 (440)

7.0 (280)

3.0 (120)

0.6 (24)

0.3 (12)

Gas

volume

m3/h (scf/h)

8.0 (280)

12.0 (420)

28.0 (990)

52.0 (1,840)

60.0 (2,120)

68.0 (2,400)

Cutting

speed

m/min (in/m)

7.0 (280)

4.0 (160)

3.0 (120)

1.5 (60)

1.0 (40)

0.5 (20)

Note: The thicker the material, the higher the nitrogen pressure. See page 19(*)

Consumption

(m3/h)

d = 2.5 mm/0.1 in d = 2.0 mm/0.08 in

d = 1.8 mm/0.08 in

d = 2.2 mm/0.08 in

d = 1.6 mm/0.06 in

d = 1.4 mm/0.05 in

d = 1.2 mm/0.04 in

d = 1.0 mm/0.03 in

0 5 10 15 20 25

Pressure (bar/psi)

60

50

40

30

20

10

Fig.12: Consumption of nitrogen depends on the gas pressure in the nozzle and the size of the nozzle, d

0 100 200 300 400

Consumption (ft3/h)

1,400

1,750

2,100

350

0

700

1,050

15

4.2 Stainless steels and other high-alloysteels

In industry, both oxygen and nitrogen areused as cutting gases for stainless steeland other high-alloy steels.

High cutting speeds are possible whenusing oxygen, due to the exothermic ener-gy contribution. The maximum thicknesstoday is 16 18 mm (3/4 in). The optimalfocal position is situated at or just belowthe upper surface of the sheet. By contrast,with mild steels, relatively high oxygenpressure approx. 5 bar (75 psi) is alsoadvantageous when cutting thicker sheets.Greater oxygen purity (99.9 %99.95 %)than standard oxygen purity (approx.99.5 %) can be used to increase the cuttingspeed. A cutting table for laser cutting ofstainless steel with oxygen is shown inTable 2.

The disadvantage of oxygen-cutting is thatthe cuts always exhibit burrs and that the cut surfaces are discolored due to chromium and iron oxides3). These oxidesobstruct the subsequent welding pro-cedure. TIG welds, for example, have blackoxide spots on the root side, which some-times cause incomplete penetration.Beyond that, the oxidised cut surface facilitates corrosion of the cut edges. Thesedrawbacks in oxygen-cutting are serious,as they often require expensive finishingoperations.

Oxide-free and burr-free cuts can be ob-tained by high-pressure nitrogen-cutting,although the cutting speed must be reducedconsiderably compared with oxygen-cut-ting. Higher cutting speeds can be usedwith higher power lasers. CO2 lasers with apower of at least 2.5 kW are commonly usedfor this application. Today, the maximumsheet thickness range where burr-free cuts are possible is about 12 16 mm(0.50 0.64 in).

The focal point should be close to the back surface of the sheet during high-pres-sure nitrogen-cutting in order to assureburr-free cuts. The kerf then becomeswider so that a larger part of the nitrogenflow can penetrate into the kerf and flushout the molten material. The lower focalposition results in a larger cross-section ofthe laser beam inside the gas nozzle than

in oxygen-cutting (see Fig.11). The diameterof the nozzle must also be enlarged inorder to allow the laser beam to passthrough the nozzle without being clipped.A nozzle diameter of at least 1.5 mm (0.05in) is usually required for high-pressurenitrogen-cutting.

Table 3 contains a cutting table for high-pressure nitrogen-cutting of stainlesssteel. Fig. 12 examines the gas consump-tion.

The purity of the cutting nitrogen has little effect on the cutting speed, at least aslong as nitrogen purity is better than99.5 %. Even small amounts of oxygenimpurity, though, have an oxidising effect.In stainless steel this means a discolorationof the cut surface. Corrosion resistance ofthe workpiece may also be impaired. Theoxygen impurity level would have to bebelow 20 PPM (0.002 %) in order to avoidall traces of oxidation. This specification,however, is desirable when laser cuttingstainless steel, but is normally not requiredin other industrial applications.

The oxygen impurity level is importantwhen using gaseous nitrogen in cylindersor bundles as there are various purity grades available. On the other hand, theuse of liquid nitrogen in tanks, whichgenerally has a very low impurity level, doesnot lead to discoloration of cut surfaces.

Fig. 11: Focus position for oxygen-cutting compared tohigh-pressure nitrogen-cutting

Laser beam Laser beam

Cutting gas:

oxygen

Cutting gas:

nitrogen

15

16

4.3 Aluminium and aluminium alloys

Aluminium is a term used for both purealuminium and aluminium alloys. Purealuminium is soft, and it is often alloyedwith small amounts of magnesium, cop-per, manganese, silicon or zinc to improveits mechanical strength.

Cutting of aluminium with CO2 lasers isconsidered difficult on account of its highreflectivity and thermal conductivity.Aluminium alloys, however, are usuallysomewhat easier to cut than pure alumini-um and cutting speed is higher.

Anodised aluminium is also easier to cut,due to enhanced laser light absorption in

2.5 5.0 7.5 10.0 12.5 15.0

Pressure (bar/psi)

Nitrogen Oxygen

5.0

4.0

3.0

2.0

1.0

0.0

5.0

4.0

3.0

2.0

1.0

0.02.5 5.0 7.5 10.0 12.5 15.0

Pressure (bar/psi)

Aluminium alloy (AIMg 3)

Velocity Velocity

(m/min) (ft/min)

5.0

10.0

100 200 100 200

5.0

10.0

Pure Aluminium (AL 99.5)

Velocity Velocity

(m/min) (ft/min)

Fig. 13: Dross-free parameter ranges when using nitrogen andoxygen up to 15 bar (217.5 psi) for cutting 2 mm (0.08 in) purealuminium and 2 mm (0.08 in) aluminium alloy, laser power1500 W

17

the thick surface layer of aluminium oxide.High laser powerover 2 kWand a goodlaser mode help improve the cutting suit-ability of aluminium.

A small focal length, of about 63 mm (2.5 in), is advantageous for thinner sheetsdue to the higher power density in thefocal spot. The maximum sheet thicknessthat can be cut is about 68 mm (0.240.32 in).

Aluminium can be cut with both oxygenand nitrogen as cutting gases, but cuttingspeed with oxygen is not significantly higher than with nitrogen. This can beattributed to aluminium oxides extremelyhigh melting point, which is 2072 C

(3762 F). Solid or viscous aluminiumoxide forms a seal on the cut front, whichprevents oxygen from penetrating into themetal itself. The oxide seal frequentlybursts as a result of turbulent melt flow,and the oxidation reaction may still proceed, although at a lower rate.

Low-pressure oxygen-cutting less than 6 bar (85 psi) is frequently used for cutting aluminium nevertheless. The laserbeam should be focused on the upper sur-face of the sheet. Standard oxygen purity(= 99.5 %) is sufficient, as higher oxygenpurity does not improve cutting speed. Thedrawbacks of low-pressure oxygen-cuttingare cut edges with dross and a rather roughcut surface.

Dross-free cuts can be obtained by usinghigh-pressure cutting with nitrogen oroxygen. It appears that nitrogen is the better alternative when cutting aluminiumalloys, whereas oxygen is better for pure aluminium4). This is demonstrated in principle in the diagrams in Fig. 13,which show the parameter ranges wheredross-free cuts are obtained for 2 mm(0.08 in) sheets of pure aluminium (Al 99.5) and an aluminium alloy (AlMg 3).When aluminium is cut with high pres-sure, the laser beam should be focusedclose to the lower surface of the sheet.Typical parameters for the cutting of AlMg 3 are summarised in Table 4.

Material

thickness

mm (in)

1.0 (.04)

2.0 (.08)

3.0 (.12)

4.0 (.16)

6.0 (1/4)

8.0 (3/8)

Laser

power

W

1,800

1,800

3,000

3,000

4,000

4,000

Nozzle

standoff

mm (in)

0.30.6 (.01 .02)

0.30.6 (.01 .02)

0.30.8 (.01 .03)

0.30.8 (.01 .03)

0.51.0 (.02 .04)

0.51.0 (.02 .04)

Nozzle

diameter

mm (in)

1.21.5 (.05 .060)

1.21.5 (.05 .060)

1.21.5 (.05 .060)

1.82.2 (.07 .085)

2.02.4 (.08 .090)

2.02.5 (.08 .100)

Nitrogen

pressure

bar (psi)

12.0 (180)

14.0 (210)

14.0 (210)

14.0 (210)

16.0 (240)

16.0 (240)

Gas

volume

m3/h (scf/h)

11.0 (390)

15.0 (530)

15.0 (530)

35.0 (1,240)

40.0 (1,410)

45.0 (1,590)

Cutting

speed

m/min (in/m)

8.2 (320)

3.6 (140)

2.5 (100)

1.8 (70)

1.8 (70)

0.8 (30)

Table 4: Parameters for laser cutting of aluminium alloy AlMg 3, the tablerefers to dross-free cuts, cutting gas: laser cutting nitrogen

See page 19(*)

18

4.4 Titanium

Oxygen and nitrogen are not suitable ascutting gases for titanium and titaniumalloys, because these gases, like hydrogen,are absorbed into the surface, where a hardand brittle layer is formed. This layer maycrack, and propagation of the crack cancause component failure.

High-pressure cutting with a completelyinert gas is therefore the preferred methodfor the laser cutting of titanium. In addition,the workpiece is often fixed in a vessel atinert atmosphere during the cutting pro-cess. High-purity argon and argon-helium-mixtures with a very low oxygen content(99.996 % 99.999 %) are mostly used.Helium content in the cutting gas isadvantageous as high intensities in thefocus cause process disturbing plasma for-mation. The cuts exhibit no significantdross adhesion. A deep focal point position,beneath the upper surface of the sheet is advantageous.

4.5 Nickel alloys

Nickel is the base metal in a number of industrially important alloys: Inconel(Ni-Cr), Nimonic (Ni-Cr-Co), Hastelloy (Ni-Mo-Cr) and Monel (Ni-Cu).

Low-pressure oxygen-cutting lower than6 bar (85 psi) can be used to obtain highcutting speeds, but the cut edges exhibitdross and the cut surfaces are oxidised.

Although burr-free and oxide-free cutsmay be achieved with high-pressure nitro-gen, the cutting speed is reduced signifi-cantly compared with oxygen-cutting. Thefocal point should be positioned deep with-in the material, below the upper surface ofthe sheet.

4.6 Copper alloys

The high reflectivity and thermal conduc-tivity of copper make cutting of this mate-rial quite a challenge. Precautions must betaken to avoid re-reflections of the laserbeam and subsequent damage to the reso-nator.

Brass is one of the important copper alloys (copper-zinc alloy), and has lowerreflectivity and thermal conductivity thancopper. It is therefore easier to cut withCO2 lasers. Very high laser power and ashort focal length of the lens about 63 mm(2.5 in) are beneficial.

Oxygen-cutting is preferable when cuttingbrass and other copper alloys with CO2lasers. Oxygen is better suited as a cuttinggas, as the oxide layer at the cut frontimproves absorption of the laser beam.Both low oxygen pressures of up to 6 bar(85 psi) and high oxygen pressures of up to20 bar (300 psi) are used. When cuttingbrass at high oxygen pressures of up to20bar (300psi), proper ventilation of theworking area must be provided to avoiddangerous enrichment of the atmospherewith oxygen. The maximum sheet thick-ness that can be cut is 45 mm (0.160.20 in). Sometimes, high-pressure nitro-gen is also used to cut copper alloys.

19

4.7 Non-metals

Laser application is widespread in the cut-ting of non-metals.

CO2 lasers are used for cutting plastics,rubber, textiles, wood, ceramics andquartz. When using a 1,000 W CO2 laser,the maximum thickness of plastics andplywood is about 25 mm (1 in).

Nd:YAG lasers are sometimes used inindustry for cutting certain types of ceram-ics, such as silicon carbide and siliconnitride.

Compressed air free from oil/grease andmoisture is the cutting gas predominantlyused for practically all of the above-men-tioned materials. Inert cutting gases likenitrogen are only used in a few cases whena plastic material or a fabric is highlyflammable. Oxygen can be used to avoiddiscoloration of the cut surface of ceramicssuch as aluminium oxide and zirconiumoxide.

During laser cutting, certain non-metals,like for example plastics, synthetic textilesand ceramics, toxic fumes or dust, are for-med, thereby making good fume/dustextraction systems necessary.

1) M. Faerber: Appropriate Gases for LaserCutting of Stainless Steel, InternationalCongress Stainless Steel 1996, Dssel-dorf, VDEM (1996), Pages 282283

2) John Powell: CO2 Laser Cutting, Springer Verlag (1993)

3) M. Faerber and W. Schmidt: Laser cuttinggases, DVS-Berichte Band 185, DVS-Verlag (1997), Pages 7274

4) T. Kristensen and F.O. Olsen: Cutting ofAluminium Alloys, Proc. of the 4th

NOLAMP Conf., Sonderborg (1993),Pages 121129

Picture creditsImage on page 3, K. Moder GmbH, Wiesmoor, Germany

Image on page 11, AUDI AG, Ingolstadt, Germany

* Values listed in tables are indicative andmay vary depending on the cuttingsystem.

5. Literature

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