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Harris and Brandt

Materials Forum (2001) 25, 88-115

Laser Cutting of Thick Steel Plate

J. Harris and IVI. Brandt

Cooperative Research Centre for Intelligent Manufacturing Systems and Technologies

Industrial Laser Applications Laboratory IRIS, Swinburne University of Technology

PO Box 218 Hawthorn, Melbourne VIC 3122 Australia

ABs-rRACT

Of the many machining processes used in manufacturing to cut metals and non-metals, laser cutting offers unique advantages in terms of cut quality, speed, absence of tool wear and minimal or no clamping of parts. In industry, the lasers used for cutting are CO2 (carbon dioxide) and Nd:YAG (Neodymium-doped Yttrium Aluminium Garnet) lasers each with their own characteristic output properties such as wavelength, power) mode of operation and beam quality. These properties, together with the optical and thermophysical properties of workpiece material and workpiece handling system, control and determine the cutting performance of any material. Presented in this paper are laser, materials and system properties, and parameters influencing the cutting of metals in particular. The operation of the CO2 and Nd:YAG lasers and their dominant features is also discussed. The cutting mechanism is described in terms of the energy balance within the workpiece. This is then used to show the difficulties with cutting thick (>10mm) steel plate with a laser, leading into a number of novel laser methods explored for the cutting of thick plates. Finally, recent work supported by the CRC for Intelligent Manufacturing Systems and Technologies (CRCIMST) on laser cutting with a "spinning laser beam" is presented and its potential for cutting thick steel plate discussed.

© Institute of Materials Engineering Australasia, Ltd (2001)

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1. INTRODUCTION

Many methods are available to cut materials and metals in particular. Among these are the high energy processes of laser, electron beam, plasma, water jet and oxy-fuel. Oxy­fuel or flame cutting, is still largely used for cutting very thick workpieces of metals, typically 100 mm to 500 mm thick (Rickfalt 1995). The edges of the cut are oxidised and the kerf produced is large. The heating fuel is usually propane, natural gas, a hydrocarbon mixture or acetylene, and is delivered through a concentric nozzle with the oxygen nozzle, so that the oxygen jet is surrounded by the fuel gas.

The development of plasma tools began in 1941 in the US aircraft industry for welding applications. This process uses an electric arc between an electrode and the metal, and a gas shielding around the arc, blown through the electrode. An innovation by scientists at Union Carbide's welding laboratory in the early 1950s reduced the gas nozzle opening. The result was that the electric arc and the gas were constricted to a small volume in space and the gas velocity increased dramatically, together with the arc temperature and voltage. A cutting tool was thus obtained and a new technology developed that has been continuously improved over the years. Plasma jets can cut through thick materials, but not as thick as those cut with the oxygen flame. They possess very high temperatures and thus can be used to cut materials with very high melting points that cannot be cut with other techniques.

Unlike the two cutting technologies mentioned above, abrasive water jet cutting does not melt the material. The operating principle is the acceleration of abrasive particles by mixing them in a stream of water. Passing through a nozzle, a highly focused jet of water is obtained that can cut

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through almost any material by erosion. The kerf is rather large ('"'"' 1 mm) and the cutting speeds are modest compared to the thermal cutting methods.

The use of the laser as a tool for materials processing began as early as 1963 and is still developing. The first experiments in laser cutting with a gas assist nozzle were made in May 1967 (Hilton 1997) at the Welding Institute in Cambridge, UK, using a 300W CO2 laser and oxygen assist gas. A 2.5 mm thick plate was cut with remarkable accuracy. The idea of using the laser as a cutting tool for metals belongs to Peter Houldcroft who was then the deputy scientific director at The Welding Institute. He thought of it when confronted with cutting trials using a plasma torch for body panel trimming at the British Motor Company in 1965. The plasma cutting was not accurate enough and produced burning of the edges.

With the first industrial laser cutting system supplied in 1970, the progress of the new technology has been fast. In the last three years some 18,000 lasers for metal sheet cutting have been installed world-wide and this number is increasing yearly by some 6000 units (Belfore 2001). The value of these laser cutting systems in 2001 is estimated at about US$1.0 billion. This high level of adoption of lasers for cutting is associated with a number of advantages the technology offers compared with the competitive cutting technologies described above (Steen 1998). These include:

• The non-contact nature of the machining process requiring light or no clamping of the workpiece. This absence of cutting forces also allows precise cutting of light or flimsy materials.

• The absence of cutting tool wear and tool chatter as a result of no contact.

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• Machining operations are not governed by workpiece hardness, with lasers able to cut many hard materials.

• Minimal noise pollution as the laser cutting process is quieter than the competitive processes of water jet and plasma.

• The narrow kerf width, typically 0.1 to 1.0 mm, requires little or no radial compensation for beam diameter. Furthermore, the possibility of nesting of components in production allows minimal wastage of raw material.

• The clean nature of the cut allows, in most cases, no subsequent cut face processing. Molten or oxidised material is removed by assist gas during the cutting process or falls away during the separation of the part at the completion of the cut.

• Given that the laser cutting mechanism is fast and the area heated is small, there is a minimal heat affected zone and consequently minimal heat distortion.

The disadvantages of laser cutting compared to other cutting technologies are:

• The process effectiveness reduces as the workpiece thickness increases. Commercially, workpiece thicknesses greater than about 15 mm are generally not cut with a laser.

• Laser cutting produces a tapered kerf shape. This is a result of the divergence of the laser beam and is more pronounced in thicker materials. The taper can be reduced by positioning the focus below the surface of the \vorkpiece.

• The heat affected zone can be a problem in some applications because of the changed microstructure.

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Given the long time that commercial laser cutting technology has been available to industry and the significant advancements achieved in that time, laser cutting is now considered a mature technology. Consequently, cutting technology and knowledge of the cutting mechanism for thin and medium thickness «10mm) steels have remained basically unchanged during the last ten years and are considered well understood. There is, however, significant demand by industry to increase cut thickness and the quality of cut at those thicknesses. This paper discusses issues associated with the laser cutting of thick (> 1 Omm) steel plate and methods currently being examined to cut it. Presented in Section 2 are general aspects of laser cutting and in Section 3 a brief description is given of the laser systems used for cutting. Section 4 summarises the chemistry of laser cutting and Sections 5 and 6 address the issues of, and novel laser technologies for the cutting of thick plate.

2. GENERAL ASPECTS OF LASER CUTTING

The process of laser cutting may be considered as a sequence of the following mechanisms:

• absorption of laser radiation by material;

• heating and melting of the material, and

• removal of the molten material by the coaxial assist gas jet. If the assist gas is oxygen the exothetmic chemical interaction with the molten metal will accelerate the cutting process.

The key features of laser cutting are illustrated in Figure 1. In laser cutting, the workpiece is placed on a CNC table and moved at a predetermined speed and path designed on a CAD system and converted to machine code instructions. The laser beam

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is typically focused to a diameter of 0.1 to 0.3 mm on the workpiece surface (or just below it) with a lens or a lens system held in a specially designed nozzle. The most popular focal length range is 100mm to 150mm giving a good balance between spot size and depth of field. Incident laser power densities are of the order of 106 W/cm2

.

Once the laser beam has penetrated through the material, an erosion front is established in the cutting direction. The erosion front is covered by a layer of molten material which is continually removed by the assist gas at the lo\ver surface of the cut and created by the absorption of energy from the laser beam (Schuocker 1993). The assist gas, gas pressure, nozzle shape and position above workpiece are all critical process parameters and affect strongly the removal of material from the kerf (Powell 1993). Nozzle diameters vary from 0.8 mm to 3 mm depending on the material being cut and the nozzle position above the workpiece, which influences gas pressure in the kerf. This position is carefully chosen because of the strong variation of the melt removal rate with the complex pressure patterns produced in the kerf due to the supersonic speed of the assist gas (Fieret 1987; Na 1989).

3. LASER SYSTEMS USED FOR COMMERCIAL CUTTING

A typical laser system used for cutting consists of:

• the laser beam source along with its utilities and control unit,

• a beam guidance arrangement from the laser beam source to the focusing optics,

• the focusing optics with cutting nozzle and process gas supply system,

• the workpiece and its support plus fume extractors for vapors, and

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• means of producing movement of workpiece relative to cutting nozzle, generally CNC controlled.

(a)

Cutting direction

.....--_ Assist gas

Narrow kerf Cutting nozzle

(b)

Figure 1. Illustrated in (a) is laser cutting arrangement and key cutting features and in (b) CO2 Jaser cutting a component (courtesy Rofin Sinar) ,

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Table 1. Principal characteristics of C02 and Nd: Y AG cutting lasers.

Active Wavelength Laser power Medium

CO2 10.6 Jlm ~ 4kW (pulsed or molecule CW)

1.06Jlm Ndion ~ 2 kW (pulsed or

CW)

The lasers used for cutting include CW CO2

lasers, pulsed Nd:YAG lasers and more recently CW Nd: YAG lasers. Typical characteristics of these two lasers are shown in Table 1. The CO2 laser is by far the most widely used laser for cutting because of its high power, beam quality and reliability. Typically, for cutting applications the CO2 lasers range in power up to 4.0kW. For the cutting of relatively small, detailed and delicate parts a pulsed Nd:YAG laser is normally used (van Dijk 1993). At power levels up to 500W this type of laser yields very low thermal input into the material and produces kerf widths of 0.1 mm to 0.2 mm compared to the 0.2 mm to 1.0 mm kerf widths produced with a CO2

laser. Some typical cutting parameters for the two types of lasers are illustrated in Figures 2 and 3.

3.1 The C02 Laser

The CO2 laser is a gas laser, whose active medium is a mixture of about 5% carbon dioxide, 10% nitrogen and the balance helium. There are several different designs of CO2 lasers depending on the method of exciting and cooling the gas mixture in the resonant cavity, however, the principle of the lasing action is the same for all CO2

lasers (Laos 1983). The active laser species is the carbon dioxide molecule. The nitrogen acts as a catalyst, transferring

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Beam delivery and Electrical efficiency focussing

Mirrors and lenses 10-15%

Mirrors~ optical fibres 4% (lamp pumped) and lenses

10% (diode pumped)

energy to the CO2 molecule and enabling it to remain in the upper laser leveL The helium cools the gas mixture through collisions and transfer of stored energy from the CO2 molecule. The CO2 laser produces output at a wavelength of 10.6 flm (invisible part of the spectrum) at power levels from tenths of a watt to forty five kilowatts (Convergent) at an electrical efficiency of about 10%. For conventional cutting applications lasers with powers up to 4kW are used. Higher power lasers normally have inferior beam quality which adversely affects the cutting process.

The output power and beam quality of a CO2 laser are primarily determined by the method of gas flow. The flow method determines how quickly the carbon dioxide can be removed from the optical cavity so that new ground state carbon dioxide can be introduced for excitation and stimulation. The CO2 lasers used for cutting can be classified as axial flow and no flow devices. In slow flo\v axial lasers the gas moves slowly in the direction of the laser beam through a glass tube (typically 10 to 14 mm in diameter) which is surrounded by another, co-axial water or oil-cooled tube. The length of the laser tube is typically 1 m and it produces about 50 to 70 W of laser power. The advantages of this type of design include very good beam quality for focusing, high peak power when pulsed and

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(a)

14~--------------------------~

13

12

11

9

8

~ 7 Q)

g. 6 Ol E 5 :;) () 4

3

2

• -·-700W -·-1200W -e-1700W

:" ':' . '-: • ': -:. '":' - • 3"

o~~~~~~~~~~~~~~~

o 2 3 4 5 6 7 8 9 10 11 12 1314 1516

Material thickness (mm)

(b)

"0 Q) Q) 0.. rJJ 0> c: E :;)

()

7

6 • \

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-'-1700W -.-1200W

\ \

\,

"~~ ..... --~-~

:'-.. . . ; '-...:

O~~L-~~~~~~~-L~~

o 2 3 4 5 6

Miterialthickness (nm)

Figure 2. Representative cutting speed as a function of material thickness for C02 laser cutting of (a) mild steel with oxygen and (b) stainless steel with nitrogen assist gas.

(a)

'2 :€ .s ""0 Q) Q.) 0.. rJJ 0) c E ::J t.)

7

6

5

4

3

2

..

-.- 500 W pulsed

\\~.- 1000 W pulsed

-:"'" ".--- .. ~ 1- ':~. o~~~--~~~~~~--~~~

o 1 234 5

Material thickness (mm)

(b)

3.0 ,.---------------------------.....

A -.-500 W, pulsed, 0.6 mm fibre

2.5 -1..- 1000 W, pulsed, 0.6 mm fibre -a- 500 W, pulsed, 0.4 rom fibre

"2 2.0

"E 1 '"0 1.6 Q) Q) a. tJi 0) c

1E 1.0

()

0.5

0.0 0 234567 B 9

Material thickness (mm)

Figure 3. Representative cutting speed as a function of material thickness for Nd:YAG laser cutting of (a) mild steel with oxygen and (b) stainless steel with nitrogen assist gas and optical fibres.

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reduced system complexity and maintenance costs. The main disadvantage is the laser size and the maximum output power. Lasers employing this approach are large because of the necessarily long laser tubes. The maximum power generated is about 2kW from some 20 laser tubes placed in series. This number of laser tubes can present a challenge in keeping the optics aligned.

CroQl1d potenllil

Guhdd

Figure 4. lllustrated is a schematic of a RF excited fast axial·flow C02 laser (courtesy Rofin Sinar).

Rear mIrror

Figure 5. The basic design of a diffusion cooled C02 laser (courtesy Rofin Sinar).

In fast axial flow lasers (see Figure 4), as the name suggests, high powers are achieved by transporting the gas very quickly through the laser tube using Roots or a centrifugal compressor thus reducing the time it spends in the heated volume. The gas travels at close to the speed of sound, and before it is recirculated through

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the tube it is cooled with a heat exchanger. Laser tube length is considerably reduced compared to slow flow tubes resulting in a more compact device. The advantages of fast axial flow lasers include, in addition to the compact size, high output power and an output which can be electronically pulsed. The main disadvantage is that the Roots or centrifugal compressor needed to circulate the gas is complex and costly to maintain.

In no-flow or diffusion cooled lasers the gas mix in the optical cavity is cooled by conduction to the walls of the optical cavity. Shown in Figure 5 is a schematic of a diffusion cooled laser. The design involves two flat copper plates placed close together that act as both the RF electrodes and a heat sink. Since the gas transport system is not required the unit size is very compact. These devices are now available in the power range from 1 kW to 3.5 kW with very good beam quality for cutting applications.

3.2 The Nd: YAG Laser

The Nd:YAG laser is a solid state laser, usually in the shape of a rod, operating at 1.06 J!ffi (Koechner 1988). The active species are neodymium ions present in small concentrations in the Y AG crystal. Both continuous wave and pulsed laser outputs can be obtained at an overall efficiency in the 3 to 5% range. The laser is used in industry because of its efficiency, output power and reliability compared to other solid state lasers. The crystal is grown using the Czocbralski crystal growing technique (Dawes 1995) which involves slowly raising a seed Nd:YAG crystal from the molten crystal constituents to extract a Nd:YAG boule. A single boule typically yields several laser rods. The concentration of Nd ions in the boule is carefully controlled and is not greater than about 1.10/0. Increasing the Nd doping further in

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order to increase the laser po\ver produces unacceptable strain in the crystal and leads to a dramatic reduction in laser power.

Figure 6. Schematic of a Nd:YAG laser (courtesy Rofin Sinar).

Laser rods are typically 6 mm in diameter and 100 mm in length with the largest commercial size rods being 10 mm in diameter and 200 mm in length. As a consequence of the crystal's small size Nd:Y AG lasers tend to be much more compact than CO2 lasers. Illustrated in Figure 6 are the main components of a single-rod Nd:YAG laser.

Laser action is achieved by optically exciting the crystal by lamps placed in close proximity to it. The lamps have an emission spectrum which overlaps the absorption bands of the Nd: Y AG crystal at 700 nm and 800 nm. In order to couple the maximum amount of lamp light into the rod and extract the maximum laser power from it, the rod and the lamp are enclosed in specially designed and manufactured cavities. The two most common pump cavity configurations are elliptical and close coupled. In the case of elliptical cross­sections the rod and the lamp are placed along the nvo foci, and in the case of close­coupled cavities the rod and the lamp are

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placed close together at the axis. The inside surface of the cavity is normally coated with gold in order to maximise the coupling of lamp light into the rod. Some laser manufacturers also manufacture ceramic cavities which produce more uniform pumping of the rod but at the expense of lower efficiency (some 5% lower) compared to that of the gold-coated cavities.

For continuous operation, krypton arc lamps are most widely used while for pulsed operation high pressure xenon and krypton flashlamps are used. Lamp lifetime dominates the service requirement of modern Nd:Y AG lasers. For arc lamps the lifetime ranges between 400 and 1000 hours while for pulsed lasers it is about 20 to 30 million pulses depending on operating conditions.

Figure 7. Rofin Sinar 2.5 kW Nd:YAG laser with four pump chambers.

As only a fraction of the emitted spectrum is absorbed by the laser crystal the rest of the emitted light is dissipated as heat in the cavity and has to be removed for efficient laser operation. This is usually achieved by flowing deionised water around the rod and lamp in a closed loop cooling system. The loop is coupled to a heat exchanger for efficient heat removal.

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To increase the laser power above 500 to 600 W typically obtained from a single rod, requires an increase in the laser volume. However, increasing the rod volume has fundamental limitations. Heat generated within the rod causes large thermal gradients which lead to variations in the refractive index, lowering beam quality, as well as large mechanical stresses, which can cause rod fracture. To obtain higher laser powers involves using multiple laser rods (see Figure 7). The rods are ananged in senes and located either within the resonator or some are placed outside the resonator to act as amplifiers. These configurations are discussed and described in more detail by Emmelmann (1995). There are now several systems on the market all giving in excess of 2 kW of laser power with the highest power commercial device producing 6 k W from 8 cavities (Trumpf).

Figure 8. 2D CO2 laser cutting system manufactured by Laser Lab.

While lamps have been an integral part of the Nd:YAG laser technology to date and will remain so for the foreseeable future because of their relatively low cost, another technology is now emerging for high power laser applications both as a pumping source for Nd:YAG lasers and as a laser source in its own right (Bachmann 1998; Emmelmann

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1999). This technology is the high power laser diode and its main advantage lies in having a very narrow spectral output compared with that of the lamp. The diode output is matched to the absorption bands of the Nd: Y AG laser rod thus increasing considerably the efficiency of the laser system. Diode-pumped Nd:YAG lasers have much better beam quality because of lower induced thermal stresses, are more compact, require smaller chillers and have much longer lifetimes compared to that of the lamp pumped systems. Rofin-Sinar and Trumpf are now offering commercial 6 kW diode-pumped Nd: Y AG lasers with guaranteed 15,000 h diode operation.

3.3 Bealn DelivelY and Focusing

To guide a CO2 laser beam to the workpiece, mirrors with precise mechanical guides are used to direct the beam along lightweight, rigid, protective tubes to the optical components near the work surface. Different mirror arrangements are used for 2D and 3D motion. To focus a CO2 laser beam both reflective and transmissive optics are used. Focusing minors tend to be made from copper as it is highly reflective at the laser wavelength and can withstand high energy densities without damage. Copper mirrors are usually cooled with water to minimize thermal distortion.

Transmission lenses can be made from gallium arsenide, potassium chloride or zinc selenide. Today, the most common material is zinc selenide. Typically, reflective optics are used with powers in excess of 4kW in order to minimise losses and imaging problems associated with the thermal distortion of the zinc selenide lenses.

In the case of Nd: Y AG lasers, mirrors and lenses made from borosilicate crown glass, designated BK7, are used to guide the beam. This material is relatively cheap and has

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excellent thetmal and optical properties. The components are coated to minimize reflection losses. The Nd:YAG laser beam, in addition to being able to be directed by mirrors, can also be directed to the workpiece through an optical fibre (diameters typically 0.3 - 1 mm). The laser may be easily switched between optic fibres to share one laser between several workstations or to share between workstations concurrently. Fibres for Nd:YAG transmission are typically of step index design, meaning a high refractive index core surrounded by a cladding of low refractive index materiaL The change of refractive index ensures total internal (and highly efficient) reflection. The optic fiber allows flexible and indeed remote operation by robots with losses of 8% with no end coatings and as little as 2% occurring in the fibres with quartz windows (Rofin 2000). Fibres as long as 100 m can be used effectively (Ishide 1990). Fibres are surrounded by a metallic sheath to provide mechanical protection and typically contain continuity detection in the case of accidental bum through, whereupon the laser is automatically turned off.

3.4 Motion Systems

The focused laser beam is positioned on the workpiece either by moving the optics and so the beam itself (flying optics, optical fibres, or galvo optics) or by movement of the workpiece or a combination of both. One-dimensional systems process only in one direction and are typic all y used in the manufacture of pipe, strip or section. Minor process adjustments occur through additional linear positioning axis. Two­dimensional systems are used for the processing of flat surfaces. By moving the beam and beam guiding components less inertia needs to be overcome than by moving the workpiece. This allows high speeds and high acceleration to be achieved

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while giving good positioning accuracy and repeatability.

Figure 9. 3D C02 laser cutting system.

An example of a 2D system is shown in Figure 8 where a 2 axis hybrid system using a CO2 laser and flying optics can achieve speeds exceeding 5 Om/min and accelerations exceeding Ims·2 with an accuracy of ±0.01 mm. This approach offers great flexibility in processing large sheets and allows complex shapes with intricate detail to be cut. The main drawback of these systems is the changing focal spot position as it traverses the working area. Even though this change is slight it is sufficient to affect the spot diameter on the workpiece and hence process efficiency.

Three-dimensional systems use 5 or 6 axis gantries or robot systems. For gantry systems, positioning is performed using three linear axes (i.e. X,Y,Z); beam guidance can be by flexible arm or optical fibre. Often the optics are moved in 2 axes

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with the third performed by movement of the workpiece. The laser optics are also equipped with two rotational axes to ensure the working surface is perpendicular to the beam path. Figure 9 shows a 3D cutting system with a 5 axis CNC hybrid system using a CO2 laser with mirror beam guidance. Such systems can achieve speeds exceeding 15m/min and accelerations exceeding Ims·2 with an accuracy of ±O.Ol mm.

Figure 10. Robotic Nd:YAG laser cutting at Volvo (courtesy Rofin Sinar).

Robot systems are more effective for the processing of 3D components· (e.g. in the automotive industry) and are now becoming increasingly common because of the ease of manipulating the processing head. Robot solutions are typically less accurate than gantry systems but are considerably cheaper to implement. Figure 10 shows a robot cutting system using a 6 axis robot and Nd:Y AG laser coupled to the robot using optical fibre.

4. THE LASER CUTTING MECHANISM

The three dominant ways to cut materials commercially are (Steen 1998):

• Sublimation cutting where the focussed laser beam evaporates the material and the co-axial assist gas removes the

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vapour or the material vapour pressure itself shears and ejects molten materiaL This technique generates narrow kerfs and high quality surfaces for thin sections.

• Fusion cutting where the incident laser beam melts the workpiece and high­pressure inert gas blows the molten material out of the kerf. This method gives increased cutting rate over sublimation cutting but includes the formation of striations on the cut surface and the adherence of dross to the lower cut edge. The use of inert gas (typically at pressures between 1 and 2 MPa) allows the generation of non-oxidized cut surfaces.

• Reactive fusion cutting where a reactive gas such as oxygen is used in conjunction with the laser heating of material. The resultant exothennic reaction aids the cutting process and high cutting rates can be achieved.

Figure 11. Laser cut edge produced in mild steel in the presence of oxygen assist gas. Note the oxide layer.

4.1 Reactive Fusion Cutting of Mild Steel

Reactive fusion cutting is normally used when cutting mild steel. The interaction between molten iron and oxygen produces

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an additional source of heat due to the exothennic nature of their interaction. The exothermic reaction typically provides 40% (Ivarson 1991) of the overall heat input to the cut. This is done by the conversion of approximately 50% of Fe removed to FeO and small amounts ofFe203.

The exothennic reaction between oxygen and iron is represented by:

Fe+~02--+ FeO

MI = -257.58 kJ/mol (at2000K)

2Fe + 3/2 O2--+ Fe203

AH = -826.72 kJ/mo! (at 2000K)

The process involves melting the material in the upper section of the workpiece by the focused beam from the laser. This molten material transfers its heat to lower areas~ which in tum also melt. All the molten material is free to react with oxygen present as the assist gas creating further heat by the exotheffi1ic reactions shown above. Molten material is removed through the bottom of the cut by the shear with the assist gas. As there is continuous interaction between the energy provided by the focused beam and the workpiece during cutting, a dynamic steady state condition is established. The parameters that influence the cutting process include laser power (continuous or pulsed), focus position, surface condition of material, assist gas pressure and cutting speed. The interaction between each of these variables is not completely understood and the influence of lesser factors (eg beam mode, material temperature (Powel1 1993), nozzle clearance (O'Niell 1992») has also been shown to be significant. An example of a reactive fusion cut surface is shown in Figure 11.

The cutting mechanism is shown to be periodic in nature and can be described by a

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series of steps shown in Figure 12 (Powell 1993). The cutting mechanism involves the following steps:

(i) The laser beam and co-axial oxygen move to the edge of the steel sheet and the area of beam illumination on the plate increases and temperature rises to a point where initially ignition and subsequently melting will occur.

(ii) The melt front once established moves away from the center of the illuminated area until reaching the outer regions of illuminated area.

(iii) Having left the energetic area at the site of illumination, the melt front cools and extinguishes.

(iv) The laser then initiates ignition and melting in the next area and the process repeats itself.

butrJing extinction reinitiation striation genuation

Figure 12. The periodic nature of the reactive fusion cutting of steels.

This model clearly explains the regularly spaced striations left on the cut edges. In the top section, the cut striations are more regular, however, in the lower half of the cut random ripples are created by the flow of the molten material out of the cut zone. This random pattern increases with increasing material thickness reflecting the greater disturbance of the liquid metal through the cut zone. The final cut surface is therefore the result of predominantly thermodynamic interactions near the top of the cut and fluid dynamic interactions towards the base (powell 1986).

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4.2 The Effect of Oxygen Purity

The purity of the oxygen assist gas is a critical aspect of the reactive fusion process. The relationship between oxygen purity and cutting speed is shown in Figure 13. Studies by Ivarson (1993) show that a build up of impurity gas at the gas-molten metal interface during cutting is the main factor responsible for the observed reduction in cutting speed. It can be seen from Figure 13 that a 2% reduction in oxygen purity results in a 50% reduction in cutting speed, demonstrating the sensitivity of the process to oxygen purity.

The build up of impurity gases (pre40minantly nitrogen) at the cutting face is a consequence of the turbulent mixing of the oxygen jet with the surrounding atmosphere (O'Niell 1992). This process is called entrainment. This effect increases with increasing distance downstream of the nozzle exit. A 2D numerical model and subsequent experimental studies found that changing oxygen concentrations occur at different distances along the cut front when using various nozzle diameters as illustrated in Figure 14.

- -1 - - --I

90

Oxygen purity {%}

Figure 13. Cutting speed as a function of oxygen purity (Ivarson 1993).

100

0.11&

0.84

0.8

J

,. ,

I

-Omm • ••• 3mm

--- 6mm

-.-20mm -- IOmm ,:; .. ", h,",,,"'''''''' ._ ................. '

,

1.5 1 2.5

Nuult' dtamfttr (mm)

J

Figure 14. Theoretical nonnalised oxygen concentration at the cut front as a function of nozzle diameter \vith cut front location depth as a parameter (O'Nie1l1992).

Given that a critical oxygen purity threshold is approximately 98% (Figure 13), then a Imm nozzle is effective to a steel thickness of 6nun, 1.5mm nozzle to a thickness of 10mm and 2.5mm nozzle to a thickness of at least 20 mm. Further, the use of lower pressure assist gas such as that described in Powell (1987) presents a less turbulent or possibly laminar jet with lower consequential entrainment. As a result, the effect of entrainment of ambient gases on reducing the oxygen purity is a serious inhibitor of cut efficiency when cutting thick steel plate.

4.3 Laser Cutting of Stainless Steels

Reactive fusion cutting of stainless steels is similar to that of mild steel, however, in addition to the oxidation of Fe, oxidation of Cr in particular plays a part (Iv arson 1991,1993). The oxidation of these two elements contributes some 40% of the overall energy input to the cut zone. Approximately 30% of removed Fe in stainless steel is oxidized to Fe203 and 30% of Cr to Cr203 with negligible formation of NiO. The reactions involved are given as:

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2Fe + 3/2 O2--''''' Fe203 Llli =·826.72 kJ/mol (at 2000K)

2Cr + 3/2 O2 --..... Cr203

Llli - 1163.67 kllmol (at 2000K)

Ni + 1/2 O2 --..... NiO

Llli = • 248.23 kJ/mol (at 2000K)

The presence of Cr also hinders the cutting process. The formation of Cr203 on the surface of the melt makes further oxidation difficult because of the inability of oxygen to diffuse through it (Powell 1993). Furthermore, the increased surface tension of the Cr203 when compared to mild steel inhibits melt removal and is frequently the cause of significant dross on the lower edges of the kerf. During the resolidification of the outer surface of the kerf a chromium rich oxide layer (5-10 ~m thick) and a deeper chromium depleted layer (50-150 ~m) is fonned. This oxide layer and decreased chromium layer at a laser cut edge can lead to corrosion or wear issues.

While higher material removal rates are achieved through reactive fusion cutting due to the exothennic reactions, the fusion cutting process produces a better cut surface quality and dimensional accuracy of the kerf. In addition, the surface can be welded because of the absence of an oxide layer. In fusion cutting of stainless steels, high pressure nitrogen is used to eject the molten material through the kerf. The nitrogen pressures usually range from 1 to 2 MPa. The reduction in cut energy (due to the absence of reactive fusion) and increased heat losses due to forced convection of the high pressure nitrogen gas yield cutting speeds 25% lower than those of reactive fusion under similar cutting conditions. Illustrated in Figure 15 is the cut surface of

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stainless steel fusion cut with high pressure nitrogen.

Figure 15. Laser cut surface produced in stainless steel in the presence of nitrogen assist gas.

5. THE CUTTING OF TIDCK STEEL PLATE

The laser cutting of steel sheet and steel plate up to 15mm in thickness is now a ,veIl established machining process (Heidenreich 1996; O'Niell 1995). The ability to CNC a laser, its cutting accuracy, clean cut edges and low heat input mean that if thicker steel plate could be reliably cut, the laser would become a serious competitor to plasma and oxy-fuel cutting technologies. This is evidenced in industries such as ship building where a laser was used to cut precisely machined ship steel up to 16mm thick to DIN 2310 Grade II quality (Heidenreich 1996). The successful use of laser cutting allowed considerable savings in time, as less post processing of the parts was required. With careful control of system parameters it is possible to cut mild steels to 40 mm but with a significant decline in cut quality and reproducibility (O'Nie1l1992).

When cutting thick steels with a fixed laser power it has been observed that, in order to achieve a given quality of surface finish, the width of the kerf remains approximately

Harris and Brandt

constant, however, the cutting speed does not reduce proportionately to the thickness (Powell 1987). Figure 16 shows the cutting speed as a function of material thickness for two different qualities of cut (maximum cutting speed is often achieved at the expense of cut quality). It can be observed from the graph that a reduction in cut efficiency occurs with increasing material thickness.

80

0' 70

E 60 .§. -g 50 Gl

g. 40 en E 30 ::s o 20

~' .\\

-.- Highest speed -.- Optimal speed

.~ .. ~\~ •

10 . : ~, ... . . ;~.

o~~~~~~~~~~~~~~

o 2 4 6 8 10 12 14 16 18 20 22 24

Cut thickness (mm)

Figure 16. Influence of material thickness on cutting speed using a 2.5 kW CO2 laser (Po\vell 1987).

This reduction in efficiency as the material becomes thicker is attributed to a reduction in the effectiveness of the assist gas as a melt remover and the loss of heat at the bottom of the cut due to conduction. Further, as the material thickness increases the gas stream travelling downwards experiences greater "fanning out" as a result of the greater reflection off the inclined cutting front thus progressively reducing the melt shearing capability of the assist gas (Powell 1987).

When cutting thick steels by fusion, the assist gas pressure must be increased to allow the ejection of molten materiaL A typical reported gas pressure (Beyer 1990) for fusion cutting of 15mm thick stainless steels with a 5 kW CO2 laser source is 2

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MPa of N2 . When using oxygen assist gas however, increasing the oxygen pressure with increasing material thickness results in too much oxygen being present within the kerf, which leads to excessive burning of the steel due to the exothennic nature of the reaction. Consequently, to counter this effect, oxygen pressure is reduced with increasing material thickness. For example} 8mm mild steel can be cut at a speed of 1800 mmlmin using an oxygen pressure of 0.2 MPa but when cutting 20 mm thick plate at 1000 mmlrnin the oxygen pressure is reduced to 0.08 MPa (Powell 1987). However, the lower the oxygen pressure, the more difficult it becomes to shear the molten material from the kerf thus limiting the cut thickness.

Close control of oxygen pressure is essential to prevent uncontrollable burning of the steel away from the heated area. Studies (Gabzdyl 1992) indicate that when cutting mild steel an increase of only 5 kPa in oxygen pressure from the optimal pressure can result in excessive bUlning and excess dross formation at the base of the cut. Similarly, work by Heidenreich (1996) using PrandtI boundary flow theory on thick ship steel plate demonstrated that by careful selection of oxygen assist gas pressure excessive burning could be controlled. Here a kerf width of more than 0.5 mm was produced and oxygen pressure of 0.12 MPa was used when cutting 12mrn thick steel. However, solutions for satisfactory cutting conditions using parameters of power, cutting velocity and oxygen assist gas pressure show that these three parameters must have certain values and cannot be varied independently (e.g. cut speed cannot be altered without altering beam power and oxygen pressure). Further to this, Heidenreich (1996) also shows that changes in oxygen pressure of less than 0.02 MPa are sufficient to change the cutting process from metal burning to dross adherence.

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In order to increase the thickness of the material that can be cut, one approach is to increase laser power and hence use the power of the laser to overcome control difficulties of the reactive fusion process. While there are a number of advantages to this approach there are also significant challenges. At higher laser powers (3.5 kW and higher) the beam quality beCOlnes poorer, lifetime of optical components is reduced due to thennal loading, equipment and running costs are high and cutting precision deteriorates. In view of these disadvantages, a number of alternative approaches are being investigated for cutting thick (> 10mm) steel plate.

6. INNOVATIVE PROCESSES TO CUT THICK STEEL PLATE

The following describes several of the innovative methods to further increase the depth of laser cut while still maintaining acceptable cut quality. Many of the processes described use novel laser techniques to overcome the limitations of thick steel cutting already discussed.

6.1 Laser Flame Cutting

Thennal flame cutting is an effective cutting process capable of cutting very thick steels (Heidem'eich 1993). Its main drawback, however, is that it is a slow process ("'" 400 mmlmin for 19 mm thick mild steel (Powell 1993). Lasers on the other hand cut at high speed depending on the material thickness. By combining the two techniques the advantages of each can be exploited.

In the augmented laser oxy-acetylene technique the material is heated by an oxy­acetylene flame that is inclined at an angle of 85° to the metal sheet with the laser beam heating a small workpiece area in front of the oxy-acetylene cutting nozzle. The laser heating creates a larger molten zone, which

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in tum leads to less disturbance of the gas stream penetrating the cut. Material removal occurs as a result of melt shear by the gas stream from the oxy-acetylene cutter. An additional gas nozzle is also used and directed into the kerf to further assist in the removal of molten metal and prevention of dross adherence or sealing of the kerf at high cutting speeds. The resultant quality of cut is DIN 2310 grade II with a kerf width of 2.5mm and a HAZ comparable to that of an oxy-acetylene cut. The cutting speed is greater than that for conventional flame or laser cutting alone. Experiments conducted on 20 rom and 30 mm thick mild steel plates demonstrated that they could be cut at 1200 mmlmin and 900 mm/min respectively using a 2 kW CO2 laser, a 150 mm focal length lens and the focus positioned 2.5 rom ahead of the cutting front (Heidenreich 1993).

Though successfully used to cut thick mild steel, the method has issues of the directionality of laser/nozzle assembly that may be overcome by CNC control of an additional axis. The process also involves high capital outlay, high gas consumption and lacks a niche application.

6.2 Cutting with Dual Focus Lenses

The most effective energy source shape for cutting is a line source (O'Neill 1999), because it can distribute the energy uniformly throughout the depth of the cut. During reactive fusion cutting of steel, the exothennic interaction of the melt with the narrow oxygen jet as it travels down the kerf does assist in the creation of an ideal line energy source. For mild steels greater than 10 rom in thickness the focal point is typically positioned at the top surface of the workpiece to allow a significant intensity of light on this surface to initiate the cutting process and to achieve process stability with acceptable cut quality. For fusion cutting of

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stainless steels, however, this focus is typically positioned well into the kerf. This allows maximum energy intensity to occur at the central zone of the cut where the molten material must accumulate enough heat energy to conductively melt material below it (powell 2000).

With increase in material thickness it becomes increasingly difficulty to cut as it is not possible to maintain adequate energy intensity at all positions through the depth of the kerf. To help counter this problem, higher laser powers and longer focal length lenses are used. This gives a longer depth of focus, however, this means an increase in spot size and commensurate decrease in power density within the spot. Further, higher powers degrade beam quality, leading to beams not suitable for cutting (Nielsen 1997).

A new lens developed for CO2 lasers has t\vo focal spots, one situated above the other. This produces focused energy deep within the cut but still allows high energy density at the top of the cut in order to sustain the cutting process (Powell 2000). Previously, these types of optics were based on multi-beam principles (i.e. combination of two laser beams). The new method uses an inner and outer annulus within a single lens, each with a different focal length. The inner annulus being flatter compared to the outer annulus has a longer focal length to focus the beam deeper \vithin the cut as shown in Figure 17. The area of lens dedicated to each focus governs the proportion of power distribution of the laser beam. The first focussed beam is maintained at the upper surface of the metal to create suitable ignition conditions while the second focused beam, near the lower surface, creates a lower viscosity melt resulting in a cleaner kerf throughout the thickness of the material.

104

dual focus lens

mner

outer

upper focus

lower focus

Figure 17. Schematic of dual focus meniscus lens. For illustration purposes the laser beam is divided into an inner section (clear) and an outer section (shaded).

The dual focus method shows improved cutting of thicker material (30 mm 40 mm compared to 15mm - 20mm carbon steel for a 3 kW CO2 laser) improved piercing with little surface eruption (Neilsen 1997; O'Neill 1999), improved quality of cuts (dross free edges) and improved cutting speeds. When cutting stainless steel of 12 mm thickness dual focus lenses produce a 30% narrower kerf with more perpendicular edges and less dross while cutting 23 % faster than single focus lenses (Powell 2000). The use of dual focus lenses demonstrates a more efficient method of cutting thick steels with more energy entering the cut, producing higher cut temperatures and consequently giving lower viscosity melts. This results in the removal of less material and reduction in dross and dross attachment. The disadvantage of the technique is the high cost of dual focus lenses. Also, they are designed for a particular material thickness making them less flexible when compared to conventional lenses.

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6.3 CO2 Beam Sawing with Adaptive Optics

This technique involves the oscillation of the focus position, by several millimetres, in the same direction as the material thickness as a result of one of the reflective mirrors of a CO2 laser being plastically deformed by a piezo-electric actuator (Geiger 1996). An external voltage controls the defonnation frequency from 0 Hz to the experimental limit of 100 Hz. This method has been shown to cut mild steel up to 16 mm

thickness without a decrease in cut quaHty using a 2.2 kW CO2 laser.

Using a focal length of 160 mm the initial focus position was Imm above the surface of the workpiece. The maximum focus travel distance was 4.5 mm after which the beam intensity at the top of the cut was reduced sufficiently to allow fonnation of excessive dross. Results show that striations in the top section of the cut reflect the frequency of the oscillating beam as is shown in Figure 18.

h! -meosur 119

.) mOl rJi~l()if

the tou sid~:

5 mrn

OXj'gen ossisled loser beam sawing 25H1 leser beam sowing tomb Il)~er beam Gutting

Figure 18. Cut surfaces at various frequencies (Geiger 1996).

laser beam cuU!n

~ ~ 1.0 mm ~~~115mm

Rz -measuring 3mm below the top side

cutting gas: 01 pressure: 1.1*105 Po focol shift: 4-.5 mm velocity: 0.7 m/min

Figure 19. Cut surface profile showing reduced amplitude of striations with increasing sawing frequency (Geiger 1996).

105

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Cl'ntral ox,gcn input

Central nOlzle

{h~'fD burrrr th)gtfl buffer in,.ot ltll.'U

Ring nOlzle (llatr

(a)

c 1.00 ~ g

0.96 c G) (.) c 0 0.92 (.)

c G) C') 0.88 ~ 0 "0 0.84 G)

.!:'l 'iii § 0.80 0 z

0

(b)

'" Burrer jet

Mainjct

--Buffer - theoreIicaI -.- Buffer - experirnerla

J. No buffer - theoretk::aI -e- No buffer - experirnerla

5 10 15 20 25

rxstance dO'Nn cut front (rrni

Figure 20. Illustrated in (a) is layout of buffer nozzle and (b) theoretical and experimental oxygen concentrations down the cut face for a 1.5 rom diameter nozzle (O'Neill 1995).

Significantly, the roughness of the cut surfaces is shown to be improved over those of conventional cutting at frequencies exceeding 25 Hz, with maximum improvement occurring at 100 Hz as can be seen in Figures 18 and 19. At the maximum frequency of 100 Hz, upper striations reflect the oscillation of the focal position but lower area striations are more random reflecting the liquid flow from the kerf.

106

Rectangularity of the cut edges is also affected with improvement reported in samples cut at frequencies greater than 75 Hz. No mention is made by Geiger (1996) of dross removal issues except a comment that another gas nozzle system is required to optimize the dross problem.

6.4 Laser Cutting using a Co-axial Nozzle

The flow of assist gas through the laser nozzle comprises an inner stream in which the pressure and velocity remains constant, and an outer boundary where momentum} material and heat diffusion take place. Narrow nozzles are shown to induce more mass transfer with the surrounding environment within a small distance of the nozzle exit (O'Niell 1992, 1995). Transfer is more pronounced with turbulent flow, jets typically becoming turbulent several nozzle diameters downstream. As the nozzle diameter is increased the effects of the momentum transfer are shifted further into the materiaL Typically, turbulence cannot be avoided unless low Reynolds numbers are used. The most important issue for reactive fusion laser cutting is the reduction of oxygen concentration through the entrainment of surrounding environmental gases such as nitrogen. As discussed in Section 4.2, small losses in oxygen concentration have been found to significantly reduce cutting speed.

To combat the effects of entrainment of ambient gases 0 'Niell (1992, 1995) suggested the use of a co-axial nozzle. A co­axial nozzle illustrated in Figure 20 comprises two concentric nozzles. The larger diameter, inner, nozzle can be used to maintain oxygen concentration further into the cut. As previously discussed, larger diameter nozzles ensure larger flows of oxygen resulting in a reduction and shifting of entrainment to larger distances from the nozzle. The second, sacrificial, "entrainment

Harris and Brandt

buffer " is created using an outer annulus to allow a turbulent mixing zone. This allows the primary cutting jet to experience less mixing with the atmosphere (i.e. the entrainment process still occurs but in the outer jet). A combined theoretical and experimental oxygen concentration with and without buffer is shown in Figure 20(b).

Results of cutting 10 mm, 16 mm and 20 lnln grade 43A mild steel plate with and without the buffer jet demonstrate significant improvement in cut quality for a given cut speed when the buffer jet is used. There is little change in cut quality for the 10 nun plate because at a depth of 10 nun there is only a 1 % reduction in oxygen concentration for a single nozzle. There is, however, a significant improvement in cut quality for the 16 and 20 mm thick plate using a central pressure of 0.05 MPa and an outer flow greater than 30 Vmin.

Processing with the co-axial nozzle has allowed the cutting of thick steel plate producing clean cut edges and effective removal of dross as a result of increased exothermic energy at the base of the cut.

.----­Cutting depth, Oct-

The device shows promise for commercial exploitation because of the ease of integration with standard cutting head designs. The main drawback of the device is the increase in oxygen consumption rate.

6.5 Dual-bealn CO2 Laser Cutting of Thick Metallic Materials

When focusing laser light onto a workpiece there may be coupling problems if plasma is formed at the surface. By using two laser beams separated by a small distance there can be improvement in coupling efficiency due to minimization of the laser plasma interactions. There is also an advantage due to the increased absorption associated with the molten surface (Molian 1993). The dual beam approach was adopted by Molian and used in laser cutting experiments. The process involves beams from two CO2

lasers of po\ver 1.5 kW each delivered through a single beam duct. Both focused beams were 0.1 mm in diameter, with foci positioned 6 mm apart and set 1/3 of the material thickness below the top surface. Nozzle diameter was 10 mm with oxygen pressure set at 137 kPa (Malian 1993).

G\.\on \lOt;>°f\l

C uttinq direction, speed V

Erosion front thi(kne~S, De

t Dri I ling depth,

Molten melt/slog

Dd~_""""""-'--L---.t'!'!!'

Figure 21. Schematic of dual-beam laser cutting (Malian 1993).

107

Harris and Brandt

The first beam partially penetrates the moving workpiece and fonns a blind cylindrical keyhole. The second laser impinges on the molten region created by the first beam and further heats it, vaporizing some material and superheating the remainder. The oxygen assist gas aids the cutting process with its exothermic reaction and in the ejection of molten droplets. A schematic of the technique is given in Figure 21.

40 • 35

~ 30

S 25 'tI III

20 III Co 1/1 m 15 c E

10 :;:, 0

5

0 6 8 10 12 14 16 18 20

Thickness (mm)

Figure 22. Effect of single (1.\) and dual (0) 1.5kW C02 laser beams on cutting speed and material thickness. Focal length of lenses was 127 mm and oxygen assist gas pressure was 413 kPa.

The workpiece preheating improves the 'deep penetration effect' mainly due to the improvement in absorption and modification of conduction characteristics. This method was tested by cutting mild steel (ATST 1018) and superalloy (Hastalloy C) plates. For a fixed laser power, improvement in cutting speed and cut depth was observed for the dual beam configuration when compared to single beam as shown in Figure 22.

A mathematical heat flow model for the dual beam cutting process was developed by Molian and shown to fit well with experimental data. It was experimentally

108

and analytically demonstrated that the maximum cutting speed increases as beams are separated in the direction of the cutting velocity, the optimum depth could be gained by combining the two beams into one single, more powerful beam. Technical difficulties were reported however in achieving a stable beam coincidence within one spot diameter (0.1 mm). Evidence of cut quality was not presented, the paper instead being significant in the development of and verification of heat flow models through experimentation.

Again, this approach demonstrates an ability to increase the thickness of steel plate that can be cut but its major disadvantage is that it has directional dependence thus making it difficult to use for profile cutting.

Figure 23. Schematic of Lasox cutting process (O'NieU1998).

6.6 Laser Assisted Oxygen Cutting - Lasox

As discussed in Section 4.1, during the conventional reactive fusion cutting of mild steels, approximately 40% of energy is provided by the exothennic reaction and in excess of 97% of combustion products is FeD (the remainder being Fe2D3 and Fe2D4 (Ivarson 1993). Assuming that FeD is the only reaction product produced and with AH

257.58 kJ/mol then if complete

Harris and Brandt

cOlnbustion of the iron takes place, the power generated in such a process is given in Table 2. Hence, large power is available to the cutting process. However, to generate this power there must be sufficient oxygen delivered to the kerf. It is well documented that during the cutting of thick steel sections, side burning becomes an issue. To minimize this effect oxygen pressure 1S

reduced to less that 100 kPa.

In the Lasox process (see Figure 23) a laser beam is employed to assist the oxygen cutting process. The laser is used to preheat the material while the cutting is performed

and controlled by the oxygen assist gas delivered co-axially with the beam.

The Lasox method employs a nozzle assembly and a short focal length lens where the focal length is within the nozzle/lens housing. This results in a highly divergent laser beam exiting the nozzle with a co-axial oxygen gas stream. The nozzle diameter is such that the laser interaction area is greater than the gas jet interaction area but the laser interaction still generates temperatures well above the critical 900°C (O'Niell 1998) required for initiation of the exothermic reaction.

Table 2. Calculated exothermic power generated in thick section laser cutting (O'Niell 1998).

Cut Depth Kerf Width Cutting Speed Exothermic Power

(rom) (mm) (mmfmin) (kW)

20 1.5 700 12.5

30 2 500 18.5

40 2.5 400 23.6

50 3 300 21.52

Table 3 .. Plate thickness and corresponding cutting speeds (O'Niell1998).

Plate Thickness (nun) Optimum Speed Speed Range (mmfmin) (mmlmin)

20 300 200 500

30 230 200 350

50 175 150 - 220

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The laser/oxygen jet interaction ensures that a reaction can be initiated across the whole of the oxygen jet, maximizing reaction yield and ensuring that the Lasox condition is met. Heating of the workpiece occurs at the surface and the ensuing exothermic reaction occurs through the depth of the cut. If the above laser/gas condition is not met, a poor quality cut is generated as a result of uncontrolled metal burning. It is essential to realize that the laser is depositing very little energy into the depth of the cut and that oxygen is the facilitator of melt production. Results of cutting trials using a 2 kW CO2 laser and 20 mm, 30 mm and 50 nun mild steel plate (O'Niell 1998) sho\v that the cutting action is driven almost exclusively by the oxygen jet reaction and that the laser contributes little to the overall energy of the process. lliustrated in Table 3 are the cutting speeds obtained.

It was observed that as the cut speed increases above optimum) dross levels increase dramatically. Under optimum cutting conditions the cut taper was shown to be less than 1 degree with the top edge square as compared to conventional oxy­flame cutting which produces a round edge. The HAZ was of similar thickness to flame cutting in the body of the workpiece but without any increase near the top surface as produced by flame impingement in flame cutting. Widening of the HAZ occurs at the base of the cut if optimum conditions are not sustained due to a reduction of melt flow velocity. Also, piercing is found to be possible along with trepanning though some amount of taper is expected. The cutting of profiles was shown to be feasible as illustrated in Figure 24.

Consequently~ Lasox is a relatively simple process that shows promise for cutting of thick section steel plate using a controlled ignition process and only moderate laser power. It may also be feasible to retrofit

110

this system to existing lasers making its implementation attractive.

Figure 24. Lasox profile cuts in 40 mm thick EN9 steel (O'Neill 1998).

6.7 Thick Plate Cutting with a Spinning Laser Beam

Another approach that has shown potential for cutting thick section steels involves spinning the laser beam while performing a conventional laser reactive fusion cut. The feasibility of this approach was demonstrated by Arai (1997) who used a 1.8 kW CO2 laser to cut mild steel and stainless steel plate up to 25 mm thick while maintaining high cut quality. It was also demonstrated that there was minimal HAZ and distortion. To produce the desired effect, a window is placed in the beam path at an angle to the beam axis as can be seen in Figure 25. This produces a lateral shift of focal position. Rotating the window causes the laser beam to form a spiral path as it is moved across the workpiece.

While Arai demonstrated that relatively lo\v power lasers can be used to cut thick steel plate with acceptable quality, little infotmation was presented as to the mechanisms responsible for the increase in thickness. In order to understand the

Harris and Brandt

processes occuning during spinning beam laser cutting, a project was initiated to determine the key process controlling factors and determine the maximum material thickness that can be cut with a fixed laser power. The project is part of a larger project involving the cutting of thick steel plate with laser beams supported by the Co-operative Research Center for Intelligent Manufacturing Systems and Technologies (CRCIMST).

Lens

Figure 25. Spinning beam apparatus (Arai 1997).

A spinning beam apparatus \vas designed and constructed as shown in Figure 26. This was attached to a fibre-optic delivered 2.5 kW Nd:YAG laser. This allowed a maximum offset ( eccentricity) of beam of 450 Jlm at a maximum spin speed of 3000 RPM. In preliminary trials, the apparatus demonstrated the spiral cut path as shown in Figure 27. Here a 200W Nd:YAG beam was used to mark the surface of a mild steel plate from left to right in (a) conventional mode and (b) spinning beam mode.

In cutting trials involving the device and mild steel plate thicknesses of 10 mm, 12 mm) 15 mm and 20 mm, it was demonstrated that the spinning beam results

111

in an increase in maximum cut depth from 12 to 15 mm when compared to conventional cutting. This is illustrated in Figure 28 where a comparison between conventional cutting and spinning beam cutting of the various plate thicknesses is shown. Attempts at cutting the 20 mm thick plate were pal1ially successful. While the cut depth was increased, excessive dross accumulated at the base of the kerf and there was excessive burning of the sides. From the figure it can be observed that a continuity of the conventional cutting characteristics extends into the spinning beam area. That is, as cutting progresses into those thicknesses cut by the spinning beam, the one characteristic curve can describe both processes for a given laser power.

Figure 26. Spinning beam apparatus cutting 12 mm thick mild steel plate.

The figure also demonstrates that although there is improvement in cut depth with the spinning beam, there is only modest

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400um

L r

(a)

920um

t

(b)

Figure 27. Beam marking at 200W representing (a) a conventional cut and (b) spinning beam cut at 440 RPM} 220 ~m offset and 150 mmlmin cutting speed.

improvement in cut speed. When cutting the 12 mm mild steel plate at 1488 watts incident on the workpiece and a cutting speed of 700 mmlmin an average kerf width of 1.2 mm was produced for conventional cutting and 2.4 rom for a 450 Jlm offset beam at 2000 RPM. Hence, during the cutting process similar process limitations to cut speed are being imposed even though cutting with the spinning beam produces a far larger kerf resulting in a larger volume of material being removed and a larger amount of oxygen entering the cut.

Current research into the possible causes of limitations to cut speed for any given power indicate that the two main contenders are (i) limitations to the amount of energy available per mole of material removed, and (ii) amount of oxygen available per mole of material removed. Distinct thresholds appear to exist for both parameters for successful cuts to be perfonned.

The spinning of the beam also affects the shape of the kerf as seen in Figure 29. The spinning beam kerf is non-symmetric and results from the difference between the rotational velocity of the beam and the translational velocity of the workpiece. Consequently, one side of the kerf

112

experiences an increase in effective cut speed and the other a corresponding decrease.

2400

2200 2000

C 1800 :§ E 1600 .§. 1400 " ~ 1200 ~ Q.

(J) 1000 11 0 800

600

400

-_.-

5 6

-·-1339W .. -.t.-1637W

-y-1769W -'-1339W SpIn -+-1637W Spin -l(- 1769 W Spin

7 8 9 10 11 12 13 14 15 16 17 18 19 20

Plate Thickness (mm)

Figure 28. Dependence of cut speed on plate thickness for different incident Jaser powers.

Illustrated in Figure 30 are cut surface profiles obtained using a spin speed of 2000 RPM and conventional cutting conditions (no spin). It is noted that the spinning beam uses larger nozzle diameters consequently when the spinning beam is stationary it does not imitate the conventional cut. Furthermore, the offset of the beam when stationary approximates a poor nozzlelbeam alignment which is a crucial cut quality parameter.

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2000 RPM

Stationary (no spin)

Figure 30. Changes in cut surface characteristic using spin speed of 2000 RPM and stationary beam for 12mm thick mild steel plate. Laser power 2000W; cut speed 600 mm/min and oxygen assist pressure 40kPa.

From the research conducted to date, the spinning beam cutting approach is showing itself to be significant in increasing the depth of cut that can be achieved, but not significant in improving the cut speed. In an

113

Figure 29. Kerf shape for (a) spinning and (b) conventional laser cut in 12mm mild steel plate.

age of ever increasing energy consciousness this method of cutting thick steel plate may prove an advantage in the next generation of more efficient laser cutting machines. Further work is being carried out to understand the mechanisms responsible for the observed increase in cut thickness.

7. SUMMARY

Today, the laser cutting of 2D and 3D steel components is a well established and flexible non-contact machining process producing narrow kerfs and minimal heat affected zones. Currently, commercial lasers cut steel plate up to 15mm thick with relative ease. There is, however, industry interest to increase this thickness which would allow lasers to be a significant competitor to plasma and oxy-fuel technology in thick plate cutting.

The cutting of steel plate using lasers is achieved by the continuous interaction of the energy supplied by the focussed beam, the feeding of the workpiece and assist gas. Using the reactive fusion technique oxygen provides momentum transfer for shearing of the melt as well as for the exothermic reaction with the molten steel. There are

Harris and Brandt

limitations inherent with the reactive fusion cutting of thick steel plate. These limitations are primarily attributed to a reduction in shear forces produced by the assist gas as the cut depth increases and a need to reduce oxygen assist gas pressure to control burning. These two complementary factors appear to limit the thickness of material that may be cut. To overcome these factors a number of innovative cutting techniques described above are being investigated. The spinning beam method shows potential for cutting thick steel plate, demonstrating significant increase in cutting depth when compared to conventional cutting at the same laser power. Further work is being undertaken to fully understand the mechanisms responsible for this increase in cut thickness.

ACKNOWLEDGEMENTS

The authors wish to thank CRCIMST for funding the spinning beam project. The CRCIMST was established under and is supported in part by the Australian Government's Cooperative Research Centres Scheme.

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