(cid:36)(cid:54)(cid:48)(cid:47)(cid:3)(cid:20)(cid:20)(cid:20)(cid:25)6
`
`LMSA
`
`

`
`Handbook of Laser Technology and Applications
`
`Volume III: Applications
`
`Edited by
`
`Colin E Webb
`
`University of Oxford
`
`and
`
`Julian D C Jones
`
`Heri0r—Watt University
`
`I0P
`
`Institute of Physics Publishing
`Bristol and Philadelphia
`
`

`
`© IOP Publishing Ltd 2004
`
`All rights reserved. No part ofthis publication may be reproduced, stored in a retrieval system or transmitted
`in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior
`permission of the publisher. Multiple copying is permitted in accordance with the terms of licences issued
`by the Copyright Licensing Agency under the terms of its agreement with Universities UK (UUK).
`
`The publisher has attempted to trace the copyright holders of all the figures reproduced in this publication
`and apologizes to them if permission to publish in this form has not been obtained.
`British Library Cataloguing—in—Publicati0n Data
`
`A catalogue record for this book is available from the British Library.
`
`ISBN 0 7503 0960 1 (Vol. I)
`
`0 7503 0963 6 (Vol. II)
`
`0 7503 0966 0 (Vol. III)
`
`0 7503 0607 6 (3 Vol. set)
`
`Library of Congress Cat‘ai0ging-in—Publicari0n Data are available
`
`
`
`Development Editor: David Morris
`Production Editor: Simon Laurenson
`
`Production Control: Sarah Plenty
`Cover Design: Victoria Le Billon
`
`Marketing: Nicola Newey and Verity Cooke
`
`Published by Institute of Physics Publishing, wholly owned by The Institute of Physics, London
`
`Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS1 61313, UK
`
`US Office: Institute of Physics Publishing, The Public Ledger Building, Suite 929, 150 South Independence
`Mall West, Philadelphia, PA 19106, USA
`
`Typeset in DTEX 25 by Text 2 Text Limited, Torquay, Devon
`Index by Indexing Specialists (UK) Ltd, Hove, East Sussex
`Printed in the UK by MPG Books Ltd, Bodmin, Cornwall
`
`

`
`Contents
`
`VOLUME III: APPLICATIONS
`
`PART D
`
`APPLICATIONS: CASE STUDIES
`
`D1
`
`D1.1
`
`D12
`
`D13
`
`D1.4
`
`D1.5
`
`D1.6
`
`Dl.7
`
`D1.8
`
`D2
`
`D2.1
`
`D22
`
`D2.3
`
`D24
`
`D25
`
`D2.6
`
`D2.7
`
`D2.8
`
`D3
`
`D3.1
`
`D32
`
`Materials processing
`Clive Ireland
`
`Welding
`HHiigel and C Schinzel
`Cutting
`John Powell and Claes Magnusson
`
`Laser marking
`Terry J McKee
`
`Drilling
`S Williams
`
`Photolithography
`Shinji Okazaki
`
`Laser micromachining
`Malcolm Gower
`
`Rapid manufacturing
`Gary K Lewis
`
`Pulsed laser deposition of thin films
`Ian Boyd and D B Chrisey
`
`Optical measurement techniques
`Julian Jones
`
`Fundamental length metrology
`
`J Fliigge, F Riehle and H Kunzmann
`
`Laser velocimetry
`C Tropea
`Laser vibrometers
`
`Neil A Halliwell
`
`Electronic speckle pattern interferometry (ESPI)
`
`Dave Towers and Clive Buckberry
`
`Optical fibre hydrophones
`
`Geoffrey A Cranch and Philip J Nash
`
`Optical fibre Bragg grating sensors for strain measurement
`David A Jackson and David J Webb
`
`High—speed imaging _
`Adam Whybrew
`
`Particle sizing
`
`Nils Damaschke, Maurice Wedd, Adam Whybrew and Damien Blondel
`Medical
`
`Terence A King and Brian C Wilson
`Light—tissue interactions
`Steven Jacques and Michael Patterson
`Therapeutic applications: introduction
`Reginald Birngruber
`
`D3.2.1
`
`Therapeutic applications: ophthalmology
`Reginala' Birngruber
`
`ix
`
`1557
`
`1559
`
`1561
`
`1587
`
`1613
`
`1633
`
`1653
`
`1661
`
`1693
`
`1705
`
`1721
`
`1723
`
`1749
`
`1779
`
`1805
`
`1839
`
`1881
`
`1919
`
`1931
`
`1951
`
`1955
`
`1995
`
`£999
`
`

`
`D3.2.2 Therapeutic applications: refractive surgery
`Giovanni Cennamo and Raimondo Forte
`
`D3.2.3 Therapeutic applications: photodynamic therapy
`Brian C Wilson and Stephen G Bown
`D3.2.4 Therapeutic applications: thermal treatment of tumours
`Stephen G Bown
`D3.2.5 Therapeutic applications: dermatology—selective photothermolysis
`Sean Lanigan
`
`D3.2.6 Therapeutic applications: lasers in vascular surgery
`Mahesh Pai
`
`D3.2.7 Therapeutic applications: hardtissue/dentistry
`Rairnund Hibst
`
`D3.2.8 Therapeutic applications: free—electron laser
`E Duco Jansen, Michael Copeland, Glenn S Edwards, William Gabella,
`Karen Joos, Mark A Mackanos, Jin H Shen and Stephen R Uhlhorn
`D3.3 Medical diagnostics
`Brian C Wilson
`
`D3.4
`
`Laser applications in biology and biotechnology
`
`D3.5
`
`D4
`
`D4.1
`
`D42
`
`D4.3
`
`D4.4
`
`D4.5
`
`D4.6
`
`D5
`
`D5.l
`
`Sebastian Wachsmann—Hogiu, Alexander J Annala and Daniel L Farkas
`Biomedical laser safety
`Harry Moseley and Bill Davies
`Communications
`
`John Marsh
`
`The basic point—to—point communications system
`John Gowar
`
`High—capacity optical transmission systems
`Paul Urquhart
`Local area networks
`
`J Lehman and K L Johnson
`
`Fibre—to—the—chip: development of vertical cavity surface emitting
`laser arrays designed for integration with VLSI circuits
`A V Krishnamoorrhy, L M F Chirovsky, K W Goosen, J Lopata
`and WS Hobson
`
`Optical satellite communications
`A C0ello- Vera and M Maignan
`Smart pixel technologies and optical interconnects
`Marc P YDesmulliez and Brian S Wherretr
`
`Optical information storage
`John Marsh
`
`Optical data storage
`Tom D Milsrer
`
`D5.2
`
`Lasers in printing
`
`Atsushi Kawamura, Seizo Suzuki and Yashinori Hayashi
`
`D6
`
`D6.l
`
`Spectroscopy
`Colin Webb
`
`Laser cooling and trapping
`C SAa’ams and I G Hughes
`
`Contents
`
`2009
`
`2019
`
`2037
`
`2045
`
`2055
`
`2065
`
`2075
`
`2087
`
`2123
`
`2155
`
`2181
`
`2183
`
`2231
`
`2289
`
`2321
`
`2345
`
`2363
`
`2389
`
`2391
`
`2421
`
`2463
`
`2465
`
`

`
`Contents
`
`D62
`
`D63
`
`D7
`
`D7.l
`
`D"/‘.2
`
`D8
`
`D8.l
`
`D9
`
`D9.l
`
`D10
`
`Dl0.l
`
`Dl0.2
`
`IontrapphngandlaserapphcafionstolengU1andtnneIneuIflogy
`P Gill and G P Barwood
`
`Tirne—resoIved spectroscopy
`Gavin D Reid and Klaas Wynne
`Earth and environmental sciences
`
`Lance Thomas
`
`Satelhtelaserranging
`Roger Wood and Graham Appleby
`
`Ijdarforannospherk:ozonerennnesenang
`Gérard Ancellet
`
`Lasers in astronomy
`R C Powell
`
`Lasers in astronomy
`Renaud Foy and Jean—Paul Pique
`Holography: holographic optical elements and computengcneraied
`holography
`
`Mohammad R Taghizadeh
`Holography: holographic optical elements—computer—generated
`holography—diffractive optics
`Hans Peter Herzig
`High-intensity lasers for plasma studies
`Ckdbxlflebb
`
`High-power lasers for plasma physics
`M H R Hutchinson
`
`High—power lasers and the extreme conditions that they can produce
`SJROM
`
`Index
`
`xi
`
`2485
`
`2507
`
`2529
`
`2531
`
`2563
`
`2579
`
`2581
`
`2625
`
`2627
`
`2643
`
`2645
`
`2657
`
`2665
`
`

`
`Cutting
`
`D1.2
`
`Cutting
`
`1587
`
`John Powell and Claes Magnusson
`
`D1.2.1
`
`Introduction
`
`Most laser cutting is carried out using CO2 or Nd:YAG lasers. The general principles of cutting are similar for
`
`both types of laser although CO2 lasers (see chapter B3.l) dominate the market. For this reason the following
`sections will concentrate on CO2 taser cutting and compare this with Nd:YAG laser cutting in section D1.2.5.
`(however, readers interested solely in Nd:YAG laser cutting need to read the following sections first). Most
`
`of the following information is paraphrased from two books: C02 Laser Cutting by J Powell [1] and Laser
`
`Institute ofAmerica Guide to Laser Cutting by J Powell [2].
`The basic mechanism of laser cutting is extremely simple and can be summarized as follows:
`
`(1)
`
`(2)
`
`(3)
`
`(4)
`
`(5)
`
`A high intensity beam of infrared light is generated by a laser.
`
`This beam is focused onto the surface of the workpiece by means of a lens.
`
`The focused beam heats the material and establishes a very localized melt (generally smaller than 0.5 mm
`
`diameter) throughout the depth of the sheet.
`
`The molten material is ejected from the area by a pressurized gas jet acting coaxially with the laser beam
`as shown in figure D1.2.1. (NB: With certain materials this gas jet can accelerate the cutting process by
`doing chemical as well as physical work. For example, carbon or mild steels are generally cut in a jet of
`pure oxygen. The oxidation process initiated by the laser heating generates its own heat and this greatly
`adds to the efficiency of the process.)
`
`This localized area of material removal is moved across the surface of the sheet thus generating a cut.
`Movement is achieved by manipulation of the focused laser spot (by CNC mirrors) or by mechanically
`moving the sheet on a CNC X-1’ table. ‘Hybrid’ systems are also available where the material is moved
`in one axis and the laser spot moved in the other. Fully robotic systems are available for profiling three-
`dimensional shapes. Nd:YAG lasers can utilize optical fibres rather than mirrors (see section Dl.2.5)
`but this option is not available for the longer wavelength C0; laser.
`
`Before moving on to a more detailed description of the cutting process, now is a good time to summarize
`the benefits of laser cutting:
`
`The process cuts at high speed compared to other profiling methods. For example, a 1500 W CO2 laser
`will cut 2 mm thick mild steel at 7.5 m min". The same machine will cut 5 mm thick acrylic sheet at
`~12 m min‘1_
`
`in most cases (e. g. the two previous examples) the cut components will be ready for service immediately
`after cutting without any subsequent cleaning operation.
`
`The cut width (kerf width) is extremely narrow (typically 0.1-1.0 mm). Very detailed work can be
`carried out without the restriction of a minimum internal radius imposed by milling machines and
`similar mechanical methods.
`
`

`
`
`
`1533 Cutting
`
`F light
`tube
`
`
`
`Water cooled
`
`tilt adjustable
`45° mirror
`
`Manual or
`automatic
`height
`adjustment
`
`'0' ring type seals
`
`Lens
`
`Pressure
`gauge
`
`C tt_
`u ing
`gas
`inlet
`
`--'*
`
`Lens mount
`
`Nozzle
`
`Work piece
`
`I
`\Cut
`
`Figure D1.2.1. A schematic diagram of laser cutting. The lens mount or nozzle (or both) can be adjusted from left to
`right or into and out of the plane of the sketch. This allows for centralization of the focused beam with the nozzle. The
`vertical distance between the nozzle and lens can also be adjusted.
`
`o
`
`o
`
`a
`
`The process can be fully CNC controlled. This, combined with the lack of necessity for complex jigging
`arrangements, means that a change of job from cutting component ‘A’ out of steel to cutting component
`‘B’ out of a polymer can be carried out in seconds.
`(Note: Nd:YAG lasers cannot cut most plastics
`because they are transparent to Nd:YAG laser light—see section D1.2.5.)
`
`Although laser cutting is a thermal process, the actual area heated by the laser is very small and most
`of this heated material is removed during cutting. Thus, the thermal input to the bulk of the material is
`very low, heat affected zones are minimized and thermal distortion is generally avoided.
`
`It is a non-contact process which means that material needs only to be lightly clamped or merely
`
`

`
`Cutting non—metals (CO2 laser)
`
`1589
`
`positioned under the beam. Flexible or flimsy materials can be cut with great precision and do not distort
`
`during cutting as they would when cut by mechanical methods.
`
`o
`
`Owing to the CNC nature of the process, the narrowness of the kerf width and the lack of mechanical
`
`force on the sheet being cut, components can be arranged to ‘nest’ very close together. Hence, material
`waste can be reduced to a minimum. In some cases this principle can be extended until there is no waste
`material at all between similar edges of adjacent components.
`
`a
`
`c
`
`o
`
`Although the capital cost of a laser cutting machine is substantial, the running costs are generally low.
`Many industrial cases exist where a large installation has paid for itself in under a year.
`
`The process is extremely quiet compared to competing techniques, a factor which improves the working
`environment and the efficiency of the operating staff.
`
`Laser cutting machines are extremely safe to use in comparison with many of their mechanical counter-
`parts.
`
`D1.2.2 Cutting non-metals (CO2 laser)
`
`D}.2.2.I General notes
`
`There are three groups of non—meta1lic materials which are commonly cut by C02 lasers: polymers, wood-
`based products and ceramics. These will be discussed separately in the following sections. Although
`figure D1.2.2 is a good starting point for a description of laser cutting, it does not give a complete picture.
`Lasers are capable of cutting by mechanisms other than simple melting. In some cases (acrylic, polyacetal)
`the material is vaporized rather than melted in the cut zone and in others (epoxy resins, wood products) the
`material cannot melt and must be locally burnt away. These different cutting mechanisms affect the quality
`of the eventual cut edge in ways which are described later.
`
`DI.2.2.2 Polymers
`
`Polymers can be divided up into two main groups:
`
`(1) Thermoplastics: These are polymers that can be repeatedly melted down and cast into new shapes. They
`include polypropylene, polystyrene, polyethylene (polythene), polyamide (nylon) and others.
`
`(2) Thermosets: These materials cannot be remelted once they have been made into their initial shape.
`They sometimes involve the mixing of two liquids which then set hard. This group includes epoxy and
`phenolic resins, fibreglass, kevlar and most natural rubber products.
`
`D1.2.2.2.1 Cutting thermoplastics
`
`Figure D1 .22 is a good description of how most thermoplastics are cut by a C0; laser. The laser produces a
`melt which is then blown out of the cut zone by a gas jet (usually air). This type of cutting is known as ‘melt
`shearing’ for obvious reasons (some workers also call it ‘fusion cutting’). The resulting cut edge is of good
`quality but covered in microscopic ripples.
`
`Not all the liquid is blown out of the cut zone and it is common to have a residue of resolidified melt or
`‘dross’ on the lower edge of the cut. Thermoplastics can be cut at high speed and relatively thick sections
`can be profiled (See table D12. 1).
`There are two important thermoplastics that do not cut by the melt shearing mechanism:
`
`(1) Polyvinyl chloride‘(PVC): this material degrades chemically when heated by the laser. The fumes given
`off contain high levels of hydrogen chloride which is extremely corrosive and very toxic. For this reason
`PVC must never be cut by laser unless suitable ventilation is arranged.
`
`

`
`
`
`1590 Cutting
`
`/
`
`Y
`
`Laser beam
`and gas jet
`
`workpiece
`surface
`
`X
`
`
`
`Cutting direction ~
`
`Figure D1.2.2. Laser cutting by melt shearing. The incident focused laser melts through the material and the gas jet
`
`acting with the laser removes the melt from the cut zone. Thermoplastics are cut in this way using air as the cutting gas
`jet. Metals are also cut by this mechanism if gases other than oxygen are used (e.g. argon for titanium or nitrogen for
`stainless steel).
`
`(2) Polymethyl methacrylate (acrylic, plexiglass etc): this material boils] rather than melts during the cutting
`process and, under the correct conditions can produce a polished cut edge. This process is known as
`cutting by vaporization. As the boiling vapour is blown away from the cut zone, it leaves a thin liquid
`layer on the cut edges. If the gas jet blowing the vapour away flows gently over this liquid layer, it will
`dry like paint to produce a glossy edged cut. If, however, the gas jet is above a minimum velocity, the
`solidifying liquid will become frosted.
`
`D].2.2.2.2 Cutting thermosetplastics
`
`Thermoset plastics are not cut by the melt shearing mechanism shown in figure D1.2.2 for the simple
`reason that they cannot melt.
`In this case the laser burns the workpiece, reducing the plastic to a smoke
`made up of carbon and the other constituents of the original material. This process is known as cutting by
`chemical degradation. Because this process takes more energy than simple melting, cut speeds and maximum
`thicknesses for thermosets are lower than for thermopl astics (see table D 1 2.2). The cut edge of such materials
`is generally flat, smooth and covered with a thin layer of carbon.
`
`1 To be accurate it should be mentioned that this is not really a boiling process. The laser heats up the solid polymer until it becomes a
`liquid which then depolyrnerizes giving off a vapour of the monomer (methyl methacrylate).
`
`

`
`Cutting non-metals (C0; laser)
`
`1591
`
`D1.2.2.3 W0od—based products
`
`Wood and wood—based products are cut by a similar mechanism to thermoset plastics. The laser burns through
`the material to produce a cut and the carbon—based smoke is blown out of the cut zone by a gas jet, which is
`usually air. The top and bottom surfaces of the workpiece retain their original appearance but the cut edge is
`covered in a layer of carbon which darkens it. Low density woods such as pine cut faster than high density
`material such as teak (see table D1.2.3). The cut edge also becomes darker as the density is increased.
`
`D1.2.2.4 Ceramics and glasSe.s'
`
`D1.2.2.4.1 Ceramics
`
`The ceramic of most interest to laser cutting is alumina which is used to produce micro-electronic sub-
`
`strates. This high—melting—point material cuts very slowly if full penetration cutting by the melt shearing
`(figure D 1 .22) method is employed (see table D l 2.4). Fortunately, most applications merely require cutting
`sheets of the material into rectangles and another faster technique called scribing can be used. Scribing
`involves the drilling of lines of small blind holes in the material surface using a single pulse of energy for
`each hole. The lines of holes are mechanically weak and can be used to accurately snap the workpiece into
`the rectangles required. Scribing speeds in excess of 20 m min” are common, which is a factor of ten faster
`than full penetration cutting. Full penetration cutting is useful, however, if curves or circles need to be cut.
`
`D1.2.2.4.2 Glasses
`
`Most glasses have a high absorptivity to CO2 laser light and can be cut by the melt shearing process
`(figure D1.2.2). However, the problem with these materials is that of cracking along the cut edge as a
`result of the rapid thermal cycle associated with laser cutting. Most glasses are not laser cut but quartz and
`heat—resistant glasses can be cut effectively. Scribing (see previous section) can also be used to produce lines
`for subsequent snapping.
`
`D1.2.2.5 Other non-metals
`
`The range of non-metals which can be cut by C0; laser is enormous. Cutting speeds exist in the literature
`for materials as diverse as boron epoxy composites and leather.
`Trial and error is really the only way to investigate a new material but a literature search or contact with
`
`a laser supplier may help. It is necessary to be cautious of the fumes given off from any unknown material,
`particularly if it might contain PVC (see section Dl.2.2.2).
`One final note of interest: C0; laser cutting of foodstuffs is generally pointless. Good cut speeds and
`cut qualities may be achievable but the charred nature of the cut edge makes it taste unpleasant.
`
`

`
`Cutting
`1592
`
`Table D1.2.1. Typical cutting speeds for thermoplastic polymers using a 500 W CO2 laser [1]. Notes:
`
`(1) Cutting
`
`speeds can be changed dramatically by changes in molecular weight, degrees of crystallinity, and porosity. (2) As a first
`approximation, cutting speeds and maximum material thickness can be assumed to vary in a linear manner with laser
`power (between 100 and 1500 W). Maximum material thickness for 500 W = 1530 mm depending on material type.
`(3) The cutting gas is usually air at moderate pressures (1-4 bar). Nozzle diameters are 1-2 mm.
`If a glossy edge is
`required on acrylic, gas pressures may be dropped to below 0.25 bar and nozzle diameters increased.
`
`Thickness
`(mm)
`
`Acrylic (PMMA)
`(m min-1)
`
`Polyethylene
`(rn min‘1)
`
`Polypropylene
`(m min_l)
`
`Nylon
`(m min"1)
`
`ABS
`(m mar‘)
`
`Polycarbonate
`(m marl)
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`12
`
`35.0
`15.0
`3.0
`5.5
`4.5
`3.5
`3.0
`2.3
`1.9
`1.5
`1.2
`
`11.0
`4.0
`2.2
`1.5
`1.2
`1.0
`0.8
`0.6
`0.5
`0.4
`0.3
`
`17.0
`7.0
`4.0
`2.8
`2.0
`1.6
`1.3
`1.1
`0.9
`0.7
`0.4
`
`20.0
`8.0
`4.8
`3.5
`2.6
`2.0
`1.6
`1.2
`1.0
`0.8
`0.5
`
`21.0
`8.2
`5.0
`3.6
`2.7
`2.1
`1.7
`1.3
`1.1
`0.9
`0.6
`
`21.0
`8.2
`5.0
`3.6
`2.7
`2.1
`1.7
`1.3
`1.1
`0.9
`0.6
`
`Table D1.2.2. Cutting speeds for selected thermoset plastics, rubbers and fibre-reinforced materials with a C02 laser {1].
`
`Notes: (1) Cutting speeds for materials such as fibreglass depend on the relative proportion of glass, resin and trapped
`
`air in the material. (2) Cutting gas is usually high pressure air (3-10 bar), nozzle diameters 1-2 mm. (3) Cutting speeds
`will increase dramatically if porous grades of rubber are cut+figures given here are for fully dense material.
`
`Material
`
`Formica
`
`Phenolic resin
`
`Rubber
`
`Rubber (carbon filled, black)
`
`Laser power
`(W)
`
`Thickness
`
`(mm)
`
`Cutting speed
`(In min“1)
`
`400
`1 200
`400
`400
`400
`400
`400
`400
`400
`400
`400
`400
`
`1.6
`1.6
`3.0
`3.0
`6.0
`3.0
`6.0
`9.0
`3.0
`6.0
`9.0
`12.0
`
`7.8
`14.0
`2.8
`2.9
`1.1
`4.0
`1.6
`0.9
`3.0
`1.2
`0.7
`0.4
`
`Fibreglass (glass fibre reinforced
`epoxy resin)
`
`5.2
`1.6
`450
`15.0
`1 200
`1.6
`2.4
`3.2
`400
`2.6
`3.0
`400
`Glass filled nylon
`
`

`
`Cutting non-metals (C0; laser)
`
`1593
`
`Table D1.2.3. Cutting results for wood and wood-based products using a C02 laser [1]. Note‘. Generally use high
`pressure air as assist gas (340 bar. nozzle diameter l~2 mm).
`
`Material
`
`Poplar
`Scotch Pine
`Teak
`Oak
`
`Ebony
`Pine
`Pine
`Pine
`
`Plywood
`Plywood
`Plywood
`MDF“
`MDF“
`MDF“
`Hardboard
`Hardboard
`
`Corrugated card
`Paper
`
`Laser power
`(W)
`
`Thickness
`(mm)
`
`Cutting speed
`(m min’l)
`
`500
`500
`500
`500
`
`500
`1000
`1000
`1000
`
`1000
`1000
`1000
`1000
`1000
`1000
`500
`500
`
`500
`500
`
`10
`10
`10
`10
`
`10
`6
`12
`20
`
`6
`12
`20
`6
`12
`20
`3
`4
`
`3
`0.1
`
`5.0
`3.3
`3.5
`2.9
`
`1.2
`8.0
`3.2
`1.6
`
`7.0
`3.0
`1.5
`9.0
`4.0
`2.0
`10.0
`7.0
`
`25.0
`5000+
`
`Medium density fibreboard.
`
`Table Dl.2.4. Cutting data for full—penetrati0n profiling of ceramic materials using a C02 laser [1].
`
`Material
`
`Glass
`
`Alumina
`
`Silica
`Ceramic tile
`
`Laser power
`(W)
`
`Thickness
`(mm)
`
`Cutting speed
`(m min”)
`
`500
`500
`500
`500
`500
`1000
`1200
`1200
`
`1
`2
`3
`1
`2
`2
`1
`6.3
`
`1.5
`1.0
`0.5
`1.4
`0.6
`2.0
`0.6
`0.6
`
`

`
`
`
`1 594 Cutting
`
`D1.2.3 Cutting mild and carbon steels
`
`D1.2.3.1 General,’
`
`In the previous section we have seen that CO2 lasers can cut non-metallic materials by one of three mecha-
`nisms:
`
`(1) melt shearing (melting),
`
`(2) vaporization (boiling) and
`
`(3)
`
`chemical degradation (burning)
`
`Some materials are cut by a combination of these mechanisms: for example, polycarbonate involves all
`three and a microscopically rippled, slightly charred cut edge is the result.
`Mild and carbon steels are cut by a combination of melt shearing (see figure D1.2.2 ) and a chemical
`reaction with the cutting gas jet, which in this case is pure oxygen. The cutting mechanism is, for obvious
`reasons, known as oxidation cutting. Inside the cut zone the oxygen jet reacts with the iron in the steel to
`
`produce iron oxides. The reaction has two beneficial effects on the cutting process:
`
`(1) The chemical reaction produces heat which speeds up the cutting process. Typical cutting speeds are
`given in table D1 .2.5.
`
`(2)
`
`chemical reaction produces an oxidized liquid with a lower melting point than steel, which does not
`adhere well to the solid steel and is easily blown away by the oxygen jet. This results in a dross-free cut
`edge and cut components which are ready for immediate use.
`
`D1.2.3.2 Oxidation cutting
`
`When cutting mild steel with oxygen, research has demonstrated that approximately half the energy supplied
`to the cut zone comes from the laser and the other half is produced by the chemical reaction [3]:
`
`Fe + To; = FeO (AH = —257.5s kJ mor‘
`
`at 2000 K [4]).
`
`The oxidation Cutting reaction produces regularly spaced striations on the cut edge even if the laser is used in
`its non—pulsed mode (i.e. cw or ‘continuous wave‘). An example of a typical mild steel cut edge is shown in
`figure D1 .23 and these striations are clearly visible. Much research has been expended on the cyclic nature
`of the oxidation reaction which gives rise to these striations [5—8]), but no clear answer has emerged. The
`use of pulsed or modulated laser beams can reduce cut—edge roughness or the burning of small details. This
`replaces the naturally generated striation pattern with a finer one produced by the overlapping laser pulses.
`Although the physics of striation generation is unclear the overall cutting mechanism is straightforward:
`the laser pre—heats the steel to a temperature at which iron burns spontaneously in an oxygen jet. The burning
`reaction is continuously extinguished by the surrounding cold metal and perpetuated by the encroachment of
`the moving laser beam.
`
`In areas where a lot of detail is to be cut (e.g. when cutting saw teeth), the cutting of one area can
`overheat the next area to be cut. If this happens the burning reaction can possibly cover a larger area than
`usual and the shape of the final product can be affected as well as the cut-edge quality. Reducing the laser
`power or increasing the cut speed can minimize this problem.
`
`DI.2.3.3
`
`The importance ofaxial symmetry ofthe energy input to the cutting zone
`
`It is clear that during the cutting of mild steel a delicately balanced dynamic equilibrium is established, rather
`than a steady state, where a continuous input of energy is matched by a continuous flow of material out of
`the cut zone. The striation—generation reactions take place in a circular manner around the centre line of the
`
`

`
`Cutting stainless steel and non—ferrous metals (C0; laser)
`
`1595
`
`Figure D1.2.3. A typical mild steel cut edge which clearly shows the regularly spaced striations caused by the intrinsically
`cyclic oxidation reaction.
`
`movement of the laser across the workpiece. It is therefore very important that the energy input to the area
`is axially symmetric (i.e. identical in cross section in all directions). The axial symmetry of the energy input
`can be affected in any of four ways:
`
`(1) The symmetry of the laser mode can be imperfect clue to poor tuning or damaged optics.
`
`(2) Any linear polarization of the beam can be considered as asymmetry as the beam will cut better in certain
`directions than others (see section Dl.2.4.2).
`
`(3) The symmetry of the oxygen jet can be disturbed by nozzle damage or contamination.
`
`(4)
`
`If the symmetry of the oxygen jet and the laser mode are individually very good, the symmetry of their
`combination can be upset if the two are not coaxial, due to incorrect centring of the nozzle with the
`beam.
`'
`
`Any lack of symmetry will result in inferior cutting in certain directions. Symptoms of this inferior
`cutting will take the form of increased cut edge roughness, adherent dross on the lower lip of the cut edge,
`material burning at corners and reduced cutting speeds.
`
`D1.2.4 Cutting stainless steel and non-ferrous metals (C0; laser)
`
`D1.2.4.I General
`
`Most metals can be laser cut although for any alloy there will be a limiting maximum thickness. For example,
`a high—power laser cutting machine of 3 or 4 kW power will commercially cut. mild steel in thicknesses up to
`approximately 20 mm, stainless steel up to 15 mm, aluminium up to 8 mm and copper up to 6 mm. Relative
`cutting speeds for these different materials will decrease with maximum thickness (i.e. copper will cut slower
`than stainless steel etc).
`
`The physical properties of materials which affect the laser cutting speed are:
`
`o
`
`thermal conductivity,
`
`

`
`
`
`1596 Cutting
`
`Table D1.2.5. Approximate cutting speeds for mild and carbon steels. Notes: (l)Powers shown here and on other tables
`
`are measured at the workpiece. (2) When cutting the highest thickness for each power, cut—edge quality is reduced. (3).
`Results can be improved if the focal length of the lens is increased with material thickness (e.g. 63.5 mm (2.5 in) focal
`
`length up to 3 mm thick, 127 mm (5 in) from 3-8 mm, 190.5 mm (7.5 in) above 8 mm). (4) Many highervpower (e.g.
`3 kW+) machines cut at lower speeds than expected at thinner sections (e.g. a commercially available 3.5 kW machine
`may only cut 1 mm thick mild steel at 9 m min“' ). This is because these machines are programmed to reduce their
`output power for thin sections in order to increase accuracy. At thicker sections full power is used and so, for example,
`the same 3.5 kW machine will cut 15 mm thick mild steel at 1.1 rn min"'1.
`
`Cutting
`speed at
`500 W
`(m min”)
`
`Cutting
`speed at
`1000 W
`(in min71)
`
`Cutting
`speed at
`1500 W
`(m min""1)
`
`Thickness
`(mm)
`
`Oxygen
`pressure
`(bar)
`
`Nozzle
`diameter
`(mm)
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`
`5.0
`2.8
`1.7
`1.2
`0.5
`—
`-—
`—
`—
`—
`—
`-
`—
`——
`—
`
`8.0
`5.0
`3.2
`2.2
`1.5
`1.2
`0.8
`0.6
`0.5
`0.4
`-
`-
`—
`—
`—
`
`11.0
`7.5
`5.0
`3.5
`2.5
`1.8
`1.5
`1.2
`1.0
`0.8
`0.7
`0.6
`0.5
`0.5
`0.4
`
`2-4
`2-3
`2-3
`1.5-2.5
`1.5-2.5
`1-2
`1-2
`0.5-1.5
`0.5-1.5
`0.5-1.5
`0.4—1_2
`0.4-1.2
`0.3-1.0
`0.3-1.0
`0.3-1.0
`
`1.1-1.5
`1.1-1.5
`1.1-1.5
`1.2-1.6
`1.2-1.6
`1.3-1.8
`1.3-1.8
`1.3-1.8
`1.3-1.8
`1.5-2.0
`1.5-2.0
`1.5-2.0
`1.5-2.0
`1.5-2.0
`1.5-2.0
`
`o
`
`a
`
`0
`
`a
`a
`
`refiectivity.
`
`melting point,
`
`density,
`
`specific heat and
`latent heat of fusion.
`
`As any or all of these properties rise, the cutting speeds decrease as does the maximum thickness which
`can be cut.
`
`The main process parameters which influence cutting speed are:
`
`a
`
`a
`
`laser power density and
`
`type of cutting gas.
`
`Laser power density is obviously a function of the laser power and the focused spot diameter. If this
`density is low, cutting speeds will be low. As power is increased and/or the spot size decreased, the cutting
`speed will increase until an optimum is reached. If, however, the spot size is reduced down to a few tens of
`micrometres in diameter the cutting process may suffer because the kerf width will be too small to allow the
`passage of enough cutting gas. Sufficient gas must be allowed to travel through the cut zone to remove the
`melt. Also, if the power density is allowed to rise above an optimum range, the metal may boil rather than
`
`

`
`Cutting stainless steel and non-ferrous metals (C02 laser)
`
`1597'
`
`melt. Boiling requires considerably more energy than melting and, in addition to this, the vapour generated
`absorbs the incoming laser beam. For this reason it is important to keep the power density of the laser high
`but below that level at which boiling becomes the preferred material response.
`
`The type of cutting gas used basically falls into two categories: reactive and non—reactive (or inert).
`Metals generally experience exothermic oxidation reactions which can benefit cutting speeds. However, an
`oxidized cut edge is of inferior quality to a n0n—oxidized edge. In the early 1990s stainless steel was usually
`cut using oxygen because standard laser cutting machines had powers of between 1 and 2 kW and extra energy
`
`from the oxidation reactions was needed to increase cutting speeds. However, the melt generated when using
`oxygen for stainless steel had a high surface tension and this resulted in resolidified melt or dross on the
`bottom edge of the cut. Higher laser powers were available but these machines had poor quality modes which
`were not axially symmetrical (see section D1 2.3.3). Towards the end of the 19905 3 and 4 kW machines of
`
`cutting mode quality became available. These machines could cut stainless steel quickly and at sections up
`to 12 mm thick using high—pressure nitrogen rather than oxygen. The resulting cuts were of higher quality
`and nitrogen cutting of stainless steel became the norm.
`Most nickel alloys are also cut with nitrogen but many copper alloys have to be cut using oxygen because
`of the material’s high reflectivity. An oxide layer is continuously created in and around the cut zone and this
`helps absorb the incident laser light.
`
`Titanium alloys react uncontrollably when laser heated in an oxygen jet and, in any case, the mechanical
`properties are ruined by the presence of oxygen. The material even reacts with nitrogen and must therefore
`be cut in a stream of genuinely inert gas such as argon or helium.
`
`D1.2.4.2 Polarization
`
`CO2 lasers generate a beam which is polarized and this can have a deleterious effect on the cutting process
`when cutting electrically conductive materials such as metals.
`The symptom of cutting metals with a polarized beam is that the beam is deflected in different directions
`depending on the cutting direction. If such a beam is used to cut a 100 mm square in 10 mm thick metal, the
`top of the cut product may be correctly 100 mm X 100 mm but the bottom could be 99 mm x 101 mm.
`For this reason laser cutting machines have a phase shifting mirror incorporated into them which
`delivers the beam to the workpiece in a random or circularly polarized condition. The cut front then remains
`perpendicular to the material surface at all times and the laser cuts equally well in all directions.
`
`D1.2.4.3 Cutting speeds
`
`Tables D1.2.5—Dl.2.ll present a series of cutting speed results to be expected in a job shop environment,
`i.e. where a general-purpose laser cutting machine is expected to change materials and thicknesses regularly.
`Higher speeds can be achieved on machines dedicated to thin section slitting and profiling (e.g.
`1 mm sheet
`aluminium; 40-50 m min‘1,
`1 mm stainless steel; 30-40 m min”, 1 mm mild steel; 30 m min’1, all at
`
`a laser power of 2.5 kW [12]). These high—speed, thin—sheet machines are starting to replace some of the
`traditional stamping methods employed by the automobile industry.
`
`

`
`1598
`Cutting
`
`Table D1.2.6. Approximate cutting speeds for stainless steel cut with oxygen. Notes: (1) Cuts produced with oxygen
`
`are o