FACTS ABOUT Laser technology Laser cutting

1 Laser cuttingJoachim Berkmanns, Cleveland USAMark Faerber, Unterschlei heim GermanyFACTS ABOUTL aser technology21. Introduction32. The Laser cutting Types of cutting Lasers for Laser cutting53. Parameters in Laser Pulsed or continuous wave (cw) Laser Laser power and Focal length of the Laser beam Wavelength of the Laser Focal position relative to the Nozzle size and standoff Gas type and gas pressure114. Pressure and volume requirements for different Mild steel and low-alloy Stainless steels and other high-alloy Aluminium and aluminium Nickel Copper Non-metals 195. Literature19 Table of contents31. IntroductionOver the past decade, Laser cutting hasdeveloped into state-of-the-art is estimated that more than 25,000 cut-ting systems are used for the high-powercutting of metals and non-metals world-wide. When including low powerapplications, such as plastics cutting andpaper cutting , the numbers are even examples of modern Laser cut-ting applications are: cutting of hydro-formed parts andtubesHigh speed cutting of thin-sheet metalCutting of thick section-materialDeveloping lasers with higher outputpowers without sacrificing beam qualityhas been one important goal in the efforts focused on improving thedrive technology of the motion system andenhancing material handling around thecutting are that Laser cutting based onimproved cutting speeds, little tool wearand unlimited flexibility will further replacecompeting technologies.

2 There are marketsurveys suggesting that the number of flatbed Laser cutter installations will doubleover the next ten years. In addition, lasermanufacturers will address new marketssuch as cutting tubes and gases used to generate the Laser beamand expel the molten metal out of the cutkerf are important consumables duringlaser operations. They can prolong the life-time of the optical component, increasethe cutting speed and improve the cuttingquality. All the above contribute to moreprofitable Laser operation. It is thereforethe objective of this brochure to familiarisepotential users of Laser cutting systemswith the technology and to provide guide-lines for proper use of the cutting of the results presented in this bro-chure were obtained in projects carried outin the application lab of the Linde Group orin projects that were initiated and spon-sored by the Linde Types of cutting processesAn almost parallel Laser beam, which isusually invisible, is generated in the lasersource and directed to the cutting head bymirrors, where it is concentrated (focused)by a lens to a small spot, cf.

3 Fig. 1. Depend-ing on the process, the spot is placed on the surface of the workpiece or on thematerial to be cut (Fig. 1, see also Factsabout: Laser Basics ). The intense light beam quickly heats upthe workpiece and melts the material. Theassist gas (also called: cutting gas) isapplied to protect and cool the focusinglens and to remove the molten metal fromthe cut kerf at the same time. There are two cutting processes, depending onthe type of assist gas used: When cutting with oxygen, the materialis burned and vaporised after being heat-ed up to ignition temperature by thelaser beam. The reaction between theoxygen and the metal actually createsadditional energy in the form of heat,which supports the cutting exothermic reactions are the rea-son why oxygen enables penetration ofthick and reflective materials when it isused as a cutting gas.

4 When cutting with non-reactive (inert)gases such as nitrogen or argon, thematerial is melted solely by the laserpower and blown out of the cut kerf bythe kinetic energy of the gas jet. As non-reactive gases do not react with the mol-ten metal, and no additional heat is generated, the Laser power required isusually much higher than in oxygen cut-ting of the same thickness. cutting withnonreactive gases is often referred to asclean cutting or high-pressure cutting is another cutting pro-cess. In sublimation cutting , the solidmaterial is converted into vapor withoutpassing through a liquid phase. Gases areused to support the process and cold cutting , the energy of the laserbeam breaks the chemical bonds of thematerial to be cut, thereby producing pow-dery residues. Laser beam energy andchemical bond energy must match, andcutting gas is often not The Laser cutting processLaserLaser beamMirrorMirrorCutting gasFocusing lensGas nozzleWorkpieceFig.

5 1: Principles of a Laser cutting system (schematic) Lasers for Laser cuttingMany lasers can be used for Laser cutting ,provided their beam can be focused on a small spot with sufficient intensity tomelt the material and their specific wave-length is absorbed in the material. Today,CO2gas lasers, Nd:YAG solid-state lasersand Excimer gas lasers are those mostcommonly used in the field of introduced Diode lasers are con-sequently not suitable for Laser cuttingwith respect to intensity in the focus beingnot sufficient yet. The latest member of thelaser family, the fiber Laser , in contrast,seems to provide a beam quality, laserpower, and beam intensity that opens abroad perspective in Laser cutting Pulsed or continuous wave (cw) laseroperationsThe highest cutting speeds can be ob-tained at high power levels in cw-modeoperation. Continuous wave (cw) meansthat the Laser power output is constant,without interruption over time.

6 At highspeeds, the Laser power is used almost en-tirely to melt or vaporise the material on the cut front and there is relatively little heat conduction into the base materi-al. However, some of the heat is conductedinto the base metal when the cutting direc-tion is reversed or when cutting around asharp corner. This reduces the feed rateand causes the workpiece to heat up, to theeffect that cutting quality cutting filigree structures or piercing holes into thicker materials, it canbe especially difficult to achieve acceptablecutting qualities with a high power cwlaser. Pulsed processing can produce bettercuts under such circumstances. High peakpower in the short pulses ensures efficientheating with an effective removal of hotmaterial from the kerf while low averagepower keeps the workpiece cool. The cutting speeds obtainable in pulsedcutting are much lower than with con-tinuous wave (cw) Laser beams.

7 Averagepower normally has to be reduced to somehundred watts in order to achieve a signifi-cant increase in cutting quality by puls-ing. This often results in cutting speedsthat are only 10 % of those obtainable in the cw mode. When cutting metallicmaterials, the peak power generally mustbe within a range of 1 to 10 kilowatts, andeach pulse must be long enough to melt alayer of the cutting front, which is typically1 3 Parameters in Laser Laser power and intensityLasers are usually described in terms ofpower, 1,000 watts or 6 kW. Laserpower is the total energy emitted in theform of Laser light per second. The intensi-ty of a Laser beam is equal to its power di-vided by the area over which the power isconcentrated. For example, focusing a 1,000 watt laserbeam over a diameter of mm ( in)will result in a power density of ,000 watts per mm2.

8 The high intensity causes the material toheat up rapidly so that little time is avail-able for heat to dissipate into the surround-ing material. This produces high cuttingrates and an excellent quality of cut. A Laser s intensity also determines thethickness that can be cut. The thicker thematerial to be cut, the higher the intensityneeded. Higher intensities can be reachedby increasing Laser power or by using afocusing lens with a shorter focal , focusing the beam on to a smallerspot also reduces the depth of focus and istherefore unsuitable for cutting intensity can be achieved both in pulsed and continuous beams. Accordingly,either the peak pulse power in pulsed cut-ting or the average power in continuouscutting determines the penetration. cutting speed is determined by the aver-age power level. The higher the averagepower, the higher the cutting speed (Fig.)

9 2).However, high-power lasers do not auto-matically deliver high intensity lens used to focus the beam is veryimportant in the context of cutting speed. Fig. 2: Typical cutting rates with approx. 3 kW of Laser power Laser power Intensity = Irradiated area048121620 Sheet thickness (mm/in)Mild steelStainless speed (m/min) cutting speed (ft/min)10020307 Fig. 3: Focusing a Laser beamFocusing lensDepth of focusShort focal distanceLong focal Focal length of the lensThe focal length of the lens defines the shape of the focused Laser beam. Theminimum spot size (df) is a function ofwavelength ( ), beam mode (K), the dia-meter of the unfocused beam (D) at thelens and the focal length of the lens (f )according to: A small spot diameter is favoured by: Short focal length (f )Good mode = even intensity distribution (close to Gaussian, K = 1/M2= 1) (K) Short wavelength of the Laser beam ( )A large beam diameter at the lens (D) The depth of focus Zf, which defines the level of tolerance for variation of the distance from the lens to the workpiece aswell as the thickness that can be cut,depends on the same parameters.

10 In gene-ral, a small spot size goes along with ashort depth of focus. This means that a lens with a short focallength produces a small spot size and ashort depth of focus, generally resulting inhigh speed and good cutting quality of thinsheet metal. However, careful control ofthe distance between the lens and theworkpiece (lens working distance) isnecessary. When thicker materials are cut,the depth of focus must be adapted to thematerial s thickness by selecting a longerfocal length (Fig. 3). As the longer focallength also results in greater focal spotpower, it must be increased in order to maintain intensity and cutting f 1df=x x D KLaser beamLaser beamSpot sizedfdfZfffSpot sizeFocusing Laser beam modeA Laser beam s mode refers to the distribution of energy through its crosssection (Fig. 4, see also FACTS ABOUT : LaserBasics ).