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The variation of melt flow velocity and melt film thickness along the cut depth is shown in Figure 3. The conduction heat transfer from the cutting front through the kerf walls to the substrate metal is the significant means of power loss from the lasermaterial interaction zone (i.e., cutting zone). <>
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|6v Wandera et al. Therefore, the ease with which a metallic material can be cut depends on the absorptivity of the material to the incident laser beam, and the melting temperature of the material or oxide formed when a reactive assist gas is used [15]. Copyright 2020 Elsevier B.V. or its licensors or contributors. 0000032676 00000 n
Wandera et al. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. <>
There is a reduction in the occurrence of the irregular deep grooves on the cut edge with a reduction of assist gas pressure and increase in cutting speed to optimum levels. On the cutting of a 10-mm stainless steel workpiece using the fiber laser, the tendency for dross attachment on the lower cut edge is more significant with focal point positions located on the upper half section of the workpiece thickness (see Figure 12). Wandera et al. [32] reported that for the cut edge quality in 610- mm stainless steel, the location of the boundary layer separation point moves closer to the bottom cut edge with increase in cutting speed. stream
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The melt surface propagates through the material with a velocity that depends on the energy input, thermal properties of the workpiece material, and the molten material removal mechanisms. support team Typically, the cutting speed giving the best cut edge quality is lower than the maximum achievable speed for cutting through a given material at the given laser power level. The effects of the processing parameters in the cutting of thick-section stainless steel and mild steel and medium-section aluminium have been reported; optimization of the processing parameters for enhancement of the cut edge quality has been discussed. The laser beam quality is the critical parameter that influences the focusability of the laser beam; Equation (2) gives the relationship between the BPP of the laser beam and the minimum focused spot size, dffor focusing optics of focal length, f, and a raw beam diameter on the focusing optics, D. [3, 15]. The melt front velocity increases with increasing laser power intensity which enhances the penetration speed [21]. Laser cutting and welding of metals require absorption of high power intensities to enhance cutting and welding at high processing speeds. Lappeenranta Laser Processing Center at Lappeenranta University of Technology in Finland is acknowledged for facilitation of the fiber laser experimentation. They operated much more efficiently, lost far less power from mirrors and lenses then their CO2 counterparts thus put more cutting power to the torch. In reactive fusion cutting using an active assist gas jet, there is a significant variation in kerf width with cutting speed because the exothermic reaction is very erratic at slow cutting speeds, resulting in increased sideways burning and widening of the kerf width (see Figure 8). Absorptivity depends on the wavelength of laser radiation, plane of polarization of the light beam, angle of incidence, material type, and temperature and state of the material (solid, liquid, or gas). $4%&'()*56789:CDEFGHIJSTUVWXYZcdefghijstuvwxyz ? [23, 31] tested the maximum cutting speeds with the corresponding required laser power levels for the cutting of 10-mm stainless steel, 15-mm mild steel, and 4-mm aluminium using the fiber laser as presented in Figure 5. The temperature in the fiber core is determined primarily by heat transport through the outer surface of the fiber [7]; the geometry of the fiber laser exposes a large surface area per unit volume, which aids the cooling of fiber lasers [8]. The real world comparisons or, 'Where the Rubber Meets the Road' of Fiber vs CO2 technology is in the cutting speeds and capacities. Optimization of the cutting process parameters for enhancement of the cut edge quality in thick-section metal cutting at high cutting speeds using the ytterbium fiber laser has shown that the maximum applicable cutting speed is influenced by the melt removal process. The absorption of the laser beam within the keyhole is through both Fresnel absorption (i.e., absorption directly by the material) and plasma absorption (i.e., inverse bremsstrahlung effect) by the free electrons in the metallic plasma. The poor cut edge quality dross attachment and boundary layer separation observed in thick-section stainless steel cut edges obtained using the high brightness fiber laser and an inert assist gas jet is caused by the difficulty in melt ejection through the narrow thick-section cut kerfs [32, 36]. f(`=F*"I$`km,#NQE QE QE QE4G;l Z# 91 *gg3 \B1UQL tUvg; R@Zg3 Hi2vmQPn_/\ 0=*)33M@Te}sR:?v@9LY8-z A schematic illustration of the double-clad fiber. Effect of assist gas pressure on the location of the boundary layer separation point and dross attachment on cut edges in inert gas-assisted laser cutting. We had the data to prove that increasing your wattage on a machine increased your speed and thus throughput. Consequently, the presence of plasma interferes with the laser beam delivery to the interaction zone by blocking the beam through beam-scattering effects caused by changes in refractive index and particles caught up in the plasma [15]. Andy is able to share this experience to assist you in your machining and fabrication equipment and application needs. ( In the relation given in Equation (1), is the wavelength of the laser beam and M2is the beam quality factor (i.e., times diffraction limit factor) which tells how much larger the BPP of the laser beam under consideration is compared to the lowest value of /for the basic Gaussian TEM00 mode (diffraction limit) [3, 15]. The maximum processing speeds, maximum processing depths, and resulting cut edge quality are governed by a number of parameters related to the laser system, workpiece specification, and the cutting process. Therefore, the power contribution to the cutting process by the oxidation reaction (i.e., reaction power) is estimated from either the oxygen flow into the interaction zone or the molten iron flow into the interaction zone [27]. [23, 31] and Sparkes et al. %
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Absorptivity, A (also known as absorption coefficient), of the metal surface to the laser radiation is defined as the ratio of the laser power absorbed at the surface to the incident laser power. HMk@+hJ!iJH% #J$I&Y}B}y53~" Effect of focal point position on the dross adherence on 10-mm stainless steel. 1526 0 obj <>
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Complete penetration of the thick-section stainless steel workpiece cannot be achieved at the maximum cutting speed for a given power level when the focal point position is far into the lower half section of the workpiece thickness because of the reduced power intensity on the workpiece surface. The extent of the exothermic reaction in the cutting front is limited by the flow rate of the rarer type of reactant (either oxygen or iron). There is no boundary layer separation on the cut edge with the 2.5-mm nozzle diameter [29]. The cut kerf width is usually a fraction of a millimeter and the molten metal has a high viscosity such that the melt flow can be assumed to cover the entire cut kerf. Exceedingly higher cutting speeds in thin-section to medium-section (less than 6 mm sheet thickness) metal cutting have been realized with the ytterbium fiber laser compared to the CO2 laser. The two major roles of an active assist gas jet (oxygen or compressed air) during laser cutting of a metal are to influence the energy balance at the cutting zone through the exothermic oxidation reaction and also eject the oxidized molten metal. "
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( A large kerf size, high assist gas pressure, and large nozzle diameter enhance gas flow rates in the thick-section cut kerfs. Efficient melt removal is obtained when wider cut kerfs are created with the focal position located below the workpiece top surface. endobj
How? The ridges observed on the cut edge in laser oxygen-assisted cutting of 15-mm mild steel using the fiber laser is typical of the mechanism of the sideways burning, as originally explained by Arata et al. to go back to the article page.Or contact our <<6A42C71E4328504B9A3F5A5CA18D9A3B>]>>
!(!0*21/*.-4;K@48G9-.BYBGNPTUT3? Plasma absorption occurs through absorption of the laser beam by the free electrons in the plasma (i.e., hot metal vapor) leading to plasma re-radiation [15]. With the coaxial arrangement of the assist gas nozzle with the incident laser beam, the entire melt surface in the cut kerf is in contact with the assist gas jet. Fresnel absorption occurs during direct interaction of the beam and the material in which the photons of the incident laser beam radiation are absorbed by the free electrons in the metal structure. 0000040416 00000 n
This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Schulz et al. The cut kerf width depends on the focused spot size, laser power absorbed, and the applied cutting speed. Wandera et al. ScienceDirect is a registered trademark of Elsevier B.V. Click here to go back to the article page. Molten metals that have high surface tension and high viscosity are more difficult to remove from the cutting front and have a high tendency to attach on the underside of the cut as dross. [32] reported an increase in fiber laser cutting speeds of up to 1.5 times higher than the CO2 laser cutting speeds. 2 0 obj
The dynamics of the laser cutting process affects the shape of the cutting front and the melt flow mechanism. Licensee IntechOpen. Absorption of the laser beam by metals increases for wavelengths toward the visible and ultraviolet regions and decreases towards the longer infrared wavelengths. The maximum achievable cutting speed for a given laser power level is the maximum cutting speed at which the cut edges are separated. Therefore, the active assist gas jet passing through the cut kerf plays two important roles which include exerting the necessary drag force to blow the molten material out the cut kerf and providing additional heat to the cutting process. The principle role of the inert assist gas jet e.g., nitrogen during laser cutting of a metal workpiece is to eject the molten metal to create the cut kerf. For an opaque material such as a metal, the absorptivity is given as A=1R, where Ris the reflectivity of the workpiece surface. Fiber Lasers were simply different than their cousin, the CO2 laser resonator. In cutting of 10-mm stainless steel using the 4 kW fiber laser power at cutting speed of 1.0 m/min and focal position located on the bottom workpiece surface, dross-free cut edges with lower surface roughness were obtained with the 254-mm focal length optics than when the 190.5-mm focal length optics was used. The nozzle stand-off distance distance between the nozzle and the workpiece top surface influences the gas flow dynamics at the entrance of the cut kerf and consequently affects the gas flow patterns at the cutting front. ( The maximum processing speeds, maximum processing depths, and resulting cut edge quality are governed by a number of parameters related to the laser system, workpiece specification, and the cutting process [15]. The confinement of pump light rays within the fiber core region subject to some losses through absorption or scattering maintains the pump light intensity propagating in the fiber over a fiber length of several meters. The two absorption mechanisms that prevail during cutting and welding of metal include Fresnel absorptionand plasma absorption(inverse bremsstrahlung effect); these laser beam absorption mechanisms are explained in the following sections. Open Access is an initiative that aims to make scientific research freely available to all. Our team is growing all the time, so were always on the lookout for smart people who want to help us reshape the world of scientific publishing. 2016 The Author(s). There is retardation of the melt as the melt flow progresses down the cut kerf, resulting in the melt buildup at the lower section of the cut kerf and subsequently dross attachment on the lower cut edge. The pump light from high-power laser diode arrays is guided into the inner cladding (referred to as pump cladding), and this pump light is confined in the inner cladding by the lower refractive index outer cladding (see Figure 1b). The more energetic photons of the shorter wavelength radiation of the ytterbium fiber laser can be absorbed by a greater number of electrons in the metal structure such that the reflectivity of the metal surface falls and absorptivity is greatly increased [15]. endobj
The increased cutting speeds for fiber laser is an indication of a higher absorption of the fiber laser beam by the metal workpiece compared to the absorption of the CO2 laser beam. Brief introduction to this section that descibes Open Access especially from an IntechOpen perspective, Want to get in touch? However, the plasma buildup is not very significant in cutting due to the assist gas which blows it away; therefore, plasma absorption mechanism is very limited in laser cutting. 0000040608 00000 n
The effects of cutting speed and focal point position on the surface roughness of the fiber laser cut edges are more significant when a short focal length of the focusing optics is used than when longer focal length optics is used. %&'()*456789:CDEFGHIJSTUVWXYZcdefghijstuvwxyz There is a tendency of dross adherence on the cut edge when cutting at the maximum cutting speeds using the high brightness ytterbium fiber laser; therefore, there is need to define the acceptable cut edge quality. [23, 31] and Wandera and Kujanp [29], also experimentally investigated the effects of process parameters in the cutting of 10-mm stainless steel, 15-mm mild steel, and 4-mm aluminum using the high brightness fiber laser. 0000040160 00000 n
The higher energy density loss from the cutting front when cutting at low cutting process increases the tendency of dross adherence on the cut edge. Or contact our Further with the use of Solid State Fiber Lasers the capabilities to cut other materials such as Brass and Copper add to the systems and your shops capability. The melting efficiency of a given laser increases with increase in the absorptivity since a larger proportion of the incident laser radiation is absorbed by the material and utilized in melting of the kerf volume during cutting. Professor Veli Kujanp of VTT Technical Research Centre of Finland and Professor Antti Salminen of Lappeenranta University of Technology are acknowledged for the invaluable discussions that inspired the formulation of the ideas presented in this chapter. Surface roughness is the unevenness of the cut surface profile which is observed as striations and adherent dross on the cut edge caused by the dynamical behavior of the laser cutting process. where Ais the absorptivity of the workpiece to the incident laser radiation, PLis the incident laser power, PMeltis the power utilized in melting the kerf volume, and PLossis the inevitable power loss from cutting zone. } !1AQa"q2#BR$3br 0000002099 00000 n
They observed that the cut edge quality in 610-mm stainless steel improved with increase in cutting speeds, higher nitrogen assist gas pressures, and wider cut kerfs. The maximum achievable cutting speeds increase with increase in the incident laser power used for cutting. The economic aspects of laser usage in manufacturing that form important criteria in the choice of a suitable laser system for thick-section metal cutting include: high processing speeds, high processing depths, high cut edge quality, and high wall-pug efficiency of the laser system. The presence of the boundary layer separation and dross adherence on the laser cut edge are characteristics of poor cut edge quality. During inert gas-assisted laser fusion cutting of metal, the absorbed laser power is the only incoming power contribution to the cutting zone. Only a small proportion of the oxygen jet is consumed in the oxidation reaction as part of the oxygen jet is utilized as drag to accelerate the melt out of the cut kerf while part of it is lost across the top workpiece surface or down the kerf; consequently, only about 50% of the molten iron reacts with the oxygen in the cut kerf. In fiber laser cutting of 15-mm mild steel using oxygen assist gas, the best cut quality was obtained with a 1.5-mm nozzle diameter and the 2.5-mm nozzle diameter produced the worst cut edge quality [23]. The speed of penetration of a metal workpiece during laser cutting depends on the absorbed incident laser power intensity. The two methods of welding of metal using a laser beam include: conduction limited weldingand keyhole welding. Among the rare earth ions used in fiber lasers, ytterbium is highly absorbing of pump radiation and is preferred as doping material for the high-power fiber laser operating at 1,0601,080 nm spectral range and delivering kilowatt output power suitable for material processing [2]. According to the ISO standard for classification of thermal cuts (SFS-EN ISO 9013:2002) [34], the cut edge characteristics that are used to classify thermal cuts include: surface roughness and perpendicularity (squareness) deviation. <>
Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too. The cutting speed difference between the fiber laser and the CO2 laser reduces with increase in metal workpiece thickness to the thick-section domain. Reactive fusion cutting utilizes an active assist gas jet (usually oxygen or compressed air) that is capable of reacting exothermically with the molten metal and the reaction generates an additional heat source to the cutting process [15, 25]. where: dZis the depth of focus, fis the focal length of the focusing optics, Dis the raw beam diameter on the focusing optics, and BPPis the beam parameter product of the incident laser beam. The ytterbium ion-doped core region is surrounded by an inner cladding of lower refractive index than the core and the inner cladding is in turn surrounded by an outer cladding of still lower refractive index forming a step index fiber (see Figure 1a). This means that the 1.07 m laser light is reflected less, absorbed more easily, and the shorter wavelength can be focused into a spot that is around 1/10th of the diameter of a comparable CO2 beam. The small core diameter of single-mode fibers (310 m) ensures that the power density of the output beam is very high. And although Fiber Laser cutting is used in many different areas, from medical to Southern Fabricating Machinery Sales, Inc. 10417 South County Road 39Lithia, FL 33547. Fiber Lasers - Why the Kilowatt is not the King, Southern Fabricating Machinery Sales, Inc. Fiber laser cutting machines are designed for metal fabrication. Therefore, it is sufficient to assume that molten metal oxide is removed through the bottom of the cut kerf without vaporization. [23] developed a theoretical model to estimate the power requirement for melting the kerf volume and the inevitable conduction power losses. Effect of cutting speed on striation pattern in 15- mm mild steel. The availability of high-power diode lasers as pumping sources for fiber lasers and improvements in fiber laser design have enabled power scaling of ytterbium (Yb3+)-doped fiber lasers to output powers beyond 1 kW in cwoperation with near diffraction-limited beam quality [8, 13]. Click HERE to read more articles aboutLaser CuttingClick HERE See our available Laser Systems. For many years we as manufacturers, distributors and OEMs of laser cutting systems were taught that Power (KW) IS Capacity& Speed. 0000002616 00000 n
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Therefore, the improved cooling mechanism in the ytterbium fiber laser has enabled the solid-state laser to achieve near diffraction limited beam quality at high output power [3]. ( The melt flow velocity increases with increase in assist gas pressure and increase in the cut kerf width, resulting in a reduction in the melt film thickness. The striations observed on the stainless steel cut edges during fiber laser cutting with an inert assist gas are associated with the melt flow mechanisms. Figure 2 schematically illustrates the cut kerf generated and the volume of material removed during laser fusion cutting of thick-section metal. 0
Therefore, the materials surface reflectivity, thermal conductivity, and workpiece thickness are the material parameters that critically affect the efficiency of the laser cutting process. Focal positions close to the workpiece bottom surface produce clean dross-free cuts because of the wider cut kerfs formed with these focal positions. However, the minimum focused spot size is also directly proportional to the focal length (see Equation (2)) such that use of longer focal length optics for focusing of the laser beam results in a larger focused spot size with reduced power intensity. Iron-oxide (FeO) generated in oxygen-assisted laser cutting of mild steel does not boil, but would dissociate when heated to high temperatures; the iron-oxide dissociation process which consumes much energy could lead to a collapse of the cutting process [26]. 0000002243 00000 n
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gK_zsZWou]w-d[4q~yI9x_n>o?CB/tjq6;>\q&2jSNo^|ZXP fFy!kBpBz N-GY<4(X~F >&*"CLf=8zJy3>g94o/*l [31] presented the different categories of the fiber laser cut edges in thick-section steel and medium-section aluminum for different cutting speeds at different power levels. The direct absorption of the beam by the workpiece takes place through the Fresnel absorption mechanism (i.e., absorption during reflection from the surface). *Address all correspondence to: cathywandera@yahoo.com. Furthermore, fiber laser beam wavelength offers more flexible beam handling through use of narrow optical fibers. Therefore, the cutting process parameters that affect the power intensity at the cutting front and the gas dynamics in the narrow thick-section cut kerf have a great bearing on boundary layer separation, dross adherence, and the resultant cut edge surface roughness. Therefore, the cutting speed giving the best cut quality is the optimum cutting speed, especially for applications where cut edge quality is of paramount importance [35]. The development in the output power of solid-state fiber laser source has resulted in the increasing interest in the use of the high brightness fiber laser in macro laser material processing applications, especially cutting and welding of metal [1]. startxref
The use of cladding-pumping in the high-power ytterbium fiber laser limits the thermal issues (i.e., the variation of the refractive index with temperature) that affect the stability of high-power Nd: YAG lasers [6]. The inert gas-assisted laser fusion cutting process utilizes a focused high-intensity laser beam to melt the kerf volume and a coincident high-pressure inert gas jet to blow out the molten metal to form a cut kerf. When the angle of incidence is zero (i.e., vertical incidence), the parallel polarized laser beam (RP) and the perpendicularly polarized laser beam (Rs) are absorbed equally. As the argument did not go away with the introduction of this new technology, the dealers capitalized on the need for more power as higher power and wattages also meant more sales dollars. xref
here Olsen [19, 20], in his description of the mechanisms of the cutting front formation, identified the melt surface, melt film, and melt frontas the three zones that comprise the cutting front. 3 0 obj
The melt film in the cutting front is generated by the melting action of the absorbed laser beam power and the oxidation reaction power (in the case of oxygen or compressed air assist gas).
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