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  Method for cutting stainless steel with a fiber laserUSPTO Application #: 20070119834Title: Method for cutting stainless steel with a fiber laser Abstract: The invention relates to a laser cutting method for cutting a stainless steel workpiece using laser beam generation means comprising a silica fibre with an ytterbium-doped core to generate the laser beam. Preferably, the laser beam generated by the ytterbium-based fibre has a wavelength between 1.07 and 1.09 μm, a quality factor of the laser beam is between 0.33 and 8 mm.mrad, and the laser beam has a power of between 0.1 and 25 kW. The assistance gas for the laser beam is chosen from nitrogen, helium, argon and mixtures thereof, and, optionally, it further contains one or more additional compounds chosen from O2, CO2, H2 and CH4. (end of abstract)
Agent: Air Liquide - Houston, TX, US Inventors: Francis Briand, Karim Chouf, Hakim Maazaoui USPTO Applicaton #: 20070119834 - Class: 219121720 (USPTO) Related Patent Categories: Electric Heating, Metal Heating (e.g., Resistance Heating), By Arc, Using Laser, Cutting, Methods The Patent Description & Claims data below is from USPTO Patent Application 20070119834. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of priority under 35 U.S.C. .sctn. 119(a) and (b) to French Application No. 0553607, filed Nov. 25, 2005, the entire contents of which are incorporated herein by reference. BACKGROUND [0002] The invention relates to a laser cutting method for cutting stainless steel using a laser source of the ytterbium-doped fiber type. [0003] At the present time, laser cutting using a laser source of the CO.sub.2 type to generate a laser beam, with a wavelength of 10.6 .mu.m and a power ranging up to 6 kW, is widely used in industry. This method is used in particular for cutting stainless steels. [0004] However, the cutting speeds that can be achieved and the cutting quality that results therefrom are very variable, depending on the material to be cut and, moreover, depending on the cutting method parameters adopted, such as the nature of the assistance gas, the diameter of the focused beam, the power of the incident laser, etc. [0005] Thus, CO.sub.2 lasers cannot be used with assistance gases of low-ionization potential, for example such as argon, without the risk of generating parasitic plasmas that could impair the method. [0006] Furthermore, CO.sub.2 lasers are limited in terms of power, thereby directly impacting the cutting speed. [0007] In addition, the fact of having to guide the laser beam from the laser generator right to the focusing head, that is to say the cutting head, has drawbacks, especially as regards alignment of the optics in the optical path. This is because guiding optics are generally polished and/or coated copper mirrors and the positions of the latter determine the path followed by the laser beam. Therefore, the alignment of the mirrors must be perfect in order to ensure optimum entry of the laser beam into the focusing head or cutting head. Now, the position of these mirrors is generally adjusted by mechanical means, which may easily go out of alignment according to time, the wear of parts and the environmental conditions, in particular the ambient temperature, moisture content, etc. [0008] In addition, the optical path of the beam must necessarily be kept in an inert atmosphere in order to avoid any contamination and to maintain a medium with a constant optical index, which is necessary for good propagation of the beam. These conditions make it possible for the properties relating to the beam diameter and the transverse distribution of the beam energy, and for the beam quality properties, to remain satisfactory for the method, the quality factor for beam parameter product (BPP) of the high-power CO.sub.2 laser beams used in cutting generally being between 3 mm.mrad and 6 mm.mrad. Such an atmosphere also makes it possible to preserve the guiding optics and to prevent them from deteriorating. [0009] Now, this is not practical in an industrial situation and incurs additional costs. [0010] In an attempt to alleviate these problems, it has been proposed to cut stainless steel with a laser device of the Nd:YAG type within which the beam is generated by a resonator containing a solid amplifying medium, that is to say a neodymium(Nd)-doped YAG rod, and sent via an optical fiber to the focusing head. [0011] However, this solution is not entirely satisfactory from the industrial standpoint either. [0012] This is because it has been found that cutting with a laser beam obtained with an Nd:YAG laser source with a wavelength of 1.06 .mu.m gives poor results in terms of cutting quality and cutting speed. [0013] This is because Nd:YAG-type lasers have quality factors unsuitable for the laser cutting process. The quality factors (BPP values) of these lasers are typically in the range from around 15 mm.mrad to 30 mm.mrad, depending on the source. Now, the higher the quality factor of a laser, i.e. the higher the product of the focused beam waist multiplied by the beam divergence, the less effective the laser beam for the laser cutting process. [0014] In addition, the transverse energy distribution in a focused Nd:YAG laser beam is not Gaussian but has a top-hat profile, while beyond the focal point the transverse energy distribution is random. [0015] More generally, to cut stainless steel by laser cutting with an Nd:YAG laser is far from being obvious when it is desired to achieve cutting speeds and cutting qualities that are acceptable from the industrial standpoint. [0016] The problem that arises is therefore how to provide an improved and industrially acceptable method for cutting stainless steels with a laser beam, which can achieve, depending on the thickness in question, speeds ranging up to 15 to 20 m/min, or even higher, and good cutting quality, that is to say straight cutting faces, no burrs and reduced roughness. [0017] The solution provided by the invention is therefore a laser cutting method for cutting a stainless steel workpiece, in which laser beam generation means comprising at least one ytterbium-containing fiber for generating a laser beam are used to melt the workpiece and thereby perform the actual cutting, characterized in that the quality factor of the laser beam is between 0.33 and 8 mm.mrad. [0018] The laser beam generation means comprise an exciter, preferably several exciters, which cooperate with at least one excited element, also called amplifying medium, in order to generate the laser beam. The exciters are preferably several laser diodes, while the excited elements are fibers, preferably silica fibers with an ytterbium-doped core. [0019] For the purpose of the invention, the terms "laser beam generation means" and "resonator" will be used indiscriminately. [0020] Depending on the case, the method of the invention may include one or more of the following features: [0021] the fiber(s) is(are) formed from an ytterbium-doped core clad with silica; [0022] the laser beam generated by the ytterbium-based fiber has a wavelength between 1 and 5 .mu.m, preferably between 1.04 and 3 .mu.m; [0023] the laser beam generated by the ytterbium-based fiber has a wavelength between 1.07 and 1.09 .mu.m, preferably of 1.07 .mu.m; [0024] the laser beam has a power of between 0.1 and 25 kW, preferably between 0.5 and 15 kW; [0025] the laser beam is a continuous or pulsed laser beam, preferably a continuous laser beam; [0026] the workpiece to be cut has a thickness between 0.25 and 30 mm, preferably between 0.40 and 20 mm; [0027] the cutting speed is between 0.1 and 25 m/min, preferably from 2 to 20 m/min; [0028] the assistance gas for the laser beam is chosen from nitrogen, helium, argon and mixtures thereof, and, optionally, it further contains one or more additional compounds chosen from O.sub.2, CO.sub.2, H.sub.2, CH.sub.4, etc.; [0029] the quality factor of the laser beam is between 1 and 8 mm.mrad, preferably greater than 2 mm.mrad, even more preferably greater than 3 mm.mrad and/or preferably less than 7 mm.mrad and even more preferably less than 5 mm.mrad; [0030] more generally, the assistance gas pressure is between about 8 bar and 25 bar, and is chosen according to the thickness that is to be cut; and [0031] the diameter of the gas injection orifice is between 0.5 and 4 mm, typically between 1 and 3 mm, the diameter increasing with the thickness of the workpiece to be cut. [0032] FIG. 1 appended hereto is a diagram showing the principle of an installation for implementing a laser cutting method using a laser beam 3 to cut a stainless steel workpiece 10, employing a laser source 1 with a resonator or laser beam generation means 2 formed by silica fiber with an ytterbium-doped core to generate the laser beam 3. [0033] The laser source 1 is used to generate a laser beam 3 with a wavelength between 1 .mu.m and 5 .mu.m, more precisely, at 1.07 .mu.m. Continue reading... Full patent description for Method for cutting stainless steel with a fiber laser Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method for cutting stainless steel with a fiber laser patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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