Machinery's Handbook, 31st Edition
1346 Electro-Thermal Processes industry for many years. Fiber lasers are a newer solid-state technology that uses an Yb- doped optical fiber to amplify, contain, and direct a diode-generated laser beam; when the amplified light exits the fiber, a lens system collimates and focuses the beam. Thin disk (Yb:YAG) lasers are another new solid-state technology that provides power and beam quality comparable to fiber lasers. Most laser cutting systems in industry today use either fiber lasers or CO 2 lasers, and thin disk lasers are gaining in popularity. Fiber and disk laser systems have the advantage of providing efficiency typically around 30 percent, while CO 2 lasers typically operate at 6 to 8 percent efficiency. Additional advantages of fiber and disk lasers are that they are made of solid-state components and do not require the high-speed turbines used in CO 2 laser systems, reducing periodic main- tenance costs. However, CO 2 lasers often are preferred for cutting non-metals, such as ceramics, plastics, and various composites, due to their longer wavelength. Most laser systems operate within an interlocked enclosure capable of blocking harmful light emissions, enclosing moving parts, and managing cutting fumes. Unlike CO 2 sys- tems, fiber and disk laser light is delivered through an optical fiber, eliminating the need for periodic cleaning and alignment of the delivery system. Wavelengths for common industrial lasers are as follows: CO 2 = 10.6 µ m, Nd:YAG = 1.06 µ m, thin disk Yb:YAG = 1.03 µ m, and fiber = 1.07 µ m. Due to increased absorptivity of the laser beam at shorter wavelengths, high cutting speeds are possible when cutting steel less than 0.2 in. (5 mm) thick with solid state lasers. For example, fiber-delivered systems can cut up to three times faster than CO 2 systems at the same power level. Laser Cutting Reflective Materials: Laser cutters, CO 2 systems in particular, are vulnerable to damage due to back-reflection when processing reflective workpieces. Back-reflections that reach the laser cavity can cause instabilities in laser output or even destroy laser components. Most high-power CO 2 laser manufacturers do not warranty their equipment for processing highly reflective metals, and devices available to protect CO 2 lasers from unwanted back reflection are bulky, expensive, and impractical for high-powered industrial uses. For fiber-delivered lasers, both the forward- and backward-traveling light is confined to the optical fiber. So back-reflection isolators designed to protect the laser can be compact fiber components, incorporated either near the output of the delivery fiber or at the output of the laser. When the workpiece is reflective, some portion of laser light is reflected instead of being absorbed to supply cutting energy. Metals reflect laser light at increasing percentages with increasing laser wavelength. Increasing laser power or using a shorter wavelength can help overcome reflectivity problems. Laser Cutting Assist Gas: A gas jet emitting from the laser head is used to eject molten material from the cut, protect the cutting head from splatter, and either react with the material or shield it. Oxygen assist gas reacts with the workpiece material and leaves an oxidized cut edge. The exothermic reaction adds energy to the cutting process and can enhance the depth or cutting speed; however, secondary operations may be necessary to remove an oxide layer if the laser-cut metal part is to be welded or painted. Low-pressure oxygen assist gas can be used when cutting carbon steel. It performs this function economically due to the low flow rates and increased thickness capability af - forded by the exothermic reaction. Oxygen assist can make cuts in stainless steel or alumi- num possible with power levels that would be inadequate without oxygen. This facilitated initial development of industrial laser cutting, when laser power was limited and very expensive. However, use of oxygen assist to cut stainless steel has some disadvantages: cut edge discoloration (which may be unacceptable for cosmetic reasons), reduced corrosion resistance (due to the formation of chromium oxides), and dross formation (due to the higher melting point and surface tension of chromium oxide). For these reasons, and with the increased viability of higher laser power, the use of oxygen in laser cutting of stainless steel is no longer common. In current practice, high-pressure nitrogen is the most commonly used assist gas for cutting stainless steels and aluminum with high-power lasers. This inert gas’s shielding
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