Assembly Planbook - April 2008 - (Page 8) Welding BEADS Electrons and Lasers eams of electrons and beams of coherent light are both excellent tools for welding metal parts. Both electron beam and laser welding excel at joining exotic and refractory metals and alloys, as well as common metals and alloys, and—in the case of laser welding— plastics. In many applications they operate as autogenous processes, fusing two pieces of the same type of metal without the addition of filler metal. Electron beam welding produces a weld by focusing a beam of high-energy electrons on the interface between the pieces being joined. The kinetic energy of the electrons is transformed into heat upon impact, melting the workpieces and—if used—the filler metal. The workpieces can be held in a fixture and moved under a fixed electron beam, or the beam can be moved along the weld seam. Either way, as the beam moves away from the molten metal it solidifies to form the welded joint. First used mainly in the aerospace and nuclear industries, electron beam (EB) welding became a viable production process in the 1950s and can weld a wide variety of metals, including dissimilar metals. The very high energy density of the electron beam can weld small and delicate components using just a few watts of power. It can also weld steel parts up to 12 inches thick using 100 or more kilowatts, although most applications involve parts less than 0.5 inch thick. It is ideal for welding refractory metals and combinations of dissimilar metals not easily welded with other techniques. When it was first developed, EB welding was carried out in a high vacuum, and some applications still require that environment. It can also be carried out in partial vacuum, and even in no vacuum, for mass production applications where high output is required. EB welding functions in either of two modes: conductance or keyhole. In the conductance mode—primarily used for thin metals—the joint area is rapidly heated to melting temperature at or just below the metal surface and heat is conducted throughout the joint for complete penetration. The resultant weld is very narrow because the high energy density allows rapid travel speeds, and melting and solidification B EB and laser welding excel at joining exotic and refractory metals and alloys. ■ By Don Hegland Editorial Director occur so rapidly that very little heat flows away from the joint. The keyhole mode is used when deep penetration is required, typically for thick parts. The concentrated energy enables the focused beam to penetrate below the surface, vaporizing material to drill a hole. Rapid vaporization and sputtering in this hole cavity develops enough pressure to suspend the liquidus material against the hole walls. As the hole advances along the weld joint, the molten material flows around the beam, filling the hole and coalescing to produce a fusion weld. Both modes result in narrow welds and minimal heat affected zones. Laser welding produces a weld by focusing an intense beam of monochromatic, coherent light on the interface between the pieces being joined. The large concentration of light energy is converted to thermal energy upon absorption at the surface, melting the workpieces at the surface. As in EB welding, either the workpieces or the laser beam can be moved to weld a seam. Also as in EB welding, laser welding functions in either the conductance mode or the keyhole mode. The two types of lasers most commonly used in industrial welding are Nd:YAG (neodymium-yttrium-aluminum garnet) and CO2 lasers. Both operate in the infrared, invisible to the human eye. Nd:YAG lasers produce light in the near infrared, which is absorbed quite well by conductive materials. They can produce power outputs up to about 500 watts. Nd:YAG and other solid state lasers are generally preferred for low to moderate power applications and are used extensively in the electrical and electronics industries. CO2 lasers operate in the far infrared, which is strongly reflected by most metals. This requires special optics to focus the beam narrowly enough to yield the energy density needed for welding. However, CO2 lasers can easily produce power outputs of 10 kilowatts or more. This enables the CO2 laser to overcome the high reflectance by operating in the keyhole mode. Once the metal surface approaches its melting point, the reflectivity drops and the absorption approaches blackbody. As a result, CO2 lasers are well suited for deep penetration welding of thick parts. A 8 ASSEMBLY / April 2008 www.assemblymag.com http://www.assemblymag.com
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