Welding Metallurgy. Sindo Kou

beam of very high intensity can vaporize the metal and form a vapor hole during welding, that is, a keyhole, as illustrated in Figure 1.26b.

Figure 1.26 Electron beam welding: (a) process; (b) keyhole.

Figure 1.27 shows that the beam diameter decreases with decreasing ambient pressure [1]. Electrons are scattered when they hit air molecules, and the lower the ambient pressure, the less they are scattered. This is the main reason for EBW in a vacuum chamber.

Figure 1.27 Dispersion of electron beam at various ambient pressures [1].

      Source: Welding Handbook, Vol. 3, 7th Edition, © American Welding Society.

The electron beam can be focused to diameters in the range of 0.3–0.8 mm and the resulting power density can be as high as 10¹⁰ W/m² [1]. The very high‐power density makes it possible to vaporize the material and produce a deep‐penetrating keyhole and hence weld. Figure 1.28 shows a single‐pass electron beam weld and a dual‐pass gas–tungsten arc weld in a 13‐mm‐thick (0.5‐in.) 2219 Al, the former being much narrower [16]. The energy required per unit length of the weld is much lower in the electron beam weld (1.5 kJ/cm or 3.8 kJ/in.) than in the gas–tungsten arc weld (22.7 kJ/cm or 57.6 kJ/in.).

Figure 1.28 Welds in 13‐mm‐thick 2219 aluminum: (a) electron beam weld; (b) gas–tungsten arc weld. Source: Farrell [16].

      Under high welding speeds, weld porosity results when gas bubbles do not have enough time to escape from the deep weld pool. Materials containing high‐vapor‐pressure constituents, such as Pb‐containing alloys, are not recommended for EBW because evaporation of these constituents tends to foul the pumps or contaminate the vacuum system.

      1.4.1.2 Advantages and Disadvantages

      With a very‐high‐power density in EBW, full‐penetration keyholing is possible even in thick workpieces. Joints that require multiple‐pass arc welding can be welded in a single pass at a high welding speed. Consequently, the total heat input per unit length of the weld is much lower than that in arc welding, resulting in a very narrow HAZ and little distortion. Reactive and refractory metals can be welded because there is no air in vacuum to cause contamination. Some dissimilar metals can also be welded because the very rapid cooling in EBW can prevent the formation of coarse brittle intermetallic compounds. When welding parts vary greatly in mass and size, the ability of the electron beam to precisely locate the weld and form a favorably shaped fusion zone helps prevent excessive melting of the smaller part.

However, the equipment cost for EBW is very high. The requirement of high vacuum (10⁻³–10⁻⁶ Torr) and X‐ray shielding is inconvenient and time consuming. For this reason, medium‐vacuum (10⁻³–25 Torr) EBW and nonvacuum (1 atm) EBW have also been developed. In addition, the fine beam size requires precise fit‐up of the joint and alignment of the joint with the gun. As shown in Figure 1.29, residual and dissimilar metal magnetism can cause beam deflection and result in missed joints [17].

Figure 1.29 Missed joints in electron beam welds in 150‐mm‐thick steels: (a) 2.25Cr–1Mo steel with a transverse flux density of 3.5 G parallel to joint plane; (b) SB (C─Mn) steel and A387 (2.25Cr–1Mo) steel.

      Source: Blakeley and Sanderson [17]. Welding Journal, January 1984, © American Welding Society.

      1.4.2 Laser Beam Welding

      1.4.2.1 The Process

LBW is a process that melts and joins metals by heating them with a laser beam. The laser beam can be produced either by a solid‐state laser or a gas laser. In either case, the laser beam can be focused and directed by optical means to achieve high power densities. In a solid‐state laser, a single crystal is doped with small concentrations of transition elements or rare earth elements. For instance, in a YAG laser the crystal of yttrium‐aluminum‐garnet (YAG) is doped with neodymium. The electrons of the dopant element can be selectively excited to higher energy levels upon exposure to high‐intensity flash lamps, as shown in Figure 1.30a. Lasing occurs when these excited electrons return to their normal energy state, as shown in Figure 1.30b. The power level of solid‐state lasers has improved significantly, and continuous YAG lasers of 3 or even 5 kW have been developed. YAG laser has a wavelength is 1.06 μm, and it can be transmitted through fiber glass bundles.

Figure 1.30 Laser beam welding with solid‐state laser: (a) process; (b) energy absorption and emission during laser action.

      Source: Welding Handbook [1]. Welding Handbook, Vol. 3, 7th Edition, © American Welding Society.

In a CO₂ laser, a gas mixture of CO₂, N₂, and He is continuously excited by electrodes connected to the power supply and lases continuously. The CO₂ laser has a wavelength of 10.6 μm, too long to be transmitted through fiberglass bundles. Higher power can be achieved by a CO₂ laser than a solid‐state laser, for instance, 15 kW. Figure 1.31a shows LBW in the keyholing mode. Figure 1.31b shows a weld in a 13‐mm‐thick A633 steel made with a 15‐kW CO₂ laser at 20 mm/s [18].

Figure 1.31 Keyhole laser‐beam welding: (a) process; (b) CO₂‐laser weld in 13‐mm‐thick A633 steel.

      Besides solid‐state and gas lasers, semiconductor‐based diode lasers and fiber lasers have also been developed. Diode lasers of 2.5 kW power and 1 mm focus diameter have been demonstrated [19]. Conduction‐mode