Welding Metallurgy. Sindo Kou. Читать онлайн. Newlib. NEWLIB.NET

Автор: Sindo Kou
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119524915
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welds with a depth–width ratio of 3 : 1 or better in 3‐mm‐thick sheets. Fiber lasers, on the other hand, can provide highly focused laser beams. The wavelength is 1.06 μm [20].

      1.4.2.2 Reflectivity

      The very high reflectivity of a laser beam by the metal surface is well‐known in LBW. As much as about 95% of the CO2 beam power can be reflected by a highly polished metal surface. Reflectivity is slightly lower with a YAG laser beam due to its lower wavelength. Surface modifications such as roughening, oxidizing, and coating can reduce reflectivity significantly [21]. Once keyholing is established, absorption is high because the beam is trapped inside the keyhole by internal reflection.

      1.4.2.3 Shielding Gas

      A plasma (an ionic gas) is produced during LBW, especially at high power levels, due to ionization of metal vapor by the laser beam. The plasma can absorb and scatter the laser beam and reduce the depth of penetration significantly. It is therefore necessary to remove or suppress the plasma [22]. The shielding gas for protecting the molten metal can be directed sideways to blow and deflect the plasma away from the beam path. Helium is often preferred to argon as the shielding gas for high‐power LBW because of greater penetration depth [23].

      Since the ionization energy of helium (24.5 eV) is higher than that of argon (15.7 eV), helium is less likely to be ionized and become part of the plasma than argon. However, helium is lighter than air and is thus less effective in displacing air from the beam path. He‐10% Ar shielding has been found to improve penetration over pure He at high‐speed welding where a light shielding gas may be less able to displace air from the beam path [24].

      1.4.2.4 Laser‐Assisted Arc Welding

Graph depicts the weld penetration in Gas-metal arc welding and laser-assisted GMAW using carbon dioxide laser at 5.7 kW.

      Source: Hyatt et al. [25]. Welding Journal, July 2001, © American Welding Society.

      1.4.2.5 Advantages and Disadvantages

      Like EBW, LBW can produce deep and narrow welds at high welding speeds, with a narrow HAZ and little distortion of the workpiece. (Penetration depth can be up to about 20 mm, far less than in EBW.) It can be used for welding dissimilar metals or parts varying greatly in mass and size. Unlike EBW, however, vacuum and X‐ray shielding are not required in LBW. However, the reflectivity of a laser beam can be high. As with EBW, the equipment cost is very high, and precise joint fit‐up and alignment are required.

Schematic illustration of the resistance spot welding which shows the (a) overview, (b) resistance to electric current and (c) weld nugget.

      The electrode clamping force need to be sufficient to keep the liquid in the nugget from being expelled, called expulsion or spitting. The lower density of liquid than solid causes volume expansion upon melting and promotes expulsion. Expulsion can leave insufficient liquid in the nugget to form a continuous solid weld, resulting in large voids and weakening the weld.

      The key process variables include the welding current (usually several thousands to tens of thousands of amperes), welding time, clamping force, and electrode shape. The copper electrodes are often alloyed with refractory metals to improve erosion resistance. It is desirable to have a nugget with a diameter about 3.5–4 times the sheet thickness and a height about two‐thirds the combined thickness of the sheets [27]. To reduce erosion, some electrode tips are coated with special materials.

Schematic illustration of the resistance spot welding of dissimilar metals with highly different thermal conductivities.

      1.6.1 Friction Stir Welding

Schematic illustration of the friction stir butt welding: (a) tool, (b) photo of too, (c) butt welding (d) tilting tool forward, usually by about 2 degree.

      To keep the