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

Автор: Sindo Kou
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119524915
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wine‐cup‐shaped weld is common in keyholing PAW.

Schematic illustration of a plasma arc weld made in 13-mm-thick 304 stainless steel with keyholing.

      Source: Lesnewich [8]. © Welding Research Council.

      1.3.3.4 Advantages and Disadvantages

      PAW has several advantages over GTAW. With a collimated arc, PAW is less sensitive to unintentional arc length variations during manual welding. The short arc length in GTAW can cause a welder to unintentionally touch the weld pool with the electrode tip and contaminate the weld metal with tungsten. However, PAW does not have this problem since the electrode is recessed in the nozzle. As already mentioned, the keyhole is a positive indication of full penetration, and it allows higher welding speeds to be used in PAW.

      1.3.4 Gas–Metal Arc Welding

      1.3.4.1 The Process

Schematic illustration of the gas–metal arc welding including (a) overall process and (b) welding area enlarged. Schematic illustration of the gas–metal arc welds in 6.4-mm-thick 5083 aluminum made with argon (left) and 75 percent He–25 percent Ar (right).

      Source: Gibbs [9]. Welding Journal, December 1980, © American Welding Society.

      1.3.4.2 Shielding Gases

      Argon, helium, and their mixtures are used for nonferrous metals as well as stainless and alloy steels. The arc energy is less uniformly dispersed in an Ar arc than in a He arc because of the lower thermal conductivity of Ar. Consequently, the Ar arc plasma has a very high energy core and an outer mantle of lesser thermal energy. This helps produce a stable, axial transfer of metal droplets through an Ar arc plasma. The resultant weld transverse cross section is often characterized by a papillary‐ (nipple‐) type penetration pattern [10] such as that shown in Figure 1.19 (left) and subsequently in Figure 16.3a. With pure He shielding, on the other hand, a broad, parabolic‐type penetration is often observed.

      1.3.4.3 Modes of Metal Transfer

      The molten metal at the electrode tip can be transferred to the weld pool by three basic transfer modes: globular, spray, and short‐circuiting:

      1 Globular transfer. Discrete metal drops close to or larger than the electrode diameter travel across the arc gap under the influence of gravity. Figure 1.20a shows globular transfer during GMAW of steel at 180 A and with Ar–2% O2 shielding [11]. Globular transfer is often not smooth and produces spatter. At a relatively low welding current globular transfer occurs regardless of the type of the shielding gas. With CO2 and He, however, it occurs at all usable welding currents. As already mentioned, a short buried arc can be used in CO2‐shielded GMAW of carbon and low‐alloy steels to minimize spatter.

      2 Spray transfer. Above a critical current level, small discrete metal drops travel across the arc gap under the influence of the electromagnetic force at a much higher frequency and speed than in the globular mode. Figure 1.20b shows spray transfer during GMAW of steel at 320 A and with Ar–2% O2 shielding [11]. Spray transfer is much more stable and spatter free. The critical current level depends on the material and size of the electrode and the composition of the shielding gas. In the case of Figure 1.20, the critical current was found to be between 280 and 320 A [11].

      3 Short‐circuiting transfer. The molten metal at the electrode tip is transferred from the electrode to the weld pool when it touches the pool surface, that is, when short circuiting occurs. Short‐circuiting transfer encompasses the lowest range of welding currents and electrode diameters. It produces a small and fast‐freezing weld pool that is desirable for welding thin sections, out‐of‐position welding (such as overhead‐position welding) and bridging large root openings in multiple‐pass butt welding with a V‐groove.

Schematic illustration of the metal transfer during GMAW of steel with Ar–2 percent O2 shielding: including (a) globular transfer at 180 A and 29 V shown at every 3 × 10-3 s, (b) spray transfer at 320 A and 29 V shown at every 2.5 × 10-4 s.

      Source: Jones, Eagar and Lang [11]. Welding Journal, April 1998, © American Welding Society.

      1.3.4.4 Advantages and Disadvantages

      Like GTAW, GMAW can be very clean when using an inert shielding gas. The main advantage of GMAW over GTAW is the much higher deposition rate, which allows thicker workpieces to be welded at higher welding speeds. The dual‐torch and twin‐wire processes further increase the deposition rate of GMAW [12]. The skill