Welding Metallurgy. Sindo Kou

the electric current enters the electrode. Consequently, the cathode spot moves around rather randomly and quickly in the flat electrode end. The time‐averaged diameter of the area covered by the moving spot can be considered as the effective cathode spot size. Without a conical surface at the electrode end, the resultant Lorentz force is still inward and downward, but the downward component is reduced as illustrated in Figure 3.5a. Consequently, the resultant arc can be expected to be more constricted, as shown in Figure 3.5b.

Figure 3.5 Arc produced by a tungsten electrode with a flat end: (a) Lorentz force (F); (b) fluid flow.

Figure 3.6 (left) shows the current density distribution in a 2 mm long, 200 A arc produced by a 3.2 mm diameter electrode with a flat end and a 2.4 mm‐diameter cathode spot. The Lorentz force, shown in Figure 3.6 (right), is less downward pointing than that in the case of a 60°‐angle electrode (Figure 3.3). As shown in Figure 3.7, the isotherms are not pushed outward as much, and the resultant arc is thus more constricted.

Figure 3.6 Current‐density field (left) and Lorentz force (right) in an arc produced by a tungsten electrode with a flat end.

      Source: Tsai and Kou [1]. © Elsevier.

Figure 3.7 Velocity and temperature fields in an arc produced by a tungsten electrode with a flat end. The isotherms from right to left are 11 000, 13 000, 15 000, 17 000, 19 000 and 21 000 K.

      Source: Modified from Tsai and Kou [1]. © Elsevier.

3.2 Effect of Metal Vapor on Arcs

      3.2.1 Gas−Tungsten Arc Welding

Tanaka et al. [7] showed that Fe vapor can significantly increase the electrical conductivity of the welding arc. As can be expected, the electrical conductivity of He is lower than that of Ar because He has a significantly higher ionization energy than Ar (Table 1.2). That is, He is more difficult to dissociate into electrons and positive ions to help conduct the welding current as explained in Chapter 1. Figure 3.8 shows that He has lower conductivity than Ar, and also shows that the Fe vapor can significantly increase the electrical conductivity of both Ar and He at lower temperatures. The distributions of temperature and Fe vapor were calculated for stationary arcs over 304 stainless steel at 150 A current and 5 mm arc length. They showed significantly more Fe vapor in the He arc than the Ar arc. After welding for 20 seconds, much Fe vapor deposit appeared around the He weld but little around the Ar weld, as shown in Figure 3.9. Without considering the Fe vapor, the calculated current density at the pool surface is too high and the calculated weld too deep.

Figure 3.8 Electrical conductivity of Ar and He and how they are affected by Fe vapor.

      Source: Tanaka et al. [7]. © IOP Publishing.

Figure 3.9 304 stainless steel welded by stationary gas‐tungsten arc for 20 s: (a) He as shielding gas, showing much Fe vapor deposit; (b) Ar as shielding gas, showing little Fe vapor deposit; (c) cross‐section of weld made with He shielding.

      Source: Tanaka et al. [7]. © IOP Publishing.

Kodama et al. [8] studied metal evaporation and its effect on nitrogen absorption in GTAW of steel. Figure 3.10 shows the calculated distributions of temperature and Fe vapor in gas‐tungsten welding arcs (150 A current and 5 mm arc length) with Ar shielding and with He shielding. With He the electrical resistance to the welding current is higher, the arc is warmer, the weld pool surface is warmer, and Fe evaporation is more significant. The calculated maximum pool surface temperature is 2870 K for the He arc and 2450 K for the Ar arc. Although the He arc is warmer as a whole, it is cooler immediately (0.1 mm) above the pool surface (5900 K) because the Fe vapor there helps it conduct the welding current. Thus, as compared to a shielding gas of Ar + 1% N₂, with He + 1% N₂, nitrogen dissociation (N₂ → 2N) in the arc near the pool surface and hence N dissolution in the pool (denoted as N) may be expected to be less significant. This was used to explain why nitrogen porosity is observed in a steel made by GTAW with Ar + 1% N₂ but not with He + 1% N₂.

Figure 3.10 Computer simulation of gas‐tungsten arcs considering metal evaporation from pool: (a) (b) Ar arc; (c) 1% N₂ in Ar causing porosity; (d) (e) He arc. (f) 1% N₂ in He causing no porosity.

      Source: Kodama et al. [8]. Springer Open Access.

      3.2.2 Gas−Metal Arc Welding

      Murphy [9–13] studied the effect of metal evaporation on gas−metal arc welding (GMAW) of Al alloys. In GMAW significant evaporation of metal elements can occur at the tip of the filler metal, which will be called the wire here for simplicity. The wire is usually positive in polarity, and the electrons enters the melted wire tip and release much energy equivalent to the work function (Section 1.3.2.2). At the anode spot at the melted wire tip, the boiling point of the wire material is achieved. The concentration of metal vapor in the arc can become as high as 50% [10–12].

Figure 3.11 shows in GMAW lap welding the arc temperature and Al‐vapor distributions in a vertical plane through the Al wire electrode and normal to the workpiece [13]. At the anode spot much Al is evaporated and carried downward as an Al‐rich (>50% Al) stream to the weld pool. According to Murphy [11], the presence of aluminum vapor strongly decreases the arc temperature. For instance,