Welding Metallurgy. Sindo Kou

Figure 3.31 New theory explaining effect of sulfur shown in Fig. 3.29: (a) (b) low S, i.e. dγ /dT < 0; (c) (d) high S, i.e., dγ /dT > 0.

      Source: Kou, Limmaneevitchitr, Wei [32]. Welding Journal, December 2011, © American Welding Society.

3.6 Active Flux GTAW

The use of fluxes in GTAW has been found to increase weld penetration in steels and stainless steels dramatically [56–59]. The flux usually consists of oxides and halides, and it is mixed with acetone or the like to form a suspension and painted as a thin coating over the area to be welded. Figure 3.32 shows transverse cross‐sections of bead‐on‐plate welds of 304 stainless steels made by GTAW without and with a commercial active flux called A‐TIG [59]. Two different 304 stainless steels were welded, one with 42 ppm S and the other 140 ppm S. In the case of low S (42 ppm), the weld penetration is clearly deeper with the active flux. In the case of high S (140 ppm S), the effect of the active flux is similar but smaller.

Figure 3.32 Transverse cross‐sections of 304 stainless steel welds: (a) 42 ppm S (low) without flux; (b) 42 ppm S with active flux (A‐TIG); (c) 140 ppm S (high) without flux; (d) 140 ppm S with active flux.

      Source: Yu and Kou [59].

      Howse and Lucus [56] observed that the arc becomes more constricted when a flux is used. Consequently, they proposed that the deeper penetration is caused by arc constriction and that the vaporized flux constricts the arc by capturing electrons in the cooler outer region of the arc. For the same welding current, the more the arc is constricted, the greater the Lorentz force (F in Figure 3.15c) and deeper penetration.

      Tanaka et al. [57], however, proposed that the oxygen from the oxide‐containing flux causes an inward surface flow and hence a deeper penetration. They GTA welded a 304 stainless steel containing little (0.002 wt%) S, with He for shielding and TiO₂ as the flux. They observed both an inward surface flow and a significant decrease in the surface tension when the flux is used. As mentioned previously, ∂γ/∂T can become positive and cause inward surface flow in the presence of a surface‐active agent such as oxygen. They observed a steep temperature gradient across the pool surface caused by the inward surface flow. Spectroscopic analysis of the arc showed that the blue luminous plasma appears to be mainly composed of metal vapor (Cr, Fe, etc.) from the weld pool. The steeper temperature gradient across the smaller pool surface suggests a more localized metal evaporation and hence a more constricted arc, which also help increase the penetration.

3.7 Resistance Spot Welding

      Wei et al. [60] pioneered the computer simulation of heat transfer and fluid flow in RSW. They further conducted a series of fundamental studies that better explained RSW [61–65], such as the effect of the magnetic properties of the workpiece, the contact resistance at the faying surface, the electrode‐workpiece interfaces, and the geometry of the electrode. Their studies can be a useful tool for further understanding of heat transfer and fluid flow in RSW without or with an externally applied magnetic field [66–70].

Recently, permanent magnets have been mounted on the electrodes in RSW to widen the nugget and increase the joint strength of the resistance spot weld. Such magnets, for example, can be 10 mm thick, 100 mm outer diameter and 40 mm inner diameter [66]. Figure 3.33 shows that, without the magnets the magnetic field (called induced magnetic field) in the faying surface is in the azimuthal (or circumferential) direction and the corresponding Lorenz force is inward toward the centerline of the electrodes. With the magnets, however, the magnetic field (called the external magnetic field) in the faying surface is outward in the radial direction and the corresponding Lorentz force is in the azimuthal direction.

Figure 3.33 RSW and MA‐RSW: (a) RSW; (b) MA‐RSW; (c) magnetic field and Lorentz force in RSW; (d) magnetic field and Lorentz force in MA‐RSW. MA: magnetic assisted.

      Source: Yao, Luo, Li, Yan, and Duan [66]. © Elsevier.

The Lorentz force acts as the driving force for fluid flow in the nugget. Since the size of the nugget increases with time, the flow pattern in the nugget changes significantly with time. Li et al. [67] calculated fluid flow and heat transfer in the nugget in RSW with the induced magnetic field, i.e., without external magnets. Figure 3.34a shows the calculated flow field at 0.35 s after welding starts. The maximum velocity is about 0.43 m/s outward, located on the faying surface at about three‐quarters of the nugget radius from the electrode axis. The flow pattern is sketched schematically in Figure 3.34b. It shows two toroidal‐shaped flow loops near the edge of the nugget, one above the faying surface and the other below it. Both loops show a radially outward flow at the faying surface. The two big arrowheads at the faying surface indicate the location of the maximum velocity. The temperature field in Figure 3.34c shows a maximum temperature of about 1797 °C near the nugget edge. Further computer modeling that includes external magnets may help explain their effect on heat transfer and fluid flow in RSW.

Figure 3.34 Calculated results of RSW with induced magnetic field: (a) velocity field at time = 0.35 s; (b) flow loops illustrating velocity field in (a); (c) temperature field at time = 0.35 s.

      Source: Li, Lin, Shen, and Lai [67]. © Wiley.

The nugget diameter can be increased significantly with the help of the magnets. In Figure 3.35 it increases from 4.21 to 4.70 mm [69]. With an increased nugget diameter, the bonding area between the upper and lower sheets and hence the joint strength can increase significantly. In fact, since the liquid in the nugget is stirred by fluid flow during solidification, grain refining can also occur to improve the joint strength. In tensile‐shear testing, the failure mode can change from the interfacial mode of weaker welds to the button‐pullout mode of stronger welds when magnets are used. The load‐displacement curves confirm the improved weld strength and ductility. Similar improvement was also observed in RSW of AZ31 Mg [66]. Finally, it is interesting to note that magnets can also improve Al/Ti dissimilar‐metal RSW [71]. Grain refining was observed in the weld made with magnets, again suggesting enhanced electromagnetic stirring.