WELDING RESEARCH Fig. 10 — Three successive acquisitions of an AC process a few hundred μs apart. They go from positive phase via current zero to negative. Marked is the change in the spectrum from Mn (Ref. 16, SOM9). DECEMBER 2016 / WELDING JOURNAL 497-s the tunnel (Fig. 1). The upper part in the spectra represents the upper region in the middle of the images and the lower part in the spectrum, which is the lower part in the middle of the image. Figure 10 shows frames from Ref. 16 (SOM9). The images show both high-speed images of the process and synchronized spectra in combination. The recorded spectra were dominated by iron (Fe), calcium (Ca), selfreversed sodium (Na) lines, and manganese (Mn) (compare to Fig. 9). Noticeable is the pair of Na lines at the center with around 589 nm, which appear as one dark stripe because they are strongly self-reversed. Most lines below 580 nm are from Fe vapor. Both the line groups between 610 and 620 nm, and 643 and 652 nm, and with three lines each are from Ca. In the center image of Fig. 10 there is another set of spectral lines visible at around 601–602 nm, which are missing in the frames before and after (see left and right image in Fig. 10). These lines originate from Mn (601.4, 601.7, and 602.2 nm), and are only detectable during the phase around current zero. This phase obviously has a lower arc temperature, and the composition of the plasma allows these lines to be emitted. Just before the positive phase at higher currents, there are many more Fe lines visible and the Na line is also more intense. A similar situation is visible in the negative phase. It might be because of the low boiling temperature of Mn that its lines are visible at all. Otherwise, the spectra show an Fedominated arc. This is consistent with the earlier presented observation made by the high-speed imaging. Both suggest that the main current path is situated below the droplet that is still attached to the wire. Therefore, the droplet transfer in SAW has similarities to a CO2 GMAW process, although its atmosphere is different. In CO2, the main current path exits at the wire tip in contrast to the Ardominated GMAW processes where it exits the wire above the liquid part of the wire. This mechanism is necessary in certain GMAW processes to achieve globular and spray transfer (Ref. 21). The same effect was stimulated in SAW when the shielding gas introduced into the tunnel was set to an excessively high pressure and Ar entered the cavern. Under these circumstances, the droplet transfer changed to a constricted spray transfer without any short circuits. This had to be avoided to maintain a diagnostic method with as little influence as possible. Chemical Analysis. The weld joints were analyzed by OES. The samples for the OES of the weld metal were collected from bead-on-plate welds with eight layers to avoid dilution with the base material. Only a few changes were found in the chemical composition of the main alloying elements within the varying processes (Table 1). This can be attributed to the chemically neutral character of the flux and the low amount of alloying elements in the wire. As can be observed, slight changes occur by varying polarity with most melting loss of alloying elements in DCEN and AC processes. This is especially the case for alloying elements with a high affinity to oxygen like carbon, aluminum, and titanium. Oxygen is an important element in welding metallurgy and can act both positively and negatively on microstructure formation. In a balanced, low amount, oxygen plays an important role in nucleation and can support a fine-grained microstructure formation with improved toughness and tensile strength. In interaction with titanium, boron, or other microalloying elements, this effect is enhanced (Ref. 12). In contrast, a high amount of oxygen in the weld joint leads to embrittlement and porosity. Therefore, optimized oxygen content is ideal for adequate mechanical properties. In sub- A B C
Welding Journal | December 2016
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