WELDING RESEARCH Fig. 4 — Front view illustrating material transfer of the DCEP process with 600 A (Ref. 16, SOM2). Table 1 — Main Chemical Composition of Used Materials (Values in wt%) Element C Si Mn P S Al Ti B Fe Foil 0.0038 0.0290 0.2470 0.0260 0.0110 – – – 99.2440 Base Material 0.0720 0.4060 1.4100 0.0190 0.0250 0.0230 0.0029 0.0042 97.8500 Wire (factory certificate) 0.0800 0.3500 1.6000 <0.01 <0.01 <0.01 – – >97.83 DCEP 600 A 0.1300 0.3100 1.5500 0.0130 0.0069 0.0086 0.0042 0.0055 97.8200 DCEN 0.1080 0.2970 1.4700 0.0170 0.0053 0.0028 0.0027 0.0061 97.9400 AC 0.0890 0.3070 1.4400 0.0280 0.0066 0.0030 0.0036 0.0074 97.9400 DCEP 1000 A 0.1110 0.2980 1.5600 0.0170 0.0050 0.0090 0.0040 0.0058 97.7600 DECEMBER 2016 / WELDING JOURNAL 493-s ties to be able to change the resolution. The spatial distribution could be recorded with an optical system that contained spherical and several planar mirrors, an edge filter, and an adjustable aperture. Therefore, it was possible to distinguish between the different areas inside the cavern and determine where the different species were located. Optical emission spectroscopy (OES) had been performed with a Spectromaxx by SPECTRO. The determination of oxygen had been implemented through carrier-gas melt extraction with a Bruker Elemental G8 GALILEO ON/H analyzer. After finishing the welding process, the remaining droplets were collected and carefully cleaned from the remaining scale for the carrier-gas melt extraction. In this setup, the welding head was fixed and the base material was moved by using a linear table with a constant velocity of 1000 mm/min. This was necessary to keep the arc in constant focus for the optical diagnostics. An overview of the whole setup is shown in Fig. 1. It consists of two HSCs and a spectrometer. The welding was performed with an inverter power source (Lincoln AC/DC 1000) with a maximum current of ±1000 A. A constant current welding characteristic was chosen. In this paper, the single-wire SAW process was analyzed with four varying parameter settings. The four parameter changes that were observed with the diagnostics are given in Table 2. The materials were not altered. The wire was a Lincoln Electric L50M (EN ISO 14171 S3Si) with a diameter of 4 mm, and the base material was an EN 10025 S355 J2+N. The main chemical composition of the materials is listed in Table 1. The flux used was a Lincolnweld 8500 (EN 760 – S A FB 1) with a basicity index of 2.9 and a neutral chemical behavior. The flux composition is listed in Table 3. The welding and wire-feed speed was constant. The height of the pile of flux was kept constant as well. This was necessary to keep the basic conditions steady. The pressure that the flux applied to the cavern was about 0.05 g/mm². The gas pressure that impinged on the cavern through the tunnel had to be finely tuned to the pressure inside the cavern. If the shielding gas pressure is too high, it will be injected into the cavern’s atmosphere and influence the process. In the case of argon (Ar), it would change the process to a spray transfer similar to the GMAW process. If the pressure is too low, the cavern will shrink, which is visible in the weld joint profile. In addition, the tunnel tends to be clogged with debris. With a balanced setting of the gas pressure, the influence on the process is minimized and the view into the cavern is unobstructed. The best results were achieved by using Ar at an overpressure of 25 mbar. Carbon dioxide (CO2) and Ar were investigated as applicable shielding gases. None of the gases in the preliminary trials changed the chemical composition of the weld deposit. Nonetheless, changes in the chemical compounds of the molten slag, investigated by x-ray fluorescence (XRF), were observed using CO2 — Fig. 2. Furthermore, the measured shortcircuit frequency changed from 3.6 Hz in the unaffected welding process to 4.2 Hz by injecting CO2 as a shielding gas. Short-circuit frequency was constant while using the inert gas Ar. This indicated that the use of CO2 as a shielding gas is more invasive to the process. Table 2 — Parameter Variation Process Identifier Current Voltage DC+ 600 A 30 V DC −600 A −30 V AC 600 A 30 V DC++ 1000 A 34 V
Welding Journal | December 2016
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