A B C D E F granules and dissolved in various chemicals for chemical analysis. The chemical analysis was conducted at the National Analysis Center of Iron and Steel (NACIS) in Beijing, China, according to the National Standards of China (GB/T) and NACIS standards (Refs. 11–16). Microstructural Analysis of the Cladding SXFiveFE, a state-of-the-art field emission electron probe micro analyzer (EPMA) for quantitative analysis and x-ray mapping at high-spatial resolution, was used (Ref. 17). Up to five wavelength dispersive spectrometers (WDS) could be fit into the microprobe for highprecision quantitative analysis. The beam diameter used was 80–100 nm (called “0 m”), the voltage and current being 8 kV and 20 nA, respectively. The volume below the sample surface affected by the beam was about 250 nm in diameter (Ref. 18). The samples were polished but not etched, in order not to affect the composition measurements by EPMA. A plasma cleaner (IBSS GV10X) was used to clean the sample surface. The standards for calibration for EPMA included pure WC, W, C, Ni, Fe, Si, and B. EPMA was done using Ka x-ray lines of B, O, C, and Si, whereas the W Ma line and the Ni Ll lines were used for those elements. Crystals used were: LPC2 for C and B; LPET WELDING RESEARCH for W and Si; and LLIF for Fe, TAP for Ni, and PCO for 0. Counting times were 10 s on peak and 5 s each on two backgrounds. During the initial stage of the present study, SEM and energy dispersive spectrometry (EDS) were also used to identify the particles removed from inside the tubular filler metal. A Bruker D8 Discover diffractometer along with a microfocus x-ray source and a Vantec area detector was also used to identify phases in the cladding. Results and Discussion Figure 2 shows a transverse cross section of the PolyTung NiBWC fluxcored wire and the particles removed from inside. EDS analysis indicates the tube sheath as Ni and the main particles as Ni, WC, and W. SingleBead Cladding Figure 3 shows example waveforms of current and voltage recorded during welding. Consider conventional GMAW first. Welds No. 53 was made at 19 V. Its waveforms in Fig. 3A show the voltage approaches zero periodically. This suggests the short-circuiting mode of metal transfer. Weld No. 55 was also made by conventional GMAW but at a slightly higher voltage of 21 V. As shown by its waveforms in Fig. 3B, the voltage approaches zero only occasionally and not quite as close to zero. This suggests the mode of metal transfer is more like globular than short circuiting. As for the GMAW-CSC, the waveforms of Weld No. 50 in Fig. 3C are typical of controlled short circuiting. When short circuiting occurs, the maximum current is always kept low (< 150 A) instead of being allowed to have a sudden surge to cause spatter (e.g., about 400 A in Fig. 3B). It was found that the window of welding parameters was significantly wider with GMAW-CSC than conventional GMAW. Consider conventional GMAW first. As shown in Fig. 4A, Weld No. 53 made at the arc voltage 19 V (107 J/mm) is smooth with only slight spatter. However, Weld No. 52 made at the arc voltage 18 V (99.5 J/mm) is highly irregular and discontinuous in shape. For Weld No. 55 DECEMBER 2016 / WELDING JOURNAL455-s Fig. 5 — Top views of welds made by GMAWCSC at various voltages and heat inputs. A — Weld No. 46; B — Weld No. 47; C — Weld No. 48; D — Weld No. 49; E — Weld No. 50; F — Weld No. 51. Travel speed: 15 mm/s. Window for welding is significantly wider in GMAWCSC than in conventional GMAW (Fig. 4).
Welding Journal | December 2016
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