WELDING RESEARCH A B A B cantly affected the cooling rates and that the weld penetration was maximized at an optimal laser arc separation distance (Ref. 27). Cho et al. (Ref. 6) simulated the molten weld pool geometry in laser-arc hybrid welding by solving the equations of continuity, momentum, and energy using a commercial package. They reported that the width of the weld was determined mainly by the GMA heat source and the penetration depth was strongly influenced by the laser (Ref. 6). However, there are very few systematic studies focused on the numerical simulation of A B weld profile evolution, cooling rates within the fusion zone, and the corresponding 136-s WELDING JOURNAL / APRIL 2015, VOL. 94 influence on the weld metal microstructures in complete-joint-penetration hybrid laser-GMA welding. The previous numerical studies (Refs. 6, 27) have discussed partial-penetration hybrid welding, where the fluid flow and heat transfer at the bottom of the molten weld pool are significantly different than those in complete-jointpenetration welding. In this work, the evolution of macroand microstructures of complete-jointpenetration laser-GMA hybrid welds in DH 36 steel is analyzed using fundamental transport phenomena and phase transformation theory. A threedimensional heat transfer and fluid flow model has been developed to study the effect of welding velocity and laser arc separation distance on weld geometries and cooling rates. Using the calculated cooling rates from the heat transfer and fluid flow model, a phase transformation model (Refs. 22–24) based on thermodynamics and phase transformation kinetics is used to provide a quantitative description of the final microstructures of the weld metal. The computed volume fractions of the weld metal allotriomorphic ferrite, Widmanstätten ferrite, acicular ferrite, and martensite are validated with corresponding experimental results for various welding conditions. The combined models are used to construct process maps capable of predicting the effect of welding parameters on resulting cooling rates and microstructures. Experimental Procedure Bead-on-plate complete-jointpenetration hybrid laser-GMA welds were made on 4.8-mm-thick DH 36 steel. An IPG Photonics® YLR-12000-L ytterbium fiber laser with a Precitec® YW50 welding head and a Lincoln Electric ® Power Wave 455 M/STT welding power source with a Binzel® WH 455D water-cooled welding gun were used for hybrid welding. The maximum power of the fiber laser is 12 kW, with a wavelength of 1070–1080 nm. The optics system utilizes collimating and focusing lenses with 200- and 500-mm focal lengths, respectively. The laser is transported to the welding head through a 200-mm-diameter process fiber. The focal spot of the laser beam in the absence of plasma was approximately 0.56 mm in diameter. The laser was focused 8 mm above the surface of the plate. The laser power used in the welding experiments was fixed at 5.0 kW for all the cases. The electrode was 0.045-in.- (1.1- mm-) diameter ER70S-6 wire. The chemical compositions of DH 36 steel and ER70S-6 welding wire are given in Table 1. The metal transfer mode for the welding wire was in spray mode. The shielding gas was a mixture of 95% argon and 5% CO2 with a flow rate of 95 ft3/h (44.8 L/min). The welding velocities and laser arc separation distances were varied to study their effects on the weld profiles and weld metal microstructures. The key welding parameters are listed in Table 2. The top and bottom surfaces of the plate were Fig. 1 — Top surface and symmetry plane of weld pool with temperature contours and velocity fields for the following: A — Welding speed of 20.0 mm/s, laser arc separation distance of 1 mm; B — welding speed of 30.0 mm/s, laser arc separation distance of 1 mm. Fig. 2 — Comparison of experimental and simulated weld cross sections for the hybrid laserGMA completejointpenetration welding of DH 36 steel for the following: A — Welding speed of 20.0 mm/s, laser arc separation distance of 1 mm; B — welding speed of 30.0 mm/s, laser arc separation distance of 1 mm. Fig. 3 — Top surface and symmetry plane of weld pool with temperature contours and velocity vectors for the following: A — Welding speed of 40.0 mm/s, laser arc separation distance of 1 mm; B — welding speed of 40.0 mm/s, laser arc separation distance of 5 mm. Table 1 — Chemical Composition of Base Metal DH 36 Steel (Ref. 10) and Welding Wire ER 70S6 (wt%) C Mn Si Ni Mo Cr V P S Al Nb Ti Cu Base 0.06 1.39 0.19 0.14 0.03 0.11 0.06 0.011 0.004 0.025 0.01 0.01 0.25 Metal Welding 0.09 1.63 0.90 0.05 0.05 0.05 0.05 0.007 0.007 0.000 0.00 0.00 0.20 Wire
Welding Journal | April 2015
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