WELDING RESEARCH APRIL 2015 / WELDING JOURNAL 137-s ground to remove scale prior to welding in order to avoid weld root defects during complete-joint-penetration welding. The sides of the plate were supported, so the welds were made without contacting the table below. Selected welds were sectioned, polished, etched, and photographed to reveal the weld fusion zone profile and microstructures. The volume fractions of selected microconstituents in the weld metal are determined by using the point counting method following the International Institute of Welding (IIW) guidelines (Ref. 28). Mathematical Model A three-dimensional heat transfer and fluid flow model for complete-jointpenetration hybrid laser-GMA welding was developed by modifying previous numerical simulation work (Refs. 27, 29, 31). Zhao et al. (Ref. 31) developed a transport phenomena-based numerical model to predict the keyhole geometry and temperature profiles in the weldment during keyhole laser welding. Rai et al. (Ref. 29) developed a convective heat transfer model for both partial and complete-joint-penetration keyhole mode laser welding of a structural steel based on the work of Zhao et al. (Ref. 31). Ribic et al. (Ref. 27) proposed a three-dimensional heat transfer and fluid flow model for partial-penetration hybrid laser-GTA welding. In this work, complete-joint-penetration hybrid laser-GMA welding is studied. Marangoni force-driven velocity boundary conditions at the bottom surface are assumed, which is different from that of partial-penetration hybrid welding. In addition, the heat transfer from the metal droplets during GMAW is integrated into the numerical model for hybrid laser-GMA welding. The material properties used in order to complete the welding calculations are given in Table 3. Details of the numerical simulation model are presented below. Calculation of Keyhole Profile The keyhole geometry is calculated using a model that considers material properties, welding process parameters, and specimen geometries. The detailed information about the model is available elsewhere (Refs. 30, 31) and only the salient features are presented here. The keyhole profile is calculated based on a point by point energy balance at the keyhole walls and is determined iteratively. Multiple reflections of the laser beam within the keyhole are assumed and the number of reflections is dependent on the keyhole geometry. The keyhole wall local temperature is taken as the boiling point of the alloy (Refs. 30, 31). Planar heat conduction from the keyhole wall into the workpiece is assumed due to the significantly higher temperature gradient in all directions in the horizontal plane compared to the vertical directions. Once the profile calculation is completed, the temperature distribution from the keyhole model is stored in a data file with all temperatures inside the keyhole assigned the boiling point temperature. This file is read into the heat transfer and fluid flow model, and at each horizontal x-y plane, the keyhole boundary is identified by a minimum and a maximum x value for any y value. Heat Transfer in Weld Pool and Boundary Conditions After the calculation of the keyhole profile, equations of conservation of mass, momentum, and energy are solved in three dimensions in the heat transfer and fluid flow model. Details about this model are available in the literature (Refs. 22, 23, 29) and only the Fig. 4 — 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 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 2 — Welding Process Parameters for CompleteJointPenetration Hybrid Laser GMA Welding of DH 36 Steel Weld Laser Power Arc Current Arc Voltage Welding Laser Arc Number (kW) (A) (V) Speed (mm/s) Separation (mm) 1 5.0 248 31 20.0 1.0 2 5.0 235 31 30.0 1.0 3 5.0 232 31 40.0 1.0 4 5.0 232 31 40.0 5.0 A B Fig. 5 — Calculated cooling curves at different y locations on the top surface of the fusion zone of weld 1 with welding speed of 20.0 mm/s, laser arc separation distance of 1 mm. The symbol y represents the distance from the weld centerline. Fig. 6 — Comparison of the calculated cooling rates of the top center fusion zone for weld 1 with welding speed of 20.0 mm/s, laser arc separation distance of 1 mm, and weld 2 with welding speed of 30.0 mm/s, laser arc separation distance of 1 mm.
Welding Journal | April 2015
To see the actual publication please follow the link above