WELDING RESEARCH APRIL 2015 / WELDING JOURNAL 143-s separation distance of 1 mm, compared to 5-mm separation distance when acicular ferrite is the predominant phase. It should be noted the welding conditions including the heat input are all identical when the laser arc separation distance changes from 1 to 5 mm. The reason for the large difference of the weld metal microstructures with the changing laser arc separation distance lies in the fact that the cooling condition, especially the cooling rate between 1073 and 773 K, significantly decreases when the laser arc separation distance increases from 1 to 5 mm. The cooling rate exceeds the critical cooling rate for martensite transformation with laser arc separation distance of 1 mm while it decreases to a value lower than the critical cooling rate for martensite transformation when the laser arc separation distance increases to 5 mm. Process Map In order to further understand the influence of welding parameters on the cooling rates and corresponding volume fractions of the microstructures of the weld fusion zone, a comprehensive process map is presented in Fig. 13. The map considers various combinations of welding speed, laser arc separation distance, and laser power. The arc current and arc voltage are 232 A and 31 V, respectively, for all the welding conditions. The laser powers for Fig. 13A–C are 4, 5, and 6 kW, respectively. The maps show the combinations of separation distance and welding speed that produce a given cooling rate and corresponding microstructure in terms of volume fractions of different phases and microconstituents. The critical cooling rate for martensite transformation is about 71 K/s, which is calculated by the model for microstructure evolution (Refs. 22–24). Cooling rates below 71 K/s are shown in dashed lines as a zone without martensite, while cooling rates greater than 71 K/s are shown in dotted lines as another zone with martensite in Fig. 13. The upper bound on the process maps is the partial penetration line, above which complete penetration is not possible. In general, welding speed has a greater effect on cooling rate than separation distance, and at low welding speeds of 20 mm/s, the cooling rate is almost independent of separation distance. The effect of power can also be observed in Fig. 13. As power increases, the cooling rates shift to higher welding speeds for a given separation distance. For example, at 1 mm separation distance, the required welding speeds to obtain a 70 K/s cooling rate are 32 and 39 mm/s for 4 kW and 6 kW laser powers, respectively. It is important to know how cooling rates affect the microstructure. As the cooling rate increases from 20 to 150 K/s, the volume fraction of Widmanstätten ferrite increases from 0.10 to 0.22 at the expense of the allotriomorphic ferrite, which decreases from 0.31 to 0.23. The amount of acicular ferrite decreases from 0.59 to 0.57 from low cooling rates up to the critical cooling rate when acicular ferrite disappears and martensite forms instead. These maps provide a means for understanding microstructure evolution during hybrid laser-arc welding and can be used to select welding parameters that optimize the weld microstructure or minimize welding time yet limit the formation of martensite. For example, as stated previously, if the power is increased from 4 to 6 kW for a constant separation distance of 1 mm, then the welding speed can be increased by 22% from 32 to 39 mm/s without the formation of martensite. Increasing the separation distance from 1 to 7 mm further increases the possible welding speed to 42 mm/s. Compared to the initial conditions in this example, the total increase in welding speed is 31% due to a 33% increase in laser power and a 6-mm increase in separation distance. The welding speed and the resulting time to make a weld are important. When comparing the costs of hybrid laser-GMA welding with conventional arc welding for pipe joining applications, Reutzel et al. (Ref. 45) found that weld time comprised between 24 and 41% of the total fabrication time, depending on pipe diameter, with other tasks, such as fitup, preparation of the weld, and movement of the part, making up the rest of the time. Additionally, since the welding is occurring with the same cooling rate, there would be no change in the microstructure. The utility of the process map indicates significant promise for understanding the evolution of microstructures in the fusion zone of hybrid laser-GMA complete-joint-penetration welding by a combination of phase transformation model and the thermal cycles calculated from the heat transfer and fluid flow model. Summary and Conclusions The effect of laser arc separation distance and welding speed on fusion zone geometry and microstructure during complete-joint-penetration hybrid laser-gas metal arc welding of low-alloy steel was investigated experimentally and theoretically. A heat transfer and fluid flow model was used to calculate the weld pool geometry and cooling rates, which were applied to a microstructure model to compute the phase fractions of selected microconstituents. Experimental weld geometries and microstructures were compared to the calculated values, and the two sets of data agreed well. The following conclusions can be drawn from this work: 1. The effect of welding speed and laser arc separation distance on weld pool geometry was investigated in complete joint-penetration hybrid laser-arc welding. The weld length and width both at the top and bottom of the pool increased with increasing laser arc separation distance for the same heat input. The weld pool dimensions decreased with increasing welding speed as expected. 2. Cooling rate was also affected by the hybrid welding parameters. When the welding speed increased, which changed the net heat input, the cooling rate increased. When the laser arc separation distance decreased, which altered the heat distribution of the combined power sources but not the heat input, the cooling rate increased. At high welding speeds, the decrease in separation distance can have a significant enough effect on the cooling rate to form martensite in the microstructure. 3. The experimental weld microstructures consisted of acicular ferrite, allotriomorphic ferrite, Widmanstätten ferrite, and martensite. Martensite existed only in the weld with the laser arc separation distance of 1 mm and a welding speed of 40 mm/s. Acicular ferrite formed at the expense of martensite when the separation distance increased to 5 mm. Increasing the welding speed from 20 to 30 mm/s resulted in a decrease of allotriomorphic
Welding Journal | April 2015
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