| At present,the further speed increase of high-speed trains has put forward higher requirements for the weight reduction of train bogies.For the weight reduction design of bogies,adopting the higher strength bogie steel,and reducing the thickness of the plate to achieve weight reduction is a good scheme which can shorten research and development cycle and cut down production cost.N800 CF high-strength bainite steel is a new generation of low-carbon bainite steel for welded structures developed and produced by Nangang.With this bainite steel frame,the weight can be reduced by more than 25%.In the past,such low-alloy structural steels with a plate thickness of 12 mm have been welded by multi-layer and multi-pass welding with arc welding,which could lead to the the inefficiency of welding and the large deformation of specimen.Therefore,this study uses laser-arc hybrid welding to explore the single-pass welding process of bainite steel and achieve efficient welding.There are common problems in single-pass joints for high-energy laser-arc hybrid welding of thick plates.Among them,the hump defect at the root of the weld is the most prominent.In this paper,the cause of root hump defects in a single-pass laser-GMAW hybrid welding(laser-leading)of 12 mm thick low-carbon bainite steel is studied,and the hump-free welding seam is obtained by using the arc-leading laser-arc hybrid welding mode.The difference in the process and microstructure properties of arc-leading and laser-leading laser-arc hybrid welding was analyzed under the same process conditions.Under the process conditions of laser power of 7.2k W,welding speed of 16mm/s,wire feeding speed of 12m/min,and defocus amount of 0mm,a high-speed camera was used to analyze the behavior of the weld pool on the surface of the weld and the hump pool at the root of the weld.The formation of the hump pool at the root mainly includes three stages of molten metal accumulation,nucleation and growth of the hump pool,and solidification of the pool.Using the PIVlab tool to analyze the flow velocity of the hump pool at the root of the weld,it was found that the significant difference in the flow velocity of the hump pool at the root of the weld at different positions is the key to the formation of the root hump.The flow velocity is 108 mm / s,which indicates that the flow velocity of the molten pool is much higher than the welding speed,which is also the cause of hump defects.For the hump defect at the root of the weld,laser power,welding speed,and wire feeding speed were selected as test factors.The number of humps,hump area,and cross section of the weld were selected as test indicators.The main process parameters that affected the defect were modeled and analyzed.It was found that the weld crosssectional area model had the lowest distortion rate(P=0.012),the correlation coefficient of the test process parameters was the highest(96.38%),and the values of the significant correlation between the cross-sectional area of the weld parameters was welding speed(less than 0.0001)> wire feed Speed(0.0104)> Laser power(0.0535).The optimal laser power process predicted by the weld cross-sectional area model is 6.2 to 7.6 k W,the wire feed speed is 12.2 to 12.7 m/min,and the welding speed is 15.0 to 16.5 mm/s.Based on the model prediction results,experimental research is performed.Under the process parameters of 6.5k W,wire feeding speed of 12.5m/min and welding speed of16mm/s,a well-formed weld without root hump was obtained.Based on the optimal process conditions,research shows that the main reasons for the arc-leading mode to suppress hump formation compared to the laser-leading mode are: the process has a longer droplet transition period(4.2ms> 3.4ms)and a smaller droplet diameter(1.65 mm <1.83mm),the accumulation of molten metal in the molten pool is slower(smaller gravity,the direction of the combined force indicates that the angle of the droplet and the vertical direction is larger in this mode),and the test results prove that arc-leading mode has higher electrical parameter stability.On the other hand,in the arc-leading mode,the weld has no obvious hump,undercuts and pore defects,and its hardness distribution is more stable and uniform.In both welding processes,the maximum value of weld hardness appears in the heat-affected zone(rough bainite),and the minimum value appears in part of the phase transition zone(mass ferrite).At the same time,the impact absorption energy of the arc-leading welding joint is 21.89%higher than that of the laser-leading mode. |
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