Study On Laser Welding Process Of Body Galvanized Sheet Based On Compound Exhaust Passage

In order to reduce the welding defects and improve the welding quality,a new technology of setting the exhaust channel structure was proposed in this paper.Zero-gap laminated lap welding experiment was carried out on 1.5mm thick CR340 cold-rolled hot dip galvanized steel plate.The welding process experiment was carried out on the exhaust channel structure,the exhaust channel roll depth,laser process parameters and so on,and the influence rule on the weld forming quality was obtained.And the optimization experiment was carried out on the exhaust channel structure and laser process parameters,and the process parameters interval of good weld forming was obtained.The temperature field model of laminated lap welding was established and the relative relationship between laser process parameters on the area of zinc vaporization and the area of molten pool during welding was studied.Finally,the flow field distribution of the weld pool was simulated by COMSOL software to obtain the formation reason of weld pores.The specific work and main conclusions are as follows:（1）The welding sample is obtained with exhaust channel structure,and the weld is well formed,basically without spatter,pores,pits and other defects,and the weld surface is continuous and uniform transition.Compared with the traditional method of reserving clearance and other processes,the method of adopting exhaust channel structure can be welded under zero clearance and is easier to control.（2）Through the analysis of process test,combining previous research results,this paper argues that in the process of welding,the main and second exhaust passage structure provides zinc layer surface of high-pressure steam with a buffer,the path of the release,not high pressure of zinc vapour instantly into molten pool,a large number of escape from the keyhole or under the molten pool surface,thus ensuring the stability of molten pool and holes.The exhaust channel structure is the main way to escape zinc vapor under the condition of satisfactory weld quality.（3）The process test results show that the optimal exhaust channel structure is213.05μm/62.19μm in the depth of main and secondary exhaust channels,which is obtained by using a 2.5mm pitch kempt knife and loading 2.5 times pitch.In the welding process,the main and secondary exhaust passages both play the role of escaping zinc vapor.Under the same roller pressure condition,the larger the tooth distance of knurling cutter,the greater the forming depth of the main exhaust passages,and the higher the escaping efficiency of zinc vapor.Secondly,the influence of the secondary exhaust passage on the weld forming quality is greater than that of the main exhaust passage.However,the formation of the secondary exhaust channel is not easy to control and has limitations,so the experiment pursues a more controllable main exhaust channel structure by rolling with large tooth pitch to improve the effect of zinc vapor escaping from the exhaust channel structure.（4）Laser process parameters have a significant effect on the weld width and there is an inverse correlation between the weld width and the number of welding defects to a certain extent.When the energy density is low,the high speed welding is helpful to restrain the welding defects.At high energy densities,high speed welding also helps to suppress weld defects such as pits and humps.However,the welding speed is not easy to be too high.In high-speed welding,when the laser energy density exceeds the threshold value,the higher the energy density is in a certain range,the faster the front melting width decreases,and the more unfavorable the zinc vapor entering the molten pool escapes.The best laser process parameters range is:laser power 2700～3000W,welding speed 20～25mm/s.（5）Through the metallographic analysis of the typical section of the weld,the microstructure of the fusion zone in the center of the weld is metallographic diagram.The microstructure of the weld zone is white massive ferrite,netlike pearlite and a small amount of Fe₃C_Ⅲat the grain boundary.Most of the zinc in the welding seam escapes through the exhaust channel.When a part of the residual zinc is left in the exhaust channel after cooling,a very small part of the zinc is still dissolved in the weld pool,and the zinc content（atomic percentage）is about 0.531%.At this time,the zinc content in the fusion zone is about 2 times that of the base metal zone.The highest hardness is distributed in the fusion zone,which is about 171HV,and the base metal zone is about111HV.The transition zone from the fusion zone to the base metal zone is the heat affected zone.The shear and tensile test results show that the strength of lap weld with exhaust channel is higher than that without,and the weld with exhaust channel breaks in the base metal area.（6）By Simufact.Welding software cascade lap Welding temperature field model is established,the introduction of characteristic parameter k description zinc vapor by molten pool’s ability to escape,through orthogonal simulation experiment,the optimum technological parameters of interal line energy density between 120～140 W?s/mm,not only the penetration moderation,molten pool to escape the efficiency of the zinc vapor is the highest,and support each other with the experimental results.Based on the weld pool flow field model of welding temperature field,it can be seen that eddy currents appear and disappear periodically in the molten pool,which is the reason for the formation of zinc vapor pores in the weld.It is necessary to have appropriate exhaust channel structure to reduce the pressure of zinc vapor escaping from the molten pool,and at the same time,reasonable process parameters should be used to make zinc vapor escape as far as possible.