| The development of automobile industry brings convenience to human beings.At the same time,the ensuing problems such as environmental pollution and energy shortage are also aggravating.The use of high strength steel in the vehicle body can not only improve the vehicle safety factor but also reduce the carbon emission,which is in line with the development requirements of vehicle lightweight.Laser welding has many advantages,such as narrow heat affected zone,high thermal efficiency,small welding distortion and high weld quality.It has gradually become the research focus of automotive high-strength steel welding.In this paper,the high strength TRIP and TWIP steel were prepared by the reasonable heat treatment procedure.The IPG YLS-6000 fiber laser was used for laser welding of the experimental materials.The effects of heat input on microstructure transformation and properties of welded joints of TRIP-TRIP,TWIP-TWIP and TRIP-TWIP were studied.Procedure optimization was carried out for the welded joints which have poor performance,and the following main results were obtained:For TRIP-TRIP welded joint,a large amount of lath martensite was generated in the fusion zone(FZ)and the heat affected zone(HAZ),resulting in the higher hardness and the lower of plasticity and formability than the base metal.For fully penetrated welds,the tensile test were fractured at the base metal,and the strength of the welded joint is consistent with the base metal.With the increase of heat input,the width of the welded joint increases,which leads to the decrease of the plasticity of the welded joint.When the heat input is 80J/mm2,the welded joint has the best performance,the tensile strength is 795MPa,the elongation is 25.6%,and the product of strength and elongation(PSE)is 20.4GPa·%,which reaches 91%of the base material,and the Erichsen value is 7.2mm,which reaches 82.8%of the base material.For TWIP-TWIP welded joint,the microstructure of the fusion zone(FZ)is coarse columnar austenite.There was no phase change but grain growing from 4.3?m to 8.7?m in heat affected zone(HAZ).The FZ and the HAZ were softened.Tensile specimens under all heat input conditions were fractured at the weld seam,with strength and ductility much lower than that of the base material,which was considered to be due to insufficient columnar austenite strength.When the full penetration weld was formed,the mechanical properties of the welded joint are decreased with the increase of the heat input.The welded joint has the best mechanical properties and formability at the heat intput of 60J/mm2,with the tensile strength 670MPa,the elongation 22.3%,the product of strength and elongation(PSE)14.9GPa·%,only 41%of the base material,and the Erichsen value 8.9mm,reaching 93.7%of base metal.TRIP steel was added to optimize the welding process.With the addition of TRIP steel,the microstructure of FZ changed from single phase austenite structure to a mixed structure of austenite and a small amount of martensite.On the one hand,the addition of dissimilar materials breaks the growth process of single-phase crystallization in the molten pool,refines the grain size of the FZ;on the other hand,martensite as reinforcement which also improves the performance of welded joints.After adding 0.1mm TRIP steel,the PSE of welded joints increased by 25.9%,the formability of the welded joint also improved,with the fracture mode changed from along the weld crack to perpendicular to the weld crack.For the dissimilar welded joint of TRIP-TWIP steel,the differences in the composition of the base material and the rapid solidification of the molten pool lead to the inhomogeneity of the microstructure and elements in the fusion zone.Segregation of alloying elements,especially manganese and aluminum,occurred inside the weld seam,resulting in the formation of martensite in the low Mn region and austenite in the high-Mn region.When the heat input is 60J/mm2,the welded joint has the best mechanical properties and formability.The tensile strength is 745MPa,the elongation is 44.3%,the PSE is 33.0GPa·%,Erichsen value is 6.0mm.The tensile test fractured at the TWIP steel base material,while the others fractured at weld seam.It was considered that the optimum proportion of the two phase microstructure in the weld zone was reached under this heat input condition.After the optimization of the weld procedure with the laser beam offsetting,the martensitic single phase(laser beam shift to TRIP side)and austenite single phase(laser beam shift to TWIP side)structure were formed respectively in the weld area,and the formability was increased by 8.3%and 26.7%respectively.The mechanical properties of the welded joint which the laser beam shift to TRIP side was consistent with that of welded joint with the lasser beam at the gap,the tensile strength is 746MPa and the elongation is 41.6%,with the fracture occurred at TWIP base metal.However,when the laser beam shift to TRIP side,the mechanical properties of the welded joint are greatly reduced,the tensile strength is 709MPa,and the elongation is only 18%,which is also due to the lack of austenite strength. |
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