Study On Cold Metal Transfer Welding Process And Microstructure And Properties Of DP980/DP780 Dissimilar Dual-phase Steel Joints With Unequal Thickness

With the popularity of automobiles,energy-saving and weight-loss of automobiles and improving the safety of collision parts are the current and future development trends of automobiles.The structural design of multi-materials and the use of high-strength steel are one of the main means to solve these problems.It is difficult to select and operate welding process for heterogeneous dual-phase steels with different thickness due to the difference of structure composition,physical and chemical properties,mechanical properties and thickness.At present,automobile enterprises mainly use Metal Active-gas Arc Welding(MAG)technology to improve the gap between spot welding joints,but the heat input in traditional MAG welding process can not be controlled,resulting in a large number of spatters,uneven weld formation and even penetration.Cold Metal Transfer(CMT)is a new type of gas shielded metal welding.Compared with traditional MAG welding,CMT has lower heat input,effective spatter control,and is more suitable for thin plate and heterogeneous material welding.In this paper,ER110G-S wire with diameter of 1.2mm was used as filler to weld1.2mm DP980/1.4mm DP780 heterogeneous duplex steel by CMT technology.The arc length correction coefficient was 0,wire feeding speed was 1.5m/min~5.0m/min,welding speed was 400mm/min~600mm/min.The microstructures and mechanical properties of welded joints were studied and analyzed by optical microscopy,scanning electron microscopy,energy dispersive spectrometer,microhardness tester,tensile tester and electrochemical test.In the case of different lapping sequences,the changing rules of macro-forming,micro-structure and mechanical properties of welded joints are revealed by adjusting welding parameters(wire feeding speed and welding speed).The experimental results show that under the two overlapping modes,the weld shape is uniform and beautiful,and there are no obvious defects within the suitable welding parameters.Under the condition of the same wire feeding speed,the currentand voltage increase with the increase of wire feeding speed,which leads to the increase of oxidation zone,depth-width ratio and grain size.Under the condition of constant wire feeding speed,with the increase of welding speed,the current and voltage do not change,and the oxidation zone,depth-width ratio and grain size of the weld decrease accordingly.The microstructures of DP980 and DP780 as base metal are mainly composed of martensite and ferrite.The content of martensite in DP980 is higher than that in DP780.The weld zone is composed of a small amount of bainite and lath martensite.The coarse grain region is composed of bainite,martensite and a small amount of ferrite.The fine grain region is composed of a certain proportion of bainite and martensite.The martensite content in incomplete phase transformation region is the lowest in all regions of welded joint,while the ferrite content is the highest in the welded joint region.Because of the uneven distribution of the microstructure in the welded joint area,the hardness values of the whole welded joint are different.When DP980 is above,the maximum hardness of the welded joint appears in the fine grain zone on the DP980 side,which is about 390 HV.The lowest hardness occurs in the incomplete phase transition region of DP780 side,which is about 200 HV.The maximum tensile loads of base metal DP980 and DP780 are 13.47 kN and 12.58 kN,respectively.The welded joints of the two overlap sequences were all broken in the incomplete phase transformation zone on the DP780 side,which is the microporous aggregation fracture.The maximum tensile load is about 9kN~12kN,which reaches over 85% of the base metal DP780.And that meets the application requirements.In summary,1.2mm DP980/1.4mm DP780 heterogeneous dual-phase steel with unequal thickness can be welded by CMT welding machine in two overlapping sequences,and the welded joints with beautiful weld shape and good mechanical properties can be obtained.The application range of the welding process parameters is wider when the lap order of DP980 is above rather than that of DP780 is above,and the optimal process parameters under the experimental conditions are obtained:DP980 is above,the arc length correction factor is 0,the wire feeding speed is3.0m/min and the welding speed is 500mm/min.