| Duplex stainless steel is widely used in shipbuilding,deep-sea oil fields,chemical platforms and other fields due to its advantages of good weldability,excellent mechanical properties and strong corrosion resistance.The traditional arc welding technology has many problems,such as high heat input,large deformation,many filling metals and low welding efficiency.In recent years,with the development of narrow gap laser welding technology,its advantages of small weld area and high laser heat source efficiency have been widely concerned in the industry.Due to the problem of porosity and side wall failure to fuse in the thick plate multi-layer welding,the mixing effect of alternating magnetic field on the molten pool was used to refine the grain and homogenize the composition,improving the stability of welding process.Based on 2205 duplex stainless steel as the research object,through the narrow gap laser-MIG hybrid welding with alternating magnetic field,exploring the effect of the composite plasma physics and the droplet transition behavior.Thus,the weld defects can be reduced,the two-phase ratio of the joint can be improved,and the mechanical properties can be improved.In this thesis,an experimental platform of alternating magnetic field assisted narrow-gap Laser-MIG hybrid welding was built.The influence of alternating magnetic field on the physical properties of the plasma in narrow-gap groove were compared by spectral analysis.When the intensity of the alternating magnetic field increased from 0m T to 50m T,the plasma electron temperature at the same collection point in the narrow-gap groove also increased,and the plasma electron density also increased.The maximum temperature in the narrow-gap groove is 8028K and the electron density is 11.78×1016cm-3without magnetic field assistance.When the diplomatic alternating magnetic field was applied,the maximum temperature in the groove could reach 8605K,which increased by 7.2%.The electron density also reached13.32×1016cm-3,which increased by 13.1%.When the parameters of the applied alternating magnetic field changed,the droplet transition frequency gradually increased from 26.0Hz in the case of no magnetic field to82.6Hz in the case of magnetic field intensity of 50m T.The weld depth and width did not increase or decrease with the magnetic field intensity unidirectional,which mainly depended on the specific magnetic field intensity.When the alternating frequency increased from 0Hz to 50Hz,the form of droplet transition changed from short circuit transition to droplet transition,and its transition frequency also increased gradually.When the alternating frequency increased to 50Hz,the well-formed welded joint could be obtained.Through the X-ray examination of the post-welding test plate,it could be seen that there were chain pores,poor fusion and other defects on the welding seam without external magnetic field,while the welding seam was uniform,smooth and beautiful without any welding defects after the addition of alternating magnetic field.By analyzing the proportion of austenite phase to ferrite phase in the filler pass,cover pass and overlap pass of welding joint with or without magnetic field,the alternating magnetic field made the distribution of austenite and ferrite more uniform and the proportion of the two phases closer.In this thesis,the mechanical properties of the 2205 dual-phase stainless steel joint were tested.The results showed that the average hardness values of the backing,filled and cover passes fluctuated up and down at 277HV,285HV and 272HV without magnetic field.The hardness values of the weld zone increased by 3.0%,3.6%and 1.6%while the heat-affected zone increased by 2.3%,1.9%and 0.4%when adding alternating magnetic field.With the addition of alternating magnetic field,the impact absorption energy of the upper and lower welds increases by 14.1%and 23.1%.In the tensile test,the fracture location of the samples was in the base metal,and the presence of magnetic field had little effect on the tensile properties. |
