| Quenched and tempered high strength steel HG785D has been widely used in all kinds of structural parts such as machinery,construction,vehicles,and ships because of its high strength,high reliability and weldability.Some defects such as cracks,embrittlement of the overheated zone and softening of the heat affected zone would easily occur if inappropriate welding process is used,due to the high carbon content of the base metal.Therefore,it is significantly important to develop suitable welding technology for the steel.The 10-mm-thick HG785D high strength steel was selected as the base metal.Three typical welding techniques including metal active gas(MAG)welding,electron beam welding and laser-MAG hybrid welding were employed.The droplet transfer behavior during welding process was observed.In addition,microstructure and mechanical properties of joints obtained using different welding processes were compared and analyzed.With regard to the research results of arc welding,the influence of welding current and welding speed on the droplet transfer of HG785D steel was determined by analyzing the droplet transfer process of arc welding.Based on the optimized welding parameters,the microstructure characterisation and evolution of the joints were compared and analyzed by means of EBSD.After that,mechanical properties of the joints were evaluated and the residual stress of the joints was measured.The results showed that the key factor in achieving stable droplet transfer was to realize drop-like transition before the droplet entered the molten pool.The welding speed had a great influence on the stability of the arc.The droplet transfer was unstable and prone to burst and splash at faster speed.The needle-like acicular ferrite and small-sized granular bainite were mingled at the weld zone.The average effective grain size was similar to that of fine grained zone,which could improve the impact toughness of weld metal.In addition,the proportion of large angle grain boundary in weld,especially high angle grain boundary is obviously high,which can effectively prevent the crack propagation.The average impact energy of the heat affected zone,weld zone and base metal at-20?is 83.6 J,110.2 J,113 J,respectively.The impact performance of the base metal and weld zone was significantly better than that in the heat affected zone.The hardness of the three regions was compared and the result was:HAZ>base metal>weld metal.The joint fractured at the base metal,indicating the filler wire matched well with base metal.The residual stress analysis showed the transverse stress reduced to zero with distance from weld center of 60 mm.The longitudinal residual stress turned from tensile stress to compressive stress with distance from weld center of 40 mm.The compressive stress could reach 60 MPa.Electron beam welding process was used to investigate the welding feasibility under condition of zero gap and square groove.The micro structural evolution and mechanical properties were studied under rapid heating and cooling rate.The results indicated that the heat input affected the penetration depth.Excessive heat input(beam current)would cause the undercut defect without on filler metal,even though it obtained the full penetration of the weld.The microstructure of EBWed joint was obviously different from that observed in MAG welded joint.The microstructure of weld metal consisted of coarse lath martensite and small amount of acicular ferrite.The microstructure of heat affected zone was tempered sorbite and martensite.The grain size of the weld was larger than the heat affected zone.The proportion of large grain size was higher than that of heat affected zone.The hardness of the weld was higher than that of the heat affected zone.While the tensile strength and impact toughness of the heat affected zone were better than those of the weld metal.The maximum strength of the joint could reach 869 MPa,which was 98.8%of that of the base metal.The fracture surface of joint obtained under the optimized parameters was characterized by uniform dimple,while cleavage fracture and shallow dimple was observed under non-optimized parameters.The adaptability and feasibility of welding HG785D steel with high energy beam was confirmed,since the reliable welded joint of thick plated steel without groove and gap was successfully achieved by electron beam welding.Thus,the laser-arc hybrid welding technology was investigated because of its high potential for industrial application.The results indicated that different energy ratio of laser beam to arc,as well as distance from laser to arc would exert great influence on droplet transfer.The stable droplet transfer and interaction between laser and arc was obtained when the ratio of laser beam to arc was maintained at 1:1.The joints without obvious defects were obtained when using suitable heat input,and fractured at the base metal indicating the root face had little influence on the joint strength.Based on the large root face and small filling amount,the process parameters were obtained:root face 8mm,laser power 4500 W?5000 W,wire feed speed 8?8.5 m/min,voltage 28 V,welding speed 1m/min,and gap 0.7 m/min.The microstructural difference was observed between arc zone and laser zone due to the different heating and cooling rate.The calculated results of temperature field further revealed the difference of thermal cycle between the two zones,which provided theoretical basis for the micro structure formation.To reveal the difference between arc welding and laser-arc hybrid welding,the spectral analysis was used to measure and compare the thermodynamic parameters of the plasma produced in the two welding techniques.Therefore,the electron temperature and density with the variation of process parameters were obtained to reveal the difference between two welding processes.The results indicated that the spectra produced by the hybrid welding were basically similar to those produced by the single MAG,and no new lines were generated,in terms of relative radiation intensity of the spectral line.The intensity of light decreased with the decrease of laser power and arc energy.With regard to the electronic temperature of the plasma,the electron temperature of the hybrid welding was higher than that of MAG welding when the laser power was high.The electronic temperature of the hybrid welding was linearly related to the laser power and arc energy.But when the laser power was low,hybrid welding would cause the decrease of the electronic temperature,resulting in the lower temperature than that of MAG welding.In terms of the electron density of the plasma,it decreased first and then rose with the decrease of the laser power. |
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