焊接速度对激光-电弧复合焊接焊缝成形和低温冲击韧性的影响

Effect of Welding Speed on Bead Appearance and Low-Temperature Impact Toughness in Laser-Arc Hybrid Welding

开展了440MPa级船用高强钢激光-电弧复合焊接试验,研究了焊接速度对焊缝成形和低温冲击韧性的影响规律。结果表明,焊接速度对焊缝成形具有较大影响:焊接速度较小时,焊接热输入较大,熔池易塌陷;焊接速度较大时,由于激光能量密度不足,激光小孔熔透不稳定,焊缝底部易形成驼峰。随着焊接速度增大,440 MPa级船用高强钢激光-电弧复合焊接焊缝的低温冲击韧性呈先增加后降低的趋势:当焊接速度低于1.0m/min时,焊缝的-40℃冲击吸收功较低,小孔型气孔是主要影响因素;当焊接速度为1.2m/min时,气孔倾向小,焊缝的冲击吸收功达到了175J;当焊接速度增加到1.5m/min以上时,贝氏体含量较大,焊缝的低温冲击吸收功下降。

Objective Welding technology is crucial for ship manufacture,accounting for between 30%--50%of shipbuilding costs.To further develop the shipbuilding industry,it is necessary to increase the efficiency and quality of ship welding.Gas shielded welding and submerged arc welding are both frequently employed in ship manufacturing currently.Welding at a slow pace and a high heat input leads to significant welding deformation and low efficiency.As a welding method that incorporates both a laser beam and an arc into one process,laser-arc hybrid welding offers many advantages,including rapid welding speed,low heat input and excellent weld microstructures when the filler wire is utilised.Due to these advantages,laser-arc hybrid welding is a viable choice for manufacturing sectional steel structures in the shipbuilding sector,as it may reduce welding deformation and improve welding efficiency.As a result,laser-arc hybrid welding of 440 MPa grade high-strength marine steel at various welding speeds was performed.The effect of welding speed on the bead appearance and root humping was examined,as well as its effect on the toughness of hybrid welds.Methods Laser-arc hybrid welding of 8 mm thick 440 MPa-class marine high-strength steel was accomplished by employing a welding system comprised of a 6kW fibre laser and a molten metal gas shielded welding machine(Fig.1).The laser wavelength is 1070nm and has a nominal focusing spot of approximately 0.4mm.Between the laser beam and the electrode axis,the angle was 35°.The laser power and defocus distance were 6kW and-3mm,respectively,in the welding direction of the leading laser.A constant filler wire feed rate of 6m/min was utilised.The arc current and voltage were 185A and 22V,respectively,with 98%Ar-2%O_(2) shielding gas.To capture and study the formation of root humping,a high-speed camera was used.Following welding,the porosity in weld metal was determined utilising non-destructive X-ray testing equipment.The metallographic samples were then cut,ground,polished and finally etched with 4%nitric acid alcohol.The optical microscope was used to analyse the microstructure of weld and the Vickers was used to measure the microhardness of the weld joint.The low-temperature impact test was carried out according to GB/T 2650—2008,and the fracture morphology was investigated using a scanning electron microscope.Results and Discussions Welding speed has a considerable effect on the bead appearance in laser-arc hybrid welding(Table 3).Both the weld width and bottom hump height decrease as welding speed increases from 0.8to 1.8m/min(Fig.2).When the welding speed increases to 1.5 m/min,root humping begins.The process of root humping creation is examined(Fig.3).It is discovered that when the welding speed is 1.8 m/min,the keyhole seldom penetrates an 8mm thick steel plate.Once the keyhole widens,the liquid metal rushes down to the keyhole to the molten pool’s bottom,causing significant boot humping.Conversely,welding speed has a substantial effect on the weld’s low-temperature impact toughness.When the welding speed increases,the impact toughness of the weld metal increases rapidly at first,then gradually decreases(Fig.4).When the welding speed is limited to 1.0 m/min,the impact absorbed energy of the weld metal is low,owing to the weld metal’s high porosity(Fig.6).When the welding speed increases to 1.2m/min,the porosity is reduced and the weld metal absorbs the maximum of 175J of impact energy.The impact fracture morphology changes from a cleavage fracture(Fig.7)to dimples(Fig.8)as the welding speed increases from 0.8 to 1.2 m/min,which corresponds to the change in weld toughness.When the welding speed is increased from 1.2to 1.8m/min,the number of acicular ferrite decreases while the amount of lath bainite increases,which is the primary effect determining the impact toughness of the weld metal at high welding speeds.It results in a steady decrease in the impact toughness of the weld metal when the welding speed increases from1.2to 1.8m/min(Fig.9).Additionally,as the welding speed increases,the hardness of the weld increases.Conclusions In the study,we examine the influence of welding speed on bead appearance and low-temperature impact toughness of 440 MPa grade high-strength marine steel when laser-arc welding was used.Susceptibility to root humping increases with welding speed,and severe boot humping occurs at welding speed of 1.8 m/min.At the scanning speed of 1.2 m/min,the weld metal exhibits exceptional low-temperature impact toughness.At the low welding speed of 0.8--1.0m/min,-40℃sharp impact absorbed energy of the weld metal is low,owing to a lot of porosity in the weld metal.When the welding speed increases from 1.2 to 1.8m/min,the number of acicular ferrite decreases while the amount of lath bainite increases,resulting in a steady decrease in the impact toughness.