Study On Porosity Inhibition Mechanism And Microstructure Properties Of Titanium Alloy Laser-MIG Hybrid Welding

TC4 titanium alloy is widely used in aerospace,shipbuilding and other fields because of its high specific strength,corrosion resistance,crack resistance and other advantages.Laser-MIG arc hybrid welding is a kind of efficient welding technology with coupling of two heat sources.The welding speed is fast,the heat input is low,and the stress and deformation after welding are small.However,the difficulty of protection in the welding process and the high sensitivity of porosity in the weld limit the wide application of this technology.In this thesis,the Laser-MIG hybrid welding technology of TC4 titanium alloy butt joint was studied,and the influence of technological parameters on weld formation was explored.The dynamic behavior of keyhole and molten pool during welding was observed and analyzed by means of high-speed photography,and the formation mechanism of technological porosity was clarified and the suppression measures were proposed.The butt joints of 5mm and 12 mm TC4 titanium alloy were welded by optimized welding technology,and the microstructure characteristics of the welded joints were studied.The mechanical properties of the joints were tested by tensile,impact and hardness tests.The main conclusions are as follows:Under the gas protection device designed in this thesis,the weld protection effect is good within the range of various process parameters.When the laser power is 3.4k W,the welding current is 120 A,the welding speed is 0.8m/min,and the defocus is 0mm,the weld surface forming is better.The influence of the change of optical wire spacing on the weld forming is not obvious.But there are a lot of technical porosity in the weld.When the defocus is 0,2,-2 and-4mm,the keyhole stability is poor.When the defocus is-2mm,the porosity in the weld is the highest,which is 4.21%.At 4mm and-6mm,the keyhole collapse occurred only at the root,and the porosity decreased by more than 80%.At 6mm and-8mm,the keyhole remained in a stable state,and the porosity decreased to 0%.When the defocus is 6mm,the laser power is increased from 2.6k W to 3.8k W,the keyhole shape becomes sharp and narrow,and there is no porosity in the weld.The disappearance of pores at the bottom of the molten pool is not only due to the improvement of the stability of the keyhole,but also because the keyhole depth fluctuates periodically under the action of pulsed arc,which plays a crucial role in the remelting process of the air mass.When the defocus is 6mm,the Laser-MIG joint with plate thickness of 5mm and 12 mm is well formed,and there is no porosity defect inside.The weld zone of 5mm TC4 titanium alloy joint is composed of large β columnar crystals,and there are a lot of acicular martensite α’ in the β columnar crystals.The α+β grains in the coarse-grained area are equiaxed,and a large number of acicular martensite α’ phases are precipitated in the β grains,and the size is smaller than that of the weld.The initial α phase in the fine grain region is distributed among β grains,and a small amount of acicular martensite α’ phase is distributed on α+β matrix.The initial α+β grain size in the transition zone increases slightly.The joint hardness distribution is saddle shape,and the highest hardness is 359 HV in heat affected zone.The tensile specimen breaks at the base material.When the heat input of 12 mm TC4titanium alloy joint is 5.98 k J/cm,the β equiaxed grain size and number are larger in the weld center.Compared with the heat input of 3.61 k J/cm,the acicular α’ width of the cap layer and the filler layer increased by 27% and 36% on average under high heat input,the βgrain size and the number of acicular martensite α’ in the coarse-grained region were larger,the number of acicular martensite α’ and β grains in the fine-grained region were increased,and the depth-to-width ratio of α grains in the transition region were decreased.The burning of Al element appeared in the process of multiple welding with small heat input.When the heat input is 3.61 k J/cm,the hardness in the heat-affected zone of the joint is the highest(377HV),and the hardness near the interlayer remelting zone increases obviously(417HV).When the heat input is 5.98 k J/cm,the hardness is more evenly distributed along the weld center axis.The two kinds of welding heat input have little effect on the tensile and impact properties of the joint,and the tensile specimens are all fractured in the base metal.