| TC4 titanium alloy has the advantages of low density,high specific strength,corrosion resistance and excel lent fatigue resistance.Compared with traditional welding technology,narrow-gap laser filler wire welding has many advantages,such as small linear energy,narrow HAZ,high welding efficiency,etc.However,the problems such as poor fusion of the side wa ll,welding solidification crack,high porosity,large deformation and stress are easy to occur.In addition,the distribution of microstructure and properties is not uniform in the thickness direction of laser welding joint with narrow-gap filler wire.Up to now,there are few researches in the field of thick plate titanium alloy laser filler wire welding,which will greatly limit the application of this technology in thick plate titanium alloy.In order to solve the above-mentioned problems,this paper has carried out the following research:Firstly,the welding process of TC4 titanium alloy with narrow-gap laser wire-filling was optimized,and the effect of welding parameters on weld formation and defects was summarized,when the laser power is between 3.0 k W～ 4.0 k W,the welding speed is between 0.4 m/min ～0.6m/min,and the wire feeding speed is between 3.0 m/min～ 4.0 M/min,the laser beam is in circular or vertical swing mode,when the swing range is between 1.5 mm and 2.0 mm,and the swing frequency is 100 Hz,a well-formed weld seam can be obtained without porosity or fusion defects;When the laser power is 4.0 k W,the welding speed is 0.42 m/min,th e wire feeding speed is 0.6 m/min,and the laser beam is in circular or vertical swing mode,the swing amplitude is 2.0 mm,when the swing frequency is 100 Hz,the weld seam with good shape,no porosity,crack,no fusion and other defects can be obtained.Secondly,the effects of welding heat inputs,groove type,shielding gases composition and cooling rate on the microstructure,properties,stress and strain of laser wire-filled welded joints were investigated.It is fou nd that with the increase of welding heat inputs,the evaporation loss of α phase stable element V increases,and the strength and hardness of the welded joints become more lower.With the increase of welding heat inputs,the hardness of each zone of the welded joints decreases as a whole,and the peak value of microhardness appears near the fusion line in the welded seam.The microstructure and mechanical properties of welded joint with heat input of 5.0 k J/cm are excellent.The stress distribution characteristics of single U-groove and double U-groove welded joints are different,and the single U-groove welded joint produce obvious stress concentration mainly on the side of the end welded,however,the stress di stribution of double Ugroove welded joint presents symmetrical distribution alo ng the wall thickness direction.The difference between longitudinal s tress peak value and transverse stress peak value,the difference between transverse shrinkage and angular deformation are small.By designing the proper reverse deformation and groove si ze,the single U-groove can obtain the welded joint with less stress and strain,and ensure the welding efficiency.During the cooling process of β phase at high temperature,the temperature transition point at which new phase begins to precipitate decreas es with the increase of cooling rate,the temperature transition point at which new phase ends to precipitate increases,and the transition temperature range of new phase becomes narrower and narrower.The microstructure after cooling at0.5 °c/s and 5 °c/s to room temperature is composed of α phase,a small amount of α phase and α’ martensite is composed when the cooling rate is increased to 20 °c/s,and when the cooling rate i s increased to 80 °c/s,the precipitates are composed of α’ martensite,with the increase of cooling rate,the interlamellar spacing between the new p hases decreases,the interface area increases and the hardness increases.The cooling rate after welding c an be reduced by controlling the temperature between layers,and the structure of welded joint can be adjusted effectively,so the properties of welde d joint can be improved.The interlayer temperature should be controlled between250 °C and 300 °C in the controlled of the test,considering the strong affinity between titanium alloy and atmosphere and the requirement of welding efficiency.Compared with Ar,He has a stronger cooling effect on liquid molten pool metal,the residence time of welded joint abov e phase transformation is shorter,and the width of HAZ is reduced from 1.1 mm t o0.80 mm,the time of grain growth is shorter,the grain size is smal ler,and the length-to-width ratio of the irregularly arranged acicular α’ martensite is smaller,compared with Ar,He is more suitable for laser welding of titanium alloy than Ar,because the welded seam microstructure is composed of a single bundle of α’ martensite with large aspect ratio.Based on the above basic process exploration tests and the laws obtained by means of optimizing and controlling the microstructure and properties of welded joints,a comparative study on the microstructure and properties of welded joints between laser-filled solid wire and selfdeveloped flux-cored wire was carried out,it is found that the diameter of α’martensite in the weld zone of laser filled solid wire is larger,the grain diameter is less than 3 μm,which accounts for 62% of the total microstructure,and the diameter of α’ martensite in the weld zone of laser filled flux-cored wire is smaller,the grain diameter less than 3 μm accounts for 83% of the total microstructure.The tensile strength,elongation after fracture and impact property of the welded joint by laser-filled flux-cored wire are higher than those welded seam by laser-filled solid wire,and the fracture position appears in the BM.The microhardness of the two welded joints showed the distribution law of WM hardness>HAZ hardness>BM hardness,and the value of flux-cored wire welded joint was slightly higher.The microstructure and properties of the welded joint with laser-filled fluxcored wire are better than those of welded joint with laser-filled solid wire.Finally,the application of flux-cored wire in narrow-gap laser filler wire welding of TC4 titanium alloy thick plate is realize d.Through the analysis of the microstructure and properties of t he welded joint and its distribution characteristics,it is found that the welded appearance is in the shape of a nail,and no welding defects such as blowhole,crack and non-fusion of the side wall are found.The deformation of the welded joint is well controlled,the distribution of micr ostructure and properties along the wall thickness is more uniform. |
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