Study On Microstructure And Residual Stress Distribution Of Laser-welded TC4 Titanium Alloy

The density of Titanium and Titanium alloy is only 4.5g/cm~3,which are widely used in various equipment manufacturing fields including advanced aircraft and manned submersible,for its excellent performance in strength,hardness,fracture and corrosion resistance and etc.Ti-6Al-4V Titanium alloy,namely TC4 Titanium alloy,has both excellent comprehensive mechanical properties and machinability.Therefore,the amount of Ti-6Al-4V Titanium alloy accounts for more than 80%of all Titanium and Titanium alloy materials.The complex phase transformation of TC4 Titanium alloy strongly affect the mechanical properties,such as grain size,microstructure morphology and distribution of a phase and ? phase.It is necessary to consider the influence of processing methods on the mechanical properties of Titanium alloy materials.Laser welding is simple and rapid weld method which has been widely used in industrial.The characteristic of laser results in the narrow Heat Affected Zone(HAZ)of the joint after welding.The rapid melting and solidification procedure in welded joint will not only cause the discontinuity of the structure and phase distribution,but also introduce the complex residual stress field which is far higher than that of solid welding,such as FSW.The complex residual stress field in the welded joint will affect the service performance with multi-field loading condition.Therefore,it is of great significance to study the microstructure and residual stress distribution of TC4 Titanium alloy laserwelded joint.Based on the high-energy X-ray diffraction technology of Synchrotron Diffraction,and other characterization such as OM and EBSD,the microstructure and residual stress distribution in the welded joint of 2 mm thick TC4 Titanium alloy sheet with different welding processes were studied.The main conclusions are as follows.The results show that the microstructure and phase distribution are determined by the process parameters of laser welding.There are a lot of Bainite in the Fusion Zone(FZ)of the samples whose Linear Energy is less than 30 J/mm.In the samples whose Linear Energy is between 30?40 J/mm,the microstructure of the Fusion Zone is mainly Bainite and a small amount of Martensite.Martensite,Bainite and Widmanstatten can be seen in the Fusion Zone of the samples with the Linear Energy higher than 50 J/mm.The isograms of residual stress distribution,obtained from the weld sections of 2.5 kW-3m/min,2.5 kW-5m/min and 2.5 kW-7m/min samples are drawn by using the lattice strain curve of the(1012)? plane.The three process parameters,2.5 kW-3m/min,2.5 kW5m/min and 2.5 kW-7m/min,correspond to the conduction type,the intermediate type and the convection type molten pool respectively.The results show that in both TD and LD directions,the distribution of residual stress in the conducting molten pool is relatively uniform,and the maximum value is about 220 MPa.The residual compressive stress in the convective molten pool has obvious gradient change,and the maximum value is about 330 MPa.The intermediate pool sample is between them,and the maximum value is about 270 MPa.In the direction of LD,the residual stress distribution of the three samples is relatively average.In the direction of TD,the distribution of residual stress is limited by the change of weld morphology,that is,it shows a sharp gradient change corresponding to the size of Fusion Zone in the weld section.Mechanical experiments show that the formation of Martensite and Bainite will enhance the mechanical performance of the weld.In the Fusion Zone,the samples which belong to convection type and intermediate type molten pool exist Martensite and Bainite mainly,so Vickers micro-hardness of the weld is obviously higher than that of the Base Metal(BM),reaching 450 HV.The stress-strain curve shows that the Rm and ReL of the weld are better than that of the Base Metal.However,only the samples with uniform size change of molten pool reach the elongation after fracture similar to the Base Metal,such as 2 kW-3m/min,3 kW-3m/min and 3 kW-5m/min samples.Based on the results of EBSD and Synchrotron Diffraction,the orientation information of products after phase transformation,Fusion Line and the interface of Heat Affected Zone in 2.5 kW-7m/min samples which belongs to convective melting pool,was studied by analyzing the ideal texture orientation and the relative intensity of diffraction peak.The results show that the evolution of texture exists in the welded joint.The{11 20}<10 1 0>plate texture in the Base Metal changes into {11 20}<0001>recrystallization texture in the Heat Affected Zone.The diffraction pattern shows that the diffraction peak with the highest intensity changes from(1010)? to(1011)? in TD.