Study On Microstructure And Properties Of Titanium Alloy By Activated Laser Welding

Ti-6Al-4V titanium alloy is a kind of comprehensive performance?+?double-phase titanium alloy,which is widely used in aerospace,shipbuilding industry,automobile manufacturing because of its high specific strength,low density,high temperature mechanical properties and good corrosion resistance and etc..Compared with the traditional titanium alloy welding method,laser welding has the advantages of small welding heat affected area,small deformation of welding parts,fine grain size and high welding efficiency.It has become widely used welding method of Ti-6Al-4V titanium alloy.However,in the laser welding process,the plasma generated above the workpiece will cause absorption,refraction and reflection of the incident laser,which affects the welding stability,weld penetration and welding quality.Therefore,CNTs,rare earth oxides La₂O₃and CeO₂ as activated flux were used to study the effect of single activated flux on the penetration of Ti-6Al-4V titanium alloy laser welds.The microstructure and mechanical properties of Ti-6Al-4V titanium alloy laser welding joints were studied.The optimized welding parameters were used to laser weld the Ti-6Al-4V titanium alloy coated with CNTs,rare earth oxides La₂O₃ and CeO₂,and observed the change of plasma morphology during activated laser welding.Micromorphology and microstructure of the weld were investigated.The effects of the activated flux on the phase composition of the joint were analyzed.By testing the mechanical properties of the joints,the influence of the activated flux on the properties of the joint was investigated.Under the same welding process parameters,the penetration and width of weld with CNTs is increased.The depth-to-width ratio of laser weld with no flux is 0.491;the depth-to-width ratio of laser weld with CNTs is 0.522.Coating with different mass fraction of CNTs showed that the increase of mass fraction of CNTs weld penetration depth changes is not very significant.Under the same welding process parameters,coating with La₂O₃ and CeO₂ penetration of weld increased,the melting width decreased.The depth-to-width ratio of laser weld with no flux is 0.502,and the depth-to-width ratio of weld with La₂O₃ and CeO₂ is 1.327 and 1.185 respectively.It is found that the plasma morphology of coating La₂O₃ and CeO₂ shrink significantly,while the plasma of coating CNTs has no obvious change.The microstructure of base metal of Ti6-Al-4V is the mechanical mixture,which is composed of equiaxed?and?.The microstructure of welds is mainly widmanstatte and basket-weave microstructure.Coating CNTs can refine microstructure of weld.Coating the rare earth oxide La₂O₃ can refine the weld grain size,and the size of??was small;Coating CeO₂ weld grain size did not change significantly,but the size of??was more small.The three activated fluxes have little effect on the weld phase composition.Under the same welding process parameters,the tensile strength of joints with no CNTs was 893.75MPa.Tensile strength was 983.35 MPa when coated with 1%CNTs,increased by 10.0%;When coated with 3%CNTs,the tensile strength was 800.41 MPa,a decrease of 10.4%.The tensile strength of the coated La₂O₃ joints was 973.85 MPa,increased by 9.0%.While tensile strength of the coatied CeO₂ joints was 796.34MPa,a decrease of 10.9%.