Study On Microstructures And Properties Of Laser Welded Joints Of Titanium Alloy/Stainless Steel Dissimilar Materials

Ti alloy and stainless steel are important engineering metal materials.They are widely used in various industrial fields,especially high-tech fields such as the aerospace.With the development of science and technology,the requirements for the performance of parts and components become more and more demanding.A single material?Ti alloy or stainless steel?can no longer meet the requirements of service performance.Ti alloy-steel composite structure is an effective way to solve this problem.The welding?joining?of Ti alloy/steel dissimilar materials has become an urgent problem to be solved consequently.Due to the great difference in chemical compositions and physical properties between Ti alloy and stainless steel,the weldability of them and mechanical properties of the joint are seriously deteriorated.This has become one of key scientific and technical problems that hinder the practicality of Ti alloy-steel composite structures.Therefore,the research on microstructures and mechanical properties of laser-welded joints of Ti alloy and stainless steel dissimilar materials has important theoretical significance and practical value.In this paper,the weldability of TC4 Ti alloy/SUS301L stainless steel dissimilar materials and the influences of laser offset and metal interlayer materials on microstructures and mechanical properties of Ti alloy/steel joint were studied firstly.The results showed that the weldability of Ti alloy/stainless steel dissimilar material is very poor.Direct laser welding caused brittle fracture of weld metal,which seriously decreased the mechanical properties of Ti alloy/steel joint.Therefore,the effective joining of Ti alloy to stainless steel cannot be achieved.It is mainly attributed to the metallurgical reaction of Ti and Fe forming a large amount of TiFe and TiFe₂intermetallic compounds in weld,resulting in the weld embrittlement.It is beneficial to reduce the weld Ti and Fe contents and the welding stress in the joint for improving the weldability of Ti alloy and steel and enhancing the mechanical properties of the joint.The weld Ti?Fe?content could be reduced by changing the laser offset?0.15mm,0.25 mm and 0.35 mm?,but the weld metal is still prone to brittle fracture due to the uneven distribution of Ti-Fe intermetallic compounds in the weld.The metal interlayer materials had a significant effect on microstructures and mechanical properties of Ti alloy/steel joints.During laser welding the addition of metal interlayer was beneficial to decrease the average content of Ti and Fe in the weld.When the Cu interlayer was used,the weld metal was mainly composed of?-Cu,Cu₂Ti,CuTi,?-Fe and TiFe₂ phases.As the Ti-Fe compounds were reduced and the?-Cu and?-Fe solid solutions were formed in the weld,the tensile strength of Ti alloy/steel laser-welded joint was obviously improved,and the joint strength could reach 368 MPa under the condition of optimizing laser welding parameters.With the Ni interlayer,the weld metal consisted mainly of??-Fe,Ni?and?-Ti solid solutions and NbFe₂,Fe₂Ti and TiFe intermetallic compounds.Due to the precipitation of more NiTi₂ and Fe₂Ti intermetallic compounds in the bond area of the Ti alloy,the tensile strength of the joint was reduced?112 MPa?.When the Nb interlayer was used,the weld metal was mainly composed of??Ti,Nb?and?-Fe solid solutions and NbFe₂,Fe₂Ti and TiFe intermetallic compounds.Due to the precipitation of a large number of NbFe₂ and Fe₂Ti intermetallic compounds in the bond area for stainless steel,the tensile strength of the joint decreased significantly?79MPa?.Under the experimental conditions,it is more beneficial to use the Cu interlayer filler material for improving the mechanical properties of the Ti/steel laser-welded joint.In order to further increase the mechanical properties of Ti alloy/steel dissimilar material laser-welded joint,the idea of Ti alloy/steel joint assisted by the metal transition layer and laser welding process with single pass and two-pass welding were put forward in this paper.For the single pass welding by focusing the laser beam on the Ti alloy?scheme 1?,the Ti alloy/steel joint obtained by adding Nb interlayer was composed of Ti alloy,laser weld,Nb transition layer,Nb/steel interface reaction layer and stainless steel.The laser weld consisted mainly of??Ti,Nb?solid solution,and the reaction layer was mainly composed of Fe₇Nb₆,Fe₂Nb and?-Fe phases.Because of the complete elimination of Ti-Fe intermetallic compounds and the precipitation of??Ti,Nb?and?-Fe solid solutions,the tensile strength of Ti alloy/steel joint was increased obviously,reaching 396 MPa under the condition of optimizing welding parameters.For the single pass welding by focusing the laser beam on the stainless steel?scheme 2?,the Ti alloy/steel joint obtained by adding Cu interlayer was composed of Ti alloy,Ti alloy/Cu interface reaction layer,Cu transition layer,laser weld and stainless steel.The Ti alloy/Cu interface reaction layer consisted mainly of Cu₂Ti,Cu₄Ti and?-Cu,and tensile strength of the joint was 320 MPa.The reaction layer was the weakest area in the Ti alloy/steel joint,and Cu₂Ti and Cu₄Ti intermetallic compounds precipitated in the reaction layer were the main factor affecting the mechanical properties of the joint.Ti alloy/steel dissimilar material welding joint assisted by the transition layer can also be obtained by using laser two-pass welding technology?scheme 3?.The interlayer?transition layer?material had an obvious effect on microstructures and mechanical properties of the Ti alloy/steel joint.The Ti alloy/steel laser-welded joint obtained by adding V interlayer was composed of Ti alloy,laser weld 1,V transition layer,laser weld 2 and stainless steel.The weld 1 was mainly??Ti,V?solid solution and the weld 2 was mainly composed of?Fe,V?solid solution with less?and?phases.Under the condition of optimizing welding parameters,the tensile strength of the Ti alloy/steel joint could reach 599 MPa due to eliminating?and?phases in the weld 1 and weld 2.It could be further increased by using the laser two-pass welding technology and adding the Ta/V/Fe composite interlayer.The laser weld 1 was mainly composed of??Ti,V?and?Ta,Ti?solid solutions,and the laser weld 2 was mainly composed of?-Fe and?Fe,V?solid solutions.V transition layer was formed between the weld 1 and the weld 2.Under these conditions,the tensile strength of Ti alloy/steel joint could be further increased to 627 MPa,and the joint fracture occurred in V transition layer.Based on results above,Ti alloy/steel dissimilar material welding joints assisted by the transition layer have higher mechanical properties.It is mainly attributed to the fact that the metal transition layer avoids the metallurgical reaction of Ti and Fe to form brittle and hard Ti-Fe intermetallic compounds,and the plasticity of the metal transition layer is beneficial for reducing the welding stress in the joint.In addition,the solid solutions formed in welding?joining?area of the joints,such as??Ti,Nb???-Fe??-Fe???Ti,V???Fe,V?and?Ta,Ti?etc.,have also contributed to the increase of the mechanical properties of the joints.