Research On Microstructures And Properties Of Dissimilar Welded Joints Between 4J29/4J36 Iron-based Alloy And TC4 Titanium Alloy

The composite components of 4J29 Kovar alloy/4J36 Invar alloy and TC4 titanium alloy have the characteristics of light weight,good economic applicability,etc.At the same time,these components can integrate the advantages of their performance and improve the performance of the components.They have shown good application prospects in the fields of machinery industry,electronics manufacturing,chemical industry,shipbuilding industry and aerospace.However,in the welding of composite components,brittle phases and residual stresses are easily generated in the weld,so they are prone to fracture at the weld.In order to find a way to improve the welding quality of composite components,vacuum diffusion welding of 4J29 Kovar alloy and TC4titanium alloy was first carried out using Co foil as an interlayer,and then vacuum electron beam welding of 4J29 Kovar alloy and TC4 titanium alloy was carried out using the combination of Cu foil and Nb foil as an interlayer.Finally,vacuum electron beam welding of 4J36 Invar alloy and TC4 titanium alloy was carried out,and the combination of V foil and Fe foil was used as the intermediate interlayer.Subsequently,the effects of welding process parameters and the usage of interlayer on the microstructure of the weld and the mechanical properties of welded joints were analyzed,and the formation mechanism of welded joints under different conditions was clarified.The vacuum diffusion welding of 4J29 Kovar alloy and TC4 titanium alloy was conducted by adding Co interlayer,and the influence of temperature on the microstructure of the weld and the mechanical properties of welded joints was explored.After adding Co interlayer,obvious defects were not produced in the weld.The interface between the titanium alloy and the Co interlayer was mainly composed of Ti Co₃,Ti Co₂,Ti Co and Co Ti₂.The presence of these Ti-Co phases and the formation of the Widmanstatten structure of titanium alloy severely reduced the tensile strength of welded joints.The interface between Kovar alloy and Co interlayer was mainly composed of Co solid solution.At 825°C,the welded joint had the maximum tensile strength of only 58.5 MPa.The thickness of Ti-Co phase increased as the bonding temperature increased,further damaging the tensile strength of welded joints.In the tensile test,the fracture surface of welded joints showed the characteristics of brittle fracture.As the bonding temperature increased,the position of fracture gradually approached the interface between Co Ti₂ and titanium alloy,indicating that Co Ti₂ had the highest brittleness.When the bonding temperature was too high and the holding time was too long,the quality of the 4J29-Co-TC4 diffusion welded joint was poor due to the formation of thicker Ti-Co phase and more Widmanstatten structure of titanium alloy.Therefore,lower bonding temperature and shorter holding time should be used in the future.The vacuum electron beam welding of 4J29 Kovar alloy and TC4 titanium alloy was conducted by adding Cu/Nb composite interlayer.The effect of the change of Nb foil thickness on the microstructure of the weld zone and the mechanical properties of welded joints was mainly analyzed.The results showed that the surface of the weld was relatively flat and continuous,without obvious welding defects.When the Nb foil was0.22 mm,the microstructure of the weld zone between the titanium alloy and Cu/Nb composite interlayer was mainly composed of Ti solid solution and Fe Ti.When the Nb foil was 0.40 mm,the microstructure of the weld zone between the titanium alloy and Cu/Nb composite interlayer was mainly composed of Ti solid solution and Cu Ti₂.The increase of thickness of Nb foil could promote the formation of Ti solid solution in the weld zone of titanium alloy side,which was beneficial for improving the toughness of welded joints.In both cases,the bond between the Kovar alloy and the Cu/Nb composite interlayer was better.In addition,Cu_0.5Fe_0.5Ti was formed in the weld zone of the titanium alloy side.The tensile test results showed that welded joints all fractured in the weld zone of the titanium alloy side.When the Nb foil was 0.40 mm,the diffusion of a large amount of Fe atoms into the weld zone of titanium alloy side could be inhibited,thereby promoting the formation of Ti solid solution and a small amount of Cu Ti₂,and eliminating the presence of Fe Ti.The tensile strength of welded joints was effectively improved to 150 MPa,and the hardness in the weld zone dropped sharply.Therefore,the usage of Cu/Nb composite interlayer could increase the content of solid solution in the weld zone and improve the welding quality of 4J29 Kovar alloy and TC4titanium alloy.However,due to the defect of insufficient melting in the weld,the tensile strength of welded joints was low.Therefore,it is necessary to further optimize the welding parameters to eliminate this defect.The vacuum electron beam welding of 4J36 Invar alloy and TC4 titanium alloy was achieved by adding V/Fe composite interlayer,and the effect of change of electron beam current on the microstructure of the weld zone and the mechanical properties of welded joints was explored.When the size of the electron beam current was 9 m A and10 m A,the molten metal in the weld zone between the titanium alloy and the V interlayer was mainly titanium alloy,and there was no good metallurgical bond between them.There was an unmelted Fe interlayer in the weld zone of Invar alloy and V interlayer,and the existence of cracks was found.When the electron beam current was increased to 11 m A,the welded joint obtained a good tensile strength(168.8 MPa).The microstructure in the weld zone between the titanium alloy and the V interlayer was mainly composed of Ti-V solid solution with a high V content.The microstructure of the weld zone between the Invar alloy and the V interlayer was mainly composed of Fe solid solution and a small amount of Fe-V solid solution.The average hardness value of the weld zone was only 203 HV.When the electron beam current was 12 m A,there was a large amount of brittle?phase at the interface between the weld zone of Invar alloy side and the V interlayer,which greatly improved the crack sensitivity of weld.Therefore,the addition of V/Fe composite interlayer could effectively avoid the generation of brittle intermetallic compounds in the weld,and improve the welding quality of 4J36 Invar alloy and TC4 titanium alloy.In order to further improve the welding quality,the position of the beam offset should be adjusted to reduce the effect of annealed V.