Research On Microstructures And Properties Of Dissimilar Welded Joints Between AISI316L Stainless Steel And TC4 Titanium Alloy

Stainless steel and titanium alloy composite structure can realize lightweight manufacturing and have better cost-efficient.It has important application value and theoretical significance in high-tech fields such as aerospace,petrochemical industry,nuclear industry and electronics.In this dissertation,Cu/Nb multi-interlayer was inserted between AISI316L stainless steel and TC4 titanium alloy to carry out diffusion bonding,laser welding and electron beam welding.The effect of different parameters on the welding performance was explored,based on which the interfacial microstructure and bonding mechanism were illustrated.The formation of welding seams of AISI316L stainless steel and TC4 titanium alloy by diffusion bonding,laser welding and electron beam welding using Cu/Nb as multi-interlayer was investigated.Cu/Nb multi-interlayer was applied to conduct diffusion bonding between AISI316L stainless steel and TC4 titanium alloy.The influence of bonding temperature and bonding time on microstructures and mechanical properties were examined to evaluate the joints.Cu/Nb multi-interlayer can achieve a plastic transit between TiA and SS when the bonding temperature is below 950°C,and the formed joints are consisted of TiA/?-?Ti/Nb/Cu/SS.Bonded at 950°C for 90 min,?-?Ti,?-?Ti,Nb?,Cu_0.5Fe_0.5Ti,Fe₂Ti,FeTi,?₁,TiC,CuTi,Ti₂Cu,Fe_9.64Ti_0.36 and?phases are identified at the interface.Maximum tensile strength of 489 MPa is accomplished when the joints processed at 900°C for 90 min and a minimum hardness value of 99HV was measured in the joint zones.A formation of fracture took place though Cu/Nb interface at lower parameters,and transferred to remnant Cu layer at 900°C for 90 min with extensive dimples on the surfaces,displaying a ductile nature.Bonded at 950°C for 90 min,the joints fractured along the intermetallic compounds upon tensile tests,and cleavage fracture and lamellar fracture were observed,indicating a brittle nature.Laser welding was employed to join stainless steel and titanium alloys by using Cu/Nb multi-interlayer and Nb single interlayer.When using Cu/Nb multi-interlayer,hot cracks were easily formed in the fusion zone between the titanium alloy and the Nb layer with higher laser power injected,and the cracks mainly extended along the grain boundary.At the Cu-Nb interface,Cu,Nb,and Ti were interfused to form Ti₂Cu₃,TiCu₄,Ti₂Cu,and Nb-based solid solution.With lower laser power,the main microstructures of the weld zone were TiCu₄,Cu-based and Nb-based solid solutions.Adjacent to the hot crack zone,the hardness rose to 401.1 HV,which was the highest hardness of the weld zone.Maximum average tensile strength of the weld joint is95.54 MPa,and the facture morphology is rough due to broken or removed grains.Dendritic grains and dimples can be observed at fracture surface,which is mixed fracture modes.When applying Nb as the transition layer,a large amount of brittle intermetallic phases,such as NbFe,Nb Fe₂,and Nb-based solid solution formed in the weld zone,resulting in spontaneous fracture of the joints under thermal stress.Fracture is located at the interface between Nb and stainless steel,forming cleavage steps and river patterns which present a brittle nature.Cu/Nb multi-interlayer was applied to conduct electron beam welding between AISI316L stainless steel and TC4 titanium alloy.The influence of beam offset on the microstructures and mechanical properties were analyzed to evaluate the joints.When using two pass welding,the closer the beam was to Nb layer side,the less the content of Fe and Cr was in the weld zone.With the beam nearing the Nb layer side,the phase changed from NbFe+?Nb?+NbCr₂?NbFe+NbFe₂+Cr_1-xFe_x+?Cu?to NbFe+?Cu?+?Nb?,which improved the tensile strength of the joints and the maximum average tensile strength is 148.77 MPa.Fracture morphorlogy changed from brittle mode to mixed fracture modes.Owing to the formation of Cu-Fe and Nb-Ti solid slotions,the hardness of the weld zone was increased but not exceeds 400 HV.In one pass welding,Nb-based solid slotion was formed at Nb-Cu interface.On the side of Cu layer and stainless steel,a light gray area was emerged,which was NbFe₂ and Cu-based solid solution.The melting zone between titanium alloy and Nb layer was mainly composed of NbCr₂,NbFe and Nb-based solid solution.Fracture surface included river patterns and pore defects,showing brittle cleavage fracture characteristic.