| Super-Ni/NiCr laminated material has better oxidation resistance,corrosion resistance,high-temperature creep resistance and higher toughness reserve.It also shows advantages of reducing structural weight and preventing sudden fracture.TC4 titanium alloy has high specific strength and fracture toughness.It becomes key material for the manufacturing aeroengines and aircraft frames.It will have great potential application in the field of fabricating significant component that reliable connection between Super-Ni/NiCr laminated material and TC4 titanium alloys is achieved.However,due to the unique structure of the Super-Ni/NiCr laminated material,the cover layer is easily detached from the base layer when heated and melted.Cracks and holes in the weld are serious problems,seriously limiting the application and development of the heterostructures of laminated composite with TC4 titanium.In order to give full play to the advantages of the laminated structure and promote the application of joining the laminated material with TC4 in the field of aerospace manufacturing,vacuum diffusion bonding process was used to bond Super-Ni/TC4 and Ni80Cr20/TC4,aiming to solve the problem of poor weldability of laminated composite.Based on the forming quality of laminated joint,the diffusion bonding process was optimized.The inherent relationships between diffusion bonding parameters,interfacial microstructure,interfacial behavior of elements and properties was established.In order to overcome the softening problem of titanium alloy during high-temperature diffusion bonding,the heating temperature was optimized.Under the diffusion bonding condition of temperature 950℃,the pressure 5 MPa,and the holding time 30～90 min,the integrity of the joint was assured with three different interlayers:no interlayer,Cu foil and Cu foil+Ti powder.It is known that the interfacial voids and cracks are significant factors affecting the bonding quality of joint.The large residual stress was easy to form near thin Super-Ni cover layer after cooling,causing the crack initiation and propagation.Short holding time caused crack near Ni80Cr20/TC4 interface.Lateral microcracks often initiated near the interfacial voids and propagated along the Ti2Cu+TiCu layer towards the TC4 side.The composition and orientation of the Ti2Cu+TiCu eutectic structure changed the propagation path and the crack is stopped in the Ti2Cu layer.According to the microstructural characteristics of transition zone with different interlayers,specific division schemes of transition zone were proposed.The transition zone of diffusion bonded joints without interlayer was divided into three layers including TiNi3,TiNi,and Ti2Ni reaction layers from Super-Ni to TC4 side.The transition zone of diffusion bonded joint with Cu foil interlayer was divided into Cu-rich reaction layer and Ti-rich reaction layer.Similarly,the transition zone of diffusion bonded joint with Cu+Ti composite interlayer,when the interfacial atoms sufficiently diffused and mixed,was divided into Ti-rich layer and Ti-Cu-Ni reaction layer.Ni80Cr20/TC4 interfacial transition zone included Ti2Ni layer,TiNi layer and Ti(Ni,Cr)3+Crss layer.The relationship between the holding time and the width of transition zone and the reaction layer meets the parabolic law.The TiNi layer at the Super-Ni/TC4 interface is significantly wider than the TiNi3 and Ti2Ni layers.The large difference in the diffusion coefficients ofNi and Ti(DNi in Ti》Dnin Ni)and the consume ofNi and Ti by the growth of TiNi layer resulted in the nickel-depleted and chromium-rich region at the Ni80Cr20/TC4 interface,The influence of the asymmetric diffusion ofNi and Ti on the element distribution was analyzed from the perspective of the high-temperature transition and the atomic diffusion path.The contact of Ti and Cu atoms promotes the formation of liquid at the original grain boundary to form transient liquid phase.The grain boundary and the transient liquid phase both accelerate the diffusion of Ti,forming asymmetric diffusion of Ti and Ni.As a result,Ti element can diffuse to form Ti-Cu-Ni reaction layer near Super-Ni side.The maximum shear strength of joints was obtained in the case of Super-Ni/Cu+Ti/TC4 and Ni80Cr20/Cu+Ti/TC4 as 85.4 MPa and 72.4 MPa,respectively,when the holding time was 60min.The fracture morphology of the laminated composited/TC4 joint without interlayer,Cu foil,Cu foil＋Ti powder composite interlayer shows regular polyhedron pit,dissociation steps and concentratedly distributed small-sized polyhedron pit,granular and particle-shaped brittle phase.The shear fracture mechanism of the joint can be described as following.The grain boundaries near the interface slipped under shear stress and formed slip steps,generating dislocation aggregation and acting as crack source.When the interface between TiNi3 reaction layer of Super-Ni subjected to shear stress,the coordinated grain deformation of Super-Ni with a certain plasticity can occur at a small distance along the shear surface but was limited by the densely packed hexagonal structure of TiNi3 with poor deformation ability.When shear stress on the crystal plane reaches the interface bonding strength,the joint undergoes shear fracture.A relative systemic research on the diffusion bonding laminated material to TC4 titanium alloy was conducted in the present work from the aspect of interfacial microstructure,diffusion and reaction of elements,interfacial bonding mechanism and mechanical properties.This work can provide experimental basis and theoretical foundation for further developing new laminated materials and diffusion bonding light laminated materials,showing great significant to promote the application of laminated materials in the field of aviation manufacturing. |
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