Study On Microstructure And Mechanical Properties Of TiAl/Ti₂AlNb Spark Plasma Diffusion Welded Joint

With the vigorous development of aerospace industry,improving the thrust weight ratio of aeroengine has become the key to improve the performance of aircraft.Using TiAl alloy and Ti₂Al Nb alloy as engine blade and disc materials instead of traditional Ni based superalloy can reduce the weight of the engine from the aspect of materials;Connecting the blade and disc body by welding instead of the traditional tenon and groove can further reduce the weight of the engine from the structural aspect.Therefore,the high-quality welding of TiAl/Ti₂Al Nb alloy is of great significance to improve the thrust weight ratio of aeroengine and the overall manufacturing level of aircraft.In this paper,spark plasma diffusion welding experiments were carried out on TiAl alloy and Ti₂Al Nb alloy with pure Ti as intermediate layer.The effects of process parameters on the microstructure,mechanical properties and joint formation process were studied.Carry out post weld heat treatment test on the joint to study the effect of heat treatment process on the microstructure and mechanical properties of the joint.The microstructure of welded joint is TiAl base metal/blockβ+α₂-Ti₃Al/equiaxedα₂-Ti₃Al/lamellarα+βmatrix/B₂ single-phase structure,there is a partial phase transformation zone between B₂ single-phase region and Ti₂Al Nb base metal.The welding temperature,the holding time and the thickness of the interlayer will affect the phase morphology and element diffusion behavior of the interface.The tensile fracture position at room temperature and high temperature of the joints with different process parameters is located at the interface,and the fracture mode belongs to the brittle fracture with the coexistence of transgranular fracture and intergranular fracture.The room-temperature tensile strength and 650℃high-temperature tensile strength of the joint obtained at 20μm and 900℃welding temperature reach 679 MPa and 641 MPa respectively.α₂+βPhase region andα₂ single-phase region is the weak region of the joint,and the increase of their thickness will lead to the decline of mechanical properties at room temperature and high temperature.The formation process of the joint under spark plasma diffusion welding is as follows:the contact of the sample surface under the action of pressure before power on,the local high temperature in the heating stage to promote the local plastic deformation and element diffusion,the gradual closure of the hole and the growth of the reaction layer in the heat preservation process,and the lamellar formation in the cooling processαPhase fromβprecipitation in the matrix.During joint formation,the growth mechanism ofα₂+βPhase region andαsingle phase region is dominated by element diffusion,and the square of thickness is linearly and positively correlated with holding time.O+β/B₂ duplex structure was formed in the phase transformation zone of the joint treated by 960℃solution treatment+780℃aging treatment,the microstructure morphology of the interface area changes.Solutio+aging treatment improves the strength and plasticity of the joint.The fracture position of the joint changes from the interface area as welded to the TiAl base metal area.The elongation increases after fracture,and the hardness of the phase transformation area of the joint is closer to the base metal.The phase transformation zone of the joint aged at780℃formed a basket structure composed of fine lath O phase,and the mechanical properties of the joint were not significantly improved.