Research On Spray-formed 2195 Aluminium-lithium Alloy Microstructure Evolution And Low-cycle Fatigue Behavior

The third generation aluminium-lithium(Al-Li)alloys,a new series of aluminum alloys,due to the superior performance such as low density,high strength and high stiffness,have been gradually used to replace 2xxx and 7xxx series aluminium alloys conventionally applied on the aircraft.The application of Al-Li alloys,which helps to realize the lightweight of aircraft structural components and increase the carrying capacity of aircraft,has a broad prospect in the aviation and aerospace fields.However,at present,the research on microstructures and mechanical properties,especially the fatigue property of Al-Li alloys is still insufficient in the literature,which directly affects the establishment of the relationship between microstructures and macroscopic properties for Al-Li alloys,as well as the evaluation for the reliability and the service life of alloy products.Therefore,it is necessary to study the microstructure evolution and the low-cycle fatigue behavior of Al-Li alloys.The spray-formed 2195 Al-Li alloy was taken as the research material in this work.The microstructure evolution of the alloy during the hot extrusion and heat treatment process was investigated,the deformation behavior of material under cyclic loading was analyzed and the low-cycle fatigue life was predicted,and the cyclic deformation mechanism and the fatigue fracture mode of the alloy were studied.The main work and conclusions of this study are as follows:(1)The alloy microstructure evolution(grain morphology,second phase particles and crystallographic textures)in the hot extrusion and subsequent heat treatment(solid solution and aging treatment)process was studied.The grains of spray-formed alloy were equiaxed without obvious textures.For the as-extruded,solution-treated and aging-treated alloys,the grain morphology changed from st:ripped shape near the center of extruded plate to equiaxed one near the surface.The solid solution treatment could increase the grain size,while the aging treatment had little effect on the grains.The center of extruded plate had near rolling-type textures,while the textures around the surface were rotated,which indicated that the heat treatment had no obvious effect on the textures of extruded plate.Li and Cu played a key role in the alloy strengthening.The Al-Cu phase in the alloy could be dissolved and re-precipitated during the heat treatment process,while the Fe-containing particles always existed stably.(2)Strain-controlled low-cycle fatigue tests were carried out for the heat-treated samples along extrusion and transverse directions.The stress-strain hysteresis loops of material were obtained,and the cyclic stress response curves were derived.The cyclic stress-strain relationships were acquired by fitting,and the fatigue life of the alloy was predicted and analyzed through various fatigue life models.With the increase of total strain amplitude,the area of hysteresis loops at initial cycle and mid-life cycle increased gradually.The material mainly presented a cycle stability characteristic under low strain amplitude,while the cycle hardening behavior was dominant under high strain amplitude.Under each strain amplitude,the cyclic stress amplitude was higher for the alloy along extrusion direction,but the fracture life was similar to that along transverse direction.It was found that the model based on the total strain energy density gave a good result to predict the low-cycle fatigue life for this alloy.(3)Through the observation for fatigue fracture surfaces,the fracture morphologies of the samples along two orthogonal directions under different strain amplitudes were compared,and the fatigue crack initiation and growth behaviors and the final fracture behavior of the alloy during loading were discussed.It was found that low strain amplitude fracture surfaces had a typical fatigue characteristic.while a near quasi-static tensile characteristic was presented under high strain amplitude.The deformation microstructure near fatigue fracture was observed,and the microstructure evolution under different strain amplitudes was compared.It was found that the deformation degree and the dislocation density of the alloy under high strain amplitude were obviously higher than those under low strain amplitude.The cyclic hardening behavior of material was closely related to the interaction between moving dislocation and(sub)grain boundaries,second phase particles and evolved dislocation structures(dislocation loops,dislocation tangles,etc.)during fatigue loading.