Effect Of Friction Stir Processing On Low Temperature Superplastic Behavior Of TC4 Alloy

TC4(Ti-6Al-4V)is a typical α+β duplex titanium alloy with low density and high strength,it is widely used in the field of weapon equipment manufacturing.Because the load per unit area and aerothermal intensification performance of supersonic missile’s outer wing increases significantly with the increase of flight speed,the design concept of multi-layer hollow structure supplemented by stiffeners is generally adopted,and the one-time near net forming of missile’s outer wing is realized by using the technology of superplastic forming/diffusion connection.Therefore,it is of great significance to study the superplastic deformation behavior and deformation mechanism of TC4 alloy at low temperature.In this paper targeting at the TC4 alloy hot-rolled plate,adopted the friction stir processing experiments under different rotation speeds(r/min)and processing speeds(mm/min).And the ultrafine grain structure of the alloy was prepared.During the lowtemperature superplastic tensile test,the superplastic deformation behavior of TC4 alloy was clarified,and the cavity nucleation and growth mechanism was constructed.The fracture mechanism of TC4 alloy was revealed by combining the microstructure characterization results.The main research results are as follows:(1)Heat treatment at 950～1000℃×2h,the proportion of α phase of TC4 alloy is 53%;after friction stir processing(rotation speed 210～360r/min,processing speed 30mm/min,60mm/min),the average grain size decreases with the increase of rotation speed and the decrease of processing speed.The average grain size from 0.6μm to 0.9μm.(2)Superplastic tensile at 850℃ and 5×10-3s-1,the breaking elongation of TC4 titanium alloy is 1080%.The strain rate sensitivity index m increases with the increase of temperature,and the maximum m value is 0.4.The deformation activation energy Q value is between 142.333～394.942KJ/mol,and it decreases with the increase of temperature,and increases with the increase of strain rate.(3)The superplastic fracture mechanism of TC4 titanium alloy is cavity nucleation,growth and aggregation connection,and large cracks are formed in the tensile direction leading to cause material fracture.The transition from diffusion growth mechanism to plasticitycontrolled growth mechanism of the critical cavity radius decreases with the increase of temperature;it increases with the increase of strain rate.(4)A 3-16-1 three-layer BP artificial neural network model was established,taking deformation temperature T,strain ε and strain rate ε as input parameters and deformation activation energy Q value as output parameters.The Q value of superplastic deformation activation energy of TC4 alloy was predicted:the multiple correlation coefficient R=0.9954,and the value of BP artificial network prediction was in good agreement with the value of experiment.