Key Technologies Of Isothermal Beta Forging For The Blisk Of TC17 Titanium Alloy And Its Application In Engineering

As one of the important ways to develop the structure of high thrust-weight ratio aeroengine rotating parts,aeroengine blisk of integrated structure can improve the structural efficiency of the engine.The development of integral blisk with high structural efficiency and lightweight is very important for the strategy of high-performance engine in many developed countries.TC17 titanium alloy processed by β isothermal forging technology has been used in many types of fans and compressors in China,which has broad application prospects in the manufacture of high performance aeroengine.However,the β isothermal forging process of integral blisk for TC17 titanium alloy is a complex process coupled of thermodynamics and mechanics.The engineering technologies such as heating control in β phase zone under multi-field coupling,cooling control after forging and design of preformed blank are the key of successful application of β isothermal forging for TC17 alloy to ensure homogeneous deformation.In this research,a lot of experiments combine with theoretical analysis and numerical simulation have been made to study the pre-forging heating,post-forging cooling and the design of preformed blank systematically.The followings show the main contents and the conclusions of the research.In order to solve the problem that the growth rule of β grain obtained by traditional10×10mm small sample deviates from the actual situation,the test of β grain growth for TC17 alloy is carried out by using the cylinder sample of Φ100×100 mm which is more close to the engineering practice.Meanwhile,heavy corrosion method was applied to acquire the statistical result which is consistent with reality of practical condition.The results show that β grain size increases with increasing heating time and the growth rate of β grain decreases with the increase of heating time.According to the difference of growth rate,the process could be approximately divided into three stages including rapid growth,stable growth and slow growth.The time range of three processes are0～1h,1～4h and 4～6h,respectively.The heating temperature has a significant effect on the β grain growth of TC17 titanium alloy,and the β grain size increases linearly with the heating temperature.Temperature of bar with dimension Ф250×250mm was measured with a thermocouple.Meanwhile,the temperature variation recorded was used to calculate the heat transfer coefficient by FEM（finite element simulation）method.Furthermore,the heating model of TC17 alloy was established and the effectiveness was validated based on the heat transfer coefficient.Then,law of heating curve was obtained by simulation of bars with different dimensions.Simulation results show that stage-by-stage heating process can significantly shorten the residence time above phase transition point compared with Direct heating in β region,and can effectively control the β grain size.Isothermal forging experiment was applied to bars of TC17 alloy with dimensionФ250×250mm to study the effect of deformation,strain rate and cooling method on microstructure and mechanical properties.The research shows that deformation and temperature have a great impact on the microstructure in isothermal forging.Small deformation would induce equiaxed original β grains and unbroken grain boundary αphase.Larger deformation leads to more β grains recrystallization or globularization of grain boundary α phases or intragranular α phases.If loading speed is too low,the blank with rapidly decreasing temperature will be mainly deformed in the two-phase region and induce more globularization of grain boundary α phases or intragranular α phases,meanwhile,the high speed would induce severe β grains recrystallization because of larger deformation in β phase region caused by temperature rise.According to simulation and metallographic observation,better process parameters could be obtained as equivalent strain between 0.7 ～2.0 and loading speed of 5mm/s.The cooling after forging indicates that the lamellar α is fully precipitated in the model of air cooling and the better basketweave structure is obtained.Thus air cooling is proposed as the cooling mode in practical production.Forging process and cooling mode after forging have obvious effects on mechanical properties.Tensile strength increases first and then decreases with the increase of strain,while plasticity increases steadily with the increase of strain.Strength and plasticity increase first and then decreases with the increase of deformation speed.Fracture analysis shows that ductile fracture is the main mode of tension at room temperature.With the increase of strain and deformation speed,fracture occurs,fracture toughness increases first and then decreases.In addition,the plasticity and fracture toughness of the blank under water-cooling condition are lower than those under air-cooling condition except for its high strength.The comprehensive mechanical properties of the blank after forging air-cooling are better.Fracture analysis shows that ductile fracture is the main fracture under air cooling condition,while mixture fracture is the main fracture under water cooling condition.The preformed blank has been optimized by the FEM to ensure the filling quality,good formality,better strain distribution and absence of folding defects combining with the structural characteristics of integral blisk of the engine.Finally,a preformed blank with specially shaped section is adopted as the optimized shape.The optimized preformed blank was tested in the β forging of integral blisk according to consequence of the experiment.The results show that shape and size of the billet are designed reasonably,and there are no folding and filling defects in the deformation process.There are no macro and micro structural defects in each part of the integral blisk,and all meet the requirements of technical standards.The comprehensive mechanical properties test of TC17 titanium alloy β forging integral blisk shows that the tensile properties of all parts of the integral blisk meet the standard requirements at room temperature and high temperature;the high cycle fatigue limit at room temperature is 510 MPa at the blade position;the fracture toughness at the accessory plate position and the rim position are all over 60 MPam^1/2,and the comprehensive performance of the integral blisk is excellent.