| Large and complex blades made of titanium alloys are the key power components which are widely employed in large steam turbine and turbofan engine,and play an important role in the energy transformation.Due to the large size,complex geometrical shape and poor service conditions of the blade,it always has a high requirement on manufacturing precision and mechanical performance during the forming process of the blade.In addition,owing to the fact that forging operation can improve the microstructure,mechanical properties and production efficiency,it has become the main forming method of the blade.However,unlike the small-sized and medium-sized blades(length?400 mm)which are widely manufactured by precision forging technology,forging the titanium alloy blade with large size(length?600 mm)is still in the conventional forging stage(machining allowances 4.5~6 mm for blade body).Moreover,a series of problems,such as poor geometric precision of forgings,unstable quality performance,short service life of forging dies,long manufacturing cycle and low utilization rate of the blade material exist.In order to solve the blade forging problems mentioned above,this dissertation proposes a precision hot forging technology of the blade with a small allowance(1.5mm for blade body).Then,a TC4 titanium alloy blade which has length of 1220 mm,changing cross section,large twist angle,thin wall,damper platform and tip shroud is selected as the research object.Finally,fundamental research on hot forging of the blade is carried out by using analytics,simulations and experiments.The main contributions of this dissertation are as follows:(1)According to the theory of metal plastic deformation and the characteristics of blade forging process,the design methods of forged balde,preformed billet and forging dies for precision hot forging of large complex TC4 titanium alloy blade with small machining allowance are proposed.The three-dimensional deformation laws of metal in hot forging operation of the blade are studied.The whole deformation laws of the blade preformed billet,and the distribution and evolution laws of different fields,as well as change law of die forces are revealed.The results show that metal mainly travels along the width direction of the blade.Moreover,the distribution of field variables is very uneven.In addition,it can be found that the maximum forging force in the horizontal direction is about 1/10 of the forging force in the vertical direction.The above studies provide a scientific basis for an optimization design of the balde preformed billet.(2)Based on the mean square error functions of effective strain and temperature,the quantitative criterions about deformation and temperature distribution uniformity of the large complex TC4 titanium alloy blade with small machining allowance is proposed.The effects of process parameters on the uniformity of strain and temperature distributions at the key cross section of the blade are carefully studied.The parametric study reveals a reasonable combination of process parameters is crucial for the titanium alloy blade forging with uniformity.The results show that the deformation and temperature uniformity of forged TC4 turbine blade is inversely proportional to the friction factor(m)and the transport time(DT),which is proportional to initial forging temperature(T_w).Moreover,the appropriate increase of forging speed(V)can effectively improve the uniformity.In addition,the optimum process parameters range from{V=400~500 mm/s,m=0.1~0.3,T_w=950~980?},and DT should be as small as possible according to the actual working conditions.The above studies can provide significant guidance for the design and optimization of blade precision forging process.(3)Using JMAK and Avrami models,the microstructure evolutions of the large complex TC4 titanium alloy blade with small machining allowance during the precision hot forging and cooling processes are studied.Moreover,the evolutions of dynamic recrystallization volume fraction,average grain size and phases were analyzed detailedly.The results show that the microstructure distribution of TC4 turbine blade is uneven.Moreover,the flash area and blade body near the tip shroud of forged blade will retain some?phase after the precision hot forging and cooling process.The above studies can reveal the relationship between the microstructure evolution and the macroscopic deformation law.(4)Based on the theory of metallography and materials science,the microstructures and tensile properties,such as tensile strength,elongation,shrinkage and impact properties of the large complex TC4 alloy blade with small machining allowance were investigated.The results show that the content of?phase in the blade body of forged blade is lower,where the tensile strength is slightly higher than other ereas of forged blade.However,the plasticity index of the forged blade is basically the same.Moreover,it can be found that when using the glass protective lubricant Ti-1,forging process(V=500mm/s,T_w=950?,T_d=250?)and heat treatment process(800?×2h),the excellent performance of TC4 turbine blade can be obtained.The relationship between the microstructure and mechanical properties of the blade was analyzed.(5)Using the modified Archard wear model,the die wear characteristics in hot forging process of the blade were studied.The relationship between the die wear volume and macroscopic deformation of the blade is revealed.Moreover,the effects of thermal parameters on the die wear characteristics are assessed.The results show that die wear distribution is uneven,the minimum wear is located in the middle of the the blade die cavity.Moreover,it can be found that the amount of die wear is proportional to T_w and inversely proportional to T_d.The optimum thermal parameters range from{T_w=920~980?,T_d=100~300?}.The above studies can provide a guidance for the design and optimization of blade forging dies.Through the above study,this research can provide a basis for guiding production,and further provide a foundation for the new breakthrough of precision forging technology of the blade. |
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