| As the unceasing development of aviation and aerospace technology,the new generation of high performance aircraft also put forth higher requirements for aero engine thrust-weight ratio,and the requirements of materials have become increasingly harsh.Due to their excellent combinations of mechanical and anticor-rosin properties at elevated temperatures,superalloys are the vital materials for the engine parts.Because of the special processing technology,powder metallurgy superalloy FGH4096 can not only meet the performance requirements,but also effectively prevent elements segregation caused by traditional metallurgical process.However,the hot deformation is difficult and deformation behaviors are complex,deformation cracks commonly occurred in the process of hot deformation.In this paper,the effect of deforming temperature,strain rate and strain on structure and properties of FGH4096 superalloy during isothermal-forming process is investigated,and realized the accurate control of the deformation parameters;Different cracks initiation and propagation during hot deformation process is researched,and the effective measures are established to control these crack.These results can provide some scientific guidelines for the future study on the hot deformation of FGH4096 superalloy.In this paper,the deformation behavior and microstrmcture evolution of powder metallurgy superalloy FGH4096 were researched systematically.Based on the thermal simulation compression tests,flow behaviors and microstrmcture evolution during the hot deformation were studied,and the constitutive equation was established;The internal relationship between work hardening and dynamic softening was researched based stress-stain curves;The internal microstrmcture evolution in the hot deformation process was studied,and the relationship between grain size and flow stress was established;Four different crack initiation mechanisms were summarized and the corresponding prevention measures were put forward;The relationship between the crack propagation and the microstrmcture was investigated.The main research contents and results were as follows:The true stress-stain curve of FGH4096 superalloy during hot deformation process was studied.The temperature sensitivity exponent and strain rate sensitivity exponent at different processing parameters were established,which can be used to quantitatively analyze the influence of processing parameters on the flow stress.When the y? phase exists,the flow stress is more sensitive to the change of deformation temperature;When the ?'hase completely melted,the influences of temperature on the flow stress weaken gradually.The strain rate sensitivity exponent reached its peak value at 1080-1100?.The constitutive equation based on the hyperbolic sine type Arrhenius function was established,and it could accurately predict the flow stress of FGH4096 superalloy.The internal relationship between work hardening and dynamic softening during the hot deformation process was investigated,and the quantity relationship between the peak strain and the critical strain was established.There are two different nucleation mechanisms for FGH4096 superalloy during hot deformation.The heterogeneous nucleation will occur along the original grain boundaries at a low temperature and a high strain rate.The spontaneous nucleation mechanism is dominant at a high temperature and a low strain rate.The model of post-dynamic recrystallization for FGH4096 superalloy has been obtained based on the dynamic recrystallization kinetics model.The microstructure evolution of FGH4096 superalloy during hot deformation was studied.Power dissipation efficiency at different processing parameters was analysed based on the processing map.Power dissipation efficiency shows a trend of rising with the increase of true strain at a low temperature and a high strain rate,while it fluctuates at a high deformation temperature and a low strain rate.When the deformation temperature lower than 1140?,deformation temperature has obvious influence on the average grain size,but has a slightly effect on the distribution ranges of grain size.However,it has obvious influence on both the average grain size and the distribution ranges of grain size when the deformation temperature higher than 1140?.DRX takes place in the interior of a number of grains when the deformation temperature at 1140?.It is favorable for grain boundary migration when the deformation temperature rises to 1200?.Meanwhile,the large grain absorbed the small ones,which can promote grain growth obviously.And the power function model between grain size and flow stress was established.The deformation cracks initiation of FGH4096 superalloy during hot deformation process was researched,and four different crack initiation mechanisms were summarized.Some effective measures to present the deformation crack initiation were proposed:the temperature and pressure in the process of hot isostatic pressing can be increased;A prolonged heat treatment is necessary before deformation,which is beneficial to uniformly distributed,fine grained and regularly shaped solid particles;The pre-deformation should be performed to improve the microstructure structure,and the uniformly distributed,fine grained and regularly shaped microstructure can be obtained.The deformation crack propagation behaviors and the microstructure along the crack were discussed.The crack propagation path is relatively flat and along the grain boundary at a low deformation temperature;while the crack propagation path is relatively smooth at a high temperature,and the crack mainly along the grain boundary.The crack propagation is slow and segmented at a high deformation temperature,therefore,the crack propagation path is relatively smooth.The majority of crack nucleation and emergence of micro cracks occur on the grain boundary,the greater grain orientation of the grain boundary,the more likely to occur crack.The crack front along the grain boundary is relatively sharp,which shows a cleavage characteristic.The crack front inside a grain is relatively passivated,which shows a cleavage quasi-cleavage characteristic.Both the crack propagation and dynamic recrystallization are related to the stress release,both of them have internal connection and influence each other.Some measures promoting dynamic recrystallization or eliminating stress concentration can inhibit the process of crack initiation and propagation.Therefor,improved the internal microstrmcture and obtained the fine and uniform microstrmcture,reduced the number and size of solid particles on the grain boundaries,obtained uniformly distributed,fine grained and regularly shaped solid particles all can effectively suppress the deformation crack propagation. |
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