Microstructure Formation Mechanisms and Hot Processing Windows of GH4169 Alloy Forgings

Author:Wang Jian Guo

Supervisor:liu dong


Degree Year:2018





As the most important and typical superalloy in aero engine,GH4169 alloy directly determines the working state and lifetime of the aero engine based on its quality stability.Otherwise,the quality stability of GH4169 forgings is difficult to be stabilized due to its narrow forging window and highly sensitivity to the hot working parameters.Although a great deal of research has been done on GH4169 alloy,the basic mechanisms in the following aspects are still not clearly stated,such as homogenization mechanism of non-uniform microstructure,precipitation kinetics ofδphases at grain boundary with different curvature,rate of dynamic recrystallization under different initial microstructure,mechanisms for the microstructure evolution during post deformation and critical microstructure characteristics for mechanisms transformation,et al.Based on GH4169 alloy as the research object,the microstructure formation mechanisms and hot working process window of forgings under different initial state and equipment conditions were studied by using the experiments which were designed to simulate the actual forging process.The evolution mechanisms of non-uniform and homogeneous microstructure during solid solution heat treatment were obtained for the solid solution heat treatment of the samples with uniform coarse grain,mixed crystal and fine crystal state.The effects of chemical composition,substructure,temperature/time on the precipitation of the delta phase were analyzed by combining the JMat Pro thermodynamic software with the experimental investigation.A kinetic model of the precipitation of the delta phase at the bending interface was also established.Based on the single pass hot compression test,the deformation behavior,processing maps and dynamic recrystallization rate of GH4169 alloy in different states were analyzed to confirm the deformation mechanism,the optimum Z parameter range and the transformation mechanism of the dynamic recrystallization rate.The microstructure evolution mechanism of the alloy forgings under the cooling process and interrupted deformation was investigated by multi pass hot compression test on the solution treated GH4169 alloy,and the critical microstructure characteristics of the mechanism change were also determined.Finally,the hot processing window of the alloy with different initial state was established by the compression test of double cone specimens,and the reliability of the processing window was verified by the full-scale forging production.The main research contents and achievements are as follows:Solid solution heat treatments were conducted on the deformed alloys with uniform coarse grain,mixed crystal and fine grains and it reveals that with the increase of temperature and time,the fraction of low angle grain boundary decreases gradually,and the fraction of high angle grain boundary increases gradually,and the grain size keeps stable first and then increasing later.Otherwise,there are obvious differences in the evolution mechanism of the deformed equiaxed subgrain and the deformed elongated grain during the solution process.With the increase of temperature and the prolongation of time,the grain grows only for the deformed equiaxed grain,however,the grain grows occurs after continuous static recrystallization for the elongated grain.For GH4169 alloy forgings with inhomogeneous microstructure,a relatively homogeneous microstructure can be obtained by solution treatment at 990℃1010℃,which can reduce the content of low angle grain boundary,increase the proportion of special grain boundary,and abstain the rapid growth of grain.Electrolytic extraction and high resolution SEM analysis show that the three-dimensional morphology of theδphase is lamellar,needle like and lath-shaped.The thermodynamic calculation results show that the difference between the peak temperature and the dissolution temperature of theδphase in the alloy is the highest when the contents of Nb and C are Nb5.30%and C0.03%,respectively.With the increase of aging temperature,the content ofδphase increases first and then decreases.With the extension of aging time,its content increased first and then remained stable.A kinetic model for the nucleation of theδphase besides the bending grain boundary was established.The results show that the critical radius and the potential barrier of the nucleation decreases with radius of curvature decreasing.The precipitation kinetics ofδphase at intergranular and intragranular is mainly influenced by grain boundary curvature and intragranular substructure.The deformation behavior and hot processing maps of different initial state samples were analyzed by single pass hot compression test.The deformation activation energy of GH4169 alloy with uniform coarse grain,mixed crystal and uniform fine grains was495.497kJ/mol,421.703 kJ/mol,401.111 kJ/mol,respectively.The proposed hot working parameters(temperature:980℃1020℃,and the strain rate:0.01 s-11s-1)can effectively abate the effect of the initial state on the hot processing and avert the growth of the dynamic recrystallized grain.Based on the dynamic recrystallization rate(DRXR)investigation of samples with different initial states,the results shows that the dynamic recrystallization rate increases first and then decreases with the increase of deformation.The dynamic recrystallization rate reaches the peak value while the recrystallization volume fraction reaches 50%,and the best deformation Zener parameter ranges of different initial states are also ascertained.Upon the microstructure characteristics of different deformation,it can be concluded that when the deformation is less than the peak strain for DRXR,the nucleation mechanism includes discontinuous dynamic recrystallization,continuous dynamic recrystallization,twin induced nucleation and particle induced nucleation.And the nucleation mechanisms of the alloy are continuous dynamic recrystallization and discontinuous dynamic recrystallization when the deformation is larger than the peak strain for DRX rate.The effect of temperature,strain rate,interruption time and initial pass deformation on the deformation behavior of GH4169 alloy was analyzed under multi-pass interruption deformation.The result shows that the interruption deformation behavior is mainly affected by strain rate and interruption time.A post-dynamic softening fraction calculation method based on initial stress is proposed which reveals that the dynamic softening fraction increases with the increase of strain rate and intermittent holding time.The microstructure analysis shows that the microstructure evolution mechanism of the alloy in the post dynamic process is static recrystallization,the recrystallized grains growth by swallow the original grain and the recrystallized grain grow by merged with neighbouring grains.The critical microstructure characteristics of the mechanism transformation in post dynamic process are the formation of single layer or multilayer dynamic recrystallized grains.The characterization method of hot processing window containing recrystallization volume fraction and average grain size was proposed for double cone compression test of uniform coarse grain,mixed crystal and uniform fine grain double conical specimen in hydraulic press and screw press.The results show that uniform microstructures can be obtained by low temperature with high deformation followed by high temperature with small deformation on the hydraulic press and low temperature,medium deformation,multi pass on the screw press.The two methods were applied in industrial forging process,and the forgings with uniform microstructure were obtained successfully.