Basis And Optimized Control Principles For Production Processes Of Homogenization And Cogging In Superalloy Ingots

With the boosting demand of large-scale complex superalloy components,and both energy resources and environmental pollution stresses,require:ments on economy and quality reliability of each process links during the production of superalloys become more rigorous.Homogenization and cogging processes play bridge roles during the whole production processes.However,problems of high energy consuming during the homogenization process and low yield during the cogging process are becoming increasingly prominent due to the insufficiency of research and production experience.For the improvement of economy and quality reliability during homogenization and cogging processes,microstructure evolution during the homogenization and cogging processes and the relevance of these two process links need to be studied.Influences of different superalloys and segregation elements,melting and casting factors and ingot sizes to the segregation were investigated.Elenment segregation incdex(K'=K??C?)was carried out.When the K' value is large enough,serious elements segregation and a large number of segregation phases were observed which needs the two-stage homogenization treatment.When the K'value is small,light elements segregation and barely segregation phases were found which the single-stage homogenization treatment could be considered.Segregation and microstructure evolution at different stages of homogenization processes were studied in different alloys.There showed a quick decrease in the residual segregation index in the "fast descending area",where dendritic segregation could be observed in this area.Although dendritic segregation was eliminatecd,grain sizes and oxidation increased quickly in the "steady area".Thus,the control principle of homogenization was proposed:the former stage of"steady area" is the optimal range of homogenization time.For the complete elimination of element segregation,homogenization time could not be too short.To preventing fast increases of grain sizes and oxidation thicknesses,homogenization time also could not be too long.Meanwhile,influences of homogenization degrees to the recrystallization behavior and deformation resistance were investigated.Combining with the study of the microstructure evolution during the homogenization process,influences of the homogenization degree to the deformation behavior and considerations of the relevance of homogenization microstructure and deformation behaviors during the cogging process,the control principle of homogenization and cogging was further carried out:the former stage of "steady area" is still the optimal range of homogenization time.During this time range,alloys exhibited the best high temperature ductility,low deformation resistance and high recrystallization rates.The forged bar after cogging with uniform fine-grained microstructure was observed remaining the same uniform fine-grained micro structure even if after the hot-deformation process.Deformation mechanisms of the forged bar with uniform fine-grained microstructure were also analyzed.This phenomenon illustrated the importance of gaining uniform fine-grained microstructure through the optimization of homogenization and cogging process to the subsequent hot-forging process.The fracture prediction model and recrystallization model of cogging processes were built and verified by the double-cone experiments.Fracture coefficient P was introduced in the finite elements analysis to quantitative predict the probability of cracking,the possible position of cracking,recrystallization volume fractions and recrystallization grain sizes during the cogging process.Based on investigations of grain size control through recrystallization and the acceleration of elements diffusion rates through shortening dendritic spacings,the processes improvement of economical homogenization and cogging was proposed:the optimal homogenization time range was moved forward to the later stage of"slow descending area" and the former stage of "steady area",to shorten homogenization time.Moreover,pre-deformation homogenization and cogging processes was also carried out.Small deformation was applied before the homogenization process to shorten dendritic spacings and promote dynamic recrystallization.Thereby,homogenization time was greatly shortened.In conclusion,segregation of ingots,microstructure evolution during the homogenization process and the influence of the homogenization degree to the deformation behaviors were studied through a large number of experiments.Under the consideration of relevance of homogenization and cogging processes,fracture prediction model and recrystallization model of cogging processes were built and control principles of-both homogenization and cogging processes were proposed.Therefore,evaluation systems of homogenization and cogging processes for superalloys were further improved...