Research On Hot Plastic Deformation Behavior Of GH4698 Nickel-based Superalloy

GH4698 superalloy is a kind of alloy which is widely used in aviation and space industry. This alloy shows outstanding mechanical properties, anti-oxidant and anti-fatigue performance under high temperature. Components like turbine disks which are made of this superalloy have high application reliability. The strength and hardness of this alloy are very strong because it contains abundant adding elements which intensify matrix body, so the strength of this superalloy at room temperature can be several times over steel. The plastic forming process of the alloy needs to be done under high temperature. Therefore, it is of great importance to understand the plastic deformation behavior and find out the best forging parameter of GH4698 superalloy.Researches have indicated the constitutive modeling of metal can describe the plastic deformation behavior while hot process map of metal is defined as a powerful tool to determine the best forging parameters. Thus, study on them is carried out in this dissertation.The true strain-stress curves of GH4698 superalloy between temperatures range of1223K~1423K were obtained by making use of MTS810.13 test machine in this dissertation. The stress data under different strain rates were analyzed and the relations among flow stress, deformation temperature, strain rate and other parameters of this alloy were also discussed. It turns out that the flow stress of material increases with the increase of strain rate under the same temperature; the flow stress of material decreases with the rise of temperature under the same strain rate. Additionally, the shapes of all true strain-stress curves are similar with each other.In order to describe the plastic deformation behavior of GH4698 superalloy, this dissertation builds up the constitutive modeling of the metal under high temperature by using two different mathematical models. A comprehensive analysis was also made on precision, error distribution and amount of calculation of the established constitutive modeling. The result shows that the constitutive relations, built from the Arrhenius model and modified Zerilli-Armstrong model, are all quite precise in this dissertation.However, the modified Zerilli-Armstrong model has a huge amount of calculation and wide distribution interval of error. Considering the influence of strain, Arrhenius constitutive model is more convenient to calculate and has less error in predicting every single stress.In order to determine the best forging parameters of GH4698 superalloy, based on correlation criterion, this dissertation establishes the hot processing map of this alloy under particular temperature range. Through the full analyze of the domains on theestablished hot process efficiency map, the high process efficiency domain of this metal were finally found under high temperature, moderate strain rate deformation situation.Meanwhile, the instability domains on the instability map shows that flow instability will occur when this metal is under the condition of low temperature with low strain rate or high temperature with high strain rate. Eventually, through the analysis and tests, it is confirmed this metal shows best processing property under temperature range of1300K~1423K with strain rate range of 0.01s-1~1s-1. At this time, a higher process efficiency can be guaranteed without instability flow phenomenon.So as to research the dynamic recrystallization phenomenon of GH4698 superalloy in hot deformation process, and according to some relevant theories, this dissertation founds out that the process hardening rate curve of material will show a inflection point feature at the critical strain location. Based on this feature, the equations of critical strain and recrystallization volume fraction were both established. By analyzing the relationship between grain size and different parameters, the grain growth model of this superalloy was also established. Experiments were carried out to make a comparative analysis of the actual grain size and the predicted grain size. The result shows that this established grain growth model has little error and can be used to describe the grain growth phenomenon of this metal in the process of hot deformation or heat treatment.