Basic Research On Solidification Characteristics And Single Crystal Blade Preparation Process Of Co-Al-W-Based Superalloys

Conventional Co-base superalloys(Co-Cr-Ni-W-base alloys)exhibit superior hot corrosion resistance,thermal fatigue resistance and weldability.Due to it is mainly strengthened through solid solution and carbide strengthening,the applications of the conventional Co-base superalloys at high temperature are restricted due to their inferior high-temperature mechanical properties compared to the Ni-base superalloys strengthened by ?'-Ni3Al phase.The discovery of strengthening mechanism of ?' phase in Co-Al-W-base superalloys in 2006 has significantly improved the high-temperature mechanical properties of Co-base alloys,and provides a new way to develop structural materials for aeroengine and gas turbine.However,the research of Co-Al-W-base superalloys is mainly focused on alloying to improve the high-temperature mechanical properties,oxidation resistance and other service performance of the alloy.The solidification characteristics,casting and solid solution properties of the alloy are not paid enough attention in the optimization of alloy composition.The key to the engineering application of Co-Al-W-base alloys is to clarify the process performance,especially the formation mechanism of casting defects.The influence mechanism of alloying elements on solidification characteristics,casting and solid solution properties of novel ?'-strengthened Co-base superalloys have been investigated in this study.Through the combination of numerical simulation and experiment,the behavior of directional solidification the alloy is studied,and the directional solidification processing of complex single-crystal alloy blades with High quality is realized.The innovative achievements of the thesis include:At present,the main problems of the Co-7Al-8W-1Ta-4Ti quinary alloy with excellent high-temperature mechanical properties are the complex as-cast microstructure,the unclear solidification behavior and solidification path.In this paper,the solidification path of the alloy was determined by directional solidification quenching experiments and isothermal quenching experiments as follow:L?Li+??L2+?+Laves?L3+?+Laves+(?+?')e?L4+?+Laves+(P+?')e+?'??+Laves+(?+?')e+?'.The strong segregation of W,Ta and Ti elements in the liquid phase during alloy solidification leads to the preferential precipitation of Laves phase.The(?+?')e eutectic rich in Al and Ti precipitates after the formation of Laves phase,and the excess Ti element forms a Ti-rich ?'phase in the final stage of alloy solidification.Due to there are the same forming elements in the three secondary phases,their precipitation during the solidification process will compete with each other.The as-cast microstructure and solidification behavior of alloys can be controlled by adjusting alloying elements.The relevant results provide a theoretical basis for the subsequent design of multicomponent Co-Al-W-based alloys.On the basis of Co-Al-W-Ta-Ti alloy,Co-30Ni-7Al-8W-5Cr-1Ta-4Ti alloy was designed by adding the most commonly used Ni and Cr elements in superalloys,and its solidification and solid solution behaviors were studied.The results show that the addition of Ni and Cr can reduce the segregation of Al and Ta.The addition of Ni and Cr has little effect on the solidification path of the alloy,but the ?' phase formed at the final stage of solidification will transform into(?+?')e eutectic.Due to the high concentration of refractory elements in the alloy,the Co-30Ni-7Al-8W-5Cr-1Ta-4Ti alloy is easy to form the unremovable ? phase(Co7W6).In order to avoid the formation of ? phase during solution and suppress the formation of Laves phase,the effects of A1 and W content changes on the solidification and solid solution properties of the alloy were studied without reducing the total amount of ?' phase strengthening elements.The results show that the increase of A1 content and the decrease of W content can inhibit the precipitation of Laves phase during solidification,and also inhibit the formation of ? phase during solution treatment.On the basis of the above research results,the optimal alloy composition given in this paper is Co-30Ni-11Al-4W-5Cr-1Ta-4Ti.The Laves phase is no longer formed when this alloy is solidified,and a single ?-phase structure can be obtained after the two-stage solution treatment of the alloy.Based on the Co-30Ni-11Al-4W-5Cr-1Ta-4Ti alloy,the effect of Ni content on the hot tearing susceptibility of the alloy was further studied.The results show that with the increase of Ni content,the segregation tendency of W and Ti increases,while the segregation tendency of Ta decreases.The precipitation temperature and volume fraction of(?+?')e eutectic decreases.The concentration of Al and Ti in the residual liquid phase of the alloy gradually increases,resulting in the decrease of the solidification rate of the alloy in the hot tearing sensitive area,and the hot tearing susceptibility of the alloy gradually increases.On the other hand,with the decrease of Ni content,the microstructure stability of the alloy decreases and the difficulty of solution treatment increases.Considering the solidification behavior,casting performance,solid solution process and structural stability of the alloy,the Ni content of the Co-30Ni-11Al-4W-5Cr-1Ta-4Ti alloy in this paper can vary from 20 to 30 at.%.This result can provide a larger choice of ingredients for the preparation of single crystal blades with different properties.The boundary conditions and thermal properties of the Co-30Ni-11Al-4W-5Cr-1Ta-4Ti alloy during the directional solidification process were obtained through experiments and thermodynamic calculations,and an accurate directional solidification process model for the alloy was established.The effects of process parameters such as heating temperature,placing mode and withdrawl rate on the temperature field and the grain structure of Co-Al-W-base alloys during directional solidification were studied.The formation mechanism of stray grain defects at the blade platform is analyzed.Combined with the simulation results,the processing technology of complex single crystal blade was optimized.This result can provide basic data support for further engineering preparation of complex single crystal blades of Co-Al-W based alloys.