| Nickel-based superalloys are widely used in aerospace and other important fields due to their excellent high temperature strength,good oxidation resistance and corrosion resistance.It is difficult for traditional forming methods to meet the requirements of the formability,mechanical properties and production costs of nickelbased superalloy parts simultaneously.Semi-solid processing(SSP)combines the advantages of casting and forging methods,and can fulfill the requirements of shape and performance of the formed parts at the same time.Therefore,it is of great significance to apply SSP to nickel-based superalloys.On the one hand,it can provide a new idea to form nickel-based superalloys,on the other hand,it can broaden the application scope of the materials for SSP.In this paper,nickel-based superalloy GH4037 was used as the research material,and the microstructure evolution in semisolid state and the thixoforming experiments were carried out.Through the analysis of the microstructure and mechanical properties of thixoformed parts,the feasibility of SSP in the field of nickel-based superalloys was verified and a new technical support was provided for the forming and manufacturing of nickel-based superalloy parts.The oxidation experiments of GH4037 alloy were carried out at solid and semisolid temperatures.The effects of temperature and time on the oxidation behavior of GH4037 alloy were investigated,and the oxidation phenomena of GH4037 alloy in air and protective atmosphere were compared.The results indicated that when GH4037 alloy was heated in semi-solid temperature range,the oxidation degree was very severe,the oxidation weight gain was significant and the oxide films fell off seriously.When the samples were oxidized in protective atmosphere,the oxidation degree was greatly reduced,the oxidation weight gain was small,and the surface was intact without falling off.Therefore,it is necessary to take oxidation protection measures to ensure the surface quality and internal purity of the billets during heating at semi-solid temperatures.A new method called semi-solid isothermal treatment of wrought superalloy(SSITWS)was proposed to prepare semi-solid billets of GH4037 alloy.The effects of isothermal temperature and soaking time on the microstructure evolution of GH4037 alloy were studied.The results showed that the semi-solid billets with spherical grains could be prepared by the isothermal treatment at 1350?1380 °C.At1350 °C and 1360 °C,the grain growth was consistent with the coarsening mechanism,while at 1370 °C and 1360 °C,the microstructure evolution was affected by the coarsening mechanism and the breaking up mechanism.The optimum parameters for preparing semi-solid billets of GH4037 alloy were isothermal temperature of 1380 °C and soaking time of 30 min,at this condition the average grain size was 130.2 ?m and the roundness was 0.65.High temperature compression experiments of GH4037 alloy were conducted.The stress-strain relationship of GH4037 alloy at different conditions was obtained,and the microstructure evolution and recrystallization behavior of the compressed samples were studied.The results presented that the flow stress and peak stress of GH4037 alloy decreased with the increase of compression temperature and the decrease of strain rate.At semi-solid temperatures,the relationship of apparent viscosity and shear rate of GH4037 alloy fitted the power law model of non Newtonian fluid,and the apparent viscosity decreases with the increase of shear rate,presenting obvious shear thinning behavior.Besides,the experimental process of thixoforming of GH4037 alloy was numerically simulated.Because the contact time between the billet and the die was shorter,the temperature of the die was lower and the possibility of defects was smaller when the billet was placed horizontally,it was more favorable to thixoforming when the billet was placed horizontally.Other process parameters also had different effects on the thixoforming process,so the influences of various forming parameters should be comprehensively considered,and reasonable selection should be made during the actual experiment.The semi-solid thixoforming experiments of GH4037 alloy were carried out.The quality of the thixoformed parts was evaluated according to the macro morphology,microstructure and mechanical properties.The results showed that when the billet temperature was higher than 1360 °C,the thixoformed parts were completely filled with good surface quality and compact internal structure.The isothermal temperature and soaking time had significant influence on the microstructure and mechanical properties of the thixoformed parts,while the holding time had little effect on the microstructure and mechanical properties.The optimum process parameters of thixoforming of GH4037 alloy were: isothermal temperature of 1380 °C,soaking time of 30 min and dwell time of 30 s.The effect of heat treatment on the thixoformed GH4037 parts was investigated.The results indicated that when the aging temperature was 800 °C and the aging time was 8?16 h,the optimum mechanical properties were obtained.After heat treatment,the eutectic structure disappeared completely,continuous carbides precipitated along the grain boundaries and uniform and fine ?? precipitates dispersed inside the grains.Heat treatment could greatly improve the tensile properties and creep properties of the thixoformed GH4037 parts,and the second phase strengthening mechanism caused by ?? precipitates was the main strengthening mechanism of the parts after heat treatment.Before heat treatment,the yield strength,tensile strength and elongation of the thixoformed part were 624 MPa,975 MPa,and 45.1% respectively.After heat treatment,the yield strength and tensile strength were 724 MPa and 1030 MPa,respectively,which were increased by 16% and 6% respectively,but the elongation decreased.Before heat treatment,the creep life and creep strain of the thixoformed part at 850 °C/196 MPa were 28.3 h and 3.3%,respectively.After heat treatment,the creep life and creep strain were increased to 52.7 h and 9.9%,respectively. |
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