| Nickel-based superalloy is widely used in hot end parts of advanced aeroengines because of high strength,fatigue resistance,gas corrosion resistance and other excellent high temperature comprehensive performance.During the heat treatment process of alloy preparation,the cooling rate(temperature drop per unit time),as a key parameter,has an important effect on the microstructure characteristics of the alloy,thus affecting the properties of the alloy.It is one of the important contents of the research and development of nickel-based superalloy materials to study the effect of cooling rate on microstructure and mechanical properties of nickel-based superalloy and establish the quantitative relationship.However,with the rapid development of aero-engine,the traditional method,in which only a set of discrete data obtained from a single experiment,is far from meeting the demand of research and development of nickel-based superalloy materials.Therefore,it is urgent to establish a high-throughput research method of heat treatment to accelerate the research and development process of advanced aeronautical materials.Based on the national key research and development project"Research on high-throughput preparation method and technology of materials based on gradient heat treatment and deformation"(NO.2016YFB0700304),the effects of cooling rate on microstructure and properties of a self-designed powder metallurgy nickel-based superalloy CSU-A1 is studied based on the gradient heat treatment experiment method,theoretical method of alloy phase transformation,characterization techniques and numerical simulation method of computational fluid dynamics.The main contents and conclusions are as follows:(1)Experimental study on gradient heat treatment of CSU-A1 alloy test rod.The induction heating-water cooling in situ gradient heat treatment experimental platform and the integrated resistance heating-air cooling gradient heat treatment experimental platform are built.Based on the measurement data from two experimental platforms,test rods with continuous gradient distribution of cooling rate,microstructure and properties are obtained,and the characteristics of the two experimental platforms are compared and analyzed.There are some problems in the gradient heat treatment experiment of induction heating-water cooling,such as poor temperature distribution uniformity,excessive cooling rate and large temperature gradient.The integrated resistance heating-air cooling gradient heat treatment experiment is beneficial to adjust the cooling rate flexibly,and the independently designed lifting shaft device can help to realize the fast movement of the high temperature test rod,which is conducive to reducing the temperature error caused by heat dissipation when moving the high temperature test rod.(2)Effect mechanism of cooling rate onγ′precipitates evolution of CSU-A1 alloy.Based on the formation mechanism ofγ′precipitates in nickel-based superalloy,the JMAK phase transformation kinetics model of CSU-A1 alloy with diffusion-controlled growth is established based on the relationship of phase transformation volume fraction with time and temperature obtained by thermal expansion experiments.Furthermore,combined with the cooling curves in the gradient heat treatment experiment,the latent heat model of CSU-A1 alloy is established based on the improved Newton thermal analysis method.Based on the two models,the effects of cooling rate on the phase transformation kinetic parameters and latent heat release of CSU-A1 alloy are analyzed,and the mechanism of its influence on the evolution ofγ’precipitates is analyzed.With the increase of cooling rate,the number of nucleation increases,while the activation energy of nucleation,the activation energy of diffusion and the latent heat release decreases,which lead to denser and finer precipitate.The latent heat release of CSU-A1 alloy decreases from17.10 J/g to 11.71 J/g when the cooling rate increases from 33.40 K/min to 50.15 K/min.(3)Construction of heat transfer coefficient model between fluid and solid on test rod surface during gradient cooling process.Based on the empirical formula method,inversion method of solid heat conduction and fluid-solid coupling simulation method,the model of surface comprehensive heat transfer coefficient of test rod during the gradient cooling process is established.And the model of comprehensive heat transfer coefficient of the bottom surface of alloy test rod changing with gas flow and temperature in the air cooling process is established.The results show that the comprehensive heat transfer coefficient of the bottom surface of the test rod is much larger than that of the top and side surface,which plays a major role in the gradient cooling.The surface comprehensive heat transfer coefficients of the test rod decrease with the decrease of temperature with air cooling.In the experimental temperature range,the surface comprehensive heat transfer coefficients range of bottom,top and side surface of the test rod are 387.57～510.54 W/(m~2·K),58.38～170.52 W/(m~2·K)and 17.90～29.62 W/(m~2·K),respectively.Meanwhile,with the decrease of temperature,the surface comprehensive heat transfer coefficient of bottom increases at first and then decreases due to the boiling phase transformation with water cooling,and reaches the peak of 121105 W/(m~2·K)at 400 K.This is because the bottom of the test rod experiences film boiling,transition boiling,nucleate boiling and forced convection region.(4)Construction of calculation method of cooling rate of test rod during the gradient heat treatment process.Combined with the study of gradient heat treatment experiment,the latent heat release and the surface comprehensive heat transfer coefficient,based on the numerical simulation of unsteady heat transfer in gradient cooling process of CSU-A1 alloy with alloy phase transformation,the calculation model of cooling rate of CSU-A1 alloy test rod in gradient heat treatment process is established,and the distribution of the axial cooling rate along the test rod is obtained.The results show that the cooling rate decreases with the increase of the distance from the bottom surface of the test rod.In the air cooling process and water cooling process,the cooling rate ranges are56.08 K/min～7126.94 K/min and 39.00～298.16 K/min,respectively.(5)Establishment ofγ’precipitates size model and the alloy hardness model of CSU-A1 alloy related to cooling rate.Based on characterization results of the axial microstructure and properties of CSU-A1 alloy test rod,combined with the axial cooling rate distribution,the influence of cooling rate on precipitate and hardness evolution is analyzed,and the quantitative relationships between the cooling rate,theγ’precipitates size model and the alloy hardness are established.The results show that in the cooling rate range of 39.00～1206.54K/min,when the cooling rate decreases,the morphology of theγ’precipitates of CSU-A1 alloy changes from spherical to nearly square or irregular polygon,the size and phase fraction increase in the range of 27.80～194.62 nm and 18.47%～32.92%,respectively.Meanwhile,the Vickers hardness increases at first and then decreases in the range of 429.37～494.55 Hv,and reaches its peak when the cooling rate is about 294 K/min.Based on the latent heat model of alloy phase transformation and the surface comprehensive heat transfer coefficient model,the calculation method of cooling rate of alloy in the process of gradient heat treatment is established.Combined with the gradient alloy test rod and its microstructure and mechanical properties based on the gradient heat treatment experiment,the high-throughput research method of heat treatment of nickel-based superalloy materials is formed.In this way,dozens or even hundreds of groups of data on mechanical properties,microstructure and cooling rate can be obtained through a single experiment,which significantly reduces the experimental cost and shortens the experimental time,and thus helps to accelerate the development process of nickel-based superalloy materials. |
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