| IN718 nickel-based superalloy is one of the most extensively used key metal structural materials in aerospace,power-generation,defence technology and other fields.With the development of large-scale,integrated and high-performance of new major equipments,greater demands will be placed on high-quality and large-sized IN718 castings and ingots.During the solidification process of ingots,the high content of strengthening elements in the IN718 alloy will be redistributed at the solidification front to various degrees,which inevitably causes solidification segregation,resulting in considerable difficulties in the subsequent thermal processing and in significant deterioration of the final casting properties.The larger size gives rise to an unavoidable slower cooling rate in the inner section of ingots;thus,differences in solidification characteristics under slow-cooled conditions may exist.Therefore,further work is required to clarify and understand the changing rules of solidification and segregation behaviors of as-cast IN718 alloy under slow cooling conditions,which is the main research direction and goal for the production of large-sized nickel-based superalloy castings and ingots.This study was financially supported by the National Natural Science Foundation of China(No.U 1560203),and mainly conducted fundamental research in terms of the solidification and segregation behaviors of as-cast IN718 alloy under slow cooling conditions.All experimental and theoretical programs in this series of work are undertaken with the aim of reflecting the universal information on the industrial scale IN718 electroslag ingot during solidification,to a certain extent,improving the basic theory of solidification and segregation behaviors of alloys,and revealing the limiting factors in large-sized ingots manufacturing.The findings in the work are expected to enrich further scientific understanding of related fields,supply basic reference data for actual production of large-sized IN718 electroslag ingots,and provide essential theoretical foundations for the improvement and promotion of key smelting and processing technologies such as casting,remelting and homogenization.First,based on the thermodynamic analysis results of phase transformation in the solidification process,the solidification and segregation behaviors of as-cast IN718 alloy at different slow-cooling rates and temperatures were comprehensively investigated and characterized by high-temperature in-situ observation and continuous/quenching solidification methods in combination with related theoretical calculations.The results show that the solidification process of IN718 alloy under slow-cooled conditions can be divided into three stages:the initial transient stage,the rapid solidification stage,and the late slow solidification stage.Nb and Mo are found to be the major segregation solutes,and the effective partition coefficients of them exhibit an almost linear increase with increasing cooling rate.The variation trends of Nb and Mo concentrations in the residual liquid during solidification can be characterized quantitatively by calculation using the Clyne-Kurz equation in combination with the experimental parameters.The typical segregated phase in as-cast IN718 alloy consists primarily of a large proportion of Laves phase and a small amount of MC(M=Nb,Ti)carbides.The volume fraction of them shows a continuous decrease with increasing cooling rate.The prediction formula of SDAS(μm)for IN718 under slow-cooled conditions(℃/min)is established as λ2=258(GR)H-1/3-54.23.The freckles are most likely to form in the early stage of solidification,at which the liquid fraction is between 0.3 and 0.2,and the temperature range is about 1320 to 1310℃.Second,a series of laboratory-scale experiments were designed to conduct fundamental research using a customized ESR furnace.A comparative investigation was performed on the characteristics of solidification quality at different positions of as-cast IN718 ESR ingots under various remelting currents.The evolution mechanism of nonmetallic inclusions was also proposed based on the experimental results and thermodynamic analysis.The results show that an appropriate increase in the remelting current can effectively decrease the size and quantity of nonmetallic inclusions,thereby reducing the oxygen content in the ESR ingot,while the extent of segregation effects increases synchronously.If the remelting current is set too low,nitrogen and oxygen absorptions might occur during the ESR process,as a result,a large amount of new inclusions is generated,which eventually causes instability in the contents and distributions of A1 and Ti in the ingots.The major nonmetallic inclusions in the IN718 ESR ingots are complex oxide inclusions with a three-layer structure,consisting of an MgO·A12O3 spinel core surrounded by an inner Ti-rich and N-rich(Nb,Ti)CN layer and outer NbC layer,and Ti-rich nitride inclusions(Nb,Ti)N.Both of these inclusions are formed sequentially in the liquid phase,and the volume fractions,average particle size,and quantity of them show a downward trend with the increasing remelting current.Third,based on the experimentally measured data and software simulation results,the high temperature diffusion mechanism of the alloying elements was discussed.The quantitative relationships among secondary dendrite spacing,elemental distribution,and homogenization time/temperature were also described and established.The results show that the difference in Nb concentration between the dendrite arms and interdendritic regions is the main limiting factor in homogenization of as-cast IN718 alloy.Under the premise that the temperature distribution is uniform,the relationship between the minimum time(min)required for Laves phase to completely dissolve and the as-cast SDAS(μm)at 1160℃ can be expressed as t11160=0.4671+0.0048λ2+0.0154λ22.Increasing the homogenization temperature is more beneficial to promoting the dissolution rate of Laves phase,which is more significant than the effect of prolonging the homogenization time.A quantitative relationship also exists between the second-stage homogenization time and the maximum Nb content in the interdendritic regions.When the SDAS inside the ingot is less than 220μm,the Laves phase can be completely eliminated by 1160℃ for 13 h at the first-stage of homogenization,and after the second-stage of 1200℃ for more than 72 h,the composition of the ingot tends to be uniform.Finally,the conventional ESR furnace was reformed to establish a new ESR equipment based on single-power two-circuit with current-carrying mold.Two IN718 ESR ingots of 260 mm in diameter were initially trial produced by the conventional and new remelting process under the same effective power supply,respectively.The influences of different remelting methods on the solidification structures of the ingots and mechanical properties at high/room temperature were comparatively investigated.The feasibility of producing large-sized nickel-based ESR ingots by the new duplex route was analyzed.The results show that the special current path of the current-carrying mold changes the temperature distribution of the slag bath and the molten pool,which is more conducive to removing sulfur and reducing oxygen content.Moreover,the axial crystallization characteristics of the alloy are more remarkable,and the cooling conditions inside the ingot are relatively more balanced.Therefore,the surface quality and the internal quality of the ESR ingot are improved simultaneously.After the same heat treatment processes,the mechanical properties of the alloy produced by the new remelting process at room temperature are similar to those of the conventional ESR.However,the creep life at 650℃ and 700 MPa is increased by approximately 43%,and the creep resistance and high temperature tensile properties are also significantly improved. |
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