| Ni-based superalloys are important candidate engineering materials and used in various fields including the aerospace,transportation and energy,since their excellent microstructure and mechanical stability at high temperature.However,the traditional manufacturing technologies are difficult to realize the optimization of internal structure and the weight reduction for components,which are significant to the development of modern engineering.The laser additive manufacturing(LAM)technology builds the 3D parts through progressively and consecutively adding materials directly guided by a digital model.This unique feature enables the sophisticated parts to be fabricated and repaired,which is difficult for the conventional processes.Thus intensive researches have been performed to investigate the LAM process of Ni-based superalloys in recent years.But the compositions of heritage Ni-based superalloys were designed without considering the specificity of LAM process,and the metallurgical defects including pores,lack of fusion and cracks were easily produced.The defects are difficult to be completely conquered through optimizing the process parameters,which is due to the poor weldability of alloys.Limit the addition of strengthening elements can improve the weldability of Ni-based superalloys,but this is at the expense of overall performance.Therefore,in order to balance the weldability,mechanical properties and oxidation resistance,it is necessary to design the new composition of Ni-based superalloys for the LAM process.In this paper,the general composition formula of the weldable Ni-based superalloys was built through the "cluster-plus-glue-atoms" model,and it was verified by analysis to the Ni-Cr-Al basic alloys.Based on the general formula,series alloys with varying Cr+Mo content and Cr/Mo ratio were designed,and the weldable alloy with optimum overall performance was obtained.This research also revealed the relationship between the composition,weldability,mechanical properties and oxidation resistance.The main contents of this are as follow:(1)According to the enthalpy of mixing between the solute element M and solvent Ni(ΔHM-Ni),the alloying elements of Ni-based superalloys were classified into three categories,including the Ni-like((?)),Cr-like((?))and Al-like((?))elements.The general formula of Ni-based superalloys(?) was built based on the structure of CN12 cluster and the interaction between alloying elements.The compositions of the weldable and unweldable alloys could be analyzed based on the above general formula,and expressed as(?) and(?),respectively.It is evident that the weldability of Ni-based superralloys is very sensitive to (?) elements content,the number of (?) atoms is less than one in weldable alloys and only occupies the center atom of cluster,but the (?) elements could maintain the Ni-based superalloys weldability within a wide composition range.(2)In order to investigate the influence of Ni,Cr and Al contents on the microstructure evolution,mechanical properties and oxidation resistance,five representative Ni-Cr-Al basic alloys were designed according to the original Ni-based superalloy composition of 80Ni-20Cr(wt.%)and the cluster formula of[(Cr1-xAlx)-Ni12](Cr3-yAly).The results revealed that the matrix structure of as-deposited alloys evolves from γ-Ni solid solution to γ’-Ni3Al phase with the increase of Al content.When the Al content reached 12.5at.%(Ni75.0Cr12.5Al12.5 at.%),the microhardness and strength of the as-deposited alloy increased significantly attribute to the precipitation of γ’-Ni3Al phase in γ-matrix,but the plasticity decreased with the increasing Al content.Meanwhile,the oxidation resistance above 1000℃ was obtained due to the formation of protective Al2O3 scale during exposure in high temperature.However,the excessive Al content and the precipitation of γ’ phases will deteriorate to the weldability simultaneously,and a large amount of pores and lack of fusions were formed in Ni75.0Cr25.0 alloy.Therefore,according to the synthetic analysis of the above results,Ni75.0Cr18.75Al6.25 exhibits the defect-free as-deposited microstructure,and was selected as the basic composition for LAM process owing to the single phase γ-matrix and the narrowest ΔT.Therefore,the general formula for weldable Ni-based superalloys was verified and optimized as(?).(3)In order to further optimize the general formula of the weldable alloys,five compositions with different (?) content were designed based on the formula of(?).The microstructure of as-deposited layers consists of γ-matrix and TCP phases in interdendritic regions.The TCP phases change from granular Laves to long-striped Laves phase with the increase of (?) content,and the needle-like σ phases precipitate in intedenditic regions when the (?) content is 27.8at.%.The amount of TCP phases of as-deposited alloys exhibit a trend of first decrease and then increase with increasing (?) elements addition.The best weldability was obtained when (?) content is 18.75at.%(NiCr-3 alloy),due to the fewest TCP phases(1.09vol.%),the narrowest ΔT(76.4℃)and the best plasticity(40.1%),the above factors about the weldability are all better than the heritage alloys such as IN718 and IN625.Moreover,the lower degree of elemental segregation is also helpful to the uniformly precipitation of γ" phases during the post heat treatment.By comprehensive comparison,the NiCr-3 alloy(Ni54.8Fe20.3Cr17.3Mo1.50Al1.56Nb3.47Ti1.22)presents the best weldability,and an desirable combination of room-temperature mechanical properties and high-temperature oxidation resistance,and the general formula for weldable alloys was further optimized as(?).(4)In order to investigate the synergistic effect from the Cr and Mo elements,four compositions with varying Cr/Mo ratio were designed based on the formula(?).The researches results revealed that the decreasing Cr/Mo ratio could restrain the segregation degree of Nb and the amount of Laves phases to some extent,the narrowest ΔT(73.0℃)and the highest ductility(40.3%)of as-deposited alloys were obtained when Cr/Mo ratio is 14:2.But a large amount of γ+P eutectic phases precipitated in as-deposited alloys due to the excessive Mo addition when Cr/Mo ratio is less than 14:2,the γ+P eutectic phases can enlarge the ΔT and deteriorate the weldability and ductility.After the solution treatment and aging,the microhardness and strength of as-deposited alloys were enhanced owing to the precipitation of γ" phases.The strength of post heat-treated alloys exhibit a variation trend of firstly increase and then decrease,and the highest strength was obtained at the Cr/Mo ratio is 13:3.But the excessive addition of Mo could inhibit the precipitation progress of γ" phases,and then restrain the precipitation strengthening effect when the Cr/Mo ratio is 12:4.What’s more,the decreasing Cr/Mo ratio also deteriorate to oxidation resistance under the condition of 800℃/100h.Therefore,overall consideration the weldability,mechanical properties and oxidation resistance,the appropriate Cr/Mo ratio of Ni-based superalloys for the LAM process could be set as 14:2,and its corresponding cluster formula was[(Al0.25Ti0.25Nb0.5)-(Ni0.75Fe0.25)12](Cr7/8Mo1/8)3. |
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