Microstructure And Mechanical Properties Of Nb Modified GH4169 Superalloy Produced By Laser Additive Manufacturing

Laser additive manufacturing(LAM)is a new manufacturing technology which realize rapidly near-net-shape of parts in the form of powder synchronous feeding or powder bed by laser beam.Because LAM is point-by-point forming with the same composition and the solidification rate of molten pool is so fast that there is no macrosegregation phenomenon compered with traditional casting process.Therefore,LAM is suitable for the formation of Ni-based superalloys such as GH4169 which are prone to component segregation.With the rapid development of aviation science and technology,the new generation of aircraft must meet the needs of ultra-high speed,high altitude,long-distance and long-distance voyage,which also puts forward higher requirements for GH4169 alloy processing technology,material structure and mechanical properties.Nb,as an important strengthening element,determines the micro-structure and mechanical properties of GH4169 sueralloy.By studying the regulation of Nb element on the micro-structure and the optimization of mechanical properties of GH4169 superalloy by LAM,the application of LAM in the field of forming and repairing Ni-based superalloy parts is broadened.The content of Nb in GH4169 superalloy powder was adjusted by ball milling alloying combining with LAM.The effects of Nb on the microstructures,microsegregation,precipitation transformation and room temperature mechanical properties of GH4169 superalloy fabricated by LAM were studied.The results are as follows:The macrostructure of LAM GH4169 superalloy specimens with different Nb content still has the characteristics of conventional GH4169 superalloy,consisted of coarse columnar crystal structure.Particularly,columnar crystals grow in different directions with different Nb content.With the increase of Nb content,the growth direction of columnar crystals gradually changes from deviating from the laser beam outward to inward.In terms of microstructure,with the increase of Nb content in GH4169 superalloy,the dendritic structure characteristics of the as-deposited GH4169 superalloy become more obvious,showing that the number of interdendritic eutectic structure gradually increases,and the morphology of interdendritic eutectic structure gradually becomes continuous.When the content of Nb in GH4169 superalloy reaches 11.1%,the amount of interdendritic eutectic structure is the largest,and the morphology gradually becomes network structure.The Nb content in dendrite core of LAM GH4169 superalloy with different Nb content increases,and the difference between dendrite core and interdendrite is reduced.When the Nb content is 11.1%,the Nb content of dendritic core reaches the maximum of 7.53%.Accordingly,the segregation ratio of Nb decreases from 8.59 to 4.13 with the increase of Nb content in LAM GH4169 superalloy.Therefore,the solid solution strengthening function of the Nb is enhanced,which maked the value of 2theta of the matrix(200)? decrease with the increase of the content of the Nb element.The morphology and amount of interdendritic Laves phase in LAM GH4169 superalloy with different Nb content have no obvious change compared with as-deposited samples after direct aging heat treatment.With the increase of Nb content,the precipitation of ? phase in the local region of eutectic structure after direct aging heat treatment increases.The LAM conventional GH4169 superalloy has a large number of ?? strengthening phases in interdendrites after direct aging heat treatment,and almost no strengthening phases precipitate from the dendrites core.With the increase of Nb content,the Nb content in dendritic core of LAM GH4169 superalloy with high Nb content increases,resulting in the precipitation of dendritic core ?? strengthening phase increases,and the difference of ?? strengthening phase between dendritic core and interdendritic decreases.Proper solution treatment can eliminate dendritic structure and homogenize the alloying elements in LAM GH4169 superalloy with different Nb content.With the solution temperature increases,the more obvious the dendrite structure is eliminated under each component.After the same heat treatment condition,the retained Laves phase in the solid solution LAM GH4169 superalloy increases with the increase of Nb content.When the Nb content in LAM GH4169 superalloy is low,1100 ? can be chosen as the lowest temperature for solid solution treatment.When the Nb content in LAM GH4169 superalloy exceeds 7.0%,in order to avoid local remelting of GH4169 alloy with high Nb content,the as long as possible at 1150 ? solid solution treatment can be considered to eliminate the interdendritic Laves phase and achieve the homogenization of LAM GH4169 superalloy.The average microhardness of as-deposited LAM conventional GH4169 superalloy is 228.4 HV.With the increase of Nb content,the microhardness of as-deposited samples increases gradually.When Nb content in LAM GH4169 sueralloy reaches 11.1%,the microhardness reaches 534.1 HV.After solid solution treatment,the microhardness of the solid solution sample decreases with the increase of solution temperature.At the same solution temperature,the microhardness of solid solution LAM GH4169 superalloy with different Nb content increases with the increase of Nb content.After direct aging treatment,the microhardness of the direct aging LAM GH4169 superalloy is significantly higher than that of the as-deposited specimens,and the highest value was 640.7 HV.With the increase of Nb content,the increasing trend of microhardness of direct aging samples becomes slower,and the difference between the microhardness of as-deposited samples and direct aging samples decreases gradually.The tensile strength and elongation of as-deposited LAM conventional GH4169 superalloy are 829 MPa and 41%.With the increase of Nb content,the tensile strength of as-deposited materials increases firstly up to 1009 MPa,and then decreases.The elongation of the material decreases with the increase of Nb content.Compared with the as-deposited material,the tensile strength of the material after direct ageing are improved,while the elongation decreases.The tensile strength of direct ageing LAM GH4169 superalloy material with different Nb content increases firstly and then decreases with the increase of Nb content.When Nb content is 8.5%,the tensile strength of the alloy increases to the maximum,reaching 1393 MPa,and the elongation decreases to 4%.The fracture surface of the as-deposited and direct ageing material gradually flatten with the increase of Nb content,and the secondary cracks gradually increase and deepen,showing the characteristics of brittle fracture.