Study On Microstructure And Mechanical Properties Of GH4099 Superalloy By Selective Laser Melting

GH4099 alloy is a typicalγ’phase（γ’-Ni₃（Al,Ti））precipitation strengthened superalloy.Due to its excellent comprehensive mechanical properties,the alloy is widely used in aero-engine combustion chambers,stiffeners and other related aerospace components.In recent years,the additive manufacturing of GH4099 alloy has received increasing attention.The additive manufacturing process can realize the near-net shape of GH4099 alloy parts with complex structure,which can provide a new development direction for the application and development of GH4099 alloy.However,there are still some limitations in the additive manufacturing process of GH4099 alloy.Theγ′-γeutectic phase in the GH4099 alloy cannot be eliminated in the additive manufacturing process,and it is easy to produce liquefaction cracks during the solidification process,which significantly decreases the properties of the alloy.Selective Laser Melting（SLM）technology is one of the typical processes of metal additive manufacturing.This technology uses a high-energy beam to melt metal powder point by point and layer by layer,which can manufacture complex and precise metal parts.During the selective laser melting process,the cooling rate of the material can reach 106～108 K/s.The fast cooling feature is obvious,and it is easy to form fine grains and substructures,which greatly decreases the element segregation in the alloy and is beneficial to improve the performance.Selective laser melting technology provides unparalleled design freedom for the manufacture of metal components,and provides a new idea for low-cost,short-cycle,net-shape manufacturing of complex metal components.In this paper,GH4099 alloy samples were prepared by selective laser melting technology,and then the formed parts were subjected to high temperature heat treatment.The effects of different heat treatment processes on the microstructure and properties of GH4099 alloy prepared by selective laser melting were analyzed based on comprehensive mechanical properties.The results of this experiment are as follows:（1）Under the premise of optimizing the printing process parameters,the GH4099alloy sample was prepared by the selective laser melting technology.The alloy sample has a dense structure and no defects such as cracks and pores.The microstructure of SLM GH4099 alloy is epitaxially grown columnar crystals in the deposition direction.The size of the columnar crystals is about 200μm.The dendrites inside the columnar crystals are particularly fine and the secondary dendrites are not developed.There are sperm cell-like structures with the same growth direction inside the alloy grains in the horizontal direction,and the size of the cellular structures is between 200 nm and 1μm.The spacing between the cellular structures indicates a very fast cooling rate,so the substructures within the same grain are relatively small in the fast cooling state.The as-deposited sample consists of several deposited layers,and the powder particles form a molten pool during the melting process.Due to the rapid cooling rate of the melt during the SLM process,the precipitation of theγ’phase in the GH4099 alloy prepared by SLM was suppressed,and there was no precipitation of theγ’phase in the SLM GH4099alloy.The nano-scale Cr₂₃C₆ carbides present in the alloy samples are about 15 nm in size.（2）The high-temperature solution treatment at 1110°C makes the molten pool structure in the as-deposited sample and the dendrite structure inside the grain disappear,but does not change the main phase structure of the SLM GH4099 alloy sample.When the solution time is 1h,the re-grain boundary grains appear in the alloy sample at the beginning,and the alloy grains are still dominated by<100>oriented columnar grains.When the solution time reaches 2 h,the recrystallized grains have basically completely replaced the columnar grains,and the preferred orientation gradually changes to random orientation,and the alloy grains are mainly equiaxed grains.After the solution treatment,the carbides in the alloy samples gradually changed from spherical carbides to massive carbides,and the size did not change much,and the existence ofγ’phase was not found in the alloy.（3）After the aging treatment,the grain size and morphology did not change significantly compared with the solid solution sample.A large number of fine and dispersedγ’phases are precipitated inside the alloy samples,and theγ’precipitates are in a coherent relationship with the matrixγphase.Theγ’phase produced during the aging process at 1110℃×1 h+700℃×16 h is the smallest and densest,with a size of about 10nm.Theγ’phase produced in the aging process at 1110°C×1 h+800°C×8 h is not as uniform and dense as in the samples aged at 700°C×16 h,and the size of theγ’phase is about 15 nm.However,the size ofγ’phase is the largest in the 1110℃×1h+900℃×5 h aging sample,which is about 60 nm.The size of theγ’phase precipitated during the aging process at 1110℃×2 h+800℃×8 h is 10 nm,and the most finely dispersed.（4）The tensile test results of different specimens show that the strength and plasticity of the alloy are significantly improved after aging treatment.The tensile strength and yield strength of the 1110℃×1 h+700℃×16 h specimen is the highest,which are1240.0 MPa and 970.0 MPa,respectively.The ductility of the alloy aged at 1110℃×1h+800℃×8 h is the best,and the elongation and area shrinkage are 39.1%and 57.0%,respectively.However,the tensile strength and yield strength at 1110℃×1 h+900℃×5h are obviously lower than the first two aging methods,and the performance is slightly higher than that of the solid solution sample.The tensile strength of the 1110℃×1h+800℃×8 h specimen is slightly lower than that of the 1110℃×2 h+800℃×8 h specimen,but the yield-strength ratio is high,and the mechanical properties are relatively excellent.The room temperature tensile properties of the aged SLM GH4099alloy are significantly better than those of the wrought alloy（UTS:980-1079 MPa,YS:686-726 MPa）.Through the research,this work provides some theoretical guidance for the direct forming of high-performance GH4099 alloy and nickel-based superalloy prepared by selective laser melting technology.