Microstructure And Mechanical Properties Of GH4648 Superalloy Manufactured By Selective Laser Melting

As a high temperature used alloy,GH4648 alloy(Ni-35Cr-5W-3Mo-1Al-1Ti-1Nb)is widely used to manufacture aviation turbofan engine components,including flame cylinder and swirler.In recent years,the additive manufacturing of aero-engine components has become a popular research field.Based on the status quo,this study employs a selective laser melting(SLM)prototyping technique to manufacture GH4648 superalloy components.This prototyping technique is used to replace the traditional casting method.By controlling the heat treatment process after the rapid prototyping(including aging time,solution temperature,heat exposure time,etc.),and comparision with the casting and forging fabricated GH4648 alloy,the phase transformation mechanism and the mechanical behavior at room temperature after heat treatment were investigated.The results provide a theoretical reference for the development of using selective laser melting technique to manufacture nickel-based superalloys,and to promote the replacement of aero-engine parts.The SLMed GH4648 alloy in this work has successfully been used to fabricate swirler and thus replace the traditional cast alloy in one type of the aircrafts.The strengthening mechanisms of the GH4648 alloy under various conditions were studied by mechanical property testing,X-ray diffraction,scanning electron microscope,electron backscatter diffraction and transmission electron microscope.The main conclusions are as follows:(1)GH4648 alloy with high density was manufactured by selective laser melting prototyping with specific scanning strategy.The microstructure features of the GH4648 alloy include high dislocation density,uniformly distributed nano-precipitated phase and multi-peak grains size structure.The cellular structure,which consists ofγphase,primary MC phase and primaryα-Cr,with small primary dendrite spacing was formed by SLM.By adjusting the scanning strategy to match the epitaxial growth,the GH464 formed a strong plate texture and had obvious anisotropy in the mechanical properties.Under specific tensile direction,the yield strength of the alloy is 842MPa,the ultimate tensile strength is 1039MPa,and the elongation can reach 40%.The combination of the mechanical properties of the SLMed alloy are much better than those of the cast GH4648 alloy.(2)The microstructures of the GH4648 alloy after ageing treatment with different period were analyzed.The microstructures and mechanical properties of the SLMed GH4648 alloy and the cast GH4648 alloy after the same heat treatment were compared.The SLMed GH4648 alloy exhibits non-equilibrium microstructure of"γ+σphase"and"γ+σphase+α-Cr phase+γ’phase"after solution and aging,respectively.The strength of the SLMed GH4648 alloy can be further improved after heat treatment,and the maximum ultimate tensile strength reaches 1400MPa,while the elongation decreases significantly.The dendrite segregation combined with inadequate solution treatment leads to the formation of topological close-packedσphase,which has an orientation relationship with the matrix in terms of(111)_γ//(001)_σ,[11 0]_γ//[110]_σ.There are also Nishiyama-Wasserman(N-W)and Kurdjumov-Sachs(K-S)orientation relationships betweenα-Cr phase and the matrix.(3)The microstructure evolutions and deformation mechanisms of GH4648 alloy treated by different solution temperatures were studied,and were also compared to the forged GH4648 alloy treated with the same solution process.The results show that,with the increase of the solution temperature,the number of the non-equilibriumσphase in the alloy gradually decreases,and the volume fraction of precipitated phase increases firstly and then decreases.The proportion of theα-Cr phase in the two orientations changes with the increase of the solution temperature,and more N-W typeα-Cr particleswere formed in the solution+aging treated samples at 1220°C,which reduces the plasticity of the alloy.The high yield strength of the alloy is due to the extremely high dislocation density caused by selective laser melting compared to cast and forged alloys.The pinning effect of precipitates relative to grain boundaries results in the high geometrical necessary dislocation density of the SLMed GH4648 alloy after heat treatment.(4)The structural evolution of the GH4648 alloy prepared by various processes under the condition of heat exposure were studied,which provides an important reference for the actual service components.The coarsening of precipitated phase is the main evolution during heat exposure.After thermal exposure,and only a small number of theσphase were detected in the alloys.Theα-Cr phase also changed from K-S type to N-W type during coarsening process.The N-W typeα-Cr has the same morphology and orientation relationship with theσphase,and the increase of its content will significantly reduce the resistance to crack propagation.