基于成分梯度的激光快速熔炼镍基合金显微组织形成及演化机理研究

Microstructure Formation and Evolution Mechanism of Laser Rapid Melted Nickel Based Alloy Based on Composition Gradient

航空发动机中涡轮盘及涡轮叶片的服役条件不同,通过制备梯度材料可以避免涡轮盘与涡轮叶片连接处提前失效。然而,梯度过渡区的成分变化会导致合金组织改变,是影响性能的关键。为探究合金成分变化对镍基梯度材料显微组织的影响,本团队选用镍基粉末高温合金和定向镍基高温合金为原料,利用激光快速熔炼技术,通过调整两种合金的混合比例,制备出了11种典型成分的合金锭;研究了合金成分对梯度合金晶粒形貌和析出相的影响,重点探讨了合金的枝晶形貌、析出相尺寸和含量随合金成分的变化规律。结果表明:激光快速熔炼11种成分镍基合金锭显微组织中的一次枝晶和二次枝晶臂均比较发达,合金成分改变对枝晶形貌的影响较小,平均一次枝晶间距约为110μm,显微组织均由γ相、γ’相、碳化物和γ/γ’共晶组成;随着镍基粉末高温合金含量的降低,合金锭中γ’相的含量和尺寸不断增加;由于元素偏析,γ’相形成元素Al、Ti、Ta、Nb会偏析在枝晶间,导致枝晶间γ’相的含量及尺寸均大于枝晶干。11个试样的显微硬度值相差不大,整体硬度值分布在500 HV左右。

Objective Nickel-based superalloys are typically used as turbine disks and turbine blade materials in aeroengines owing to their excellent high-temperature performance.Because of the different service conditions of the turbine disk and turbine blade,the premature failure of joints can be avoided using gradient materials.However,a change in the composition of the gradient transition zone can change the microstructure,which significantly affects the properties of the alloy.Therefore,the evolution of the microstructure and the hardness of laser melted nickel based superalloys must be investigated,providing a basis for the laser additive manufacturing of gradient nickel-based superalloys.Methods The materials used in this experiment are IC10 directional superalloy and FGH9X powder superalloy with a particle size of 0.15?0.25 mm.IC10 and FGH9X powders are prepared as nickel-based superalloy powders with different compositions,and alloy ingots with 11 typical gradient components are prepared by changing the ratio of the two alloys.(From samples F100 to F0,the mass fraction of FGH9X decreases by 10%intervals,and the mass fraction of IC10 increases by 10%intervals.For example,the FGH9X mass fraction of the F80 sample is 80%and the IC10 mass fraction is 20%).A 50 g nickel-based superalloy mixed powder is prepared and placed in a copper crucible.The laser power and opening time are set to 5 kW and 2 s,respectively.A melting experiment is performed in an argon atmosphere,and the alloy ingot samples are obtained after air cooling.The microstructures of the samples are observed using an optical microscope(Leica DM4M)and scanning electron microscope(Apreo S LoVac).The primary dendrite spacing,γ’phase size,andγ’phase content are measured using the Image Pro Plus software.Thermo-Calc 2020b is used to simulate the nucleation driving force of theγ’phase in the alloys.A hardness test is performed using a micro Vickers hardness tester(MH-6)with a load of 4.9 N and a holding time of 15 s.Results and Discussions The microstructure of the 11 nickel based alloy samples prepared via laser melting is composed of dendrites,and primary and secondary dendrites.When the sample changes from F100 to F0,the dendrite morphology of the alloys remains almost unchanged,and the primary dendrite spacing is in the range of 100?120μm.Based on literature review,the dendrite morphology and dendrite spacing of the alloys are primarily affected by the cooling conditions,and the effect of alloy composition is insignificant.An analysis of the alloy microstructure shows that the 11 types of nickel-based alloys are composed of theγandγ′phase,carbides,and theγ/γ′eutectic phase(Fig.3).When the alloy composition changes from F100 to F0,the content and size of theγ′phase increase continuously(Fig.5),which is due to the gradual increase in Al and Ta contents in the alloys.The nucleation driving force of theγ’phase increases and moreγ’phases precipitate.In addition,owing to element segregation,the content and size of the interdendriticγ’phase differ significantly from those of the dendritic trunkγ’phase.The content ofγ’phase in interdendritic zone is more than that in dendritic trunk and the size ofγ’phase in interdendritic zone is larger than that in dendritic trunk.In addition,the results show that the change in alloy composition does not significantly affect the microhardness,and that the overall hardness value is approximately 500 HV.This is because as the alloy composition changes from F100 to F0,the content of solid-solution strengthening elements and the content of carbides with high hardness and brittleness in the alloys decrease gradually,whereas the hardness and size of theγ’phase increase gradually;therefore,the hardness of the 11 types of nickel-based alloys is similar.Conclusions Primary dendrite and secondary dendrite arms are developed in the microstructures of the 11 types of nickel-based superalloy ingots prepared via laser rapid melting.The change in alloy composition does not significantly affect the dendrite morphology,and the average primary dendrite spacing is 110μm.The microstructures of the alloys are composed of theγphase,γ′phase,carbides,and theγ/γ′eutectic phase.As the powder nickel-based alloy content in alloy ingots decreases,the content and size of theγ’phases increase continuously.As a result of element segregation,Al,Ti,Ta and Nb,which are the constituent elements of theγ’phase,segregate in the interdendritic regions,thus causing the content and size of theγ’phase in the interdendritic regions are greater than those in the dendritic trunk.The microhardness values of the 11 samples are similar,and the overall hardness value is approximately 500 HV.