不同预热温度下电子束粉末床熔融K447A组织演变及力学性能研究

Microstructural Evolution and Mechanical Properties of K447A Alloy Fabricated by Electron Beam Powder Bed Fusion at Different Preheating Temperatures

摘要: 为阐明预热温度对电子束粉末床熔融(Electron Beam Powder Bed Fusion, EB-PBF)成形K447A镍基高温合金热历史、组织演化与力学响应的耦合作用，采用850℃和1000℃两种预热温度制备K447A合金试样。利用光学显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)和室温拉伸试验，对试样成形完整性、γ/γ′组织、晶体学特征、残余应变及断裂行为进行了系统研究。结果表明，两种预热条件下试样均形成沿构建方向定向生长的柱状晶和典型γ/γ′双相组织；当预热温度由850℃提高至1000℃时，熔池冷却过程和层间热累积状态得到改善，枝晶间元素偏析和碳化物连续偏聚减弱，γ′相析出更加充分且分布更加均匀。EBSD结果表明，较高预热温度促进了组织回复和取向连续化，小角度晶界比例增加，局部取向差(KAM)降低，并增强了<001>择优织构。室温拉伸结果显示，1000℃预热试样的抗拉强度达到1028.8 ± 26 MPa，断后延伸率达到10.8 ± 1%，均优于850℃预热试样。断口分析表明，随着预热温度升高，断裂模式由以解理断裂为主逐渐转变为准解理与微孔聚合韧性断裂共同作用的混合断裂。研究表明，提高预热温度能够通过“降低热应力–减弱偏析与晶界缺陷–促进γ′析出与回复–改善协调变形能力”的连续机制降低K447A合金裂纹敏感性，并实现强度与塑性的协同提升。

Abstract: To clarify the coupling effect of preheating temperature on thermal history, microstructural evolution and mechanical response of K447A nickel-based superalloy fabricated via Electron Beam Powder Bed Fusion (EB-PBF), K447A alloy specimens were produced at two preheating temperatures of 850℃ and 1000℃. Optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and room-temperature tensile tests were employed to systematically investigate the forming integrity, γ/γ′ microstructure, crystallographic characteristics, residual strain, and fracture behavior of the specimens. The results reveal that columnar grains epitaxially grown along the building direction and typical γ/γ′ dual-phase microstructure are formed in specimens under both preheating conditions. When the preheating temperature rises from 850˚C to 1000˚C, the molten pool cooling procedure and interlayer thermal accumulation state are optimized. Interdendritic elemental segregation and continuous carbide segregation are alleviated, while the precipitation of γ′ phase becomes more sufficient with a more uniform distribution. EBSD measurements demonstrate that a higher preheating temperature facilitates microstructural recovery and orientation homogenization, accompanied by an increased fraction of low-angle grain boundaries, reduced kernel average misorientation (KAM), and intensified <001> preferred texture. Room-temperature tensile testing indicates that the specimen preheated at 1000˚C achieves an ultimate tensile strength of 1028.8 MPa and an elongation after fracture of 10.8%, both superior to those of the 850˚C preheated counterpart. Fractographic analysis suggests that the fracture mode transforms gradually from cleavage-dominated fracture to a mixed fracture consisting of quasi-cleavage and microvoid coalescence ductile fracture as preheating temperature increases. This study demonstrates that elevating preheating temperature reduces the cracking susceptibility of K447A superalloy through a sequential mechanism: alleviating thermal stress - weakening segregation and grain boundary defects - promoting γ′ precipitation and microstructural recovery - enhancing coordinated deformability, thereby achieving simultaneous improvement in strength and ductility.