白斑缺陷对GH4586合金组织和力学性能的影响

Influence of White Spot Defects on Microstructure and Mechanical Property of the GH4586 Alloy

系统分析了GH4586合金白斑缺陷的组织特征、元素分布及其对室温拉伸性能和硬度的影响,结合热力学计算和感应锭凝固过程仿真模拟,探讨了白斑缺陷的形成机制及其控制方法。结果表明,GH4586合金盘锻件中的白斑缺陷为枝晶白斑,宏观上表现为枝晶花样,微观组织是由MC型碳化物、M6C型碳化物和大尺寸非共格γ′相组成的第二相团簇。由于枝晶白斑中含有较多的高硬度、易开裂的第二相团簇,使合金的室温抗拉强度和塑性显著降低,对GH4586合金盘锻件的力学性能影响较大。GH4586合金大尺寸真空感应锭凝固过程中会产生较大内应力和形成较多缩孔,直接用做电极进行自耗重熔时,易因缩孔开裂而发生电极掉块,落入熔池后形成白斑缺陷。提升自耗电极的质量和优化真空自耗工艺能有效控制GH4586合金盘锻件中的白斑缺陷。

GH4586 is a nickel-based wrought superalloy developed by China, and features high Ti,W, Mo and low Al. However, it is easy to form whit spot(WP) defects within the disk components manufactured from the GH4586 ingot with a diameter of 508 mm produced by double vacuum melting method.In present work, the macro-and micro-structural characteristics, as well as the element distribution of WP defect were systematically studied. Special attention was focused on the effects of WP on mechanical properties. Hardness, tensile properties and fracture morphology of normal and defective areas were investigated for comparison. In addition, the formation mechanism and control method of WP defects were investigated based on the thermodynamic calculation and solidification simulation, respectively. In order to analyze its dendritic segregation and second phase precipitation in the solidification process, solidification phase diagram and thermodynamic equilibrium phase diagram of GH4586 alloy were calculated with Jmat Pro software. To study the forming mechanism of WP, the solidification process of vacuum introduction melting(VIM) GH4586 ingots was simulated with Procast software. Results show that the WP defects within GH4586 components are dendritic WP which exhibits dendritic pattern on the macro level.While on micro level, it is the precipitate clusters which consisted of MC carbides, M6C carbides and large incoherent γ′ phase. Due to the clusters’ inherent rigid and fragile characteristics, the tensile strength and ductility of the alloy are significantly reduced. Therefore, the WP defects are harmful to the mechanical properties of the GH4586 disk significantly. The WP defects must be avoided during the forging process.Since excessive internal stress and shrinkage cavities form during the solidification process of large VIM GH4586 ingots, chips fall into molten pool easily if the VIM ingots are employed as vacuum arc remelting(VAR) electrode, and then give birth to WP defects within the final forging components. Therefore, it is the effective method for avoiding WP defects by improving the quality of VAR electrode and optimizing the VAR process.