Cracking on a nickel-based superalloy fabricated by direct energy deposition

Cracks have consistently been a significant challenge limiting the development of additive manufactured nickel-based superalloys.It is essential to investigate the location of cracks and their forming mechanism.This study extensively examines the impact of solidification process,microstructural evolution,and stress concentration on crack initiation during direct energy deposition(DED).The results emphasize that the crack formation is significantly related to large-angle grain boundaries,rapid cooling rates.Cracks caused by large-angle grain boundaries and a fast-cooling rate predominantly appear near the edge of the deposited samples.Liquation cracks are more likely to form near the top of the deposited sample,due to the presence ofγ/γ'eutectics.The secondary dendritic arm and the carbides in the interdendritic regions can obstruct liquid flow during the final stage of solidification,which results in the formation of solidification cracks and voids.This work paves the way to avoid cracks in nickel-based superalloys fabricated by DED,thereby enhancing the performance of superalloys.

Microstructures and mechanical properties of DZ125 directional solidified superalloy repaired by HIP technology

DZ125 directional solidified superalloys show excellent bearing temperature ability and creep resistance and have been widely used to manufacture the advanced aviation engine blades parts.However,the high temperatures and stresses for longtime service would cause a loss of mechanical properties and service life.In this paper,the damaged alloys were repaired using hot isostatic pressing combined with rejuvenation heat treatment technology,where the irregularγ'phases in damaged alloys are recovered to be cubic particles like original state and the creep cavities and casting porosities also reduce obviously compared with those in damaged state.The restorable alloys exhibit apparently higher tensile properties and hardness than the alloys in damaged state,which is close or even more than those of original alloy,and their elongation and rupture life can satisfy criterion completely.This method can be easily extent to repair other damaged superalloys.

Effect of Solidification Condition and Carbon Content on the Morphology of MC Carbide in Directionally Solidified Nickel-base Superalloys

Evolution of the morphology of MC carbides with the change of cooling rate and carbon content in two kinds of nickel-base superalloys, K417 G and DD33, has been investigated. The morphology of MC carbides evolves from faceted to script-like with increasing cooling rate. Varying the carbon content from 40X10-6 to 320X10-6, the morphology of carbides changes from blocky, rod-like into script-like. Scanning electron microscopy observation of deep-etched samples indicates that these carbides evolve from octahedral to dendritic and then into welldeveloped dendrites accordingly in a three-dimensional manner. The morphology evolution is discussed from the viewpoint of the preferential growth orientation of fcc crystals and the carbide growth rate during directional solidification.

Thermal cycling creep properties of a directionally solidified superalloy DZ125

Aero-engine turbine blades may suffer overheating during service,which can result in severe microstructural and mechanical degradation within tens of seconds.In this study,the thermal cycling creep under(950℃/15 min+1100℃/1 min)-100 MPa was performed on a directionally solidified superalloy,DZ125.The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950℃/100 MPa,950℃/270 MPa,and 1100℃/100 MPa.The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950℃/100 MPa.The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep.The thermal cycling creep curve exhibited three stages,similar to the 1100℃isothermal creep,but its minimum creep rate occurred at a lower creep strain.The overheating events caused severe microstructural degradation,such as substantial dissolution ofγ'phase,earlier formation of raftedγ'microstructure,widening of theγchannels,and instability of the interfacial dislocation networks.This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life,as the thermal cycling promoted more dislocations to cut intoγ'phase and more cracks to initiate at grain boundaries,carbides,and residual eutectic pools.This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design.

Microstructure and wear behavior of IC10 directionally solidified superalloy repaired by directed energy deposition

Directed energy deposition has been used to repair superalloy components in aero engines and gas turbines.However,the microstructure and properties are generally inhomogeneous in components because of the different processing histories.Here,the microstructures and wear behavior of different zones(substrate,HAZ,and deposit)are investigated for the IC10 directionally solidified superalloy repaired by the directed energy deposition process.It is found that the microstructure of the deposited layers is strongly textured with a<001>-fiber texture in the building direction,and the texture intensity is continuously increased along the building direction.Two kinds ofγ’phase(primary and secondaryγ’phase)can be found in the heat-affected zone(HAZ),and the average size of primaryγ’phase is smaller than that in the substrate due to liquation.In the deposit layers,the size ofγ’phase is much smaller than those in the substrate and the primaryγ’phase of HAZ;both size and the fraction of theγ’phase decreases with the increase of building height.The wear rate of the substrate is the smallest,indicating the best wear resistance;while the wear rate of HAZ is the largest,indicating the worst wear resistance in the repaired sample.The wear rates in the deposit layers increase from the bottom to the top zones,showing a decreasing wear resistance.Abrasive wear is found to be the dominant wear mechanism of the repaired alloy,and the resistance to which is closely related to the fraction ofγ’phase in the microstructure.The understanding of the influence of microstructure on wear resistance allows for a more informed application of inhomogeneous superalloy components repaired by directed energy deposition in industry.

Simulation of stray grain formation at the platform during Ni-base single crystal superalloy DD403 casting

The mechanism of stray grain formation at the platform of turbine blade simulator and the effect of withdrawal rate(V) on the stray grain phenomenon have been investigated using a macro-scale Pro CAST coupled with a 3D Cellular Automaton Finite Element(CAFE) model. The results indicate that the stray grains nucleate at the edges of platform at V =150 μm·s^(-1) and 200 μm·s^(-1). Using Pro CAST computer simulation software, it was proven that the stray grain formation is signifi cantly dependent on the undercooling and the temperature fi eld distribution in the platform. The macroscopic curvature of the liquidus isotherm becomes markedly concave with an increase in the withdrawal rate. The probability of stray grain formation at the edges of platform can be increased by increasing the withdrawal rate in the range of 70 μm·s^(-1) to 200 μm·s^(-1).

Crack Types, Mechanisms, and Suppression Methods during High-energy Beam Additive Manufacturing of Nickel-based Superalloys: A Review

Nickel-based superalloys have been widely used in aerospace fields,especially for engine hot-end parts,because of their excellent high-temperature resistance.However,they are difficult to machine and process because of their special properties.High-energy beam additive manufacturing(HEB-AM)of nickel-based superalloys has shown great application potential in aerospace and other fields.However,HEB-AM of nickel-based superalloys faces serious cracking problems because of the unique characteristics of superalloys,and this has become the most significant bottleneck restricting their application.In this review,the current research status related to the types,formation mechanisms,and suppression methods of cracks in nickel-based superalloys produced by HEB-AM is described.The initiation and propagation mechanisms of cracks and their multiple influencing factors are also analyzed and discussed.Then,several possible research directions to solve the cracking problems in nickel-based superalloys produced by HEB-AM are outlined.This review provides an in-depth and comprehensive understanding of the cracking problem in AM nickel-based superalloys.It also provides valuable references for AM crack-free nickel-based superalloy components.

定向凝固镍基高温合金叶片榫齿高效深切成型磨削

针对定向凝固镍基高温合金DZ125叶片榫齿，采用电镀成型CBN砂轮对其进行了高效深切磨削（High efficieney deep grinding，HEDG）试验，对磨削比能及工件表面完整性进行了分析。结果显示，在保持速比（Us／‰）不变时，提高磨削速度饥可有效降低磨削比能，提高平均材料去除率。磨削比能表现出“尺寸效应”，其值最终稳定在40～60J／mm^3之间；在相同的平均材料去除率下，磨削比能随着磨削深度的增大而上升；在相同的单颗磨粒切厚下，磨削深度的差异对磨削比能的影响较小。对试验中最大平均材料去除率下获得的工件表面质量进行分析发现，已加工工件表面不同区域磨削纹理均很清晰，无皱叠及犁沟两侧翻起等现象；表层金相显微组织基本无变化，未发现相变、撕裂及晶粒扭曲现象；工件表层加工硬化程度为7．7％～19％，深度为40μm。结果显示了HEDG在高效磨削DZ125叶片榫齿中推广应用的潜力，并为其实际生产中磨削参数的选择提供了参考。

CMSX-3单晶合金的微观组织研究

镍基单晶高温合金是重要的航空燃气轮机涡轮叶片材料。本文利用光学金相、SEMTEM等分析手段研究了采用定向凝固造晶法制造的CMSX－3单晶合金的微观组织。对其铸态组织与γ＇形态做了描述与分析。

CMSX-2合金位错组态与凝固组织的关系

对CMSX 2合金粗枝晶、细枝晶以及超细枝晶单晶持久变形后的位错组态进行了研究。结果表明 :超细枝晶单晶持久变形过程中在γ/γ′相界上形成的位错网络比粗枝晶单晶的均匀 ;γ相内部形成的位错密度减小 ,分布也更加均匀 ,多位错滑移程度降低 ;切过γ′的位错明显减少 ,反映出高温下合金抵抗蠕变变形的能力提高。

Effects of TiC on the microstructure refinement and mechanical property enhancement of additive manufactured Inconel 625/TiC metal matrix composites fabricated with novel core-shell composite powder

The flexible product shape of additive manufacturing(AM)is attractive,but the process suffers from a lack of material property diversity due to a limited number of printable alloys and post-processing options.To overcome this problem,the AM of metal matrix composites(MMCs)is a highly suitable solution because the properties of MMC can be tailored using various reinforcements.Therefore,extensive research has been conducted on the AM of MMCs;however,the major huddle for this process has been the difficulties in preparing feedstock powder and operating the AM process.This study introduces an easily synthesizable core-shell composite powder,which was fabricated by a recently developed process called the SMART process.The core-shell powder has a novel morphology,consisting of a metal core and composite shell,distinguishing it from the powders used in conventional AM approaches.Inconel 625/TiCp composites were fabricated using the core-shell composite powder,with various fractions of TiCp up to 10 vol.%.Compared to additive-manufactured Inconel 625,the additive-manufactured MMCs showed enhanced strength with significantly fewer defects.The results of this study may accelerate the application of MMC fabricated by AM,which offers superior properties and reliability compared to casting and powder metallurgy due to the higher degree of dislocation density and reinforcement dispersion.

一种低成本、高性能新型重型燃气轮机叶片用定向凝固高温合金的组织和性能

介绍由钢铁研究总院研制的一种新型重型燃气轮机叶片用定向凝固高温合金:G3,该合金具有低成本、高持久断裂寿命以及高抗热蚀性能等特性。合金中含有小于2%(wt.%)的Ta元素和约16%(wt.%)的Cr元素,在保证低成本的同时保持着较高的抗热蚀性能。与DZ22B定性凝固合金相比较,G3合金在具有高的抗热腐蚀性能的情况下,室温拉伸性能提高了300MPa,在930℃/270Mpa下的高温持久性能提高了20 h以上。

高温条件下DZ125缺口疲劳性能试验

研究了定向凝固合金DZ125U型平板缺口试样在850℃下的缺口疲劳行为,进行了理论应力集中因子分别为1.91,3.01和4.35下的疲劳试验,原位观察了疲劳裂纹的萌生位置和扩展方向,并进行了疲劳寿命分析和裂纹萌生与扩展分析.结果表明:理论应力集中因子关联缺口疲劳寿命.随着名义应力的增加,缺口疲劳寿命对理论应力集中因子更敏感,而在低应力范围内,正好相反.通过原位观测技术,试验中还发现缺口疲劳裂纹的萌生和扩展并没有在最大轴向应力的缺口根部,而是缺口附近最大主应力位置.

定向凝固技术及定向凝固高温合金的现状和发展

详细论述了定向凝固原理及国内外定向凝固高温合金与定向凝固技术的发展现状，简要评述了几种新型定向凝固技术，并展望了高温合金与定向凝固技术的发展。