Numerical Simulation of Transport Phenomena for a Double-Layer Laser Powder Deposition of Single-Crystal Superalloy

A turbine blade made of single-crystal superalloys has been commonly used in gas turbine and aero engines. As an effective repair technology, laser powder deposition has been implemented to restore the worn turbine blade tips with a near-net shape capability and highly controllable solidified microstructure. Successful blade repair technology for single-crystal alloys requires a continuous epitaxial grain growth in the same direction of the crystalline orientation of the substrate material to the newly deposited layers. This work presents a three-dimensional numerical model to simulate the transport phenomena for a multilayer coaxial laser powder deposition process. Nickel-based single-crystal superalloy Rene N5 powder is deposited on a directional solidified substrate made of nickel-based directional-solidified alloy GTD 111 to verify the simulation results. The effects of processing parameters including laser power, scanning speed, and powder feeding rate on the resultant temperature field, fluid velocity field, molten pool geometric sizes, and the successive layer remelting ratios are studied. Numerical simulation results show that the maximum temperature of molten pool increases over layers due to the reduced heat dissipation capacity of the deposited geometry, which results in an increased molten pool size and fluid flow velocity at the successive deposited layer. The deposited bead geometry agrees well between the simulation and the experimental results. A large part of the first deposition layer, up to 85 pct of bead height, can be remelted during the deposition of the second layer. The increase of scanning speed decreases the ratio of G/V (temperature gradient/solidification velocity), leading to an increased height ratio of the misoriented grain near the top surface of the previous deposited layer. It is shown that the processing parameters used in the simulation and experiment can produce a remelting ratio R larger than the misoriented grain height ratio S, which enables remelting of all the misoriented grains and guarantees a continuous growth of the substrate directional-solidified crystalline orientation during the multilayer deposition of single-crystal alloys.     

Development of a Single-Crystal Fifth-Generation Nickel Superalloy

The chemical and phase compositions of a rhenium-ruthenium-containing fifth-generation VZhM8 nickel superalloy, which is intended for single-crystal turbine blades of an aviation engine, are calculated using computer simulation. VZhM8 alloy <001>, <011>, and <111> single crystals are fabricated. The microstructure, the γ/γ' misfit, the segregation coefficients of alloying elements, the dissolution temperature of the γ' phase, and the solidus and liquidus temperatures of the VZhM8 alloy single crystals in the as-cast state and after heat treatment are studied. The temperature–time dependences of the static elastic modulus, the short-term mechanical properties, and the long-term strength of the alloy single crystals are determined     

Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.     

Characterization of Individual Microneedles Formed on Alloy Surfaces by Femtosecond Laser Ablation

Cross-sectional microstructural analyses of micron/nano-sized structures (termed microneedles) formed by low and high fluence pulse laser ablation of AISI 4340?steel, Ti6Al4V, and Al 5754 alloy specimens were performed. Dependence of length scale and orientation of microneedle microstructures on energy absorptance during laser irradiation, heat transfer direction, absorptivity, and thermal conductivity of the material was established. Microneedle nucleation and growth process were explained based on penetration depths, redeposition of ablated material, and ablation rates.     

Evolution of Micro-Pores in a Single-Crystal Nickel-Based Superalloy During Solution Heat Treatment

Evolution of micro-pores in a third-generation single-crystal nickel-based superalloy during solution heat treatment at 1603 K (1330 °C) was investigated by X-ray computed tomography. 3D information including morphology, size, number, and volume fraction of micro-pores formed during solidification (S-pores) and solution (H-pores) was analyzed. The growth behaviors of both S-pores and H-pores can be related to the vacancy formation and diffusion during heat treatment.     

Stray Grain Formation in Welds of Single-Crystal Ni-Base Superalloy CMSX-4

Autogenous welds on the single-crystal (SX) alloy CMSX-4 were prepared over a wide range of welding parameters and processes to investigate the formation and behavior of stray grains (SGs). The quantity and location of SGs in the welds were analyzed by orientation imaging microscopy (OIM). Heat- and fluid-flow modeling was conducted to understand the influence of welding parameters on the local solidification conditions and resultant SG formation tendency. The results indicate that constitutional supercooling and SG formation are generally reduced in low-power, high-travel-speed welds. Because of the complex effect of travel speed on temperature gradient and solidification velocity, the worst conditions for SG formation in alloy CMSX-4 for the conditions examined here occur at intermediate travel speeds of ~6 mm/s. These findings were corroborated with heat-transfer/fluid-flow modeling simulations that were coupled with SG predictions. These calculations also indicate that SG formation will be greatest where different regions of dendrite growth intersect, due to the so-called off-axis heat flow. For a given set of welding conditions, the amount of SGs will also vary with substrate orientation. This effect is attributed to differences in the number and location of dendrite growth intersection regions within the melt pool that occur with changes in substrate orientation.     

高能飞秒激光烧蚀金靶材的动态热导率影响研究

讨论了高能飞秒激光烧蚀金属金靶材过程中电子热导率对激光烧蚀性质的影响.由于电子热容量,电声耦合系数及电子热导率等热物理参数对能够表征飞秒激光烧蚀性质的电子最高温度,电声耦合时间及电声耦合温度都有一定影响,且这3个热物理参数都为电子温度或电子温度及晶格温度相关的表达式,为了更清楚了解电子热导率对飞秒激光烧蚀性质的影响,将电子热容量及电声耦合系数在合理范围内分别取常数值,而对电子热导率在合理范围内按等差规律分别取3个常系数值,通过在一维双温模型的基础上变化电子热导率常系数值模拟飞秒激光烧蚀金属靶材的热传输演化过程,详细研究了电子热导率对飞秒激光烧蚀金靶材性质的影响.结果表明:电子热导率对金靶材表面电子最高温度,电声耦合时间及电声耦合温度都有不同程度影响.由于电子热导率实质在靶材表面电子达到最高温度后反映的是电子亚系统中电子释放能量速率快慢,因此导致电子热导率对电声耦合温度的影响最为显著.     

On the Mechanical Behavior of a New Single-Crystal Superalloy for Industrial Gas Turbine Applications

The mechanical behavior of a new single-crystal nickel-based superalloy for industrial gas turbine (IGT) applications is studied under creep and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modeling are used to quantify the variation of the gamma prime (?á?) strengthening phase around the ?á? solvus temperature; these aid the design of primary aging heat treatments to develop either uniform or bimodal microstructures of the ?á? phase. Under creep conditions in the temperature range 1023?K to 1123?K (750?°C to 850?°C), with stresses between 235 to 520?MPa, the creep performance is best with a finer and uniform ?á? microstructure. On the other hand, the OP TMF performance improves when the ?á? precipitate size is larger. Thus, the micromechanical degradation mechanisms occurring during creep and TMF are distinct. During TMF, localized shear banding occurs with the ?á? phase penetrated by dislocations; however, during creep, the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalized.     

Effect of Overheating Events on Microstructure and Low-Cycle Fatigue Properties of a Nickel-Based Single-Crystal Superalloy

Micro-mechanical response to overheating events is an important factor when considering the serviceable life of aero-engine components. A key lifing consideration is the low-cycle fatigue (LCF) behavior of nickel-based single-crystal superalloy components. In this study, the LCF response was investigated at different overheating temperatures (1100 to 1300 °C) and times (30 to 120 min). The overheating events had an impact on the γ' phase content, which was observed to decrease with the increase in overheating temperature and time (fully dissolving at 1300 °C). Due to the dissolution of γ' precipitates, the average γ' size was shifted to larger values, however, the subsequent cooling post the overheating events resulted in the formation of tertiary γ' and a bimodal particle distribution. During the LCF tests, that were performed after the overheating exposure, either cyclic hardening or softening was observed. It could be concluded that overheating exposures had no significant effect on the fracture features. However, overheating events resulted in a decrease of LCF properties, which was correlated to the local dislocation response.

     

Neutron and X-Ray Diffraction Study of Internal Stress in Thermomechanically Fatigued Single-Crystal Superalloy

The relationship between internal stress and thermomechanical fatigue (TMF) in a Ni-based single-crystal superalloy is studied by neutron and X-ray diffraction. The extents of internal stress, deformation, lattice mismatch, and distortion during TMF are characterized by the determined deviatoric stress invariants and lattice parameters and compared with relevant microstructural information from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that, in general, the macroscopic stress, plastic deformation, lattice mismatch, and distortion have all increased during TMF. The lattice mismatches of the TMF samples are at a high level, where the values along [100]/[010] are negative, but those along [001] are positive. The tetragonal lattice distortion of the γ matrix is slightly greater than that of the γ′ precipitates, where the c/a values of the γ matrix are smaller than 1, but that of the γ′ precipitate larger than 1. The γ matrix yields and becomes hardened at the initial TMF cycle and gradually loses most of its strength during the earlier TMF cycles, associated with stress relaxation and homogenous deformation. However, the γ′ precipitates yield and become hardened later, bearing the most stress up to the necking of the superalloy. This process is associated with a buildup of stress and significant concentrated and inhomogeneous distribution of deformation in the γ′ precipitate. The residual deformation states of the superalloy and its component phases at the earlier TMF are basically shearing, and only become stretched at a later stage of TMF. The microstructure of the TMF samples shows an initial stage of rafting, where the dislocations are accumulated at the interfaces of the γ matrix channels, but both dislocation networks and stacking faults are inhomogeneously distributed in the γ′ precipitates. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Effect of Platform Dimension on the Dendrite Growth and Stray Grain Formation in a Ni-Base Single-Crystal Superalloy

A model of typical turbine blade shape with different platforms was designed to study the nucleation and growth of stray grains in the platforms by both experimental investigation and a ProCAST simulation based on a cellular automaton finite-element model. The results show that at the withdrawal rate of 5 mm/min, no stray grains nucleate in the small dimensional platform. However, the primary grain grows into the inner and outer sections of this platform in different manners due to different thermal conditions in these sections. Furthermore, with the increase of platform dimension, stray grains with random orientations gradually nucleate in the corners of the platforms. It is found that these stray grains tend to nucleate either in the inner corners or at a faster withdrawal rate, which is associated with the corresponding thermal condition. Based on these results, the rule of the critical platform dimension and withdrawal rate without stray grain formation has been proposed. Besides, the simulation results are in accordance with experimental findings.     

Neutron Diffraction Study of Strain/Stress States and Subgrain Defects in a Creep-Deformed, Single-Crystal Superalloy

A single crystal superalloy with initial sample axis 10 deg deviated from [001] was creep deformed at 1273 K (1000 °C) 235 MPa and its triaxial strain/stress state and subgrain defects were studied by neutron diffraction. Normal internal stresses with their directions close to the loading axis and their scales smaller than those perpendicular to the axis were observed and attributed to a lattice rotation toward [001] pole. The internal stress at a level approaching to the loading stress and mostly in the state of interphase stress was induced during the first stage of creep prior to rafting and associated to lattice rotation, microstrain relaxation and line-up of misoriented γ′-precipitates. The internal stress was diminished and released at final stage of creep associated with a reduction in unit-cell volume and a transition of strain/stress state between the two phases. The observation was explained by development of dislocations and raft structure during creep.     

An In Situ X-ray Tomography Study on the Stress Corrosion Behavior of a Ni-Based Single-Crystal Superalloy

Metallurgical and Materials Transactions A - The present study was devoted to investigating the stress corrosion behavior of Ni-based single-crystal superalloys using in situ three-dimensional...     

Microstructural Parameters Controlling High-Temperature Creep Life of the Nickel-Base Single-Crystal Superalloy MC2

The influence of both topologically close-packed (TCP) phase precipitation and pores on the creep life of a single-crystal superalloy has been studied at 1323?K (1050?°C)/160?MPa. Despite very reproducible primary and secondary creep stages, the creep life is scattered for this specific condition where a very steep tertiary creep stage is observed, corresponding to a highly localized failure process. Image processing was performed after failure to determine the stereological parameters characterizing pores and TCP-phase particles. It was determined that pores are major determinants of creep life under these temperature and stress conditions. It was also observed that the average surface area or the density of pores is not sufficient to explain creep life variability. A homogenization method including modified ??/???? microstructure area surrounding pores and TCP-phase particles was developed and correlated to creep life. It is shown that the greater the extent of the modified microstructure, the lower the creep life. Moreover, a better understanding of the TCP-phase role in controlling the creep life was obtained: TCP-phase particles modified the local stress field and disturbed the local ??/???? microstructure. They enhance the generation of vacancies and subsequent nucleation and growth of pores.     

Additive Manufacturing of Single-Crystal Superalloy CMSX-4 Through Scanning Laser Epitaxy: Computational Modeling,Experimental Process Development,and Process Parameter Optimization

This paper focuses on additive manufacturing (AM) of single-crystal (SX) nickel-based superalloy CMSX-4 through scanning laser epitaxy (SLE). SLE, a powder bed fusion-based AM process was explored for the purpose of producing crack-free, dense deposits of CMSX-4 on top of similar chemistry investment-cast substrates. Optical microscopy and scanning electron microscopy (SEM) investigations revealed the presence of dendritic microstructures that consisted of fine γ′ precipitates within the γ matrix in the deposit region. Computational fluid dynamics (CFD)-based process modeling, statistical design of experiments (DoE), and microstructural characterization techniques were combined to produce metallurgically bonded single-crystal deposits of more than 500 μm height in a single pass along the entire length of the substrate. A customized quantitative metallography based image analysis technique was employed for automatic extraction of various deposit quality metrics from the digital cross-sectional micrographs. The processing parameters were varied, and optimal processing windows were identified to obtain good quality deposits. The results reported here represent one of the few successes obtained in producing single-crystal epitaxial deposits through a powder bed fusion-based metal AM process and thus demonstrate the potential of SLE to repair and manufacture single-crystal hot section components of gas turbine systems from nickel-based superalloy powders.     

Influence of Melt Superheating Treatment on Solidification Characteristics and Rupture Life of a Third-Generation Ni-Based Single-Crystal Superalloy

The influence of melt superheating treatment on the melt properties, solidification characteristics, and rupture life of a third-generation Ni-based single-crystal superalloy was investigated to reveal the critical temperature range of melt structure evolution and its effect on rupture life. The results showed that the viscosity of superalloy decreased but the surface tension increased with increasing superheating temperature. Two characteristic temperature points where the melt viscosity and undercooling degree suddenly change were determined to be 1600 °C and 1700 °C, respectively. Similarly, the stability of the solidification interface firstly improved and then weakened with increasing superheating temperature. The dendrite arms were well refined and the segregation was reduced at 1700 °C. In addition, the rupture life obtained at 1100 °C and 137 MPa increased by approximately 30 pct, approaching the rupture life of the corresponding superalloy containing 2 pct Ru, with increasing superheating temperature from 1500 °C to 1700 °C. When the melt was further heated to 1800 °C, the rupture life decreased. The evolutions of solidification characteristics and rupture life with increasing melt superheating temperature were attributed to changes in the melt structure.     

Formation Mechanism of Stray Grain of Nickel-Based Single-Crystal Superalloy Under a High Magnetic Field During Directional Solidification

Metallurgical and Materials Transactions B - We investigate the formation mechanism of stray grain of nickel-based single-crystal superalloy under a high magnetic field. It was shown that the high...