As an advanced material forming method, additive manufacturing (AM) has been rapidly developed in many industry fields. Due to the non-weldability of nickel-based single crystal (SX) superalloys and ...complex solidification and stress conditions of AM process, hot cracks can easily form in depositions. Previous research focused on the additive manufacturing of single-track thin-walled SX samples, while effective solutions to fabricate crack-free multi-track SX block samples are still limited. Directed energy deposition (DED) technique was used to fabricate multi-track block samples in this work. The effects of the overlapping ratio, scanning velocity, and laser power on the microstructure and cracking susceptibility were analyzed. The experimental results demonstrated that improper process parameters play significant roles in the formation of high-angle grain boundaries and large internal stress, both of which will increase the cracking susceptibility. Crack-free SX multi-track block was successfully fabricated after adjusting the process parameters. These findings provide a guide to reduce the cracking susceptibility by controlling the process parameters, which aids in fabrication of large-scale crack-free SX blocks through DED technique.
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•The crack-free multi-track nickel-based single crystal superalloy block with 15 layers was successfully fabricated.•Improper overlapping ratio is the main reason for the occurrence of misoriented grains in the overlapping region•Appropriate laser power and scanning velocity effectively reduce the cracking susceptibility.
Considering the anisotropy of nickel‐based single crystal superalloy, an anisotropic creep lifetime prediction model based on slip plane damage is developed. The predicted results indicate that the ...lifetime prediction model could predict the creep lifetimes of nickel‐based single crystal superalloy with different orientations accurately. Furthermore, in order to reveal the damage mechanism of nickel‐based single crystal thin‐walled specimen, creep experiment and failure analysis of thin‐walled specimen are conducted, and there are obvious differences in damage mechanisms between surface and interior of the thin‐walled specimen. In this paper, both thickness debit effect and abnormal thickness debit effect are considered to be caused by the damage differences between surface and interior of the thin‐walled specimen, and zone‐based failure criteria considering thickness debit effect and abnormal thickness debit effect are proposed. Finally, combining the anisotropic creep lifetime prediction model and zone‐based failure criteria, the creep lifetime prediction of DD6 specimen with different thickness is conducted. The predicted lifetimes are within a scatter band of factor 2.26, and the predicted laws between creep lifetimes and thicknesses of the specimens are in good agreement with the experimental results, which verifies the rationality and accuracy of the thickness‐sensitive creep lifetime prediction method established in this paper.
Highlights
Based on continuum damage mechanics, an anisotropic creep lifetime prediction model is established.
Creep experiment and failure analysis are conducted to investigate the damage mechanism of thin‐walled specimen.
Zone‐based failure criteria for reflecting thickness debit effect and abnormal thickness debit effect are proposed.
A thickness‐sensitive creep lifetime prediction method of nickel‐based single crystal superalloy is developed.
•The femtosecond laser and Ni3Al interaction is simulated via TTM-MD approach.•The key physical parameters for femtosecond laser machining of Ni3Al are obtained.•An integral visual picture of laser ...ablation mechanisms in whole spot is created.
Detailed knowledge of the physical essence of femtosecond laser and Ni3Al interactions is of great significance for the effective processing of film-cooling holes in Ni3Al-based single crystal superalloy turbine blades. Although attempts have been made to understand the femtosecond laser processing of Ni3Al-based single crystal superalloys via experiments or mesoscopic simulation, several phenomena and material removal mechanisms at the atomic level remain obscure. The information obtained in previous studies cannot be directly related to the femtosecond laser processing of Ni3Al-based single crystal superalloys due to the differing intrinsic properties of materials. In this paper, using TTM-MD, three regimes of material response to femtosecond laser irradiation are identified in simulations with a single pulsed 500 fs laser: melting, photomechanical spallation and phase explosion. Several key physical parameters related to the thermodynamic and dynamic behaviour of Ni3Al are obtained at the atomic level, including the fluence threshold, crystal lattice thermal stability and thermodynamic critical temperature. At low fluence, the material behaviour is characterized by heterogeneous surface melting and homogeneous subsurface melting. Photomechanical spallation occurs at higher fluence, which can be interpreted as void nucleation and growth, microcrack formation and eventual large liquid layer ejection. Under extremely high laser fluence, the material behaviour follows the phase explosion mechanism, forming an ablation plume with a layered structure. The dominant driving force change under different fluences causes the mechanism change. Additionally, an integral visual picture is successfully created based on the key parameters obtained, through which the mechanisms throughout the laser spot are revealed.
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•The design of a high γ′-fraction and high γ′-stability improves the recrystallization resistance of Ni-based single crystal superalloy.•Recrystallization preferentially initiates in ...the interdendritic of high γ′-fraction as-cast single crystal superalloy due to the small-sized secondary γ′.•The γ-channel entangled dislocations may be the primary driving energy source for the static recrystallization.
The static recrystallization (SRX) of the designed high γ′-fraction Ni-based single crystal superalloy (SX) AlloyX was investigated. The three-dimensional SRX threshold of AlloyX involves plastic strain – deformation temperature – heat treatment temperature established by isothermal deformation and heat treatment tests. The influence of different dislocation configurations obtained at different deformation temperatures on critical SRX temperature was analyzed. The high-density entangled dislocations were observed to gradually transition into low-angle grain boundaries at heat treatment temperature within the γ-channel, potentially serving as the primary driving force for the SRX of AlloyX. Compared with the lower γ′-fraction SX DD32, it is further determined that the preferential dissolution of the small-sized γ′ in the interdendritic region (IDR) caused by high Al is the important factor that induces the recrystallization to occur preferentially in the IDR of AlloyX rather than the dendrite region (DR).
•The thermal fatigue failure mechanism of Ni-based single crystal superalloys with hole structure has been revealed.•The strain–stress constitutive equation under transient thermal shock based on the ...crystallographic theory was established.•The thermal stress location and evolution around the hole at different temperatures was obtained by FEM.•A thermal fatigue crack initiation life model considering transient thermal stress and cyclic oxidation has been proposed.•There is good consistency between the experimental results and the simulation results.
The transient thermal fatigue behavior of Ni-based single crystal superalloys with film cooling holes under different peak temperatures (25 °C → 980 °C and 25 °C → 1050 °C) were investigated by a combination of test and finite element simulation. The initiation position and propagation direction of thermal fatigue cracks in both single hole and multi-hole specimens exhibit the angles of ± 45° with respect to the horizontal direction at the four corners of the holes. Octahedral slip failure mainly occurred at the crack initiation location and crack tip. The cyclic oxidation around the film cooling hole promotes the initiation and propagation of cracks. Based on the crystallographic theory, the strain–stress constitutive equation of the Ni-based single crystal superalloy under transient thermal shock was established. The location and evolution of thermal stress around the hole at different temperatures were obtained by finite element calculation. The kinetic behavior of cyclic oxidation has been quantitatively characterized. A thermal fatigue crack initiation life model considering heating temperature, transient thermal stress, and cyclic oxidation has been established. The simulation results are in good agreement with the tests.
The microstructural evolution and orientation information in the plasma re-melted Ni-based single crystal superalloy are systematically researched. The bowl-shaped FZ with small depth/width ratio is ...obtained under a wide range of processing current. The re-precipitated γ' phase in the position of origin γ' phase is apparently larger than that in the position of origin γ matrix at the bottom part of HAZ, and more γ' phase re-precipitates in the origin γ matrix under larger processing currents in the HAZ. The γ' phase in the interdendrite is larger than that in the dendrite core and both increase with the processing current rising. No obvious element burning occurs during the re-melting and the segregated degree for each element hardly changes from 25 to 45 A. The transition regions exist higher plastic strain than inside the dendrite zones except at 45 A. The low plastic strains at 25 and 45 A are respectively attributed to the few transition regions and extensive stray grains formation. The 25 and 30 A are more suitable considering the position and area fraction of stray grains together with the orientation deviation in general. The results display the potential possibility for the repairing by plasma besides the current technologies.
•The Ni-based single crystal superalloys re-melted by the plasma are systematically researched.•The bowl-shaped FZ with small depth/width ratio is obtained under a wide range of processing current during the re-melting.•No obvious element burning occurs and the segregated degree for each element hardly changes from 25 to 45 A.•The plastic strain is low at 25 and 45 A due to the few transition regions and the quantities of stray grains respectively.•The dominant single crystals with weak orientation deviation are acquired under the processing current from 25 to 40 A.
Cyclic stress responses during low cycle fatigue of a Re-bearing Ni-base single crystal superalloy with 001, 011 and 111 orientations have been investigated at 980°C, and attention is paid to the ...corresponding deformation microstructure to establish a clear microstructure-mechanical relationship. It is found that deformation of the 001 specimens with increased strain amplitude is characterized by cyclic softening at the early stage, while the 011 and 111 specimens exhibit cyclic softening under large strain amplitude and cyclic hardening under small cyclic amplitude. The softening response is related chiefly to the formation of dislocation networks, γ′ degradation and the dislocation recovery process. Moreover, plenty of parallel aligned dislocations in the 011 specimens reduce the probability of dislocation interactions among different slip systems, resulting in cyclic hardening. With respect to the 111 specimens, since a considerable number of dislocations pile up in γ channels, the resistance of the dislocation movement increases and cyclic hardening is resulted. Our results throw light upon microscopic deformation mechanism responsible for the cyclic stress response behaviors of the alloy with various orientations.
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•The 011 and 111 orientations exhibit abnormal initial cyclic hardening in the low cycle fatigue at 980°C.•The differences in orientational stress response are firstly explained by dislocation structures.•Initial softening relates to the formation of dislocation networks, γ′ degradation and the dislocation recovery process.•Parallel aligned dislocations and dislocations pile-up induce hardening of the 011 and 111 orientations, respectively.
•The corrosion behavior of the Ni-based single crystal superalloy and its recast layer is investigated by electrochemical techniques in NaNO3 solution.•The recast layer surface forms a stronger ...passivation film with high corrosion resistance.•Only γ phase exists in the recast layer, the surface bugle consists of oxide, and there is shedding during the electrochemical dissolution.•The γ phase on the surface of Ni-based single crystal superalloy is preferentially dissolved, leaving the block γ' phase.
Ni-based single crystal superalloy is favored for turbine blades. In order to adapt to the high temperature of the working environment, film cooling technology is widely used. The film cooling holes of turbine blades require no recast layer residuals and high machining efficiency. Combined or hybrid methods of electrical discharge machining (EDM) and electrochemical machining (ECM) show prospective applications. Here, the recast layer generated by discharge during high-speed drilling is removed by electrochemical dissolution. But the dissolution behavior of it is not investigated. In this paper, The anodic electrochemical behavior of a Ni-based single crystal superalloy and its recast layer in NaNO3 electrolyte is reported, and the surface morphology change of electrochemical dissolution is elucidated. The results showed that the γ' phase disappeared in the recast layer after EDM. Its oxide content within the passivation film is significantly elevated, exhibiting a stronger corrosion resistance than the matrix. For the material removal characteristics, the crater on the recast layer surface preferentially undergoes corrosion, adopting an alveolate structure, with the bulges later dissolved or removed by shedding due to oxide enrichment. For the matrix, the γ phase undergoes electrochemical dissolution, leaving a block γ' phase. A qualitative electrochemical dissolution model is established.
•Cusp magnetic field can inhibit the stray-crystal formation of superalloy blades.•The suppression effect of magnetic field is weakened as withdrawal rate increases.•The inhibition effect of magnetic ...field is weakened as temperature gradient increases.
The stray crystal in the platform region is one of the common main defects in single-crystal superalloy blades. The simple and effective method to eliminate this defect is urgent to be explored. This work found that the Cusp magnetic field can effectively inhibit the stray-crystal formation in the platform. The tendency of stray-crystal formation decreases as the magnetic-field strength increases at a certain withdrawal rate and temperature-gradient. The suppressing effect decreases as the withdrawal rate or the temperature-gradient increases. Finally, the inhibiting mechanism on the stray-crystal formation from the Cusp magnetic field is proposed based on the experiments and the numerical simulation. The magnetic-field application strengthens the flow velocity and changes the flow structure near the liquid-solid interface, and further reduces the radial temperature difference. Accordingly, the secondary dendrites in the heat-conduction undercooled zone expands towards the corner in a faster speed, which reduces the stray-crystal formation in the platform corner. This study provides an effective and simple method for decreasing the stray-crystal formation during the preparation of single-crystal with platform region.
Gas turbine blades normally operate under high‐temperatures, extreme pressure, and high‐speed conditions, which leads to complex failure mechanisms and difficulties in fatigue life assessment. ...Moreover, due to the randomness of manufacturing errors, working loads, and material properties, the low cycle fatigue (LCF) life normally presents unavoidable stochastic behavior. Therefore, accurate and efficient fatigue life prediction is critical for the design of gas turbine blades. Accordingly, this paper analyzes the effects of mean stress, anisotropy, and uncertainties on the LCF life of nickel‐based single‐crystal turbine blades. The modified Hill yield criterion is employed to deal with the anisotropic damage characteristics of nickel‐based single‐crystal superalloy (NSCS) under a multiaxial stress state. Meanwhile, the effects of the temperature and crystal orientation are introduced into the walker mean stress correction term. A multiaxial LCF life prediction model is developed based on the energy criterion‐based fatigue life estimation method. Moreover, multi‐source uncertainties originating from rotation speeds, material properties, and temperature are analyzed and quantified. Comparison between the predicted and tested life indicates that the proposed method produces good accuracy and robustness, which can provide a reference for the reliability design of NSCS turbine blades.
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