Turbine Blade Alloy Research Articles

Investigation of Stress Corrosion Cracking in CMSX-4 Turbine Blade Alloys Using Deep Learning Assisted X-ray Microscopy and Correlative Imaging Workflow

Investigation of Stress Corrosion Cracking in CMSX-4 Turbine Blade Alloys Using Deep Learning Assisted X-ray Microscopy and Correlative Imaging Workflow

Modelling and optimization of residual stress induction on laser-worked X12Cr turbine blades

The energy and power industry conventionally depends on large-scale turbomachinery to meet the ever-growing global energy demands. However, unplanned in-service failures remain a threat to sustainability with safety and economic consequences. The laser shock surface treatment technique is being considered a competitive alternative in mitigating crack initiation and growth, wear and fatigue of industrial components such as turbine blades. This paper presents the modelling and optimization of laser shock treatment parameters using numerical methods and commercial codes such as ABAQUS® and MATLAB®. Model-based process optimization parameters for the induction of global optimum compressive residual stress distribution in laser-worked Chromium-12 based high strength steel alloy (X12Cr) turbine blade is established, showing parametric combinations of inputs variables within the domain under investigation, yielding maximized CRS outputs. A hierarchy of significance of the input parameters to the laser shock peening process for stress induction has also been put forward as an outcome of this study. The capacity to predict and analyze outcomes before actual treatment of the components is beneficial and imperative to cutting costs, downtimes and other economic losses associated with unplanned failure of these components.

Effect of NaCl and Na2SO4 on Low Temperature Corrosion of Vapour- and Pack-Aluminide Coated Single Crystal Turbine Blade Alloys CMSX-4 and RR3010

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na2SO4–2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na2SO4–2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl3 in the cracks.

Corrosion of Potential First Stage Blade Materials in Simulated Supercritical CO2

ABSTRACT Global power consumption is predicted to double by 2050, notably driven by the transportation and energy sectors necessitating limitations of emissions. Due to its compact turbomachinery, better thermal efficiency, and simpler layout, supercritical-CO2 cycles have received attention, with numerous variations proposed (either indirect-fired/closed cycles or direct-fired-open cycles). One technical challenge is degradation pathway quantification of turbine materials in sCO2 as selection is crucial to successfully and economically operate new plants. This requires degradation assessment in representative environments simulating the Allam cycle. Laboratory tests were conducted on a first stage turbine blade alloy, CM247, with either an environmentally resistant coating or bond coat/thermal barrier coat at one atmosphere and 800°C, with potential exposure including (O2, H2O, N2, SO2) for up to 1000 h. Weight change and metallographic measurements tracked scale development. Scanning electron microscopy/energy dispersive X-ray spectroscopy studied scales and internal precipitates. Locations of contaminant element in the CO2-rich environment were investigated.

Sustainable Hybrid Manufacturing of AlSi5 Alloy Turbine Blade Prototype by Robotic Direct Energy Layered Deposition and Subsequent Milling: An Alternative to Selective Laser Melting?

Additive technologies enable the flexible production through scalable layer-by-layer fabrication of simple to intricate geometries. The existing 3D-printing technologies that use powders are often slow with controlling parameters that are difficult to optimize, restricted product sizes, and are relatively expensive (in terms of feedstock and processing). This paper presents the development of an alternative approach consisting of a CAD/CAM + combined wire arc additive-manufacturing (WAAM) hybrid process utilizing the robotic MIG-based weld surfacing and milling of the AlSi5 aluminum alloy, which achieves sustainably high productivity via structural alloys. The feasibility of this hybrid approach was analyzed on a representative turbine blade piece. SprutCAM suite was utilized to identify the hybrid-manufacturing parameters and virtually simulate the processes. This research provides comprehensive experimental data on the optimization of cold metal transfer (CMT)-WAAM parameters such as the welding speed, current/voltage, wire feed rate, wall thickness, torch inclination angle (shift/tilt comparison), and deposit height. The multi-axes tool orientation and robotic milling strategies, i.e., (a) the side surface from rotational one-way bottom-up and (b) the top surface in a rectangular orientation, were tested in virtual CAM environments and then adopted during the prototype fabrication to minimize the total fabrication time. The effect of several machining parameters and robotic stiffness (during WAAM + milling) were also investigated. The mean deviation for the test piece's tolerance between the virtual processing and experimental fabrication was -0.76 mm (approx.) at a standard deviation of 0.22 mm assessed by 3D scanning. The surface roughness definition Sa in the final WAAM pass corresponds to 36 µm, which was lowered to 14.3 µm after milling, thus demonstrating a 55% improvement through the robotic comminution. The tensile testing at 0° and 90° orientations reported fracture strengths of 159 and 161.3 MPa, respectively, while the yield stress and reduced longitudinal (0°) elongations implied marginally better toughness along the WAAM deposition axes. The process sustainability factors of hybrid production were compared with Selective Laser Melting (SLM) in terms of the part size freedom, processing costs, and fabrication time with respect to tight design tolerances. The results deduced that this alternative hybrid-processing approach enables an economically viable, resource/energy feasible, and time-efficient method for the production of complex parts in contrast to the conventional additive technologies, i.e., SLM.

Hot Isostatic Pressing in the Manufacture of ZhS3DK-VI Alloy Turbine Blades with 50% Returns in the Charge

Hot Isostatic Pressing in the Manufacture of ZhS3DK-VI Alloy Turbine Blades with 50% Returns in the Charge

Adaptive segmentation method in radiographic testing for turbine blades based on spatial entropy

During the radiographic testing with multiple voltage exposures, the problem of image redundancy has serious influences on the speed of analysis. In this paper, by the calculation of spatial entropy and the extraction of microtopography features, a new segmentation method for an unpredictable free-form surface in turbine blades is presented to reduce the testing redundancy. First, the entropy calculation is applied to a set of radiography images of the target object. The optimized image for surface segmentation is determined according to global entropy values and exposure parameters. Then, the obtained image is set as the reference image for the segmentation process. Through the spatial entropy calculation, the reference radiographic image is divided into several regions by geometric features of the tested object. The different thickness range of free-form surfaces is indicated by those regions through the analysis of spatial entropy distribution in the reference image. By the statistical advantage of entropy calculation, the selected region is self-adaptive to the unpredictable free-form surface in the blade. A nickel-based alloy turbine blade is used to validate the segmentation method in the radiographic testing. The processed image quality is assessed by using the American Society for Testing and Materials image quality indicator to address its capability for the detection of defects, where the resolution of the image is not affected by the down-sampled effect of entropy calculation. The experimental results show that the image redundancy in the multiple exposure testing is reduced to less than 30% during the inline testing, while the dynamic range in each extracted region with an optimal image is significantly improved.

High-Efficiency Machining of Titanium Alloy Using Combined Machining Method of Driven Rotary Tool and Hale Machining

Titanium alloys are widely used as aerospace materials, especially for turbine blades, due to their excellent mechanical properties. In the high-efficiency machining of titanium alloy turbine blades, the feed rate for ball-end milling is limited to 1000 mm/min due to the low thermal conductivity and chemical reactivity of the titanium alloy. These characteristics result in tool damage and an increase in the cutting temperature, significantly reducing the machined surface accuracy. A new processing method is thus needed for achieving a high accuracy and high efficiency in titanium alloy machining. It has been reported that driven rotary machining of hardened steel improves the machined surface and increases the processing efficiency, suggesting that high-efficiency machining can be realized by employing hale machining with a rotary tool. In this study, hale machining was performed using a driven rotary tool and the effects of different cutting conditions and cutting environment on the machining characteristics were investigated. The results showed that the tool life was longest at a feed rate of 9000 mm/min among the three feed conditions because the number of times of adhesion and detachment decreased with the decreasing friction distance of the cutting edge. Furthermore, it was clarified that adhesion formation at the cutting edge was suppressed by lubrication with an oil mist in a minimum quantity lubrication environment. This lubrication effect reduced the tool damage and adherence at the cutting edge, significantly extending the tool life and improving the machined surface quality compared to the results obtained in a wet environment.

Creep resistance improvement of a polycrystalline Ni-based superalloy via TiC particles reinforcement

In order to develop highly efficient aircraft engines, the use of alloys for turbine blades that combine high creep resistance and lower density brings substantial benefits. That may be achieved by the introduction of a reinforcing phase, such as a discontinuous reinforcement. So far, most researched particle-reinforced Ni-based superalloys made use of sub-micrometer particles, such as oxides and borides, often produced by in-situ reactions. These are proposed solely as dislocation immobilizers, in a role comparable to that carried out by γ’ precipitates, while metal matrix composites (MMCs) containing micro-scale particles offer a load-transfer potential. To combine both approaches, a recently designed polycrystalline Ni-based γ’-strengthened superalloy reinforced with TiC particles was produced by low energy mixing of powders, followed by pressure sintering. The composite, along with a non-reinforced reference, was submitted to compression creep tests at 973 K (700 °C), with stresses varying from 280 to 500 MPa. Selected specimens had their microstructure evaluated with techniques such as EBSD and XRD. When simulating a working turbine, the material density influences directly the centripetal forces acting on a section of a turbine blade, and consequently the creep rates. In the proposed composite, not only an increase in the threshold stress by a load-transfer component was obtained, but also the density reduction affected remarkably the creep rates. Moreover, the presented MMC not only presented a superior creep resistance in relation to the non-reinforced alloy, but also exhibited higher stability of primary and secondary γ’ volumes after 500 h in creep tests.

Investigation of Stress Corrosion Cracking in CMSX-4 Turbine Blade Alloys Using Deep Learning Assisted X-ray Microscopy

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Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions

In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.

Porosity and surface defects characterization of hot isostatically pressed Inconel 718 alloy turbine blades printed by 3D laser metal fusion technology

Porosity and surface defects characterization of hot isostatically pressed Inconel 718 alloy turbine blades printed by 3D laser metal fusion technology

Effect of pulse duration and vibration tumbling on re-cast layer thickness and surface roughness of super alloy undergoing EDM

Many geometrically complex functional parts of turbo-pumps of Liquid Oxygen (LOX)-Hydrocarbon based rocket engines (Semi-Cryogenic Engines) which are difficult to be realized through conventional machining processes, are realized through Electric Discharge Machining (EDM). Every EDM process, deposits a re-cast layer on the spark eroded surface due to melting and subsequent quenching of the eroded surface. Studies show that structure and hardness of the re cast layer, differs from the parent material. Energy Dispersive Spectroscopic (EDS) analysis reveal that the chemical composition of the re-cast layer matches very closely to that of the parent metal itself. Re-cast layer has inherent micro cracks due to both thermal and tensile stresses during cooling. Even though re-cast layer is a superficial phenomenon, cracks once initiated on the re-cast layer, penetrate deep down to the parent metal as well, under fatigue loads causing fractures. In order to reduce the micro cracks on such spark eroded surfaces, thickness of the re-cast layer is to be reduced to the extent possible by fine tuning the EDM parameters. Failure analysis carried out on the fractured turbine blades of a LOX-Hydrocarbon based engine, realized through 4-axis EDM, which failed during the functional tests, indicated the propagation of such micro cracks from spark eroded surface into parent material causing fatigue fracture at turbine blade tips. Amongst the significant parameters governing the formation of re-cast layer in the EDM process, the effect of pulse duration is studied for its effect on re-cast layer thickness and surface roughness on super Nickel alloy turbine blades. Additionally, a vibration tumbling process is introduced and its effect on re-cast layer thickness and surface roughness is demonstrated. Turbine blades realized from the fine-tuned pulse duration, followed by vibration tumbling, show great improvements in surface finish, reduction of micro cracks, surface porosities and thickness of recast layer. This turbine successfully passes through the functional tests without any fractures.

Service damage mechanism and performance attenuation of nickel-based alloy turbine blades

Multi-scale morphology observations and fatigue performance tests were carried out to study the damage evolution mechanism and fatigue performance attenuation behavior of K403 turbine blades in service. During service in field, the dendrite separation and breakage, γ' phase polymerization and rafting, MC carbide decomposition, harmful phase precipitation and grain boundary weakening are having adverse effects on the fatigue performance of turbine blades. Meanwhile, the loss of alloying elements in matrix causes the alloy matrix to soften. In addition, a large number of pores and microcracks formed in the process of service further deteriorate the service performance of turbine blades. Therefore, after long-term operations, the solid solution strengthening, precipitation strengthening, dispersion strengthening and grain boundary strengthening effects of K403 turbine blades are all weakened, which led to serious degradation of the fatigue performance of these turbine blades with the fatigue life reduced. Besides, the crack initiation source of turbine blade is gradually transformed from subsurface metallic pore initiation to carbide initiation.

Shear thickening polishing of the concave surface of high-temperature nickel-based alloy turbine blade

High-temperature nickel-based alloy, which is a valuable crucial material for fabricating functional components, is commonly used to fabricate turbine blades. However, the concave surface of the turbine blade is much difficult to be polished. In this work, shear-thickening polishing (STP), which employed the shear-thickening mechanism of non-Newtonian power-law fluid, was utilized to achieve high efficiency and high-quality polishing of the concave surface of the high-temperature nickel-based alloy turbine blade. The finite element simulation of the pressure and velocity distribution on the concave surface during STP process was carried out by ANSYS, and the effects of different polishing angles, polishing velocities, pH values of polishing slurry and Fenton's reagent on the surface morphology and roughness of the workpiece were investigated by experiment. The comparison between simulation and experimental results indicated that the suitable polishing angle is 45°. The best surface can be obtained with a pH value of 6.5. With the increase of the polishing speed, polishing efficiency improved gradually. The workpiece surface was improved by adding 0.5% hydrogen peroxide (H2O2) and 0.15% FeSO4 in the polishing slurry. In the final, surface roughness of the turbine blade was reduced rapidly from Ra = 72.3 nm to Ra = 4.2 nm after 9 min polishing under the appropriate conditions. The research results provide reference and basis for high-temperature nickel-based alloy turbine blade polishing.

Accounting for crack closure effects in TMF crack growth tests with extended hold times in gas turbine blade alloys

Crack closure effects are known to have a large impact on crack growth behaviour. In this work, tests were performed on Inconel 792 specimens under TMF loading conditions at 100–850 °C with extended hold times at tensile stress. The effective stress-intensity range was estimated experimentally using a compliance-based method leading to the conclusion that crack closure appears to have a primary impact on the crack growth behaviour for this material under the conditions studied. The crack closure behaviour for the tests was successfully modelled using numerical simulations, including creep.

Probabilistic Life Assessment of Gas Turbine Blade Alloys under Creep

Deformations occur gradually in the gas turbine components since they are working under high temperature and stress. In the turbine blade alloys, creep is the most significant failure mechanism. In this research, creep life has been estimated for the blade alloys by considering humidity. A method is proposed to estimate the creep life by direct consideration of humidity on the creep life of the gas turbine blade. In the proposed model, the humidity factor is added to the classic Larson Miller creep life estimation method. This model is capable of predicting creep life with known dry temperature (Water Air Ratio=0), mechanical stress, and humidity. In this approach, there is no need to measure blade temperature variation during operation. As a case study, the creep life of first-stage turbine blade alloy is predicted using the proposed method and benchmarked with published (Finite element analysis) FEA results. The reliability of the blades was estimated by considering different success criteria using Monte Carlo simulation. The reliability of the creep rupture life of Nimonic-90 steel was carried out using SCRI mode based on the Z-parameter. The scattered data has been considered for creep rupture of materials in this part. The results show that creep life increases with humidity increase. It is also shown that with an increase in mechanical stress and temperature fluctuations, the reliability of the turbine blade creep life decreases sharply.

Mysterious failure in load-free superalloys under repeated thermal shocks

The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

A new process chain combining cross-wedge rolling and isothermal forging for the forming of titanium alloy turbine blades

Titanium alloy blades are significantly important parts for aero-engine, and their special outline and production status requires an efficient and low-cost production process. In this paper, a new sequence for forming the titanium alloy blade using cross-wedge rolling (CWR) and isothermal forging was proposed. A finite element (FE) numerical model which contains the entire sequence, including the transfer process, CWR process, and isothermal forging process, was developed. And the axial displacement, strain-stress state, and temperature variation were analyzed based on its material properties of titanium alloy. The results showed that the billet experiences a smooth flowing in the center along the axial direction, and no large concavity occurs due to the higher viscosity. Moreover, the chilling effect of the dies was effectively counterbalanced by the deformation-heat near the surface, and there is no larger temperature difference. During the isothermal forging, the flash is symmetrically distributed around the fully filled blade and no obvious forming defects occur. Moreover, the novel sequence contributes to improving the deformed microstructure by accelerating the globularization of the long-strip alpha phase and increasing the volume fraction of the primary alpha phase. The technical feasibility of the novel sequence was verified by the corresponding experiments.

Mysterious Failure in Load-Free Superalloys Under Repeated Thermal Shocks

Abstract The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.