Superalloy DD6 Research Articles

Improved oxidation resistance of AIP NiCrAlYSi coatings by applying positive bias

The NiCrAlYSi coatings with a bias voltage of −30 V (N-coating) and + 5 V (P-coating) applied on the DD6 superalloy substrate were prepared by arc ion plating (AIP). The microstructure evolution and the oxidation resistance at 1100 °C of the two coatings were investigated. The results show that the positive bias could maintain the Al content from the target material through to the coating by enhancing droplet deposition proportion and resulting in better oxidation resistance. The as-deposited P-coating consists of γ-Ni, γ’-Ni3Al and α-Cr phases with significantly lower pores and ~ 1.1 wt% higher Al content than N-coating. The larger size (1.5 to 3.5 μm) block β-NiAl grains formed within P-coating after annealing at 1050 °C for 2 h, while it was not found in the N-coating. The oxidation parabolic rate constant at 1100 °C of annealed P-coating was confirmed as 3.54 × 10−3 mg2/(cm4·h), ~ 27.6 % lower than N-coating.

Grinding surface and subsurface stress load of nickel-based single crystal superalloy DD5

Nickel-based single crystal superalloys have broad application prospects in the aerospace field. Their excellent physical properties enable the workpiece to serve in complex environments for a long time, and are often used in the most complex stress and the worst working conditions. However, the damage caused by mechanical material removal deteriorates the wafer surface flatness and die strength, while its surface and subsurface formation principle has yet to be revealed. This study aims to analyze the influence of abrasive particles on the load of the workpiece during machining was scrutinized through the lens of stress tensor, yield performance, and third invariant. The results indicate that, a substantial stress load is exerted on the front end of the abrasive grain, leading to the yield deformation of the workpiece. Conversely, the stress load at the bottom of the abrasive grain is comparatively lower, resulting in tensile deformation and the formation of a discernible plastic damage layer. To delve deeper into the workpiece's damage phenomenon, this study conducts a comparative analysis from the standpoints of chips, surface morphology, and subsurface characteristics of DD5. Experimental results indicate that the chips on the contact surface exhibits a relatively smooth texture, while the grinding surface manifests grooves of varying depths. Concurrently, subsurface analysis reveals the presence of slip deformation and elemental flow. This investigation elucidates the intricate interplay between the grinding process and subsurface loading, offering valuable insights to enhance the quality and precision of nickel-based single-crystal superalloy workpiece manufacturing.

Microstructure analysis and life prediction of DD6 superalloy under 760 °C combined high and low cycle fatigue conditions

The impact of high and low cycle combined loads on the fatigue performance of DD6 alloy was investigated in this work. Specifically, the combined high- and low-cycle fatigue response of DD6 alloy was examined at a temperature of 760 °C under different low cycle stress amplitudes (960, 1000, and 1040 MPa). The findings revealed that higher low cycle stress amplitudes result in shorter combined high and low cycle fatigue life and less dispersion in life. Correlations between the microstructural features (crystal plane distribution, fracture elevation difference, and the area proportion of crack propagation zone) and fatigue performance were accurately established, demonstrating a linear relationship between these microstructural features and the logarithm of fatigue life. The study also uncovered the evolution rule of microstructural features in high- and low-cycle combined fatigue. Furthermore, the fracture mechanism was analyzed and the distribution of high and low cycle combined fatigue life of DD6 alloy at 760 °C was statistically validated using the dual-parameter Weibull distribution theory. Building upon the crystal plasticity theory, a fatigue life prediction model was developed, and the combined cycle fatigue lifespan under various low cycle stress amplitudes was compared. The comparison between the predicted life and actual fatigue life indicated that the predicted lifespan falls in three times the scatter band.

Grinding subsurface damage mechanism of nickel-based single crystal superalloy based on stress-strain

This study investigates the microstructure damage mechanism of the difficult-to-machine material nickel-base single crystal superalloy DD5 by analyzing the working condition load and crystal deformation. The complexity of the macroscopic and microscopic deformation phenomena during the machining process makes it challenging to study the damage mechanism of the workpiece. Hence, crystal kinematics equation is established using true stress-true strain theory, and the internal force balance and constitutive model of the crystal are analyzed. The proposed theory is applied to finite element processing and experimental analysis of the workpiece, and the subsurface deformation behavior is studied from a mechanistic perspective.The stress model is employed to explore the plastic flow phenomenon of the material during the machining process, and the influence of stress and shear force on the subsurface of the workpiece is discussed. The theoretical analysis of crystal deformation, based on the principles of materials science and mechanics, uncovers the relationship between subsurface deformation curvature and stress. The study delves into the effects of the grinding process on subsurface damage and deformation by analyzing the phenomena of plastic flow, plastic slip, and plastic winding in the subsurface microstructure. The results indicate that at grinding speed vs = 25 m/s and grinding depth ap = 60 μm, the damage to the subsurface deformation layer is minimal. The grinding process considerably impacts the stress exerted on the workpiece, with an amplitude ranging from 1.255 × 103 to 1.286 × 103. Conversely, the strain on the workpiece is relatively less affected by the grinding process, exhibiting an amplitude of 2.366–3.568. Moreover, when the normal stress and shear force of the abrasive particles form 45° angle, the subsurface deformation of DD5 manifests a distinct slip distortion at an approximate 45° curvature.

Design and characterization of BSAS-polyester abradable environmental barrier coatings (A/EBCs) on SiC/SiC composites

This study aims to develop an abradable coating suitable for silicon carbide-based ceramic matrix composites (SiC-CMCs) and further clarify the coating performance. BSAS-polyester abradable environmental barrier coatings (A/EBCs) are designed and prepared on SiC-CMCs using atmospheric plasma spraying technology, which consists of four layers: a Si bond layer, mixed layer (mullite/BSAS), BSAS layer, and additional porous BSAS coating as the abradable layer. The microstructure, bond strength, and rubbing behavior of the coating are characterized. A stable monoclinic-BASA phase is obtained via the heat treatments at 1175–1200 °C, and no phase transformation occurs at 1200 °C for 300 h. The minimum coating hardness is (57.7 ± 2.0) HR45Y with a porosity of approximately 30 % when the polyester content is 8 %. The abradability of CMC and BSAS-polyester A/EBCs with DD6 superalloy at 1100 °C is evaluated by rubbing rig test. Under an incursion rate of 30 μm/s and blade tip speed of 450 m/s, the incursion depth ratio (IDR) of naked CMC is 86.1 %, which is insufficient to provide acceptable abradability to the DD6 superalloy owing to the cracks in the substrate. By contrast, the IDR of the A/EBCs with polyester content of 4 % and 8 % decreases to 69.2 % and 22.4 %, respectively. This study confirmed that BSAS-polyester A/EBCs could improve the abradable performance of CMC significantly at high temperatures up to 1100 °C, and the work can promote the practical application of A/EBCs.

Effect of thermal exposure on subsurface microstructure evolution of nickel-based single crystal superalloy DD5 after milling

Thermal exposure experiments were performed on the second-generation nickel-based single crystal superalloy DD5 under different conditions, and the evolution of microstructure and mechanical properties in the milling subsurface were systematically investigated. The results show that when the thermal exposure temperature is lower than 1000 °C, the thickness of the heat-affected layer is 5–20 μm, which does not affect the performance of DD5. When the thermal exposure temperature is 1100 °C, the milling subsurface of DD5 is composed of a surface recrystallization (SRX) layer and a cellular recrystallization (CRX) layer both with lower nano-hardness than the DD5 matrix. The thicknesses of both SRX and CRX layers increase monotonically with the extension of holding time, and the thickness of the heat-affected layer (composed of SRX and CRX layers) is 20–40 μm. As the thermal exposure temperature increases to 1200 °C, the milling subsurface of DD5 is composed of an SRX structure with large size topologically close-packed (TCP) precipitates and a transition layer with refined γ/γ′ structure and TCP precipitates. The transition layer with higher nano-hardness than other heat-affected layers is formed between the recrystallization region and DD5 matrix, and its thickness increases from 44.5 μm to 66.8 μm with the extension of holding time increasing from 6 h to 24 h. The results will provide an experimental basis and a theoretical reference for the determination of removal allowance in subsequent processing and the applicable service conditions of DD5.

An investigation on the effect of change in experimental sequence of warm laser shock peening and laser shock peening on the performance of DD6 nickel-based single-crystal superalloy

Incorporated with the benefits of dynamic strain aging (DSA) and laser shock peening (LSP), warm laser shock peening (WLSP) is able to induce a compressive residual stress (CRS) layer with high thermal stability on the near-surface of the material. It has been widely used in recent years as a new surface modification technique to prolong the fatigue life of aero-engine turbine blades. Single WLSP has limited effect on material modification, while multiple WLSP when the confining layer is quartz glass will affect the thickness and thermal stability of the CRS layer during replacement of the confining layer, absorbing layer, and cooling and heating. To address this issue, this study investigates the mechanism of the CRS response, mechanism of strengthening, mechanism of action, and change of electrochemical corrosion performance of DD6 superalloy samples to different surface treatments after double LSP, single WLSP plus single LSP and double WLSP surface treatments by changing the experimental sequence of WLSP and LSP.The experimental results show that all three surface treatments deform the sample by inducing 〈110〉 cross-slip bands on the sample surface. Also all three surface treatments strengthen the sample surface by pipeline diffusion of elements to form a dislocation network similar to the γ/γ′ substructure.Meanwhile, single WLSP plus single LSP treatments can effectively prevent the thermal relaxation of the previously induced CRS layer. The final induced CRS layer thickness, dislocation distribution and dislocation density are better than those of double LSP and double WLSP. Pipeline diffusion of elements can improve the electrochemical and thermal corrosion resistance of the single-crystal superalloy.

Study on the chip formation in grinding of nickel-based single-crystal superalloy DD5

Study on the chip formation in grinding of nickel-based single-crystal superalloy DD5

Experimental investigations into effect of milling parameters on milling force and burr micromorphology for Zr-based bulk metallic glass by using groove milling

Zr-based bulk metallic glass is an amorphous alloy. In order to study the burr problem in the groove milling process of Zr-based bulk metallic glass, the milling force in the process is studied experimentally. Then the effect of milling parameters on the burr of Zr-based bulk metallic glass was studied, and the relationship between milling force and burr was analyzed. In order to further study the milling burr of amorphous alloy, under the same test parameters, the single crystal alloy material (nickel base single crystal superalloy DD5) and the traditional polycrystalline metal material (304 stainless steel) were selected for comparative test and analysis. The results show that Zr-based bulk metallic glass has good groove milling machinability. The test results provide some experimental basis for groove milling of Zr-based bulk metallic glass.

Study on milling material removal mechanism and surface integrity of nickel-based single crystal superalloy DD5

Study on milling material removal mechanism and surface integrity of nickel-based single crystal superalloy DD5

The porosity distribution of nickel‐base single‐crystal superalloy DD5 and its fatigue life property

AbstractThe porosity of nickel‐base single crystals (NBSXs) is still an unavoidable casting problem that has a significant influence on the fatigue life of NBSXs. In order to quantify the porosity effect, a porosity‐distribution‐related life prediction method is proposed in this paper. The porosity size in a NBSX was measured, and the size distribution was regressed. The results show that the porosity size that is characterized by Feret's diameter follows a modified logarithmic normal distribution. The relation among the applied stress, porosity size, and lifetime is expressed by using the proposed method. The disperse band of S–N curve is investigated based on the probability distribution of the porosity size. The results show that the predicted S–N curve agrees well with the experimental data.

Effect of remelting solution heat treatment on microstructure evolution of nickel-based single crystal superalloy DD5

The microstructure evolutions of nickel-based single crystal superalloy DD5 after solution treatment at different temperatures as well as heat treated with various routes were systematically investigated by differential scanning calorimeter (DSC), optical microscope (OM), scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The formation and elimination mechanisms of unfavorable incipient melting microstructure emerged during heat treatment were explored in detail. Microstructure observations reveal that a higher solution temperature can speed up the element diffusion, resulting in a more uniform element distribution. A reduced degree of lattice mismatch between γ phase and γ' phase would be obtained, which leads to an increase in the average size and content of γ' phase. However, excessively high solution temperature would cause the pre-melting occurred at the interface of γ matrix and γ' phase, causing the formation of incipient melting microstructure. The incipient melting microstructure can be eliminated by the subsequent homogenization process under the conditions that the solution temperature exceeds the solvus temperature of γ' phase. The formation course of incipient melting microstructure includes the pre-melting at the interface of γ matrix and γ' phase, the expansion of molten liquid, and the formation by the resolidification of molten liquid. The elimination of incipient melting microstructure begins with the dissolution of fine eutectic, followed by the dissolution of coarse γ' phase and the filling of pores, and eventually the dissolution of residual eutectic. Compared with standard heat treatment, remelting heat treatment can provide better homogenization effect of element for DD5 superalloy.

Oxidation behaviors and compressive strength evolution of DD6 Ni-based single-crystal superalloy at 1100 ℃

In this work, the oxidation behaviors of DD6 Ni-based single-crystal superalloy and its compressive strength evolution after oxidation were investigated. The results showed that the thermal exposure time has a significant effect on the oxidation products, oxidation kinetics, Al elements diffusion and compressive strength evolution of DD6 superalloy. Firstly, the relationship between grain size change of (Ni0.9,Co0.1)O, the thickness of γ′-free zone and the exposure time was approximate power exponential law, while the relationship between weight change of DD6 superalloy and exposure time was approximate parabolic law since the densification of inner continuous Al2O3 layers hindered the diffusion of oxygen ions. Moreover, the black phases of AlN in the γ′-free zone formed during the oxidation process, and its quantity and size increased first and then decreased with the oxidation time extending, since the inner Al2O3 not only blocked the ion but also reacted with AlN. In addition, with the increase of oxidation time, the compressive yield strength of DD6 in [001] direction first decreased rapidly and then gradually stabilized at about 740 MPa.

Microstructure evaluation and fracture mechanism of dissimilar diffusion bonded joint of single crystal superalloy DD5 and polycrystalline superalloy GH4169

The reliable joining of DD5 single crystal superalloy and GH4169 polycrystalline superalloy was obtained by diffusion bonding method at 1120 °C for 90–180 min under 10 MPa with or without 730 °C /180 min aging process. The joint was divided into four distinct regions: GH4169 base metal zone, the diffusion zone, the structure affected zone, and DD5 base metal zone. The size, distribution and morphology of γ' phases at the whole joint were distinguishing under different bonding conditions. The morphology of γ' particles changed from nearly cubic shape into irregular sunflower shape, and nearly cubic shape growing up to cubic at the diffusion zone and DD5 base metal zone, which resulted from the operation of additional aging process at 730 °C for 180 min after bonded at 1120 °C for 180 min. Furthermore, the optimal tensile strength of the joint was 1058 MPa when the joint was bonded at 1120 °C/10 MPa/180 min and then heated at 730 °C for 180 min, of which the fracture was occurred at the base metal zone. The in-situ tensile test indicated the damage and fracture behavior of the joint resulted from the increasing slip lines and cracks in carbides.

Study on machining performance in grinding of Ni-base single crystal superalloy DD5

The Ni-base single crystal superalloy DD5 is commonly employed in critical parts of turbine blades and turbine disks in aeronautical engines. To improve the surface machining quality, single-factor experiments were carried out to investigate the changes of grinding force, surface morphology, and chip morphology under different grinding conditions (dry, conventional flood, minimum quantity lubrication). The experiments showed that the conventional flood lubrication and minimum quantity lubrication (MQL) could enormously reduce the surface roughness, grinding force, and work-hardened layer thickness of DD5. At the depth of 5 ~ 15 μm, the DD5 subsurface microhardness decreased sharply with the increase of depth, and the microhardness under different grinding conditions was in the order of dry grinding, conventional flood, and MQL. However, at the depth of 20 ~ 150 μm, the DD5 subsurface microhardness tended to equilibrate and fluctuate around 540 HV. In addition, compared with dry grinding, the degree of chip serration was obvious in flood or MQL environment, and its chip formation frequency fluctuated less and chip formation was stable. Within the experimental parameters, the MQL grinding machining method obtained the lowest surface roughness and the highest surface quality with the lowest associated cost, which is the ideal grinding and cooling lubrication method for the Ni-base single crystal superalloy.

Modeling of burr height in ultrasonic-assisted drilling of DD6 superalloy

Modeling of burr height in ultrasonic-assisted drilling of DD6 superalloy

Influence of γ/γ′ morphology evolution on the mechanical properties of Ni-based single crystal superalloys formed by laser metal deposition

The variation in hardness and tensile properties of Ni-based single crystal (SX) superalloy DD6 during laser metal deposition processing has been investigated, and the effects of laser scanning velocity and post heat-treatment on the evolution of microstructure and its interaction with mechanical properties of deposited region were determined. It is found that the microstructure of as-deposited Ni-based single crystal superalloy mainly consists of columnar dendrite epitaxially growing from the substrate. The fine γ′ phase microstructure and dense γ/γ′ phase interfaces contribute more to the microstructural evolution and mechanical properties of deposited part formed by laser metal deposition. Furthermore, the heat treatment reduces the mechanical properties of deposited sample due to the introducing of stray grains that destroys the original single crystallinity. The mechanism unraveled here could shed light on the reliability evaluation and parameter selection of the laser metal deposition technique and other directed energy deposition technology.

Thrust force model for ultrasonic-assisted micro drilling of DD6 superalloy

Thrust force model for ultrasonic-assisted micro drilling of DD6 superalloy

Quantitative Characterization of the γ’ Phase Distribution in the Large-Scale Area of the Second-Generation Nickel-Based Single Crystal Blade DD5

Nickel-based single crystal superalloy blades have excellent high-temperature performance as the hot end part of the aero-engine turbine. The most important strengthening phase in the single crystal blade is the γ’ phase, and its morphology and size distribution directly affect the high temperature performance of the single crystal blade. In this work, scanning electron microscopy (SEM) was used to obtain the microscopic images of the γ’ phase in multiple large continuous fields of view in the transverse sections of single crystal blades, and the quantitative statistical characterization of the γ’ phase was performed by image segmentation method based on deep learning. The 20 μm × 20 μm region was selected from the primary dendrite arm, the secondary dendrite arm, and the interdendrite to statistically analyze the γ’ phases. The statistical results show that the average size of the γ’ phase at the position of the interdendrite is significantly larger than the average size of the γ’ phase at the position of the dendrite; the sizes of the γ’ phase at the primary dendrite arm, the secondary dendrite arm and the interdendrite all obey the normal distribution; about 3.17 × 107 γ’ phases are counted in 20 positions in the 5 transverse sections of the single crystal blade in a total area of 5 mm2, and the size, geometric morphology and area fraction of all γ’ phases are respectively counted. In this work, the quantitative parameters of the γ’ phases at 4 different positions of the section of the single crystal superalloy DD5 blade were compared, the size and area fraction of the γ’ phases at the leading edge and the trailing edge were smaller, and the shape of the γ’ phase of the leading edge and the trailing edge is closer to the cube.

Study on microstructure and mechanical properties of single crystal/powder superalloy pulsed current diffusion bonded joints

To optimize the overall structure design of the aero-engine turbine disks, the study of pulsed current diffusion bonding on the second generation nickel-based single-crystal superalloy DD5 and the third generation powder superalloy FGH98 was performed, and high-quality joints were obtained after post-welding heat treatment(PWHT). The microstructure and element diffusion were analyzed by using SEM and EDS, and the microhardness and room / high temperature tensile strength were analyzed. It can be seen from the results that with the pulse current, the diffusion bonding produced a high-temperature gradient from the interface to the base metal, which protected the base metal. The precipitation phase of γ′ phase within the connection interface near the FGH98 side within 200 µm exhibits very small granular-large-size unstable petals-larger granular similar to the base metal. The γ′ phase in DD5 grows abnormally at the interface. Large granular γ′ enhancement phases occurred on the FGH98 side after heat treatment with no significant changes on the DD5 side. The microhardness of the joints is not less than 400HV; and from DD5 side to FGH98 side, the hardness decreases first and then increases. The average tensile strength of the joints at room temperature is 1078 MPa, which is 99% of the tensile strength of the base metal with low performance. The average tensile strength of joints at high temperature (650 °C) is 1089 MPa, which is equivalent to the strength of the base metal.