Stray Grains Research Articles

Simulation and experimental investigation of grain structure, residual stress,γ′ phases in single crystal blade

Ni-based superalloy turbine blades have been required components in contemporary aero-engine. Knowing the solidification behavior, residual stress at grain defects, and microstructure of directionally solidified turbine blades is a required condition to improve the service performance of directionally solidified turbine blades. Firstly, the temperature field evolution of the blade under the withdrawal rate of 3 mm/min was studied. The deviations in temperature distribution in the high-rate solidification (HRS) procedure, particularly near the platform, can lead to transformations in the mushy zone, potentially resulting in solidification defects. Secondly, the grain growth of hollow turbine blades was calculated using the cellular automaton-finite factor method. The simulated grain framework was essentially consistent with experimental results. A method of process bar addition based on physical field distribution is also proposed. This method involves designing a combination of one Y-shaped and two I-shaped rods to decrease the cooling rate of blade edges and eliminate stray grains (SG). Then, the residual stress distribution at the locations of stray grains and low-angle grain boundaries (LAGBs) was analyzed before and after the addition of process bars. Finally, discussions were held regarding the distribution of γ′ phases in grain defects and blades.

The Effect of Optimized Substrate Orientation on Layer Step in Laser Metal Deposition of Single-Crystal Nickel-Base Superalloys.

Laser metal deposition is a promising way to repair the surface defects of single-crystal components in turbo engines. Understanding the mechanisms and improving the efficiency of the repair have been long-standing problems. In this study, the influence of the substrate orientation on the laser metal deposition (LMD) was investigated and its effect on repair layer-step was examined. LMD experiments were conducted on single crystal superalloys with a normal substrate orientation (001)/[100] and with an optimized substrate orientation (101)/[101¯]. It reveals that the laser cladding with the optimized orientation leads to a larger height of the [001] dendrite region than that with the normal orientation. The calculated results of the growth velocity, thermal gradient, and susceptibility to CET in the dendrite-preferred growth direction indicate that, for the (101)/[101¯] orientation, the [001]/[100] boundary is located at relative high position in each layer, which not only decreases the formation ability of stray grain significantly, but also eliminates the appearance of the maximum susceptibility. This makes the necessary dilution position much higher, and thus, a large cladding step can be selected. Our findings could find potential applications in laser repair of single-crystal components.

Research on the effect of remelting on the epitaxial growth process of single crystal superalloy

In this paper, the remelting process is applied to Laser Directed Energy Deposition (L-DED) to repair single crystal superalloys. The main formation modes of stray grains in single crystal epitaxial growth were analyzed, and the inhibitory effect of remelting process on stray grains in single crystal epitaxial growth using L-DED was studied. Comparative experiments between remelting and non-remelting processes have been conducted. Three process parameters in the remelting process: power, z-axis lift and cladding-remelting interval time were studied. The impact of different remelting process parameters on single crystal epitaxial growth and the formation of stray grains has been studied. Remelting has been shown to effectively prevent the formation of stray grains during the solidification of the melt pool, which are otherwise caused by the presence of unmelted powder during the powder feeding phase of the L-DED process. A cladding layer without stray grains on both sides is obtained. After remelting, the epitaxial growth structure can achieve larger cellular and columnar dendritic zones and smaller primary dendrite arms. A favorable epitaxial growth structure was obtained in this paper, under the optimized remelting process window.

The Microstructure characteristic and Its influence on the stray grains of Nickel-based single crystal superalloys prepared by Laser directed energy deposition

The microstructure adjustment and the stray grains inhibition are important for the preparation of Nickel-based single crystal (SX) superalloys by laser directed energy deposition (L-DED). In this work, the single channel monolayer and single channel five-layers DD6 SX superalloys have been prepared by L-DED. The macroscopic simulations in the deposition processing and multiscale characterizations of deposition structure were used to analyze the formation and evolution mechanisms of unique precipitations and stray grains. Results shows that the larger laser power and higher powder feed rate have a positive impact on the epitaxial growth of columnar dendrite and the elimination of stray grains. The residual stress caused by the complex thermal cycles is not the direct reason that induce the formation of SGs, but the rapid movement of dislocations result in the formation of sub-grain boundary. According to the columnar transition to equiaxed theory (CET), temperature field simulation and analysis of element segregation behaviors, the element segregation results in the unique microstructure, include petal-shaped γ/γ′ eutectic and demarcation band with a thickness of a single γ/γ′ eutectic. Besides, the geometric size of γ/γ′ eutectic decrease with respect to the deposition height. With the change of height at deposition region, dendritic orientation deviations affect the uniformity of element segregation, resulting in variations in γ′ phase and γ/γ′ eutectic dimensions between dendritic core and the inter-dendrite zone at the microscopic level and manifesting as grain boundaries at the mesoscale. At the same time, the formation mechanisms of carbide and adjacent γ/γ′ eutectic leads to the misorientations between the dendrite core and the inter-dendrite zone, which also appears as stray grains at the mesoscopic scale.

On the control of epitaxial growth and stray grains during laser-directed energy deposited Ni-based single crystal superalloy

Laser-directed energy deposition (L-DED) is considered as an effective repair method for damaged single-crystal (SX) superalloy turbine blades. However, controlling stray grains (SGs) and epitaxial growth is still the major problem. This study investigated the influences of pre-solution treatment and intrinsic heat treatment (IHT) on the formation of SGs and epitaxial growth of L-DED René N5 SX superalloy. The γ/γ’ eutectics and coarse γ’ precipitates at the inter-dendrite region can be eliminated after pre-solution treatment procedures before the L-DED experiment. A lower temperature in pre-solution treatment resulted in the retention of residual γ/γ’ eutectics and coarse γ’ precipitates. Thus, the aggravated element segregation within the local melt pool and interface collapse during the L-DED process can result in stray grains (SGs) formation. Improved pre-solution treatment procedures can also help reduce SGs caused by the carbides in the substrate. Besides, the SG formation in the deposited layers can be attributed to the recrystallization as a result of high thermal stress during the IHT in thermal cycles. The optimized pre-solution treatment procedure reduced the elemental segregation of the substrates, resulting in a larger primary dendrite arm spacing (PDAS) from the bottom of the deposited layers. Due to IHT during thermal cycles in the deposited layers, the equivalent heat treatment process promoted the re-precipitation and growth of γ’ phase. Meanwhile, the optimized pre-solution treatment procedure decreased the size of the heat-affected zone (HAZ) caused by thermal cycles. In summary, by optimizing the substrates' pre-solution treatment procedures to eliminate the γ/γ’ eutectics,coarse precipitaes and decrease the carbides in the subsrtates, the formation of SGs resulting from substrate segregation and heterogeneous microstructure can be controlled. This work provides the potential guidance of substrate pre-treatment for L-DED repair of SX superalloys.

Simulation and Experimental Investigation of a Combined Control Method for Stray Grain During Single Crystal Blade Solidification

Simulation and Experimental Investigation of a Combined Control Method for Stray Grain During Single Crystal Blade Solidification

Orientation control of multiple single crystal blades using a novel high-throughput mold via seeding-grain selection technique

As the increasing demand for high temperature performance of the Ni-based single crystal blades, the orientation consistency of the blades has become more and more important. In this work, a novel high-throughput mold capable of producing four single crystal blades with exactly the same orientation was proposed. The effects of the two important parameters as connector height and blade placement angle on single crystal growth were investigated by ProCAST simulation. The results showed that the 90° connector with a height of 50 mm could obtain the most appropriate temperature gradient for the production of single crystal blades, and when the blades were placed at 120° and 270°, the flattest liquid isotherm could be obtained. However, due to the difference in the distance of the nucleation points from the mold center and the direction where the thin edge of the blade platform faced, the blade placed at 120° had a larger platform area occupied by the stray grains. Therefore, the structure of this mold was determined by using a 90° connector and placing blades at 270°. At last, using this mold, four single crystal blades with the same three-dimensional orientation and no stray grains were successfully produced. These results fully demonstrated the feasibility of this novel mold and also provided ideas for using simulation methods to guide the design of the new mold structure.

Microscale stray grains formation in single-crystal turbine blades of Ni-based superalloys

Microscale stray grains (MSGs), which are composed of misoriented and fragmented high-ordered dendrite arms, small-sized equiaxed grains, and columnar grains, are a novel grain defect that occurs within the shrouds of single-crystal (SC) blades made from Ni-based superalloy. State-of-the-art non-destructive testing methods are incapable of detecting this defect, resulting in a high-risk application of SC blades. This study aims to control this defect by systematically investigating directional solidification processing parameters, shroud dimensions, and rhenium (Re) content in the formation of MSG defects. The results reveal that the defect forms exclusively at the overhanging extremities of the upper shrouds on the back side of the blades that face the heater. Increasing the withdrawal rate reduces the occurrence of MSG defects. As the shroud dimensions increase, MSG defects appear in the transition area between the downward suspended extremity of the blade shroud and the side of the blade body, as well as near the corner of the downward suspended extremity of the shroud. The occurrence of MSG defects increases with increasing shroud dimensions. Varying Re content sharply decreases the formation of MSG defects. A concentration-attached Rayleigh-Taylor instability (RTI) superheating (CARTISH) model, considering solidification shrinkage, was proposed to comprehend the formation of MSG defects. Simulation results based on this model are consistent with the experimentally observed distribution and degree of MSG defects under different conditions. Effective control of the CARTISH is critical to managing MSG defects.

Formation Mechanism of Stray Grain in Laser Remelting Zone of DD6 Nickel‑Based Single Crystal Superalloy

Formation Mechanism of Stray Grain in Laser Remelting Zone of DD6 Nickel‑Based Single Crystal Superalloy

Epitaxial growth of single crystal superalloy by a novel spot-cutting laser cladding method

A spot-cutting laser cladding process was proposed to repair the thin wall single crystal superalloy in this paper. The effects of spot cutting and its gap width on the dimensions and microstructure of cladding layers were studied. The results demonstrate that the cladding layers exhibit directionally epitaxial dendrites throughout the layer, reducing the formation of “stray grains” and significantly decreasing the machining allowance compared to samples without spot cutting. By optimizing the parameters, continuous epitaxial grains were achieved in the cladding layer, reaching a height of 1173 μm. The layer dimensions increased initially with weld height, but then decreased with a flattening morphology as the base metal width increased.

The Stray Grains from Fragments in the Rejoined Platforms of Ni-Based Single-Crystal Superalloy

Nickel-based single crystal superalloy is the most important material for blade preparation. However, some solidification defects inevitably occur during the process of preparing single-crystal blades through directional solidification. In this study, in order to study the origin of misorientation defects during solidification, a model with rejoined platforms was designed according to the geometry of single-crystal guide vanes. Electron Back-Scattering Diffraction (EBSD) was used to quantify the orientation deviation of the dendrites and identify the solidification defects in the rejoined platforms. The results showed that stray grain defects appeared in the platforms and their misorientation changed gradually, not abruptly. Combined with the simulation results, it was proposed that the stray grains formed as the result of the dendrites fragment, which was induced by solute enrichment in the mushy zone during solidification. Meanwhile, it was accompanied by a obvious dendritic deformation, which was caused by solidification shrinkage stress. This suggested that the fragmentation was induced by multiple factors, among which, the concave interface shape provided favorable conditions for solute enrichment, and the dynamic variability in the local thermal gradient and fluctuations of the solidification rate might play catalytic roles.

Effect of pulling speed on directional solidification structure of the single crystal blade

In this paper, the influence of withdraw speed on the directional solidification structure of the single crystal blade was studied. The influence of 2.7 mm/min, 5.4 mm/min, and 9.5 mm/min drawing speed on temperature field, temperature gradient, and microstructure were analyzed. The results show that as the casting speed increases, the temperature gradient G decreases, the degree of downward depression of the temperature field also increases, and more stray grains appear at the single crystal blade platform. In addition, it is also found that the increase in the withdrawal speed helps to reduce the secondary dendrite arm spacing and refine the grains. This paper provides a theoretical basis for optimizing the directional solidification process of single crystal blades.

Numerical simulation of directional solidification process of single crystal blade based on precast

A finite element method was applied to establish a mathematical model of a single crystal blade’s directional solidification process. The model had different angles between the axis of the mold group and the blade. The effects of different radiation angles on temperature variation laws at different blade positions were investigated. The temperature near the center was lower than that near the wall on the upper side of the baffle, while it was higher on the lower side. Moreover, the grain growth of stray particles in blade casting defects was influenced by blade orientation. The grain growth of scattering particles in blade casting defects was faster. The isolated solid-liquid mixing zone was more prone to form at the platform tip near the center when the blade orientation angle was smaller. The 0° orientation angle resulted in the blade’s most severe stray grain phenomenon.

In situ observation of crystal rotation in Ni-based superalloy during additive manufacturing process

Understanding the dynamic process of epitaxial microstructure forming in laser additive manufacturing is very important for achieving products with a single crystalline texture. Here, we perform in situ, real-time synchrotron Laue diffraction experiments to capture the microstructural evolution of nickel-based single-crystal superalloys during the rapid laser remelting process. In situ synchrotron radiation Laue diffraction characterises the crystal rotation behaviour and stray grain formation process. With a complementary thermomechanical coupled finite element simulation and molecular dynamics simulation, we identify that the crystal rotation is governed by the localised heating/cooling heterogeneity-induced deformation gradient and recognise that the sub-grain rotation caused by rapid dislocation movement could be the origin of granular stray grains at the bottom of the melt pool.

Epitaxial Growth and Stray Grain Control toward Single‐Crystal Metallic Materials by Additive Manufacturing: A Review

Additive manufacturing (AM) possesses tremendous potential in controlling epitaxial growth of deposited materials, and thus attempts have been made to employ AM technology to fabricate or repair single‐crystal (SX) metal parts in recent years. One of the key issues in producing SX structures is the elimination of stray grain (SG) formation, but the layer‐wise fabrication nature of AM processes makes SGs difficult to avoid. Herein, the state‐of‐the‐art developments in this emerging research area are surveyed. It introduces the fundamental mechanisms of epitaxial growth in AM, summarizes the recent innovations and findings on using AM techniques to assist epitaxial growth and avoid SGs for SX metallic materials, discusses the relation of stray‐grain avoidance and crack‐free microstructure, and reviews the analytical and numerical modeling efforts for epitaxial growth and texture control. In addition, the challenges in achieving defect‐free SX microstructure are discussed, such that future research directions are pointed out. It is believed that this critical review provides insights on microstructure control for AM‐produced metallic materials and lays a solid foundation to stimulate more rigorous research.

Influence of macroscopic interface curvature on dendritic patterns during directional solidification of bulk samples: Experimental and phase-field studies

The envelope of the solidification front naturally develops a macroscopic curvature in a multitude of solidification processes. However, its effect on dendritic microstructure formation remains poorly understood. Here we exploit a microgravity environment where convection is suppressed to investigate quantitatively the effect of curvature on dendritic array growth during directional solidification of a transparent succinonitrile – 0.46 wt% camphor alloy. In addition, we interpret the results using both theoretical analyses and phase-field simulations. In situ observations reveal that even a weak macroscopic interface curvature can have a major effect on both the array pattern evolution and grain structure. First, convex and concave interfaces lead to a continuous increase and decrease in time of the average primary spacing, respectively, which only attains a stationary value for a flat interface. We show that this results from the formation of drifting velocity gradients along the interface under the combined effect of the crystal misorientation and the interface curvature. Second, interface concavity is accompanied by “stray grains” of larger misorientation. These grains form at sample boundaries and overgrow pre-existing interior grains of smaller misorientation to the detriment of well-oriented crystal growth. In addition, they induce a change of array structure when the direction normal to the sample boundary is parallel to one of <100> axis of secondary branches. These results provide new insights for controlling and optimizing directionally solidified alloy microstructures.

Comparative Investigation of the Undercooling Capacity and Single-Crystal Castability of Some Ni-Based Superalloys

The undercooling capacity of a superalloy is an essential physical property to determine its single-crystal (SC) castability, because stray grains (SGs) will be formed if the geometrical undercooling established at the platform extremities exceeds the undercooling capacity of the applied alloy. In the present work, both the undercooling capacity of eight Ni-based superalloys and their SC castability were experimentally investigated. The liquidus temperature, the critical temperature for grain nucleation, and hence the undercooling capacities of the investigated alloys were evaluated based on the temperature evolution during the heating and cooling processes. The current experimental study revealed a significant difference in undercooling capacity for the superalloys. In the production of SC blade castings, the tendency to form SG defects was found to be highly related to the alloy’s undercooling capacity. The alloys having a low undercooling capacity of around 10 K were very prone to the formation of SGs. In comparison, the alloys with a moderate undercooling capacity from 20 K to 30 K could be easily cast into SC blades without SGs, exhibiting the best SC castability. Other factors influencing the SG formation were also analyzed. As a result, a criterion for the formation of geometry-related SG defects was proposed, in which the influence of the alloy undercooling capacity, casting geometry, and solidification condition are involved.

Quantifying Equiaxed vs Epitaxial Solidification in Laser Melting of CMSX-4 Single Crystal Superalloy

The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as practiced in additive manufacturing. Single-track laser scans were performed on a powder-free surface of directionally solidified CMSX-4 alloy with several combinations of laser power and scanning velocity. Electron backscattered diffraction (EBSD) mapping facilitated identification of new orientations, i.e., “stray grains” that nucleated within the fusion zone along with their area fraction and spatial distribution. Using high-fidelity computational fluid dynamics simulations, both the temperature and fluid velocity fields within the melt pool were estimated. This information was combined with a nucleation model to determine locations where nucleation has the highest probability to occur in melt pools. In conformance with general experience in metals additive manufacturing, the as-solidified microstructure of the laser-melted tracks is dominated by epitaxial grain growth; nevertheless, stray grains were evident in elongated melt pools. It was found that, though a higher laser scanning velocity and lower power are generally helpful in the reduction of stray grains, the combination of a stable keyhole and minimal fluid velocity further mitigates stray grains in laser single tracks.

Microstructure characteristics of a René N5 Ni-based single-crystal superalloy prepared by laser-directed energy deposition

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

Laser deposited superalloys with rotated substrate orientations: Microstructures and single-crystal formation

One- and multi-layer laser powder depositions were performed on single crystal (SX) Ni-based superalloy DD6 by rotating the substrate orientation around the [100], [010], and [001] crystallographic directions with the angle ξ[hkl] (hkl = 100, 010, 001) from the initial orientation (001)/[100], and the substrate orientation effect on the microstructure and SX nature were determined. It shows that for the one-layer deposition, the substrates with different rotation manners produce various dendritic domain distributions and stray grain (SG) susceptibilities in the deposited bead. Remarkably, one epitaxial dendrite domain can be only obtained for the [010]-axis rotation with ξ[010] = ±45°, which can suppress the formation of SGs effectively. However, at least two dendrite domains exist for [100]- or [001]-axis rotations, leading to slight dependence of SG susceptibility on the orientation. The multi-layer deposition experiments reveal that the SX nature can be retained with ξ[010] = 45°. The underlying mechanism of the orientation-dependent SG susceptibility is that different substrate orientations cause various columnar-to-equiaxed transition (CET) sensitivities. The difference in SG susceptibility between laser deposition and laser remelting is also discussed. Our results can provide an in-depth insight into controlling SG via substrate orientation in the laser processed SX superalloy.