Stray Grains Research Articles

Multi-scale characterization of stray grain in the platform of nickel-base single crystal turbine blade

Through comparatively characterizing the platforms with and without stray grain in multi-scale levels, it is determined that stray grain (SG) originates from heterogeneous nucleation with randomly distributed orientations with respect to the matrix grain. This independently nucleated deleterious grain constitutes distinct macro, dendritic and γ′ morphologies and forms a high angle boundary (HAB) in the conjunction area of SG and the matrix grain. Compositional distribution reveals that severe segregation could be identified in GB area due to compositional enrichment and deprivation during the last stage of solidification.

Effects of Substrate Crystallographic Orientations on Microstructure in Laser Surface-Melted Single-Crystal Superalloy: Theoretical Analysis

A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed. And the effect of welding conditions on the tendency of stray grains formation during solidification was researched. In the present work, these analytical methods were further extended. Combined with an original vectorization method, a 3D Rosenthal solution was used to determine thermal conditions of the welds. Afterward, the dendrite growth orientation, the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted. Finally, the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot. The results indicate that substrate orientation has some impacts on the crystal growth pattern, dendrite growth velocity, distribution of thermal gradient and stray grain. Based on the research methods proposed in this work, any substrate crystallographic orientation can be studied, and predicted stray grains distribution can be visualized.

Prediction of Dendrite Orientation and Stray Grain Distribution in Laser Surface-melted Single Crystal Superalloy

A vectorization analysis technique for crystal growth and microstructure development in single-crystal weld was developed in our previous work. Based on the vectorization method, crystal growth and stray grain distribution in laser surface remelting of single crystal superalloy CMSX-4 were investigated in combination of simulations with experimental observations. The energy distribution of laser was taken into consideration in this research. The experimental results demonstrate that the simulation model applies well in the prediction of dendrite growth direction. Moreover, the prediction of stray grain distribution works well except for the region of dendrites growing along the [100] direction.

Grain structure evolution in Inconel 718 during selective electron beam melting

Selective electron beam melting (SEBM) is an additive manufacturing method where complex parts are built from metal powders in layers of typically 50µm. An electron beam is used for heating (about 900°C building temperature) and selective melting of the material. The grain structure evolution is a result of the complex thermal and hydrodynamic conditions in the melt pool. We show how different scanning strategies can be used to produce either a columnar grain structure with a high texture in building direction or an equiaxed fine grained structure. Numerical simulations of the selective melting process are applied to study the fundamental mechanisms responsible for differing grain structures. It is shown, that the direction of the thermal gradient during solidification can be altered by scanning strategies to acquire either epitaxial growth or stray grains. We show that it is possible to locally alter the grain structure of a part, thus allowing tailoring of the mechanical properties.

Effect of a High Magnetic Field on Microstructures of Ni-Based Single Crystal Superalloy During Seed Melt-Back

The effects of a high magnetic field on microstructures during seed melt-back of superalloy were investigated. Experimental results indicated that the high magnetic field significantly modified the melt-back interface shape and the melt-back zone length. In addition, stray grain on the edge of sample was effectively suppressed in the high magnetic field. Based on experimental results and quantitative analysis, the above results should be attributed to the increasing temperature gradient in a high magnetic field.

Effect of a High Static Magnetic Field on the Origin of Stray Grains during Directional Solidification

Effect of a High Static Magnetic Field on the Origin of Stray Grains during Directional Solidification

Effect of substrate orientation on the formation of equiaxed stray grains in laser surface remelted single crystal superalloys: Experimental investigation

Substrate orientation could play a key role in the stray grain (SG) formation in laser welded single crystal (SX) alloys, which is critical to a successful repair of SX components. In a previous work [Acta mater 2015; 88:283–292], we studied theoretically the substrate orientation effect on equiaxed SG susceptibility in laser melt pools by altering the orientation via rotations around x-, y-, and z-axis, which coincide with the [100], [010], and [001] crystallographic directions, respectively. It was found that y-axis rotation leads to a strong variation in SG ability whereas x- and z-axis rotations nearly have no effect. In this study, experiments were conducted to verify the calculated results. DD6, a second-generation Chinese SX superalloy was chosen as the object of research. The orientation-dependent SG formation was determined and the area-average volume fraction of SGs was employed to evaluate the overall CET tendency. The experimental tendency of the orientation dependent SG agrees well with the theoretical prediction. The effect of the substrate orientation on SG formation was further revealed by cosψ distribution, where ψ is a geometrical parameter which is the angle between normal vector to the solidification front and preferred crystallographic direction in dendrite domain. Our results provide an in-depth insight into the control of laser processing window for SX repair via adjusting the substrate orientation.

Control of crystal orientation and continuous growth through inclination of coaxial nozzle in laser powder deposition of single-crystal superalloy

The effects of inclining angle of the coaxial nozzle in the longitudinal section of deposited bead on the molten pool geometry and the corresponding crystal growth in laser powder deposition of single-crystal superalloy are studied through the coupling of a numerical FLUENT program and a three-dimensional transient transport phenomena mathematical model. Systematical experiments with single-crystal nickel-based superalloy were conducted to verify the computational results. The results show that the inclination angle of the coaxial nozzle in the longitudinal section of deposited bead has a predominant effect on the molten pool geometry and the solidified microstructure. The inclination of coaxial nozzle reduces the height of the molten pool while increases the melting depth compared to the normal-direction deposition. The epitaxial grain growth in the deposited bead is enhanced when the coaxial nozzle inclines toward the laser scanning direction, while it is restrained as the nozzle inclines toward the opposite direction. When the coaxial nozzle inclines to a +45°, the ratio of melting depth to the height of equiaxied stray grain on the top of the previous layer of deposited bead exceeds 1.0, which implies that the deposited layer can completely remelt the stray grains in the previous layer. The capacity of continuous-epitaxial grain growth can be therefore achieved through the nozzle inclination effectively. This method can be used to optimize and control the processing parameters and broaden the processing window for a single-crystal turbine blade tip repair.

The process analysis of seeding-grain selection and its effect on stray grain and orientation control

A combined technique denoted as seeding-grain selection (SGS) was designed for fabricating single crystal turbine blades. The microstructural evaluation during SGS process was metallographically analyzed in both starter section and spiral section of this technique. Four zones with distinct dendritic morphologies were identified in seed melt back region. After epitaxial propagating from melted seed and branching through the spiral passage, the lined-up dendritic pattern was observed at the outlet of the passage. The effectiveness of the technique on stray grain and orientation control was also assessed by Electron Back-Scattering Diffraction (EDSD) technique. The results indicated that the technique can control stray grain with no detrimental effect on crystal orientation.

A synchrotron study of defect and strain inhomogeneity in laser-assisted three-dimensionally-printed Ni-based superalloy

Synchrotron X-ray microdiffraction was employed to investigate the inhomogeneous distribution of defect and residual strain in the transitional region between the dendritic and stray grains in a laser-assisted 3D printed Ni-based superalloy. The dendritic region was found to be under tensile strain transversely to the primary dendrite arm directions. The dendrite edges, where high level of strains and geometrically necessary dislocations were detected, were discerned as low angle grain boundaries. High angle grain boundaries were observed in the stray grain region, and the orientation of the strain tensor in this region varied dramatically at the micron scale, in contrast with the more or less homogeneous distribution in the dendritic region.

Investigation of the Undercoolability of Ni-Based Alloys Using High Temperature Thermal Analysis

During the single crystal (SX) solidification of turbine blades, grain defects can form in the platform regions which have abruptly varying cross-sections. A high undercoolability of a Ni-based alloy prevents the growth of stray grains in the thermally undercooled platform area. To evaluate the undercoolability of different Ni-based alloys, temperature measurements were conducted using the same thermal conditions in an Al2O3-SiO2 investment casting shell mold system. Furthermore, the results were compared with stray grains in directionally solidified components.

A synchrotron study of microstructure gradient in laser additively formed epitaxial Ni-based superalloy

Laser additive forming is considered to be one of the promising techniques to repair single crystal Ni-based superalloy parts to extend their life and reduce the cost. Preservation of the single crystalline nature and prevention of thermal mechanical failure are two of the most essential issues for the application of this technique. Here we employ synchrotron X-ray microdiffraction to evaluate the quality in terms of crystal orientation and defect distribution of a Ni-based superalloy DZ125L directly formed by a laser additive process rooted from a single crystalline substrate of the same material. We show that a disorientation gradient caused by a high density of geometrically necessary dislocations and resultant subgrains exists in the interfacial region between the epitaxial and stray grains. This creates a potential relationship of stray grain formation and defect accumulation. The observation offers new directions on the study of performance control and reliability of the laser additive manufactured superalloys.

EXPERIMENTAL AND SIMULATION STUDY OF DIRECTIONAL SOLIDIFICATION PROCESS FOR INDUSTRIAL GAS TURBINE BLADES PREPARED BY LIQUID METAL COOLING

Advanced aero and power generation industry needs high-performance gas turbine. As key parts of gas turbine directionally solidified (DS) columnar grain and single crystal (SX) blades operate in heavy stress and high temperature conditions. The continuous demand for increasing turbine inlet temperature and aggressive environment has pushed alloy designers to develop DS and SX Ni-based blade alloys that contain high amount of alloying elements. DS process of blades using such alloys has become a challenging task. The small DS and SX blades are usually produced by high rate solidification (HRS) process. However, the growth of large DS and SX blades requires directional solidification with a sustained thermal gradient along the DS direction. By increasing the thermal gradient, the dendrites are refined, which results in a mechanically-superior DS and SX with reduced defects. One method to achieve consistent and higher thermal gradients is the utilization of the liquid metal cooling (LMC) process. In this method, heat extraction from the outer surface of the mold during DS relies on heat conduction rather than radiation in the conventional HRS process. The optimization of the LMC process is difficult and costly by experimental methods, especially for the complexly shaped industry gas turbine (JOT) blades because of the complicated process parameters associated with the technique. Numerical simulation is an efficient method to solve this problem. In this work, directionally solidified industry gas turbine hollow blades were prepared by high gradient LMC process. Liquid Sn was used as cooling medium. The temperature fields, macrostructures, primary dendrite arm spacing (PDAS) at various withdrawal rates during LMC process have been calculated with ProCAST software. The impact of withdrawal rate on formation of stray grains and freckles was predicted. The calculated results and the experimental observations agreed well. The solidification rates and cooling rates were found to increase with the increase of withdrawal rate. The axial thermal gradient was high and stable during the LMC process. It was found that stray grains would not block the growth of original grains at optimized withdrawal rate. No freckles were observed in the industry gas turbine hollow blades prepared by LMC technique due to the high cooling rate. Though the mean diameters of columnar grains in LMC blades were almost identical to that observed in HRS blades, the PDAS were more than 50% refined in LMC blades than those in HRS blades.

Lamellar Microstructure Alignment in Ti-47Al Alloy by Electromagnetic Confinement and Directional Solidification Using a Seed

As a promising electromagnetic process to obtain TiAl samples without contamination, electromagnetic confinement and directional solidification was successfully applied for lamellar microstructure control in Ti-47Al alloy. Seeded by a Ti-43Al-3Si seed, columnar α grains grew stably and the lamellae within these grains were aligned parallel to the growth direction. However, stray α or β grains, in which the lamellae were complex, nucleated and grew in the sample edge. The possible reasons for the formation of stray grains were discussed by analyzing the temperature gradient at the solid/liquid interface and the macrosegregation of Ti5Si3 particles and Al solute along the sample radius. Moreover, the fluid flow induced by electromagnetic force, which led to the macrosegregation, was also discussed by using a simple model.

Experimental Evidence of the Effect of a High Magnetic Field on the Stray Grains Formation in Cross-Section Change Region for Ni-Based Superalloy During Directional Solidification

The effect of a high magnetic field on the stray grains in cross-section change region was investigated experimentally during directional solidification of superalloy. The microstructures showed that the high magnetic field significantly suppressed the stray grains formation in cross-section change region. Meanwhile, the temperature curves indicated that the nucleation undercooling was significantly increased in a high magnetic field. The effect of a high magnetic field on the stray grains was discussed based on the nucleation mechanism.

Detection of an Intermediate Layer Containing a Rhenium-Rich Particle at Grain Boundaries Formed Within Single Crystal Nickel-Based Superalloys

A stray grain exposed on the surface of a Re-containing single crystal Ni-based superalloy turbine blade was examined by high-resolution analytic electron microscopy. An intermediate layer with the size range of 3 to 4 μm, composed of elongated γ′ phase, was clearly detected forming a boundary between a normal matrix grain and a stray grain. Beyond the intermediate layer, the {100} direction of the γ′ phase in the matrix changed slightly, and a stray grain was formed. In each γ′ grain in the intermediate boundary, a Re-rich region of the size range of 100 to 200 nm was detected, suggesting a role for rhenium in the formation of stray grains.

The Wyckoff positional order and polyhedral intergrowth in the M3B2- and M5B3-type boride precipitated in the Ni-based superalloys

Ni-based single superalloys play a crucial role in the hottest parts of jet engines. However, due to the complex geometry and macro-segregation during the solidification process, the cast defect such as stray grains is inevitable. Therefore, the transient liquid phase (TLP) bonding which can join several small single crystalline castings together is gradually believed to be an effective method for improving the yields of production of the complex components. The melting point depressant element B is always added into the interlayer filler material. Consequently, borides including the M3B2 and M5B3 phase usually precipitate during the TLP bonding process. So a comprehensive knowledge of the fine structural characteristics of the borides is very critical for an accurate evaluation of the TLP bonding process. In this work, by means of the aberration-corrected transmission electron microscopy, we show, at an atomic scale, the Wyckoff positional order phenomenon of the metal atoms in the unit cell of M3B2- and M5B3-type boride. Meanwhile, the defect along the (001) plane of the above two types of boride are determined to be the polyhedral intergrowth with complex configurations.

Mathematical Modeling of Crystal Growth and Microstructure Formation in Multi-layer and Multi-track Laser Powder Deposition of Single-crystal Superalloy

A self-consistent three-dimensional mathematical model was developed to predict the crystal growth and microstructure formation in multi-layer and multi-track laser powder deposition of single-crystal superalloy. Laser powder deposition experiments with nickel-based single-crystal superalloy were conducted to verify the computational results. The results indicated that the overlapping ratio was the key parameter to ensure the continuity and consistency of the epitaxial columnar dendrite growth. Both small and large overlapping ratios lead to residual stray grain region of the previously deposited layer. Scanning with alternating X and Y direction method produces better result with the continuous columnar dendrite growth compared to that of the one direction scanning method. With proper deposition parameters and the alternating scanning method, the epitaxial growth of columnar dendrite microstructure can be achieved in a multi-track and multi-layer deposit.

Method of Stray Grain Inhibition in the Platforms with Different Dimensions During Directional Solidification of a Ni-Base Superalloy

A model of typical turbine blade shape with different platforms was designed to study the method of stray grain inhibition in the platforms by both experimental investigation and a ProCAST simulation based on a Cellular Automaton Finite Element model. The results show that stray grains with random orientations increasingly nucleate and grow in these platforms with the increase of platform dimension. To inhibit these stray grains, assistant bars are designed to introduce the primary grain into these platforms according to the theory of seeding method. The results indicate by the help of these assistant bars, the primary grain is introduced to the large dimensional platforms by developing secondary and tertiary dendrites, which form “crisscross” structure in these platforms. And, the stray grains in the large dimensional platforms are successfully and effectively inhibited. Further more, it is found that the thermal condition and the solidification sequence are changed in the large dimensional platforms by the use of assistant bars, which result in the introduction of primary grain and the inhibition of stray grains. Besides, the simulation results are in accordance with experimental findings.

Formation of stray grains during directional solidification of a superalloy AM3

Stray grains were found during the preparation of single crystal superalloy AM3 by seeding technique and the researching on the competitive growth of bi-crystal. Stray grains were mainly observed at the diverging 〈001〉 corner of the mold wall. Therefore, increasing orientation deviation angle would intensify the possibility of the formation of stray grains. This was because the solute was inclined to enrich at the diverging 〈001〉 corner of mold wall, leading to the relatively large undercooling, and accordingly resulted in the formation of stray grains.