Formation Of Stray Grains Research Articles

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.

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.

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.

Elimination of the Stray Grain Defects of Single Crystal Blade by Variable Wall Thickness Based on Integral Ceramic Mold

The stray grain defect is often found at the platform in the process of directional solidification of single crystal blades because local undercooling easily occurs in this area. A variable wall thickness mold based on stereolithography and gelcasting technology was proposed to solve the local undercooling at the platform. The influence of variable wall thickness mold on the temperature field of the platform during directional solidification was studied via simulation. The numerical simulation results show that variable wall thickness can effectively prevent heat dissipation at the platform, thereby reducing the undercooling and avoiding the formation of stray grains. At the same time, the influence of molds assembly angle on the formation of stray grains was analyzed, and the appropriate molds assembly angle is suggested.

Temperature Field Evolution of Seeding during Directional Solidification of Single-Crystal Ni-Based Superalloy Castings

By combined numerical simulations and unidirectional solidification experiments, the temperature field evolution of seeding for nickel-based single-crystal superalloys was studied. At the steady state, the position of the mushy zone in the seed moves down as the melt is poured into the mold. The holding time required for the temperature field of the seed segment to reach the steady state is less for seeds with casting than for those without casting. The holding time required to completely eliminate randomly oriented broken dendrites in the melt-back zone is much longer than that required for the temperature field to achieve a steady state. A short incubation stage is required before the temperature field evolution process; then, the migration rate of the isotherm gradually increases until it is the vicinity of the withdrawal rate. Finally, the effect of temperature field evolution on the formation of stray grains in seed segments is discussed.

A new strategy to inhibit stray grain formation during laser directed solidification of single crystal superalloys

Although laser surface remelting displays excellent potential for repairing the nickel-based single crystal (SX) superalloy components, controlling the grain structure while avoiding the stray grain (SG) formation is still a challenge in the industry. In order to inhibit SG formation for nickel superalloys, a novel rotated laser irradiation approach is introduced in this paper and the study of the origin of the SG formation in laser directed solidification process. Besides, a numerical model of combined alloy composition with the tendency of equiaxed grain formation during epitaxial growth processing was developed in this paper to reveal the mechanism of SG formation at the bottom of the molten pool. Results indicate that SGs formed at the bottom of the molten pool are mainly due to the remarkably increased possibility of the equiaxed grain transformation with the rough fusion interface. During the laser process, the number of SGs gradually decreases with the increase of the angle (θ) between the laser irradiation and gravity direction. For θ = 90°, well-aligned epitaxial growth of [001] dendrites without any SGs are observed in the pool with the smooth fusion interface. Furthermore, applying the new laser deposition technique based on this phenomenon during laser directional solidification will be an effective method to control the grain-oriented growth in industrial manufacture for the single crystal components.

Effects of melt convection on stray grain formation in single crystal superalloys during directed energy deposition

During laser repairing of Ni-based single crystal (SX) superalloys, stray grain (SGs) formation leads to a deterioration of high-temperature properties and hot cracking. The main goal is to prevent SGs formation from forming, which controls the local solidification front thermal gradient ( G ) and solidification velocity ( V ). Meanwhile, the melt pool geometry, which is influenced by melt convection, has a significant impact on G and V . Here, experiments and theoretical calculations were used to study the evolution of the melt pool geometry during melt convection and its effects on the formation of SGs. Melt convection, i.e., Marangoni convection, outperformed thermal conduction in the SX superalloys melt pool during directed energy deposition, as evidenced by the estimated Peclet number and Prandtl number. The integrative impact of laser power, scan velocity, and beam diameters exacerbated Marangoni convection as the laser energy density distribution increased. Marangoni convection caused the melt pool shape to change from a near-semicircle to an undulating morphology, promoting the oriented-to-misoriented crystal growth transition and resulting in high stray grain sensitivity. The heat and mass transport in front of the dendrite tips was altered by Marangoni convection, which aggravated the deviation of the epitaxial dendrite orientation with respect to the substrate. This work provides an in-depth insight into controlling SG formation during additive manufacturing of SX superalloys. • The melt pool geometry transformed from a near semicircle to an undulating morphology by Marangoni convection. • The interaction between the thermal conduction and thermal convection was calculated for SX superalloys. • The mechanism of the melt pool geometry evolution resulting from Marangoni convection was proposed. • Marangoni convection promoted the oriented-to-misoriented transition leading to high sensitivity of stray grains. • Marangoni convection aggravated the epitaxial growth deviation by altering the heat and mass transfer.

Effect of mould baffle technology on stray grain formation in single crystal blades by integral fabrication based on 3D printing

Stray grains, the most serious casting defect, mainly occur in the platform because of the abrupt transition of the cross-section in the directional solidification of superalloy single-crystal blades. A new mould baffle technology based on 3D printing and gelcasting is proposed herein to reduce the formation of stray grains in the platform. The influence of the proposed mould baffle technology on the temperature field in the platform during solidification was investigated by simulation and experiment. The numerical simulation results indicate that the proposed mould baffle technology can effectively hinder the radiation and heat dissipation at the platform extremities, and therefore, reduce undercooling in the platform and the formation of stray grains during directional solidification. Casting trials of a hollow turbine blade were conducted using CMSX-4 superalloy. The trial results demonstrate the potential of the proposed approach for manufacturing single-crystal superalloy blades.

Influence of platform position on stray grain nucleation in Ni-based single-crystal dummy blade clusters

Stray grains are the most severe of the solidification defects that occur in the industrial single-crystal blade preparation process. In this study, a single-crystal dummy blade cluster with different crystal orientations controlled by the seeding method was prepared, and the influence of the position of the circular platform (relative to the sample and furnace body) on stray grain nucleation was investigated. Results show that the microstructure of the circular platforms could be divided into the center, expansion, and stray grain regions. The inside of the circular platform facing the center of the cluster is more prone to stray grain formation than the outside of the circular platform facing the furnace body. With an increase in the distance between the circular platform and the bottom of the dummy blade cluster, the stray grain region expands, whereas the expansion region narrows. The stray grain is slightly aggravated with increase of the misorientation. Finally, the mechanism underlying the influence of platform position on the formation of stray grains in single-crystal dummy blade clusters is discussed based on the temperature evolution during directional solidification.

Formation mechanisms and control method for stray grains at melt-back region of Ni-based single crystal seed

An experimental study together with numerical simulation was conducted to investigate the formation mechanisms and control methods for stray grains at melt-back of seed in this paper. The numerical simulation results showed that the convex solid/liquid (S/L) interface changed very slowly into a concave interface during the initial solidification stage and the undercooling was slightly affected by the S/L curvature change. The formation of stray grains below melt-back interface at seed perimeter could be suppressed with the decrease of clearance between the seed and mold. However when the clearance was further decreased to 0, an orientation deviation layer formed originated from the deformation of un-melted seed caused by different expend coefficient between the seed and mold. The nucleation of stray grains above the melt-back interface at the seed perimeter could be suppressed using the mold with small surface roughness. The isolated un-melted dendrite stem at semi-solid mushy of the seed with the large original primary dendrite spacing transfer to complex network structure, suppressing the formation of the stray grains, when the primary dendrite spacing of seed decreased or the seed underwent solution heat treatment. Then, the formation mechanisms of the stray grains at the melt-back region are discussed based on above results, and a method is proposed to suppress the formation of the stray grains for preparing single crystal components by re-used seed.

Effect of substrate cooling on the epitaxial growth of Ni-based single-crystal superalloy fabricated by direct energy deposition

The columnar-to-equiaxed transition (CET) or the formation of stray grains in the laser melting deposition is the least desirable for the repair of single-crystal blades. In this work, the forced water-cooling was conducted on a single-crystal René N5 substrate during the direct energy deposition (DED). The single track remelting, one-layer, two-layer, and eight-layer depositions were investigated to explore the grain growth mechanism. The solidification conditions of the DED process, including temperature field, temperature gradient, and solidification speed, were numerically analyzed by a finite element model. The single-track remelting results showed that the fraction of columnar crystal regions increases from 55.81 % in the air-cooled sample to 77.14 % in the water-cooled one. The single-track deposits of one- and two-layer have the same trend, where the proportion of columnar crystal height was higher under the forced water-cooled condition. The electron backscattered diffraction (EBSD) grain-structure maps of an eight-layer deposit show that the epitaxial growth height increases from 1 mm in the air-cooling sample to 1.5 mm in the water-cooling one. The numerical results showed that the temperature gradient in [001] direction was significantly increased by using forced water-cooling. In conclusion, the in-situ substrate cooling can become a potential method to promote epitaxial growth during DED via the influence on CET occurrence.

Epitaxial laser deposition of single crystal Ni-based superalloys: Repair of complex geometry

Abstract A multi-track single crystal structure produced by laser cladding can be succeeded on a flat but failed on complex geometry for single crystal superalloys repair. It was always accompanied with stray grains and cracks when the complex shape defects were repaired by laser cladding. According to analysis, the geometrical profile of the defect may change the heat dissipation condition during laser cladding so that it promotes to form stray grains locally. In this paper, the off-axis heat flow angle (β) which was defined by the heat flow direction and the dendritic growth direction, was used to evaluate the formation of stray grains. Results showed that dendrites grew epitaxially along the preferred crystal orientation which was closer to heat flow. In addition, stray grains could not be formed in the epitaxial columnar dendrite region until the β was more than the critical value of 30°. Two preferred orientations competed for growth perpendicular to each other when the β was close to 45°. Therefore, the repair principle was proposed that the β was not more than 30° ensured by geometrical profile design, and a v-groove defect was successfully guided and repaired.

Epitaxial laser deposition of single crystal Ni-based superalloy: Variation of stray grains

A successful repair of single crystal Ni-based superalloy components requires maintaining the stability of the laser cladding process. So, it is critical to understand the relationship between stray grains and the actual repair condition including cooling condition, laser beam scanning pattern and unstable powder feeding. In this study, the variation of stray grains was investigated in terms of volume, crystal orientation, and distribution through experimental approaches in the laser cladding of single crystal superalloy. The results indicated that the formation of stray grains, low angle boundaries, and dendritic deflections or rotations could be suppressed on the condition of continuous substrate cooling and unidirectional laser beam scanning. In particular, a single crystal deposit with no stray grains was fabricated in a liquid nitrogen cooling condition. Besides, the dendritic deflection and rotation, which were harmful to epitaxial growth, were affected by the laser scanning pattern. Additionally, it was proved that stray grains preferably appeared at the entrance of powder into the molten pool. The offset of powder relative to the laser beam reshaped the distribution of stray grains in the molten pool, which would provide a new strategy to repair single crystal with laser cladding.

Inhibition of stray grains at melt-back region for re-using seed to prepare Ni-based single crystal superalloys

Assisted by the mold preinstalled an alumina tube into seed segment, the influence of the original primary dendrite spacing of seed on the formation of stray grains at melt-back region was investigated during Ni-based single crystal casting using seeding method. The results showed that the interface reaction between the seed and mold as well as the formation of stray grains at the surface of seed were avoid using an alumina tube with surface roughness of 0.35 μm as the mold inner wall. As the original primary dendrite spacing of the seed decreased to less than 201 μm, the morphology of un-melted solid phase in semi-solid zone changed from isolated dendrite stem to the complex network, resulting in the inhibition of the formation of stray grains inside of the seed. The seed with original primary dendrite spacing of 201 μm was also successfully re-used to fabricate single crystal casting.

Systematic Phase-Field Study on Microstructure Formation During Brazing of Mar-M247 with a Si-Based AMS4782 Filler

Although advanced solidification processes like high temperature brazing have gained more and more industrial importance, they still are often not well understood, and defining optimal process conditions for practical use in many cases can be achieved only by expensive experimental trial and error methods. On the other hand, the phase-field method has become more and more powerful for the simulation of solidification and phase transformations in technical alloys. This paper presents phase-field simulations for high temperature brazing of the Ni-base superalloy Mar-M247 using a Si-based AMS4782 filler. The applied phase-field model can be online-coupled to Calphad databases which deliver the thermodynamic and mobility data for this complex multicomponent multiphase alloy. Unknown or uncertain physical parameters like diffusion coefficients have been calibrated by comparing to optical micrographs and EDX element mappings obtained from laboratory braze experiments for the same material combination. By variation of the process conditions the effects of the brazing temperature, the brazing time, and the thickness of the braze gap on precipitation of brittle phases and formation of stray grains as well as changes in the microstructure of the base material itself have been systematically investigated. Results are presented and discussed.

Formation Mechanism of Stray Grain of Nickel-Based Single-Crystal Superalloy Under a High Magnetic Field During Directional Solidification

We investigate the formation mechanism of stray grain of nickel-based single-crystal superalloy under a high magnetic field. It was shown that the high magnetic field induced the formation of stray grains on the side that the dendrite diverges the wall. In addition, results showed a sloping solid/liquid interface and a further increased sloping magnitude under a high magnetic field. Further, the formation process of stray grain in the high magnetic field is analyzed in detail.

Quantitative analysis of withdrawal rate on stray grain formation in the platforms of a Ni-Based single crystal dummy blade

Quantitative analysis of withdrawal rate on stray grain (SG) formation in the platforms of Ni-base single crystal (SX) dummy blades was carried out through experiments and simulations. The results showed that the tendency of SG formation becomes higher with the increase of withdrawal rates from 3 to 12 mm min−1. Considering that SG formation is governed by a competition on the amount of time used for the dendrites branching and the cooling process, the underlying mechanism associated with the influence of withdrawal rate on SG formation was then investigated based on the effect of withdrawal rate on the thermal profile development and the macrostructural evolutions. Finite element simulation indicated that the thermal profile in the platform is wave-shaped, which results in two kinds of SGs formation; one being isolated in small areas in the platform ends, and the other connecting to a large area across the upper and lower edges of the platform.

Effect of Construction Manner of Mould Cluster on Stray Grain Formation in Dummy Blade of DD6 Superalloy

AbstractA dummy blade of Ni-based single crystal (SX) superalloy was modelled to investigate the stray grain (SG) defect by both experiment and simulation. Three construction manners of mould cluster (CMMC) with 0°, 45° and 90° assembling angle were considered. The experimental results reveal a strong dependence of SG sensitivity upon CMMC. Profuse SGs took place in the case of 0°, but almost no one shown in those of 45° and 90°. The FEM simulation results indicate that SG occurrence is not only determined by critical nucleation undercooling, but also the geometrical characteristic, shape of liquidus isotherm and other three important effects (shadow effect, channel effect and dimension effect). The tendency of SG formation increases with the increase of number of these effects. By means of the combination effect consisting of the above three effects, SG occurrence can be qualitatively predicted and process window is more accurately modified.

Simulation of stray grain formation in Ni-base single crystal turbine blades fabricated by HRS and LMC techniques

The simulation models of the thermal and macrostructural evolutions during directional solidification of Ni-base single crystal (SX) turbine blades under high rate solidification (HRS) and liquid metal cooling (LMC) have been constructed using ProCAST software, coupled with a 3D Cellular Automaton Finite Element (CAFE) model. The models were used to investigate the tendencies of stray grain (SG) formation in the platform region of turbine blades fabricated by HRS and LMC techniques. The results reveal that the LMC technique can prohibit SG formation by smoothing the concaved isotherm and in turn alleviating the undercooling in the platform ends to let the dendrites fill up the undercooled zone before SG nucleation. The simulation results agreed well with the experimental results, indicating that these models could be used to analyze the macrostructural evolution or to optimize process parameters to suppress SG formation. Using these models, the critical withdrawal rate for casting SX turbine blades without SG formation were determined to be around 75 μm·s-1 and 100 μm·s-1 for HRS and LMC respectively, suggesting that LMC can be used as an efficient technique in fabricating SX turbine blades without any SG defect formation.

不同工艺条件对激光熔化多道沉积DD5单晶高温合金杂晶的影响

不同工艺条件对激光熔化多道沉积DD5单晶高温合金杂晶的影响