Dendrite Arm Research Articles

Effect of cladding speed on the microstructure and hardness of high-hardness B-bearing Cr17Ni2 coating

The B-bearing iron-based coating produced by laser cladding exhibits super-high hardness. However, the strengthening mechanism of B-bearing Cr17Ni2 coating and non-equilibrium phase transformation during the cooling stage require further investigation. The study focused on the B-bearing Cr17Ni2 coatings produced by laser cladding at four speeds ranging from 10 mm/s to 200 mm/s. The results show that the thickness of the single layer in the coatings gradually decrease as the increase of scanning speed. The secondary dendrite arms spacing and grain size also decrease with increasing scanning speed. The microstructure of the coating consists of a martensitic phase and a eutectic phase consisting of Cr2B compounds and austenite. Combined with G\\R diagram and finite element analysis, the microstructure refining mechanism of the coating has been concluded. The effect of fine grain strengthening, solid solution strengthening, precipitation strengthening, and dislocation strengthening on hardness is investigated. It has been observed that the coating hardness increases with increasing scanning speed. This change in the hardness of the coatings is mainly because of the grain and precipitate refining.

Study on the characteristics and high-temperature dissolution mechanism of eutectic carbides in medium-alloy steel

In this study, the high-temperature evolution of eutectic carbides in a medium-alloy steel was investigated as well as their precipitation mechanism. Results revealed that MC and M6C eutectic carbides in the steel appeared during the final solidification due to the segregation of Mo and V and the melt undercooling. These eutectic carbides consist of blocky shapes with varying orientations, and rod-like shapes with the same orientation. Further investigation was confirmed that the carbides (MC and M6C) exhibited an intergrowth behavior. The cooling rate had a direct influence on the secondary dendrite arm spacing (SDAS), which decreased from 180 to 130 μm as the rate ranged from 0.6 to 21.9 °C/min. After holding for 0, 25, 50, and 80 h, respectively, the volume percentage of eutectic carbides exhibited a significant decreasing trend from 2 to 0.2%. Additionaly, the carbides exhibited irregular morphology after holding for 0–50 h, respectively, and which was transformed into the rod-like shape with a small size distributed along the grain boundaries after holding for 80 h. This phenomenon was attributed to the mutual diffusion between Mo, V, and other alloying elements in the carbides and Fe element in the steel matrix.

Impact of Aging Time on the Metallurgical Properties and Hardness Characteristics of an Al-Si-Mg-Cr Hypoeutectic Alloy Intended for Automotive Applications

This research investigates the microstructural evolution and mechanical properties of LM25 (Al-Si-Mg) alloy and Cr-modified LM25-Cr (Al-Si-Mg-Cr) alloy. Microstructural analysis reveals distinctive ε-Si phase morphologies, with Cr addition refining dendritic structures and reducing secondary dendrite arm spacing in the as-cast condition. Cr modification results in smaller-sized grains and a modified ε-Si phase, enhancing nucleation sites and reducing ε-Si size. Microhardness studies demonstrate significant increases in hardness for both alloys after solutionising and aging treatments. Cr-enriched alloy exhibits superior hardness due to solid solution strengthening, and prolonged aging further influences ε-Si particle size and distribution. The concurrent rise in microhardness, attributed to refined dendritic structures and unique ε-Si morphology, underscores the crucial role of Cr modification in tailoring the mechanical properties of aluminium alloys for specific applications.

Carbon Macrosegregation Pattern in 30Cr15Mo1N Ingot: Impact of Pouring Rate

The effect of the pouring rate on the carbon macrosegregation in 30Cr15Mo1N ingots has been clarified by investigating changes in the content of carbon‐containing precipitated phases, cooling rate, secondary dendrite arm spacing (SDAS) and the flow characteristics of molten steel. During the solidification process, the solute‐enriched liquid steel migrates toward the center of the ingot under the action of convection, resulting in an increasing segregation ratio from the edge to the center. However, the growth of equiaxed grains partially restricts the flow of solute‐enriched liquid steel, which leads to a maximum segregation ratio not at the center but at approximately a quarter radius from the center. Furthermore, due to the upward flow of molten steel in the center, positive segregation occurs at the top. An increment in the pouring rate reduces the SDAS by enhancing the cooling rate, and thus the permeability decreases. It aggravates the carbon macrosegregation horizontally. However, increasing the pouring rate decreases the carbon macrosegregation vertically. This is due to the more intense flow of molten steel and the larger liquid phase region at higher pouring rate, which reduces the solute enrichment degree and promotes compositional homogeneity above and below the ingot.

Microstructure and Tensile Properties of Ni-Base Superalloy IN738LC according to Solidification Rate and Heat Treatment

The strength of Ni-base superalloys mainly depends on the γ' precipitates that improve the strength of the materials at high temperatures. The presence of γ' particles within the matrix restricts dislocation movement, and optimized heat treatments can tailor the size, shape, and volume fraction of γ'. In this study the effects of solidification rate and solution temperature on the tensile properties of IN738LC superalloy were investigated. The secondary dendritic arm spacing of casting materials with different diameters was measured and the solidification rate of the casting materials was derived by comparing the results of the solidification microstructure obtained from a directional solidification experiment. The D17 material, which had a faster solidification rate, showed higher values of tensile strength and yield strength than the D60 material, which had a slower solidification rate. The study also concluded that the monomodal γ' precipitates in the S80 material have higher tensile strength and yield strength at room temperature and 760℃ than the bimodal γ' precipitates in the S20 material. As for the deformation behavior at 760℃, an isolated stacking fault was observed in the S20 material only within the large γ’ precipitates. In the S80 material, the high dislocation density increased the yield strength due to the strong interaction between dislocations and fine γ’ precipitates.

Predicting the effect of cooling rates and initial hydrogen concentrations on porosity formation in Al‐Si castings

AbstractAl‐Si alloys are widely used in automotive casting components while microporosity has always been a detrimental defect that leads to property degradation. In this study, a coupled three‐dimensional cellular automata (CA) model has been used to predict the hydrogen porosity as functions of cooling rate and initial hydrogen concentration. By quantifying the pore characteristics, it has been found that the average equivalent pore diameter decreases from 40.43 to 23.98 μm and the pore number density increases from 10.3 to 26.6 mm−3 as the cooling rate changes from 2.6 to 19.4°C/s at the initial hydrogen concentration of 0.25 mL/100 g. It is also notable that the pore size increases as the initial hydrogen concentration changes from 0.15 to 0.25 mL/100 g while the pore number remains stable. In addition, the linear regression between secondary dendrite arm spacing and the equivalent pore diameter has been studied for the first time, matching well with experiments. This work exhibits the application of CA model in future process optimization and robust condition design for advanced automotive parts made of Al‐Si alloys.

A Numerical Time Integration Procedure for Secondary Dendrite Arm Spacing in Hyper-Peritectic Steel Alloys

AbstractA numerical time integration procedure for the calculation of the secondary dendrite arm spacing (SDAS) in FeC hyper-peritectic alloys is presented, a preferred group of low-carbon casting steel. The procedure incorporates a three-stage thin arm dissolution model, which is solved at each time step using Newton–Raphson iteration. This is coupled to a coarsening model based on the integral forms of the dissolution model, which are solved using Gaussian quadrature, as well as a growth model for solid fraction evolution. The procedure can easily be embedded into numerical models for solidification, in which the space–time evolution of the SDAS is required for determining the dynamic permeability in the mushy zone. Higher order approximations for both growth and solute concentration evolution can easily be incorporated. Temperature dependence of thermophysical parameters is taken into account using inter-dendritic solidification empirical models, and an alloy-specific peritectic reaction constant is used to determine the isothermal peritectic holding time. The procedure is validated against experimental data presented in literature. Various cases of SDAS as a function of local solidification time, cooling rate and carbon composition are investigated. The method is compared to experimental results of SDAS obtained from test castings of a hyper-peritectic steel alloy and can be used to iteratively determine the alloy-specific peritectic reaction constant by comparing the solid fraction evolution during the peritectic reaction with that found from the experimental cooling curve.

Microstructure, Hardness, Wear Resistance, and Corrosion Resistance of As-Cast and Laser-Deposited FeCoNiCrAl0.8Cu0.5Si0.5 High Entropy Alloy

FeCoNiCrAl0.8Cu0.5Si0.5 high-entropy alloys were fabricated using vacuum induction melting and laser deposition processes, followed by a comparison of the structural and mechanical properties of two distinct sample types. The as-cast FeCoNiCrAl0.8Cu0.5Si0.5 alloy is comprised of BCC1, BCC2, and Cr3Si phases, while the laser-deposited alloy primarily features BCC1 and BCC2 phases. Microstructural analysis revealed that the as-cast alloy exhibits a dendritic morphology with secondary dendritic arms and densely packed grains, and the laser-deposited alloy displays a dendritic structure without the formation of granular interdendritic regions. For mechanical properties, the as-cast FeCoNiCrAl0.8Cu0.5Si0.5 alloy demonstrated higher hardness than the as-deposited alloy, with values of 586 HV0.2 and 557 HV0.2, respectively. The wear rate for the as-cast alloy was observed at 3.5 × 10−7 mm3/Nm, with abrasive wear being the primary wear mechanism. Conversely, the as-deposited alloy had a wear rate of 9.0 × 10−7 mm3/Nm, characterized by adhesive wear. The cast alloy exhibited an icorr of 4.062 μA·cm−2, with pitting as the form of corrosion. The laser-deposited alloy showed an icorr of 3.621 μA·cm−2, with both pitting and intergranular corrosion observed. The laser-deposited alloy demonstrated improved corrosion resistance. The investigation of their microstructure and mechanical properties demonstrates the application potential of FeCoNiCrAl0.8Cu0.5Si0.5 alloys in scenarios requiring high hardness and enhanced wear resistance.

Joint Characteristics of Cu-Ni Alloy Fabricated by GTAW and MPAW Processes: A Comparative Study

The current work presents a comparative analysis of the joint behavior of Cu-Ni alloy weldments fabricated by Micro-Plasma Arc Welding (MPAW) and Gas Tungsten Arc Welding (GTAW) processes. The Cu-Ni alloy thin sheets are fabricated at different values of heat input (~40-135 J/mm) by MPAW and GTAW processes, respectively. Further, to evaluate their characteristics, joints are subjected to metallurgical, mechanical, and electrochemical testing. The joints fabricated with a higher magnitude of heat input resulted in deteriorated surface quality with a value of Ra~ 6.13 μm. The increased surface roughness value of the joints resulted in a higher corrosion rate (1.273 mm/year). A finer microstructural morphology is achieved for lower heat input condition. Accordingly, the weldment exhibited higher joint efficiency of ~91%.The prominent reason for achieving higher joint strength is related to the presence of lower secondary dendritic arm spacing (SDAS), which enhances the joint strength and ductility for the joints as compared to higher SDAS value. Further, the micro-fractography analysis reveals the presence of micro/macro-voids for high heat input, whereas the existence of numerous dimples of varying size and depth is observed for low heat input condition, implying the role of heat input of utmost importance.

Fabrication and Characterization of Nickel‐Aluminide Cladding by Dual‐Wire Arc Additive Manufacturing

Nickel aluminide is a widely utilized intermetallic compound, prized for its high strength, low density, and resistance to both corrosion and creep. This study investigates the impact of heat input during the fabrication of nickel‐aluminide intermetallic compounds using the dual‐wire gas tungsten arc welding process. The results indicate that an increase in clad layer dilution from 28.9% to 33.4% reduces the amount of aluminum in the molten pool from 32.46 to 21.96 wt% while increasing the presence of the Ni3Al phase. Additionally, the NiAl phase decreases when moving from the upper side of the coating to the fusion line. When the arc current is increased from 110 to 150 A, a coarse dendritic structure forms, and the dendritic arms increase from 2.6 ± 0.4 to 5.5 ± 0.3 μm. This increase in current also results in yield strength and tensile strength values of 500.21 ± 14.56 and 752.32 ± 25.12 MPa, respectively, representing decreases of 15.2% and 5.1%. Furthermore, as the arc current increases from 110 to 150 A, both the friction coefficient and the wear rate increase to 0.43 and 0.29 ± 0.02 μg m−1, respectively. However, corrosion resistance improves by 66.3%.

Effect of purity on solidification structure and micro-segregation in a nickel-based single crystal superalloy

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.

Thermal-induced evolution of microstructure as a plasma arc coating Direction-Dependent phenomenon

This study elucidates the effects of different directed energy deposition trajectories on the microstructure and properties of powder plasma transferred arc welding (PPTAW) fabricated coatings. Two trajectories, straight (STT) and complex (CXT), were explored to deposit NiCrBSi powders onto a low alloy steel substrate. Finite element thermal analysis revealed distinct heat distribution zones, the middle zone (MZ) and transition zone (TZ), showing temperature variations between the STT and CXT coatings. Microstructural examination indicated lower dendrite arm spacing (DAS) in the STT coating (11 μm) than in the CXT coating (16 μm). Electron backscatter diffraction (EBSD) analysis demonstrated significant grain refinement in the STT coating (17.4 μm) versus the CXT coating (68.7 μm). X-ray diffraction phase analysis identified three reinforcing phases − γ-Ni, Cr23C6, and Cr3C2 – enhancing corrosion resistance and tribological properties. The STT coating exhibited superior corrosion resistance with a corrosion current density (icorr) of 12.8 µA/cm2, while the CXT coating showed icorr of 34.3 µA/cm2. Additionally, the STT coating exhibited higher hardness (832 ± 33 HV1.0) and better wear resistance with a wear rate of 10.46 ± 0.29 × 10-3 m3/m, over the CXT coating (hardness: 450 ± 36 HV1.0; wear rate: 12.67 ± 0.54 × 10-3 m3/m).

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.

Research on directional solidification combined with electron beam melting and refining for recycling of FGH4096 alloy scraps

The constraints of traditional manufacturing techniques have led to the generation of a considerable amount of waste superalloy, causing substantial resource wastage. The primary concern with waste superalloys lies in the excessive presence of inclusions and impurity elements, rendering them unsuitable for further utilization. This study proposes the use of directional solidification combined with electron beam melting and refining (EBMR) to prepare FGH4096 superalloy ingots (Φ120 mm), which offers a promising approach to produce large-sized powder superalloy master alloy ingots with high-purity and high homogeneity. It is found that the secondary dendrite arm spacing and the size of the γ’ phase in the alloy are significantly reduced after EBMR treatment. The concentration of O and N can be decreased to 5.7 and 0.6 ppmw, respectively. The size of inclusion can be reduced below 8.69 μm and the inclusion numbers reduce by over 50 % compared with both conventional ingots. A droplet transfer mode and melt convection mode mechanism have been proposed, which accurately predicts the melting temperature and melt flow. A quantitative study is conducted on the influence of inclusion density, the contact angle between inclusions and melt, and the surface tension of the melt on capillary forces. The results indicate that the capillary forces between inclusions in the melt range from 1.8 × 10−19 N and 1.9 × 10−17 N. Furthermore, the collision-aggregation behavior of inclusions in the melt is discussed, and an estimate of the tendency of inclusion collision-aggregation is provided. The enrichment of inclusions in the EBMR ingot is primarily the result of turbulent collision. The Marangoni effect and the extremely high surface temperature of the melt promote the collision and agglomeration of inclusions, causing them to accumulate rapidly at the melt surface. Moreover, the thorough removal of inclusions through the decomposition and dissolution process of the electron beam offers theoretical backing for the preparation of superalloys through electron beam melting and high-purity refining.

Effects of Superheating Treatment on the Microstructure, Tensile and Thermal Conductivity Properties of Fe-Bearing Al-10Si-Mg Casting Alloy

In this study, we designed and manufactured a new Fe-bearing Al-10Si-Mg casting alloy (F alloy) and investigated its microstructure, mechanical properties, and thermal conductivity. Two types of Fe-bearing Al-10Si-Mg alloys were used: the Conventional-F alloy, injected at 720 ℃ and cooled by water quenching, and the Superheated-F alloy, heated to 820 ℃ and maintained at that temperature for 1 hour. Subsequently, it underwent a degassing process at 720 ℃ before being cooled by water quenching. Both the Conventional-F alloy and the Superheated-F alloy exhibited dendritic microstructures and Fe-intermetallic compounds. The Secondary Dendrite Arm Spacing (SDAS) of the Conventional-F alloy measured 32.4 μm, whereas the Superheated-F alloy measured 28.6 μm. Additionally, the average eutectic Si sizes were 10.3 μm for the Conventional-F alloy and 7.7 μm for the Superheated-F alloy. Fe-rich IMCs were observed in the eutectic region, with their size decreasing due to the superheating treatment. Tensile tests at room temperature were conducted at a strain rate of 10<sup>-3</sup>/s. The Conventional-F alloy exhibited a yield strength (YS) of 93.4 MPa, ultimate tensile strength (UTS) of 183 MPa, and an elongation (El.) of 6.4%. Conversely, the Superheated-F alloy displayed a YS of 115.4 MPa, UTS of 218.2 MPa, and an El. of 5.1%. The mechanical properties notably improved with the superheating treatment. Regarding thermal conductivity, the Conventional-F alloy exhibited 114.9 W/m·K, while the Superheated-F alloy displayed 153.7 W/m·K. This represents a roughly 14% increase compared to the thermal conductivity of the commercial Al-10Si-Mg material (Silafont36: 135.1 W/m·K). The effects of the Superheating Treatment on microstructural characteristics, deformation behavior, and thermal conductivity of the Fe-bearing Al-10Si-Mg casting alloys were discussed.

Dimension Prediction and Microstructure Study of Wire Arc Additive Manufactured 316L Stainless Steel Based on Artificial Neural Network and Finite Element Simulation.

The dimensional accuracy and microstructure affect the service performance of parts fabricated by wire arc additive manufacturing (WAAM). Regulating the geometry and microstructure of such parts presents a challenge. The coupling method of an artificial neural network and finite element (FE) is proposed in this research for this purpose. Back-propagating neural networks (BPNN) based on optimization algorithms were established to predict the bead width (BW) and height (BH) of the deposited layers. Then, the bead geometry was modeled based on the predicted dimension, and 3D FE heat transfer simulation was performed to investigate the evolution of temperature and microstructure. The results showed that the errors in BW and BH were less than 6%, and the beetle antenna search BPNN model had the highest prediction accuracy compared to the other models. The simulated melt pool error was less than 5% with the experimental results. The decrease in the ratio of the temperature gradient and solidification rate induced the transition of solidified grains from cellular crystals to columnar dendrites and then to equiaxed dendrites. Accelerating the cooling rate increased the primary dendrite arm spacing and δ-ferrite content. These results indicate that the coupling model provides a pathway for regulating the dimensions and microstructures of manufactured parts.

Effect of processing parameters on the microstructure of laser-clad Stellite 6 on the X19CrMoNbVN11-1 stainless-steel substrate

This investigation aims to study the effect of laser cladding parameters on the microstructure of Stellite 6 on the X19CrMoNbVN11-1 stainless-steel substrate. First, Stellite 6 powder was coated on the X19CrMoNbVN11-1 substrate using the laser cladding method. The effect of laser cladding parameters (i.e., laser power, scanning speed, and powder feed rate) was studied on the microstructure of deposits. The secondary dendritic arm spacing was assessed, and the structural defects were studied (e.g., lack of bonding, porosity, and crack). The results revealed that the microstructure has changed from coating/substrate interface to coating surface, from plate-cellular to columnar and equiaxed dendrites. Also, an increase in the laser power increased the cellular structure in the coating/substrate interface and equiaxed dendrites in the coating surface. The cooling rate (G × R) increased by increasing the scanning and powder injection rates. The microstructure of the Stellite 6 was composed of cobalt solid-solution γFCC.

Improving Microstructural Homogeneity of Investment‐Cast Single‐Crystalline Ni‐Base Superalloys by Using Fluidized Carbon Bed Cooling

One impeding factor for faster solidification in typical investment casting processes is the limited heat removal capability. This study compares the classical Bridgman technique and the fluidized carbon bed cooling (FCBC) casting method. In particular, the influence of the thermal design is investigated in terms of microstructural and compositional homogeneity in single‐crystalline CMSX‐4. These investment casting processes mainly differ in the baffle type used to insulate the hot and cold zones inside of the casting vessel. While the Bridgman process is based on radiation cooling, the FCBC casting process relies on a dynamic baffle made of expanded graphite that is buoyant on fluidized bed of glassy carbon beads. Such a dynamic insulation layer can adapt to the cross‐sectional variation of ceramic shell molds and provide a better thermal insulation, which results in more uniform solidification conditions. This enables a reduction in the primary dendrite arm spacing by up to 26%, an increase in the homogeneity of the microstructure and composition, as well as a reduction in overall porosity and mean pore size of 41%.

Study on solid‐air dendrite growth and motion with thermosolutal convection‐diffusion using non‐isothermal PF‐PSLBM model

AbstractThis study unveils a numerical paradigm that amalgamates the partially saturated lattice Boltzmann method (PSLBM) with the non‐isothermal quantitative phase‐field (PF) model. This innovative integration equips us with a prognostic tool ready to elucidate the progression and motion of solid‐air dendritic growth in the presence of both natural and forced convection. The PSLBM is employed to compute the flow of the solution and the interaction forces between the fluid and solid dendrites. Concurrently, the PF model is utilized to simulate the formation of solid‐air dendrites. The reliability of calculating of interaction forces between the fluid and solid was confirmed through a numerical case study involving fluid flow around a stationary cylinder. The results indicate that this model is applicable for simulating the growth and evolution of single/multiple solid‐air dendrites under the influence of convection, whether they are stationary or in motion. The promotion of the upstream side dendritic arms and the inhibition of the downstream dendritic arms increase with the intensification of natural convection. As the initial undercooling is raised, the capacity of natural convection to reshape dendritic morphology gradually diminishes. With the enhancement of forced convection intensity, due to alterations in the flow pattern, the downstream dendritic arms do not consistently exhibit growth suppression. The motion of solid‐air dendrites induced by forced convection counteracts the influence of convection, resulting in slightly faster growth of the downstream dendritic arms compared to the upstream arms. Simultaneously, it fosters the formation of secondary dendritic branches in the upstream zone.

Multi-physics simulation of non-equilibrium solidification in Ti-Nb alloy during selective laser melting

This paper underscores the critical role of temperature distribution in the molten pool on the resulting solidification microstructure during selective laser melting (SLM) process. A powder-resolved computational fluid dynamics (CFD) model of Ti-Nb alloy is established to investigate heat transfer and fluid flow in the molten pool. When comparing the bulk model to the powder bed model, discrepancies in temperature gradient and cooling rate can reach up to 26.29 % and 18.32 %, respectively. Utilizing the powder bed CFD model, temperature gradient and cooling rate data at the molten pool boundary are integrated into the finite interface dissipation phase-field model. The simulation results illuminate the non-equilibrium solidification microstructure in the SLMed Ti-Nb alloy. Results indicate that the rapid solidification fosters a pronounced solute trapping effect, mitigating solute microsegregation. The microstructure manifests as cellular and dendritic structures, with Nb concentration in cellular cores and depletion in intercellular region. Predicted primary dendrite arm spacing (PDAS) or cell size range between 0.4 to 0.6 μm, consistent with experimental observations. In particular, the study reveals a transition from dendritic to cellular and from cellular to planar structures, with an accompanying analysis of the underlying mechanisms.