Primary Dendrites Research Articles

Phase-field modeling and Experimental investigation for rapid solidification in wire and arc additive manufacturing

The rapid solidification speed and significant temperature gradient observed in the wire and arc additive manufacturing (WAAM) process result in a deviation of the solid-liquid interface from the equilibrium state assumed by the conventional directional solidification phase field (PF) model. This, in turn, limits the ability of the traditional directional solidification PF model to accurately depict the microstructure evolution in additive manufacturing, where non-equilibrium effects dominate. In this work, we employed a 3D finite element (FE) model to simulate the temperature distribution during the wire and arc additive manufacturing (WAAM) process. Additionally, a modified PF model that applies to additive manufacturing is also used to simulate the dendrite morphology and solute segregation behavior in the WAAM process at different scanning speeds. Experimental validation of the simulation results was also conducted. We observed a significant reduction in dendrite growth velocity and solute segregation with increasing scanning speed, attributable to the combined effects of the temperature gradient G(t) and solidification speed Vp. Furthermore, higher scanning speeds were found to increase the primary dendrite arm spacing (PDAS) and average dendrite width. Importantly, our results demonstrated a quantitative agreement between the simulation results using the modified directional solidification PF model and the experimental observations. This approach provides a foundation for selecting and predicting processing parameters and microstructural characteristics under additive manufacturing conditions, facilitating a comprehensive analysis of dendrite growth and solute segregation at different scanning speeds in the WAAM process.

Effect of secondary orientation on the microstructures in thin-walled castings of nickel-based single-crystal superalloys

With the increasing turbine inlet temperature of aero-engine, the requirement of temperature capacity of turbine blades is more stringent. A variety of complex cooling structures have been designed, among which the micro cooling represented by lamilloy is the latest development. At present, the thickness of lamilloy turbine blade is 0.5 mm or less. The reason for performance degradation caused by thin-walled is still controversial and the understanding of dendrite growth behaviour under space constraints is insufficient. In this study, 0.75 mm wall thickness nickel-based superalloy DD403 samples were cast by high-rate directional solidification. The growth and evolution of dendrites in plank-shaped specimens with different secondary deviation angles were investigated. The variation of dendrite spacing and the arrangement of dendrites under different secondary deviation angles were studied. It is found that the primary dendrite arm spacing in the thin-walled region decreased with the increase of the secondary deviation angle, but there was no significant change in the average size of γ′ phases and elements segregation.

Investigation of microstructure evolution and mechanical properties of cold crucible directional solidification Nb-Si alloy

In this work, the microstructure and properties of Nb-16Si-24Zr alloy directionally solidified by electromagnetic cold crucible under different process parameters were investigated. As the pulling rate increases, the size of both primary Nbss and eutectic Nbss decreases, resulting in a straighter primary dendrite γ-Nb5Si3 and a fishbone like Nbss/γ-Nb5Si3 coupling structure of secondary eutectic. As the power supply increases, the size of the nascent Nbss and γ-Nb5Si3 increases, and the coupling growth trend of Nbss and Nb5Si3 weakens. As the pulling rate and power supply increase, the toughness shows a trend of first increasing and then decreasing, while the room temperature compressive strength gradually increases.

Development of a rapid solidification model for additive manufacturing process and application to Al-Si alloy

Accurate prediction of the solidification microstructure produced by additive manufacturing (AM) process is indispensable for the precise manufacturing process control. In the present study, a simple solidification model that can be applied to the powder bed fusion – laser beam (PBF-LB) process has been developed by integrating conventional and rapid solidification theories in literature. Based on the solidification parameters calculated by Rosenthal equation, the present solidification model can predict the primary dendrite arm spacing (PDAS), primary cell fraction, and solute profile using solute trapping, dendrite tip undercooling, and eutectic undercooling. Single track experiments for Al-12Si binary alloy were also performed to validate the accuracy of the model.

The Mechanical and Functional Behaviors of Mn–15 wt%Cu Alloy Produced by Selective Laser Melting

Mn–Cu‐based alloys possess excellent mechanical and functional properties (damping capacity and shape memory effect). This work utilizes a typical additive manufacturing technique, selective laser melting (SLM), to fabricate Mn–15 wt%Cu alloys to explore the mechanical and functional behaviors. The results indicate that the γ‐(Mn, Cu) and a small amount of γ′‐(Mn, Cu) phase are the main phase compositions in the as‐SLMed Mn–15 wt%Cu alloy, as well as twins and second‐phase precipitation particles. The microstructure shows a fine cellular γ‐(Mn, Cu) dendrite with primary dendrite spacing of ≈0.9 μm in the columnar grains with a size of ≈8 μm. The martensitic transformation start temperature (MS) and phase transformation hysteresis are ≈132.6 and ≈5.1 °C, respectively. The existing nanoscale chemical segregations of Mn and Cu are attributed to the spinodal decomposition of γ‐(Mn, Cu). A compositional modulation is observed with a wavelength of ≈20 nm and an amplitude fluctuation of Mn (55–85 wt%) and Cu concentrations (15–45 wt%). Finally, the as‐SLMed Mn–15 wt%Cu alloy boasts good tensile properties (359, 268 MPa, and 3.6%), damping (internal friction of 0.032 when the strain amplitude is 9 × 10−4), and shape memory performances (one‐way ηow = 37–48%, and two‐way ηtw = 16–20% under the pre‐deformation strain of 2–3%).

Melt pool evolution and microstructure simulation of SLM 316L based on SPH-PFM coupling model

The study in this work fully utilized the computational advantages of various numerical methods and established a SPH-PFM coupling model for the 316 L SLM process. The temperature field, melt pool changes, and microstructure evolution of the 316 L SLM process were simulated and calculated. The correctness of the simulation results was verified through the comparison with experiments, and a calculation platform for melt pool evolution and microstructure simulation of metal material SLM process with independent intellectual property rights was developed. (1) Establish the mathematical model required for the dynamic simulation of the molten pool in the metal material SLM process based on the SPH method. (2) Extract the temperature field data and molten pool shape from the calculation results based on the SPH method, and then substitute them into the phase field model for molten pool modeling. The formation mechanism of the microstructure morphology inside the molten pool and the micro-segregation mechanism of solute elements at different positions during solidification were analyzed and discussed. The article compared the simulation results with the experimental results. The simulation results showed good consistency with the experimental results in terms of primary dendrite spacing, secondary dendrite spacing, and melt pool microstructure morphology, proving the correctness of the model established in this article. At the same time, the SPH-PFM coupling model established in this article can effectively solve the problem of predicting the microstructure of the melt pool in the SLM process, providing a new method for the study of microstructure prediction in the SLM process.

Microstructure dependence of electrochemical corrosion resistance for rapidly solidified Ti50Al48Mo2 alloy

The rapid solidification of undercooled liquid Ti50Al48Mo2 alloy was achieved by the electromagnetic levitation (EML) technique. At small and medium undercoolings, primary (βTi) dendrite reacted with surrounding liquid to drive a peritectic transformation into the (αTi) phase. The solutal Mo and Al segregations were located within the dendrite center and the grain boundary during peritectic transformation, consequently B2 phase in the dendrite center and γ phase at the grain boundary formed. Once undercooling exceeded 253 K, the peritectic transformation was completely inhibited, and the formation of the B2 phase and γ phase was completely suppressed. The ultrafine eutectoid structure was formed and a complete solute trapping effect was realized. Homogeneous solute distribution facilitated the formation of thicker passivation film with lower defect density and higher film resistance on the alloy surface. Moreover, this weakened micro-galvanic effect reduced the susceptibility to pitting corrosion, and consequently the corrosion resistance of the alloy was improved.

The Effect of Precipitates on the Stress Rupture Properties of Laser Powder Bed Fusion Inconel 718 Alloy

Improving the high-temperature stress rupture properties of Inconel 718 (IN718) alloys is crucial for enhancing aircraft engine performance. By using the laser powder bed fusion (LPBF) technique, IN718 alloys were crafted at varying volumetric energy densities (VED) in this study. The dendrite growth mode, reinforcing phase distribution and high temperature stress rupture properties of various VED samples were investigated. The results showed that the stress rupture life and the uniform elongation of the samples both first increased and then decreased with the increase in VED. When the VED was 60 J/mm3, the maximum rupture life and elongation of the sample were 43 h and 3.8%, respectively. As the VED increased, the angle of dislocation in the dendrite decreased while the spacing between primary dendrite arms increased, resulting in an increase in the size and volume fraction of the Laves phase. Following a heat treatment, the δ phase would nucleate preferentially around the dissolved Laves phase causing an increase in the volume fraction of the δ phase with the increase in VED. The creep voids readily formed around the δ phase are distributed along the grain boundaries, while the inhomogeneous δ phase and fine grains facilitated crack initiation and propagation. Furthermore, a significant quantity of the δ phase consumed the Nb element, thereby hindering adequate precipitation in the γ″ phase and causing cracks.

The Effect of the Distance between Ultrasonic Horn and Torch on the Microstructure of Ultrasonic-Assisted Gas Tungsten Arc Welded Inconel 690 Alloy Joint

The study focuses on investigating the relationship between the ultrasonic effect and microstructure of ultrasonic-assisted gas tungsten arc welding (UA-GTAW) Inconel690 alloy joints. The influence of ultrasonic vibrations on Inconel690 plates was examined, while also clarifying the distribution pattern of the ultrasonic effect across the plate. Furthermore, actual welding experiments were performed by varying the distance between the ultrasonic horn and the welding torch. The results revealed that there were changes in both grain growth direction within the weld zone and refinement effects achieved under different distances. The optimal refinement of primary and secondary dendrite arm spacing was observed at distances of 60 mm and 180 mm between the welding torch and ultrasonic horn. The hardness of weld zone reached 235HV1 when the distance between ultrasonic horn and welding torch is 180 mm.

Molten pool free surface solidification behavior in laser remelted near-β titanium alloy

Free surface solidification (FSS) is a common physical phenomenon that has been found by various studies in metallurgy field while targeted studies are rare. This work investigated special structure and formation mechanism of surface grains generated by free surface solidification through laser remelting a near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55,511). A thin layer crystal film covers molten pool surface through two-dimensional constrained solidification, which can be classified into flake-like crystal film located at molten pool edge and lotusleaf-like crystal film at molten pool center. Numerous cells grow from overflowing melt chill zone around molten pool edge and undergo two-dimensional constrained growth on surface, forming flake-like crystal films. TiNx particles formed from surface melt and N2 reaction can act as heterogeneous crystal nuclei, forming lotusleaf-like crystal films, whose secondary dendrite arm growth direction transforms from 〈100〉 of primary dendrite arm to 〈110〉. Equiaxed lotusleaf-like crystal films and columnar flake-like crystal films have a competitive growth relationship. Inward growth cells growing from two kinds crystal films form flake-like surface grains and central surface grains, respectively. This study can provide FSS theoretical guidance for laser additive manufacturing and surface modification.

Effect of boron content on microstructure evolution and crystallization behavior of Fe7(CoNi)93-xBx (x=15, 18, 21) eutectic high-entropy alloys

The effect of boron content on microstructure evolution, glass-forming ability and crystallization behavior of Fe7(CoNi)93-xBx (x = 15, 18, 21) eutectic high-entropy alloys (EHEAs) was investigated. Bulk Fe7(CoNi)93-xBx EHEAs and ribbons were prepared by vacuum frequency induction melting furnace and melt-spinning method, respectively. The results show that the microstructure of as-cast hypoeutectic Fe7(CoNi)78B15 alloy consists of primary (Fe, Co, Ni) dendrite and lamellar eutectic. The eutectic Fe7(CoNi)75B18 alloy shows a fully lamellar eutectic structure consisting of (Fe, Co, Ni) phase and B-rich phase. The microstructure of hypereutectic Fe7(CoNi)72B21 alloy exhibits primary B-rich dendrite and lamellar eutectic. The as-spun Fe7(CoNi)75B18 and Fe7(CoNi)72B21 alloys show an amorphous structure. By comparison, the hypereutectic Fe7(CoNi)72B21 alloy shows an excellent glass-forming ability and thermal stability, which is attributed to the increase of crystallization activation energy induced by B addition. In addition, the increase of B addition inhibits the crystallization behavior, which is caused by the formation of stable covalent bonds between boron and other elements. This work provides an alternative route for the preparation of metallic glass by introducing the non-metallic boron component.

On the formation of gradient-distributed dendrites in a single crystal nickel-based superalloy directionally solidified under transverse static magnetic field

Directional solidification (DS) of alloys under static magnetic field has attracted extensive attention due to the development of thermoelectric magnetohydrodynamics (TEMHD). Assessing the influence of static magnetic field on solidification microstructures in single crystal (SX) superalloys is of great significance for both the development of SX superalloys and the understanding of external field-assisted solidification. In this contribution, under different transverse static magnetic fields, the evolution of dendritic structure in a SX nickel-based superalloy directionally solidified over a range of solidification velocities is systematically investigated. The results show that applying a transverse static magnetic field (TSMF) during DS does not disrupt the crystal orientation of SX superalloys. The distribution of dendrites or local primary dendrite arm spacings within the cross section of SX superalloys shows a gradient variation in the direction perpendicular to the TSMF. Additionally, the formation of gradient-distributed dendritic structure is closely related to the applied TSMF intensity and solidification velocity. It is easier to form a gradient-distributed dendritic microstructure at low solidification velocities and high magnetic fields. Based on the experiments and simulations, the formation of gradient-distributed dendritic microstructure is supposed to attribute to the variation of interdendritic constitutional undercooling created by the change of solute concentration in interdendritic region, which is induced by the TEMHD. The SX superalloy with a gradient distribution of dendrites may have potential applications due to its gradient properties. Meanwhile, these findings also provide a new method for preparing alloys with gradient microstructures.

An interpolation-based transient solidification conditions model for numerical calculation of grain growth during laser welding

The weld microstructure is highly related to the performance of welded joints during the laser welding. The temperature gradient and pulling velocity are critical solidification conditions which play a major role in determining the grain evolution in the weld formation. Therefore, an interpolation-based transient solidification conditions model is proposed in this paper for numerical calculation of the grain growth during laser welding. The transient temperature gradient obtained in the model is compared with that from the previous method and the accuracy of transient temperature gradient is improved. The corresponding phase field (PF) model based on the obtained transient solidification conditions is developed to calculate the evolution process of grain growth. It is found that the morphology and spacing of the primary dendrite arm from the numerical calculation agree well with the experimental results. In the competitive growth stage, some grains are suppressed, which provides more space for neighboring grains to grow coarser. The solute redistribution and insufficient solute diffusion demonstrate significant effects on the solute concentration distribution. The proposed method is of great importance for controlling microstructure and improving the laser welding quality.

Casting of film-cooling holes for single-crystal blades

The flawless formation of film-cooling holes (FCHs) is critical for manufacturing single-crystal blade. Presently, high-energy beam drilling is mostly used to process gas film holes, which introduces the limitations of complex hole structures that are difficult to form and processing thermal defects that are difficult to eliminate. This study demonstrates an approach to form FCHs using casting, where an indirect additive manufacturing technology using a core/shell integral ceramic mold is applied for casting. Cylindrical 0.4, 0.8, and 4.0 mm-diameters hole are cast. The influence of fine ceramic cores on the microstructure of single-crystal blades during directional solidification is investigated using simulations and experiments. The fine ceramic cores of the FCHs affect the growth morphology of dendrites around the hole, which is determined by the hole diameter, primary dendrite spacing, and local temperature filed. Owing to the thermal conductivity difference between the alloy and ceramic, the local temperature filed around the holes changes abruptly, leading to the formation of shrinkage porosity and cavities. Furthermore, the process window for the casting of FCHs is determined using the longitudinal temperature gradient (GZ) and solidification rate (VZ). The proposed approach is of practical significance for the defect-free casting of tiny structures in single-crystal blades.

Effects of the thermal history on the microstructural and the mechanical properties of stainless steel 316L parts produced by wire-based laser metal deposition

Laser metal deposition with coaxial wire feeding is a novel technology for additively manufacturing near-net-shape metal parts, as well as for repairing and modifying existing components. The microstructure and, thus, the mechanical properties of the deposited material significantly depend on the thermal history of the part, which is influenced by the process parameters and the amount of dissipated heat energy. In order to investigate these correlations, stainless steel 316L samples were produced under different thermal conditions and thoroughly characterized. Solid cuboids were built on a substrate plate at room temperature as well as on a preheated substrate plate. Subsequently, the influence of the different thermal conditions on the melt pool temperatures was evaluated, and the microstructures, the microhardness, and the tensile properties were analyzed. It was found that the mean melt pool temperatures and the primary dendrite arm spacing (PDAS) increased with the build height, while the cooling rates showed an inverse relationship with the build height. The determined PDAS values were correlated with the microhardness profile of the specimens using a Hall-Petch type relationship. An increased heat accumulation in the parts was thereby associated with a coarser microstructure and diminished mechanical properties. The presented results pave the way for tailoring the properties of additively manufactured components to specific requirements.

Microglia are dispensable for developmental dendrite pruning of mitral cells in mice.

During early development, neurons in the brain often form excess synaptic connections. Later, they strengthen some connections while eliminating others to build functional neuronal circuits. In the olfactory bulb, a mitral cell initially extends multiple dendrites to multiple glomeruli but eventually forms a single primary dendrite through the activity-dependent dendrite pruning process. Recent studies have reported that microglia facilitate synapse pruning during the circuit remodeling in some systems. It has remained unclear whether microglia are involved in the activity-dependent dendrite pruning in the developing brains. Here, we examined whether microglia are required for the developmental dendrite pruning of mitral cells in mice. To deplete microglia in the fetal brain, we treated mice with a CSF1R inhibitor, PLX5622, from pregnancy. Microglia were reduced by >90% in mice treated with PLX5622. However, dendrite pruning of mitral cells was not significantly affected. Moreover, we found no significant differences in the number, density, and size of excitatory synapses formed in mitral cell dendrites. We also found no evidence for the role of microglia in the activity-dependent dendrite remodeling of layer 4 neurons in the barrel cortex. In contrast, the density of excitatory synapses (dendritic spines) in granule cells in the olfactory bulb was significantly increased in mice treated with PLX5622 at P6, suggesting a role for the regulation of dendritic spines. Our results indicate that microglia do not play a critical role in activity-dependent dendrite pruning at the neurite level during early postnatal development in mice.Significance StatementSynapse elimination is essential for activity-dependent circuit remodeling in the developing brains of mammals. Recent studies suggested that microglia play a critical role in the synapse elimination. This study found that microglia are dispensable for the activity-dependent dendrite pruning in developing mitral cells and layer 4 neurons in the barrel cortex. Thus, microglia are not critical for activity-dependent dendrite pruning at the neurite level during normal developmental process.

Interaction between growing dendrite and rising bubble under convection

Prediction and control of gas porosity defect during solidification is challenging because of multiphase and multiphysics characteristics. In this work, a multiphase-field lattice Boltzmann method is employed to study the interaction between solidifying dendrite and rising bubble. The influencing factors are divided into two categories including external and internal factors, and the effects of the multiple factors are discussed and compared. The bubble changes the dendrite skeleton by hindering the solid growth, including entrapped in the dendrite root, tilting growth towards the neighboring bubble, forming the knot structure, and growing along the radial direction of the bubble. The bubble in the columnar dendrite array with random orientation distribution has larger effects on the average primary dendrite arm spacing. Based on the summary of different interaction results, a phase diagram is proposed to distinguish different interaction regimes. Further, the size of the closed liquid phase region is measured to evaluate the porosities tendency and guide how to reduce the porosities.

Influence of the temperature gradient and the pulling velocity on solidification cracking susceptibility during welding: A phase field study

The influence of the temperature gradient and the pulling velocity on solidification cracking susceptibility (SCS) during welding is studied by simulating the liquid channel evolution with a quantitative phase field model. Increasing the pulling velocity or decreasing the temperature gradient increases the pressure drop from the dendrite tip to the coalescence point, leading to an increase in SCS. Decreasing the primary dendrite arm spacing (PDAS) decreases the permeability of the liquid channel and the liquid channel length at the same time, resulting in a decrease in the pressure drop and SCS when the PDAS is small. Consideration of the PDAS dependency on the temperature gradient and the pulling velocity influences the value of the pressure drop but does not change the tendency of SCS. The conclusions are also valid for an alloy with strong back diffusion. As the temperature gradient and the pulling velocity within the melt pool are controlled by process parameters, the findings from this work provide a theoretical basis to optimize process parameters to avoid solidification cracking.

Solidification, α-Al morphologies, and tensile properties of low-Sc modified Al-Mg alloys

Al alloys are very attractive in engineering applications that require a combination of low density and mechanical strength, such as in aeronautical and aerospace industry devices. Therefore, the search for alloys with higher specific strength is justified from environmental and economic points of view. This work aims to study the influence of cooling rates on the phase morphology and tensile properties of ternary Al-Mg-Sc alloys, whose composition covers the spectrum of low-Sc Al-Mg-Sc alloys. The ternary Al-3, −5, and − 10 wt% Mg-0.1 wt% Sc alloys were processed by transient directional solidification. This process induces a wide range of cooling rates which causes the formation of a microstructural variety along the longitudinal cooling direction and, consequently, different tensile properties. Several characterization methods were employed, including: thermal analysis, x-ray diffraction (XRD) structural analysis, SEM-EDS, optical microscopy, CALPHAD, tensile tests, and fractography. Two major findings should be noted after fully characterizing the microstructures, the formed phases, and the tensile properties. Solidification evolution revealed that more Mg added resulted in higher solidification rates due to higher fluidity with more Mg. Furthermore, cell growth was mapped in the three alloy compositions while competing with dendrites. Solidification scaling relationships between dendritic or cellular spacing and cooling rate for all alloys were examined. Although the Al-10 wt% Mg-Sc alloy had more Mg in solid solution, the Al-5 wt% Mg-Sc alloy showed superior tensile strength due to the predominance of cells and thinner primary dendrites trunks forming its microstructure.

Processing of Bimetallic Inconel 625-16Mo3 Steel Tube via Supercritical Bend: Study of the Mechanical Properties and Structure.

Incineration is currently the standard way of disposing of municipal waste. It uses components protected by high-temperature-resistant layers of materials, such as Inconel alloys. Therefore, the objective of the current paper is to study the mechanical properties and structure of a bimetallic Inconel 625-16Mo3 steel tube. The Inconel 625 layer was 3.5 mm thick and was applied to the surface of the tube with a wall thickness of 7 mm via the cold metal transfer method. The bimetallic tube was bent using a supercritical bend (d ≤ 0.7D). This paper is focused on the investigation of the material changes in the Inconel 625 layer areas influenced by the maximum tensile and compressive stresses after the bend. The change in layer thickness after the bend was evaluated and compared to the non-deformed tube. In addition, the local mechanical properties (nanohardness, Young modulus) across the indicated interfacial areas using quasistatic nanoindentation were investigated. Subsequently, a thorough microstructure observation was carried out in areas with maximum tensile and compressive stresses to determine changes in the morphology and size of dendrites related to the effect of tensile or compressive stresses induced by bending. It was found that the grain featured a stretched secondary dendrite axis in the area of tensile stress, but compressive stress imparted a prolongation of the primary dendrite axis.