Interdendritic Liquid Research Articles

Circumventing cracking in grading 316L stainless steel to Monel400 through compositional modifications in directed energy deposition

In joining Fe-alloys and Cu-containing alloys to access the high strength of steels and corrosion resistance of Cu-alloy, cracking is widely observed due to the significant Cu microsegregation during the solidification process, resulting in an interdendritic Cu-rich liquid film at the end of solidification. By fabricating functionally graded materials (FGMs) that incorporate additional elements like Ni in the transition region between these terminal alloy classes, the hot cracking can be reduced. In the present work, the joining of stainless steel 316L (SS316L) and Monel400 by modifying the Ni concentration in the gradient region was studied. A new hot cracking criterion based on hybrid Scheil-equilibrium approach was developed and validated with monolithic multi-layer samples within the SS316L-Ni-Monel400 three-alloy system and a SS316L to 55/45 wt% SS316L/Ni to Monel400 FGM sample fabricated by direct energy deposition (DED). The new hot cracking criterion, based on the hybrid Scheil-equilibrium approach, is expected to help design FGM paths between other Fe-alloys and Cu-containing alloys as well.

A Kinetic Model for Oxide–Carbonitride Inclusion Heterogeneous Nucleation and Precipitation during Superalloy Solidification

Complex oxide–carbonitrides (MgO-Ti(CN), Al2O3-Ti(CN), and MgO·Al2O3-Ti(CN)) are the most common non-metallic inclusions presented in cast and wrought superalloys. In this work, a coupled kinetics model was proposed to predict the complex oxide–carbonitride inclusion’s precipitation behavior during the solidification of superalloys. This model takes into account thermodynamics, micro-segregation, heterogeneous nucleation in the inter-dendritic liquid, and growth controlled by the diffusion of solute elements and kinetics of interfacial reaction. The results demonstrated that both the cooling rate and nitrogen content take significant effects on the final size of complex oxide–carbonitride inclusions, as the former controls the total growth time and the latter determines the initial precipitation temperature. In comparison, the particle size of primary oxides shows a negligible impact on the final size of complex inclusions. The practice of an industrial vacuum arc remelting confirmed that the inclusion size variation predicted by the present model is reasonably consistent with the experimental results.

Eutectic Solidification Morphologies in Rapidly Solidified Hypereutectic Sn–Ag Solder Alloy

The effect of rapid solidification upon hypereutectic Sn–Ag solder alloy has been investigated using a 6.5 m drop tube. Powder sizes ranging from > 850 to < 38 μm were produced, with equivalent cooling rates of 250 to 14,800 K s−1 for 850 and 38 μm droplets, respectively. At all cooling rates investigated, dendritic β-Sn was observed as the primary solidification phase, not proeutectic Ag3Sn as predicted by the phase diagram. The volume fraction of interdendritic eutectic was observed to decrease with increasing cooling rate, with the Ag concentration in the residual interdendritic liquid estimated at 12.5–15 wt pct Ag, far in excess of the eutectic concentration of 3.8 wt pct. Much of the Ag3Sn observed within the eutectic had a blocky, divorced eutectic appearance. A model is proposed which can explain these observations in terms of sluggish nucleation of the Ag3Sn intermetallic, coupled with a metastable phase diagram that permits significant supersaturation of Ag at modest undercooling.

The morphology features, formation mechanism and elimination of channel segregation in industrial-scale Ti–Nb ingots produced by vacuum arc remelting

In this study, the morphology characteristics and formation mechanism of channel segregation in the industrial-sized Ti–Nb binary ingots produced by vacuum arc remelting (VAR) are researched based on experiments and calculations. The results show that the channel segregation is usually located at the 1/4–3/4 radius of the ingot and the morphology is closely related to the VAR process. The Ti-enriched channel segregation stripes tend to parallel to the ingot centerline and columnar dendrites in steady-state remelting region and hot-top region, respectively, while along the molten pool profile in residual molten pool region. For the first time, the Ti2O3 particles precipitated in the channel segregation were observed, following the orientation relationship of <001>β//<133>Ti2O3 and {11‾0}β//{011‾}Ti2O3 with β matrix in coherent boundary. First-principles calculations evident that the enrichment of Ti-solute tends to decrease the formation enthalpy, and hence makes for the precipitation of Ti2O3 particles. The numerical simulation and the experiment results manifest that the channel segregation originates from the convergence of interdendritic Ti-enriched liquid, resulting in the density inversion between the Ti-enriched initiation site and the surrounding liquid. The density gradient and the solidification rate dominate the growth direction and the radius distribution of the channel segregation. Based on the above, an optimized technique with helium cooling and the external stirring magnetic field was conducted and the channel segregation was eliminated successfully.

Solidification of the Ni-based superalloy CMSX-4 simulated with full complexity in 3-dimensions

In this work, we present phase-field (PF) simulations directly coupled to thermodynamic and kinetic databases in three dimensions. The direct coupling allows consideration of the full alloy complexity of the CMSX-4 superalloy over a large range of temperatures. The simulation conditions are chosen for additive manufacturing utilizing Electron Beam Melting (EBM). Transformation of interdendritic liquid into eutectic γ′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma '$$\\end{document} is considered. The simulation results confirm the unique segregation behavior of all the alloying elements. It is demonstrated that the treatment of the full complexity of alloy composition is superior to all approximations with quasi-binary or -ternary approximation and justifies the significantly increased computational effort. Our results demonstrate that multi-component simulations must become a standard for phase-field applications to real material systems.

Laser additively manufactured crack-free aluminum-bearing high entropy alloys: alloy design, synthesis, cracking inhibition and microstructure evolution effects on their tensile properties

ABSTRACT Developing high-performance high-entropy alloys (HEAs) fabricated by laser additive manufacturing (LAM) is the pursuit of the metallic community. In the present work, we designed a series of [(Al6-xNbx)-(FeCoNi)12]Cr3 HEA compositions using a high-entropy alloying strategy based on a cluster-plus-glue-atom model. And their thin-wall-sharped bulks were fabricated by LAM and post-aging treatment. The effects of cracking inhibition and microstructure evolution on the tensile properties were researched in detail. The results show that as the Nb substitutes for Al atoms, the cracking behaviour is ameliorated, ascribed to the tiny Laves phase refined the dendrite spacings and back-filled in the inter-dendritic liquid film. Also, introducing Nb atoms improves the strength but deteriorates the ductility. Significantly, the Nb4 HEA possesses the best tensile-property combination (i.e. σs ∼ 419.2 MPa, σb ∼ 787.4 MPa, and δ ∼ 15.5%) with a strain mechanism of dislocation slip mode. After post-aging for 72 h, the microstructure comprises fully recrystallized equiaxed FCC grains and many tiny needle-like D019 precipitates, leading to high strength and sufficient ductility (i.e. σ0.2 ∼ 535.9 MPa, σb ∼820 MPa and δ value of 8.9%). These findings provide a new paradigm for the LAM of crack-free HEAs with excellent mechanical properties.

Microstructure evolution and phase transformation of the mushy zone in a quenched β-solidifying TiAl alloy

This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique. The results indicate that the mushy zone consists of unmelted β dendrites and interdendritic liquid, whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent. The β dendrite is composed of numerous subgrains with various orientations. During quenching, the β dendrite transforms into Widmanstätten α via a precipitation reaction, owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid. Additionally, after quenching, the interdendritic liquid is transformed into γ plates. Within the single β phase field and the lower part of the mushy zone, a massive transformation of β to γ occurs . Conversely, in the β+α phase field, both β and α phases are retained to ambient temperature. During the heating process, the transformation of α → β gives rise to the formation of β variants, which affects the orientation of β dendrites in the mushy zone. The growth kinematics of the α → β transformation was elucidated, revealing the preferential growth directions of 〈111〉 and 〈112〉 for β variants. Furthermore, this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.

A new process for efficient purification of 6061 aluminum alloy scrap under semi-solid and super-gravity conditions

A new method for efficient and sustainable recovery of pure Al from 6061 aluminum alloy scrap based on semi-solid processing and supergravity separation was proposed in this study. Firstly, the microstructural variations of the alloy scraps after semi-solid isothermal treatment (SSIT) were investigated, and the liquid phase accumulated with a large amount of impurity elements would gradually aggregate at the grain boundaries and eventually form a dendritic network, while the spherical Al particles within the grain had high purity. Subsequently, the inter-dendritic liquid was successfully extruded from the intergranular by super-gravity separation technology and the pure Al was efficiently recovered. In this process, the effects of separation time (i.e., isothermal treatment time, t), separation temperature (i.e., isothermal treatment temperature, T) and gravity coefficient (G) on the purification effect were evaluated. The recovery rate of pure aluminum (Al ≈ 99.3 wt%) was 81.98% at the optimum conditions of t = 15 min, T = 640 °C, G = 500. In addition, it was found that increasing the separation time and temperature was beneficial to the purification effect of the alloy scrap but had a negative impact on the recovery rate. Therefore, this study has designed a new process for super-gravity purification and recovery, which realized cyclic recycling and significantly improves the recovery efficiency of aluminum scrap.

New alloy design approach to inhibiting hot cracking in laser additive manufactured nickel-based superalloys

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni11Zr9 segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni11Zr9 network layer was able to act as a “skeleton” to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties.

In-situ experimental investigations of the morphological characteristics of plumes and chimneys formed during the directional solidification of a binary alloy

The defects such as freckles formed during the directional solidification of binary alloys severely affect the performance of cast products. The phenomenon of freckle formation is strongly influenced by the convective flows generated during solidification. The rejection of solute during the solidification at the solid/liquid interface induces the concentration gradients in the liquid. These concentration gradients in combination with existing temperature gradients, may induce buoyancy-driven convection in the mushy zone (plumes) and the parent liquid region (fingers). As the mushy zone evolves, small chimneys are formed, which leads to the formation of discrete plumes of interdendritic liquid. These plumes have a very complex flow structure which leads to the structural and compositional heterogeneities in the final cast product. The current study is an effort to investigate the morphological characteristics of plumes and chimneys formed during the directional alloy solidification. For this, controlled solidification experiments in a bottom-cooled rectangular cavity with an analogue transparent aqua ammonia system (NH4Cl + 74 wt% H2O) are performed. Particle Image Velocimetry (PIV), High Speed (HS) imaging and thermocouples temperature measurement techniques are used for real-time measurement of flow field, solidified and mushy zone dynamics and local temperature during the course of solidification. The dynamical nature of plumes is captured and the morphology of chimneys (shape, size, spacings, etc.) and plumes (velocity magnitude, shape, size, etc.) are quantified. The phase-locking with 180° phase difference due to the coupling between nearby plumes is observed. Also, the evolution of double-diffusive layers (DDLs) is captured and the critical value of thermal Rayleigh number for the onset of DDLs is estimated.

Effective evaluation of permeability for interdendritic fluid flow using adaptive mesh refinement: Phase-field lattice Boltzmann study

An effective permeability prediction method was developed by applying an adaptive mesh refinement (AMR) scheme to the lattice Boltzmann (LB) simulations to compute interdendritic liquid flow. The developed method can automatically create adaptive meshes for LB simulations based on dendrite morphologies obtained by phase-field (PF) simulations. The AMR scheme drastically reduces the number of grid points and accurately computes the liquid flow around dendrites. We found that the reduction rate of the number of meshes was remarkable when using coarser meshes compared to those used in the PF simulation. This method will enable the systematic evaluation of the permeability for liquid flow in various dendritic structures.

Interfacial gradient M7C3 carbides precipitation behavior and strengthening mechanisms of stainless steel/carbon steel clad plates

The mechanical incompatibilities between ferrite and martensite at the cladding interface of stainless steel (SS)/carbon steel (CS) clad plates can result in stress concentration and interface cracks. In this paper, 304 SS/Q235 CS clad plates were prepared by liquid-solid (L-S) composite casting, and the mechanical incompatibilities between ferrite and martensite reduced by constructing gradient carbides in the interfacial ferrite. The precipitation behavior of the gradient carbides was analyzed, and the strengthening mechanism was revealed. The ultimate strength of the SS/CS clad plates was 370 MPa when stretched perpendicular to the cladding interface, and the SS/CS clad plates fractured in the CS that was far from the cladding interface. The interfacial microstructures from the CS to the SS were ferrite, M7C3 carbides, and lath martensite, and the carbides content gradually decreased toward the CS. During the L-S composite process, a micro-melting region with decreased volume fraction of interdendritic liquid to the CS formed at the cladding interface. The C, Cr, and Ni mixed and diffused in the interdendritic liquid, and the gradient distribution of elements resulted in the precipitation of coherent M7C3 carbides whose volume fraction decreased toward the CS. The coherent strengthening and precipitation strengthening of carbides strengthened the interfacial ferrite and resulted in a gradient strengthening, which reduced the mechanical incompatibilities between ferrite and martensite at the cladding interface. Additionally, the gradient carbides changed the stress state at the cladding interface and improved the interfacial bonding strength.

Probing interdendritic flow and hot tearing during solidification using real time X-ray imaging and droplet tracking

Despite the well-known importance of controlling shrinkage-induced liquid flow in alloy castings to avoid the formation of catastrophic hot tears during the final stages of solidification, there has been little direct experimental measurement of liquid metal flow and hot tear formation under practical conditions. We use synchrotron X-rays to obtain radiographic video sequences of the solidification of monotectic Al-Pb alloys in which Pb droplets form as a fine-scale emulsion. We track and measure the velocity of thousands of Pb droplets as they move through interdendritic regions due to the effect of liquid to solid shrinkage during the final stages of solidification, up to the point of hot tear formation. Based on the droplet velocities, we present an analysis to estimate the interdendritic liquid velocity as solid fraction increases, and thus the shrinkage pressure drop driving the flow. The analysis is applied for video sequences obtained for both equiaxed and columnar microstructures, each under a range of cooling rates. Our measurements of the critical shrinkage-induced pressure for hot tear formation agree well with prior model-based and theoretical suggestions. The limitations and prospects for droplet tracking measurements of liquid metal flows are discussed.

Numerical Studies on the Effects of Axial Magnetic Fields and Cooling Intensities on the Macrosegregation of Directionally Solidified Ni-Based Alloys: Elimination of Thermal Convection and Stabilization of Interdendritic Liquid Flow

Numerical Studies on the Effects of Axial Magnetic Fields and Cooling Intensities on the Macrosegregation of Directionally Solidified Ni-Based Alloys: Elimination of Thermal Convection and Stabilization of Interdendritic Liquid Flow

Cellular growth during rapid directional solidification: Insights from quantitative phase field simulations

In this paper, columnar cellular growth with kinetic effects including kinetic undercooling and solute trapping in rapid directional solidification of alloys was investigated by using a recent quantitative phase-field model for rapid solidification. Morphological transition and primary spacing selection with and without kinetic effects were numerically investigated. Numerical results show that doublon structure is an intermediate state in the primary spacing adjustment of cellular arrays. It was found that the inclusions of kinetic effects result in the increase of the solute in the solid phase and the solute enrichment in the interdendritic liquid channel. Moreover, predicted results indicate that the growth directions of the cellular arrays in rapid directional solidification with and without kinetic effects are independent of the Péclet number. Therefore, the kinetic effects play important roles in numerical simulations of the growth pattern selection and solute distribution during rapid solidification. Neglecting them will result in the inaccurately predicted results.

Phase-field study on an array of tilted columnar dendrites during the directional solidification of a binary alloy

An array of columnar dendrites is important to the microsegregation and permeability of interdendritic liquid flow. In this study, an array of tilted columnar dendrites growing during the directional solidification of a binary alloy was investigated via large-scale phase-field simulation. The main conclusion is that the hexagonal array is the dominant array for reasonably tilted dendrites as well as dendrites that grow along the temperature gradient. It is also concluded that the array ordering gradually improves with growth for all tilt angles, and the ordering rate is faster for a small tilt angle.

β Grain refinement by yttrium addition in Ti-6Al-4V Wire-Arc Additive Manufacturing

Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium parts has the potential to reduce costs in aerospace applications. However, with titanium alloys, such as Ti-6Al-4V, conventional WAAM processing conditions generally result in epitaxial solidification from the melt pool fusion boundary, which over many layers can generate coarse cm-scale,<001>//ND fibre textured, columnar β grain structures within the deposited metal. The mechanical anisotropy caused by this coarse primary grain structure cannot be eliminated by subsequent solid-state phase transformations. In order to attempt to refine the size of the solidified β-grains and reduce their strong texture, the growth restriction efficiency of low addition levels of the strongly partitioning element (k = 0.1) yttrium (Y) has been investigated. Less than 0.8 wt.% Y was sufficient to reduce the widths of the solidified columnar β grains from 1 to 2 mm to 100–300 µm. Y was also found to induce a columnar-to-equiaxed transition (CET) in the latter stages of melt pool solidification, which benefits from a lower liquid thermal gradient and higher solidification velocity. Inter-dendritic segregation of Y was also found to be significant and oxygen scavenging led to the formation of Y2O3 particles in the inter-dendritic liquid, with a previously unreported irregular eutectic morphology. High-resolution EBSD analysis showed these particles exhibited specific orientation relationships with the solidified β grains, which were confirmed experimentally.

Quantifying the effects of cooling rates and alloying additions on the microporosity formation in Al alloys

Microporosity in aluminum alloy castings is detrimental to the final mechanical properties such as fatigue life. Previous studies have shown that alloying additions in sand casting have a large impact on the percentage of porosity but no systematic studies on the effects of cooling rates and alloying additions have been reported. In this study, we investigate the porosity formation in three alloys with different cooling conditions using X-ray computed tomography (XCT). The microporosities of the three aluminum alloys at different cooling rates were visualized and statistically analyzed. When the cooling rate increased from 0.7 to 4.2 °C/s, the average equivalent diameter of microporosity in Al-7.3Si-1.5Cu decreased from 149.8 μm to 61.3 μm, with a size reduction of about 60 %; At a cooling rate of 0.7 °C/s, the average equivalent diameter of microporosity increased nearly three times with the increase of Si content to 7.3 wt.%. Comparing to three-dimensional (3D) cellular automata (CA) model predictions of porosity evolution in Al-Cu and Al-Si-Cu alloys, it was found that high cooling rates can increase the nucleation rate of microporosity and adding Cu and Si will give rise to the low solubility of hydrogen in the interdendritic liquid region during solidification. Therefore, robust casting components can be achieved by controlling the chemistry and applying non-equilibrium solidification conditions which change the interdendritic liquid flow and hydrogen supersaturations.

Improving mechanical properties of wire arc additively manufactured AA2196 Al–Li alloy by controlling solidification defects

Additive manufacturing has advantages of cutting the time from design to making new parts but it suffers from metallurgical defects such as porosity. For example, wired arc additive manufacturing(WAAM) of new alloys such as Al–Li alloys is very challenging due to the fact that the porosity level is so high as to degrade not only fatigue life but also the static mechanical properties. In this study, we found that large chains of porosity could form at the interdendritic liquid during WAAM of an AA2196 Al–Li alloy. After T6 heat treatment, those chains of porosity actually grow bigger than the as-deposit state, with the microporosity size over 50 µm increasing from 2.8% to 5.8% and the maximum size grow up to 107 µm. Using X-ray Computed Tomography (XCT), we have quantified the spatial distribution of microporosity as a function of hot deformation after WAAM. It was found that the chain of microporosity along the direction of deposition could be effectively eliminated by controlling the strain. Combining the 42% hot deformation and T6 heat treatment, the closure of microporosity together with nano-sized T1 precipitates can be achieved and yield tensile strength (YTS) can be improved by 199%, the ultimate tensile strength (UTS) can be increased by 168%, and the elongation can be extended by 460%, reaching up to 372 MPa, 439 MPa, and 6.9%, respectively. Therefore, the solidification defects during WAAM can be remediated by applying proper hot deformation which is extremely useful for the Al–Li alloys with a long solidification range and high susceptibility to solidification cracking.

In-situ evidence for impurity-induced formation of eutectic colonies in an interdendritic liquid

Eutectic colony formation in near-eutectic alloys is often attributed to independent nucleation of eutectic grains on the surface of primary dendrites. In this paper, we report an in-situ study of eutectic solidification in hypoeutectic Zn–Al alloys using synchrotron X-radiography. While the α-Al/η-Zn eutectic is observed to grow from a single nucleus in the interdendritic liquid, a cellular instability at the eutectic front is observed for the first time. The cellular instability is induced by an impurity dissolved in the liquid, while a flow effect is negligible. This observation confirms an alternative mechanism of eutectic colony formation.