Dendritic Solidification Research Articles

Influence of the Heat Input on the Dendritic Solidification Structure and the Mechanical Properties of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal

The alloy 2.25Cr-1Mo-0.25V is commonly used for heavy wall pressure vessels in the petrochemical industry, such as hydrogen reactors. As these reactors are operated at elevated temperatures and high pressures, the 2.25Cr-1Mo-0.25V welding consumables require a beneficial combination of strength and toughness as well as enhanced creep properties. The mechanical properties are known to be influenced by several welding parameters. This study deals with the influence of the heat input during submerged-arc welding (SAW) on the solidification structure and mechanical properties of 2.25Cr-1Mo-0.25V multilayer metal. The heat input was found to increase the primary and secondary dendrite spacing as well as the bainitic and prior austenite grain size of the weld metal. Furthermore, it was determined that a higher heat input during SAW causes an increase in the stress rupture time and a decrease in Charpy impact energy. This is assumed to be linked to a lower number of weld layers, and therefore, a decreased amount of fine grained reheated zone if the multilayer weld metal is fabricated with higher heat input. In contrast to the stress rupture time and the toughness, the weld metal’s strength, ductility and macro-hardness remain nearly unaffected by changes of the heat input.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

Multicomponent dendrite growth is theoretically predicted to optimize solidification cracking susceptibility during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification. The distribution of dendrite trunk spacing along the weld pool solidification interface is clearly symmetrical about the weld pool centerline in beneficial (001)/[100] welding configuration. The distribution of dendrite trunk spacing along the weld pool solidification interface is crystallography-dependent asymmetrical from bottom to top surface of the weld pool in detrimental (001)/[110] welding configuration. The smaller heat input is used, the finer dendrite trunk spacing is kinetically promoted by less solute enrichment and narrower constitutional undercooling ahead of solid/liquid interface with mitigation of metallurgical contributing factors for solidification cracking and vice versa. Vulnerable [100] dendrite growth region is predominantly suppressed and epitaxial [001] dendrite growth region is favored to spontaneously facilitate single-crystal columnar dendrite growth and reduce microstructure anomalies with further reduction of heat input. Optimum low heat input (both lower laser power and higher welding speed) with (001)/[100] welding configuration is the most favorable one to avoid nucleation and growth of stray grain formation, minimize both dendrite trunk spacing and solidification cracking susceptibility potential, improve resistance to solidification cracking, and ameliorate weldability and weld integrity through microstructure modification instead of inappropriate high heat input (both higher laser power and slower welding speed) with (001)/[110] welding configuration. The dendrite trunk spacing in the [100] dendrite growth region on the right side of the weld pool is considerably coarser and grows faster than that within the [010] dendrite growth region of the left side in the (001)/[110] welding configuration to deteriorate weldability, although the welding conditions are the same on the either side. Furthermore, the alternative mechanism of crystallography-dependent solidification cracking as consequence of asymmetrical microstructure development and diffusion-controlled dendrite growth of γ phase is therefore proposed. The theoretical predictions are comparable with experiment results. The reliable model is also useful for welding conditions optimization for crack-free laser processing.

Quantitative phase field simulations of polycrystalline solidification using a vector order parameter.

A vector order parameter phase field model derived from a grand potential functional is presented as an alternative approach for modeling polycrystalline solidification of alloys. In this approach, the grand potential density is designed to contain a discrete set of finite wells, a feature that naturally allows for the growth and controlled interaction of multiple grains using a single vector field. We verify that dendritic solidification in binary alloys follows the well-established quantitative behavior in the thin interface limit. In addition, it is shown that grain boundary energy and solute back-diffusion are quantitatively consistent with earlier theoretical work, with grain boundary energy being controlled through a simple solid-solid interaction parameter. Moreover, when considering polycrystalline aggregates and their coarsening, we show that the kinetics follow the expected parabolic growth law. Finally, we demonstrate how this vector order parameter model can be used to describe nucleation in polycrystalline systems via thermal fluctuations of the vector order parameter, a process that cannot be treated consistently with multiphase or multi-order-parameter based phase field models. The presented vector order parameter model serves as a practical and efficient computational tool for simulating polycrystalline materials. We also discuss the extension of the order parameter to higher dimensions as a simple method for modeling multiple solid phases.

Dimension effect of Y2O3 nanomaterial on microstructure and tensile properties of laser metal deposited stainless steel coatings

Yttrium (Y) is an engineering material used in refinement, and the physical properties of Y 2 O 3 nanomaterials play key roles in thermal-effect of refinement. In this study, the experimental thermal-efficiencies induced by dimension of Y 2 O 3 nanomaterials and their influence on microstructures and tensile performances of graded stainless steel coatings were investigated. To this end, 16Cr2NiSiMo stainless steel coatings were fabricated by laser metal deposition (LMD) technology in the presence of small amounts (<1 wt%) of Y 2 O 3 nanoparticles (diameter 30–50 nm) and Y 2 O 3 nanosheets (thickness 20–30 nm) without additional heat-treatment. Compared to Y 2 O 3 nanosheets-modified coatings, more decline in the interlayer heat-affected zone, dendritic solidification microstructure, pore, microcrack , and taenite phase were all observed for coatings prepared in the presence of Y 2 O 3 nanoparticles. The reason for this difference had to do with the dimension of Y 2 O 3 nanomaterial , which directly influenced the dissociation of Y 3+ ions in dendrites by decreasing the melting point of the material and transition temperature of the metastable eutectic . As a result, Y 2 O 3 nanoparticles promoted the hardness and tensile properties of the coatings, while Y 2 O 3 nanosheets increased microcracks and pores but declined the tensile properties. The ultimate tensile strength and elongation of the sample prepared by adding 0.75 wt% Y 2 O 3 nanoparticles enhanced by 17% and 47.9%, respectively. • Nano-Y 2 O 3 declines ILHIZ of coating without additional heat treatment. • Nanoparticles are more effective than nanosheets of Y 2 O 3 in refinement effect. • Nanoparticles decline dendrite with gradient orientation above ILHIZ. • Nanosheets refine graded dendrites into cellular dendrites with metastable eutectic. • Y 2 O 3 nanosheets increase microcracks and decline tensile properties of coatings.

An In Situ High‐Energy Synchrotron X‐Ray Diffraction Study of Directional Solidification in Binary TiAl Alloys

Phase formation and microstructure selection during solidification in γ‐TiAl alloys are highly relevant, both with respect to the microstructure and texture of cast alloys and to directional solidification, which allows to obtain a unique combination of properties. However, during cooling to room temperature, the solidifying phases transform to low‐temperature phases, which mask the prior situation during solidification. A new inductive crucible‐free zone melting furnace is used in this work, which is specially designed for in situ investigations of solidification using synchrotron radiation. Herein, alloys with 45 and 48 at% Al are studied, and it can be shown that with varying withdrawal rate, a change from α solidification to synchronous β + α solidification occurs in Ti–48Al, as it is predicted in the literature. Furthermore, it is observed that by increasing the withdrawal rate, a preferred &lt;001&gt; growth of the β phase occurs in alloys with 45 at% Al, which is interpreted as a transition to dendritic solidification. The observations are compared with a microstructure selection map calculated according to the nucleation and constitutional undercooling (NCU) model proposed in the literature.

In situ synchrotron diffraction and modeling of non-equilibrium solidification of a MnFeCoNiCu alloy

The solidification mechanism and segregation behavior of laser-melted Mn35Fe5Co20Ni20Cu20 was firstly investigated via in situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn35Fe5Co20Ni20Cu20 solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3 to 65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.

Laser metal deposited steel alloys with uniform microstructures and improved properties prepared by addition of small amounts of dispersed Y2O3 nanoparticles

Abstract Steel is the most common engineering material used in additive manufacturing. In this work, 100-layers wall structures of 17Cr2NiSi steel alloys were successfully fabricated by laser metal deposition (LMD) in the presence of small dispersed amounts of Y2O3 nanoparticles without additional heat treatment. The effects of added amounts of Y2O3 nanoparticles on the microstructure, formability, microhardness, and tensile properties were all investigated. The results indicated a decline in many features, including formation of columnar and dendrite solidification structures, elongated grains in the build direction, pronounced crystallographic textures, and interlayer heat-affected zone. The reason for this was associated with the dissociation effect of Y3+ ions on dendrites by decreasing the melting point of the material, combined with the low-energy heterogeneous nucleation effect of Y elements on the grains through segregation into Si–Y–O particles. Meanwhile, minor amounts of Y element (

Correlative Synchrotron X-ray Imaging and Diffraction of Directed Energy Deposition Additive Manufacturing

The governing mechanistic behaviour of Directed Energy Deposition Additive Manufacturing (DED-AM) is revealed by a combined in situ and operando synchrotron X-ray imaging and diffraction study of a nickel-base superalloy, IN718. Using a unique DED-AM process replicator, real-space imaging enables quantification of the melt-pool boundary and flow dynamics during solidification. This imaging knowledge was also used to inform precise diffraction measurements of temporally resolved microstructural phases during transformation and stress development with a spatial resolution of 100 µm. The diffraction quantified thermal gradient enabled a dendritic solidification microstructure to be predicted and coupled to the stress state. The fast cooling rate entirely suppressed the formation of secondary phases or recrystallisation in the solid-state. Upon solidification, the stresses rapidly increase to the yield strength during cooling. This insight, combined with the large solidification range of IN718 suggests that the accumulated plasticity exhausts the ductility of the alloy, causing liquation cracking. This study has revealed the mechanisms that govern the formation of highly non-equilibrium microstructures during DED-AM.

Quantitative three-dimensional phase-field modeling of dendritic solidification coupled with local ensemble transform Kalman filter

Prediction of dendrite growth during alloy solidification is important for controlling the microstructures and mechanical properties of alloys. To this end, quantitative phase-field (QPF) models have been developed to accurately predict dendrite growth. However, a comprehensive method to calibrate the parameters of the QPF models has not been established. In this study, QPF modeling of dendritic solidification coupled with a data assimilation method based on the local ensemble transform Kalman filter (LETKF) is proposed. This modeling could simultaneously estimate multiple parameters and improve the prediction of dendrite growth. The modeling was applied to simulations of isothermal dendritic solidification in Fe–C–Mn ternary alloys. The results show that the solid–liquid interfacial energy and diffusion coefficients of solute atoms in a liquid phase can be estimated from the shapes of the growing dendrites. The accuracy of parameter estimation depends on the growth rate of the primary dendrite arm instead of the dendrite shape. This study demonstrates that LETKF-based data assimilation improves the prediction of solute concentration fields. The proposed QPF modeling paves a promising way to improve the prediction accuracy of conventional QPF models and derive new knowledge from the experimental data obtained by in-situ observations of growing dendrites.

The local tangential lifting method for moving interface problems on surfaces with applications

In this paper, a new numerical computational frame is presented for solving moving interface problems modeled by parabolic PDEs on static and evolving surfaces. The surface PDEs can have Dirac delta source distributions and discontinuous coefficients. One application is for thermally driven moving interfaces on surfaces such as Stefan problems and dendritic solidification phenomena on solid surfaces. One novelty of the new method is the local tangential lifting method to construct discrete delta functions on surfaces. The idea of the local tangential lifting method is to transform a local surface problem to a local two dimensional one on the tangent planes of surfaces at some selected surface nodes. Moreover, a surface version of the front tracking method is developed to track moving interfaces on surfaces. Strategies have been developed for computing geodesic curvatures of interfaces on surfaces. Various numerical examples are presented to demonstrate the accuracy of the new methods. It is also interesting to see the comparison of the dendritic solidification processes in two dimensional spaces and on surfaces.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-based Single-crystal Superalloy Part IV: Welding Conditions Optimization

The thermal-metallurgical modeling of columnar/equiaxed transition(CET), multicomponent dendrite growth and nonequilibrium solidification behavior was further developed during single-crystal superalloy weld pool solidification over a wide range of laser welding conditions (laser power, welding speed and welding configuration). It facilitates the unprecedent understanding of the effect of welding conditions on overall stray grain formation, dendrite trunk spacing and solidification temperature range throughout the weld pool. In order to eliminate the solidification cracking through microstructure control, it is imperative to optimize the welding conditions for successful defect-free laser welding. Crystallographic orientation of dendrite growth plays an important role than heat input in microstructure development and solidification cracking susceptibility. Optimum low heat inputs (low laser power and high welding speed) of solidification condition simultaneously prevent stray grain formation, refine dendrite size and mitigate the solidification temperature range to improve the weldability and weld integrity and vice versa. Nonequilibrium solidification behavior of the weld pool suppresses the solidification temperature range and dendrite size, and improves the solidification cracking resistance. The overall stray grain formation, dendrite trunk spacing and solidification temperature range in (001) and [100] welding configuration are beneficially reduced than that of in (001) and [110] welding configuration regardless of heat input. The theoretical predictions agree well with the experiment results. Moreover, this promising model is also available to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

Numerical Analysis of Aerospace Nickel-Based Single-Crystal Superalloy Weldability Part III: Solidification Cracking Susceptibility

The thermal-metallurgical model of primary γ gamma phase through couple of heat transfer model, dendrite selection model, columnar/equiaxed transition (CET) model, multicomponent dendrite growth model and nonequilibrium solidification model is further developed on the basis of criteria of minimum growth velocity, constitutional undercooling and marginal stability of planar front during nickel-based single-crystal weld pool solidification. It is clearly indicated that crystallographic orientation plays more important role than heat input in microstructure development and solidification behavior. The dendrite trunk spacing and solidification temperature range along the solid/liquid interface are symmetrically distributed about the weld pool centerline in (001) and [100] welding configuration, while they are asymmetrically distributed in (001) and [110] welding configuration. The dendrite size and solidification temperature range are beneficially smaller in (001) and [100] welding configuration than that of (001) and [110] welding configuration regardless of heat input. The mechanism of asymmetrical solidification cracking because of crystallography-dependent growth kinetics and solidification behavior is proposed. Optimum low heat input (low laser power and high welding speed) refines dendrite size and suppresses the solidification temperature range to minimize the solidification cracking susceptibility and ameliorate the weldability through microstructure control, while high heat input (high laser power and low welding speed) deteriorates the weldability and weld integrity. It is therefore imperative to optimize the welding conditions for successful defect-free laser welding. Moreover, the promising theoretical predictions agree well with the experiment results. The useful model is also applicable to other single-crystal superalloys with similar metallurgical properties by laser welding or laser cladding, and provide a more accurate and reliable way of solidification cracking susceptibility evaluation.

Interface kinetics modeling of binary alloy solidification by considering correlation between thermodynamics and kinetics

Abstract By considering collision-limited growth mode and short-range diffusion-limited growth mode simultaneously, an extended kinetic model for solid−liquid interface with varied kinetic prefactor was developed for binary alloys. Four potential correlations arising from effective kinetics coupling the two growth modes were proposed and studied by application to planar interface migration and dendritic solidification, where the linear correlation between the effective thermodynamic driving force and the effective kinetic energy barrier seems physically realistic. A better agreement between the results of free dendritic growth model and the available experiment data for Ni−0.7at.%B alloy was obtained based on correlation between the thermodynamics and kinetics. As compared to previous models assuming constant kinetic prefactor, a common phenomenon occurring at relatively low undercoolings, i.e. the interface migration slowdown, can be ascribed to both the thermodynamic and the kinetic factors. By considering universality of the correlation between the thermodynamics and kinetics, it is concluded that the correlation should be considered to model the interface kinetics in alloy solidification.

Structural Factors and Electrochemical Behaviors of 3D Architected Carbon Electrodes

Cycling of lithium-ion batteries composed of intercalation materials and electrolyte involves interfacial reactions and transports, governed by differences in ion concentrations, currents, and potentials. Those electrochemical behaviors have been investigated by characterizing simplified electrode structure such as thin film and single particle, and bridging the knowledge toward commercial batteries with modeling and simulations. However, the stochastic structure of slurry electrodes commonly used in commercial batteries makes it hard to investigate local environments in the battery system and understand relations between electrochemical factors and resulting phenomena such as solid electrolyte interface formation and lithium dendrite formations at local sites. Here, we address this challenge by developing deterministic 3D architected carbon electrode with micron-to-centimeter form factors and mechanical integrity, which enables us to prescribe the electrode architecture with the consideration of distribution of ion concentrations, currents and potentials and investigate electrochemical responses in battery operation and then conduct post-characterization of the electrode.The 3D architected carbon electrodes were fabricated by the combination of direct light processing (DLP) 3D printing and pyrolysis process. The uniform shrinkage of 3D printed UV-cured polymer during pyrolysis led to the fabrication of 3D architected carbon with the prescribed structure. The electrode is fully controllable in micron-to-centimeter length scales to prescribe porous structure, electrode thickness and relative position to the counter electrode.To demonstrate the 3D architected carbon as an electrode of lithium ion batteries, we constructed a half-cell with beam-based 3D architected carbon (Figure 1) using a 2032 coin cell. To compare effects of structural factors, we chose to vary two structural factors independently, either sample thickness or beam diameter, which directly relate to diffusion length in electrolyte or electrode and locked all other parameters. Those structural factors (i.e. beam diameter and thickness) were varied in the estimated electrode diffusion-limit regime. The galvanostatic cycling at different current densities showed improved rate performance as decreasing electrode diffusion length, which agreed with the estimation that electrode diffusion was the rate-limiting process. In the regime, furthermore, slurry electrodes made by pulverizing 3D carbon electrode into particles showed similar rate performance to the 3D architected carbon electrode for the same mass loadings. These results also indicate that non-tortuous porous structure of the 3D architected carbon did not take advantage of effective lithium ion transport trajectories in electrolyte because of the electrode diffusion-limiting regime.In addition, we used the 3D architected carbon electrode to investigate solid electrolyte interface (SEI) variation in a cell scale. The deterministic 3D architecture can prescribe local ion concentrations in the porous structure and its mechanical integrity enables us to investigate SEI on the 3D architected carbon electrode at different locations as post-characterization in nanoscale. This multi-scale tunable 3D architected electrode with mechanical integrity enables to elucidate relations between structural factors of battery electrodes and electrochemical behaviors, which may lead to designing rational battery electrode in multi-scales.

Modeling of gas porosity and microstructure formation during dendritic and eutectic solidification of ternary Al-Si-Mg alloys

A two-dimensional (2-D) multi-component and multi-phase cellular automaton (CA) model coupled with the Calphad method and finite difference method (FDM) is proposed to simulate the gas pore formation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys. In this model, the pore growth, and dendritic and eutectic solidification are simulated using a CA technique. To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification, the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method. The diffusion equations of hydrogen and two solutes are solved by FDM. The developed CA-FDM coupled model can be used for simulating the evolution of gas microporosity and microstructures, involving dendrites and irregular binary and ternary eutectics, of ternary hypoeutectic Al-Si-Mg alloys. It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics, the competitive growth among the growing gas pores of different sizes, together with the time-evolving concentration fields of hydrogen and solutes. The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation. The influences of the initial hydrogen concentration and cooling rate on the microporosity formation are investigated. It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time. The varying features of simulated porosity percentage, the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores, while the size of gas pores will significantly reduce with increasing cooling rate, leading to a lower final porosity percentage.

Effect of the laser power on the microstructure and mechanical properties of the laser-MIG hybrid welding joints of the 2195 Al–Li alloy

ABSTRACT In the present study, 2195 aluminium–lithium (Al–Li) alloy joints were welded by laser-metal inert gas (laser-MIG) hybrid welding. The effect of different laser powers (700, 1000, and 1300 W) on the microstructures and mechanical properties of the weld joints was investigated. The dendritic solidification structure of the weld joint comprised the α-Al, θ-(Al2Cu), and T-(AlLiSi) phases. When the laser power increased, the grains coarsened, and the amount of the precipitated phase decreased. Consequently, the micro-hardness of the weld decreased. The heat input also increased with the increase in laser power. This increased the fluidity and solidification time of the liquid metal at the bottom of the molten pool. Therefore, the deposition of the particles of the refractory metal compounds and the formation of the equiaxed grain zone (EQZ) was hindered.

Flow-solidification interaction: A numerical study on solidification of NH4Cl – 70 wt.%H2O solution in a water-cooled mould with a large sample thickness

This study focuses on the flow-solidification interaction during the solidification of NH4Cl – 70.43 wt.%H2O solution in a water-cooled mould with a large sample thickness (100 × 80 × 10 mm3). First part of the paper is to use a volume-average-based dendritic solidification model to reproduce an experiment in which transient flow is monitored in-situ using particle image velocimetry, and the solidification sequence is recorded with a light camera. The striking features of the solidification experiment are that the crystal morphology is dominant in a columnar structure, and four typical flow regimes appear in the bulk solution during cooling/solidification, namely pure thermal convection, unstable turbulent flow caused by double diffusive convection, turbulent-stratified flow, and meandering flow. The results show that there is quantitative agreement between the simulation and experiment results. The focus of this paper (second part) is to study the flow-solidification interaction by correlating the above flow regimes with the solidification quantities in the mushy zone. Owing to the enhanced mass (solute) and energy transport by advection, flow suppresses the growth of the mushy zone, while speeding up the solidification inside the mushy zone, leading to the formation of dense mush and massive columnar dendrite trunks. The crystalline morphology of NH4Cl, as presented by a dendritic envelope and described by morphological parameters, is an important factor that influences the flow; as a feedback, the flow further influences the solidification inside the mushy zone. The numerical model also provides valuable information about the formation of freckle trails.

Phase-field simulations of dendritic morphologies in hot-dip galvanized Zn-Al coatings

The growth morphologies of dendrites in thin Zn-0.2 wt% Al coating layers are investigated as a function of the inclination axis and angle of the Zn basal plane with respect to the coating plane using three-dimensional (3D) phase-field simulations under realistic process conditions. In addition to the well-known growth direction families of 〈101¯0〉 and 〈0001〉, we incorporate a third preferred growth direction family into interface kinetic anisotropy based on the recent suggestion by Kim et al. [Metall. Mater. Trans. A 50 (2019), 3186–3200]. When this interface kinetic anisotropy is combined with the isotropic interface energy in the phase-field model, the 3D simulations realistically reproduce most of the morphological characteristics of the dendrites observed in experiments. These include the asymmetric morphologies about the inclination axis, morphological changes with the inclination angle, and angular variations between the primary aims. However, simulations using the experimental estimates of the interface energy anisotropy fail to reproduce characteristic morphologies, such as the four- and eight-fold dendrites, even when combined with strong interface kinetic effects. The present study establishes the existence of a third preferred growth direction family, which is close to the directions normal to the 12¯11planes, and the importance of the interface kinetic anisotropy over the interface energy anisotropy in the dendritic solidification of Zn-rich alloys.

Correlative Synchrotron X-Ray Imaging and Diffraction of Directed Energy Deposition Additive Manufacturing

The governing mechanistic behaviour of Directed Energy Deposition Additive Manufacturing (DED-AM) is revealed by a combined in situ and operando synchrotron X-ray imaging and diffraction study of a nickel-base superalloy, IN718. Using a unique process replicator, real-space phase-contrast imaging enables quantification of the melt-pool boundary and flow dynamics during solidification. This imaging knowledge informed precise diffraction measurements of temporally resolved microstructural phases with unprecedented precision during transformation and stress development with a spatial resolution of 100 µm. The diffraction quantified thermal gradient enabled a dendritic solidification microstructure to be predicted and coupled to the stress orientation and magnitude. The fast cooling rate entirely suppressed the formation of secondary phases or recrystallisation in the solid-state. Upon solidification, the stresses rapidly increase to the yield strength during cooling. This insight, combined with IN718’s large solidification range suggests that the accumulated plasticity exhausts the alloy’s ductility, causing liquation cracking. This study has revealed additional fundamental mechanisms governing the formation of highly non-equilibrium microstructures during DED-AM.

On the mechanism responsible for unconventional thermal behaviour during freezing

Abstract