Solidification Front Velocity Research Articles

Nickel-based superalloy microstructure obtained by pulsed laser powder bed fusion

A methodology using pulsed laser powder bed fusion to produce crack-free AM parts from Superalloy was demonstrated on Inconel 718. In the as-deposited condition, columnar grains with epitaxial growth were observed. The texture was observed to be towards (100) planes. Dendritic structure with average dendrite arm spacing of 0.37±0.12μm was observed, which suggested a cooling rate in the magnitude of 106K/s. The solidification front velocity ranging from 0.17 to 0.75m/s was calculated from KGT model. This high cooling rate and rapid solidification prevented the precipitation of both γ′ and γ″, and broke down the continuous feature of eutectic phases formed from the residual liquid phase during terminal solidification. The eutectics consisted of mainly discrete Laves phases and small portion of NbC. The discrete Laves phases contributed to the good solidification behavior of the Inconel 718 build. After solution annealing and two-step aging treatment, partial recrystallization occurred and resulted in the nucleation of small equiaxed grains. Laves phase dissolved and δ phase with needle-like shape formed. Precipitates also developed through the solid-state transformation during heat treatment.

Refining a complex ZhS32-VI nickel alloy from silicon and phosphorus by unidirectional solidification of the melt at low solidification front velocities

The possibility of decreasing the silicon and phosphorus contents in a high-temperature ZhS32-VI nickel alloy by directional movement of solidification front at a velocity V = 6 mm/h has been studied. As a result, the contents of the impurities have decreased as compared to those in the starting alloy; the decrease in the silicon and phosphorus contents is from 2 to 4 and from 14 to 20 times, respectively. Thus, nonmetallic inclusions, in particular, those containing silicon and phosphorus, are moved to the top of the ingot.

Erratum to: Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Laser powder deposition was performed on a substrate of Inconel 738 using blended powders of Mar M247 and Amdry DF3 with a ratio of 4:1 for repairing purposes. In the as-deposited condition, continuous secondary phases composed of γ-Ni3B eutectics and discrete (Cr, W)B borides were observed in inter-dendritic regions, and time-dependent nucleation simulation results confirmed that (Cr, W)B was the primary secondary phase formed during rapid solidification. Supersaturated solid solution of B was detected in the γ solid solution dendritic cores. The Kurz–Giovanola–Trivedi model was performed to predict the interfacial morphology and correlate the solidification front velocity (SFV) with dendrite tip radius. It was observed from high-resolution scanning electron microscopy that the dendrite tip radius of the upper region was in the range of 15 to 30 nm, which yielded a SFV of approx 30 cm/s. The continuous growth model for solute trapping behavior developed by Aziz and Kaplan was used to determine that the effective partition coefficient of B was approximately 0.025. Finally, the feasibility of the modeling results were rationalized with the Clyne–Kurz segregation simulation of B, where Clyne–Kurz prediction using a partition coefficient of 0.025 was in good agreement with the electron probe microanalysis results.

Quantifying hydrate solidification front advancing using method of characteristics

AbstractWe develop a one‐dimensional analytical solution based on the method of characteristics to explore hydrate formation from gas injection into brine‐saturated sediments within the hydrate stability zone. Our solution includes fully coupled multiphase and multicomponent flow and the associated advective transport in a homogeneous system. Our solution shows that hydrate saturation is controlled by the initial thermodynamic state of the system and changed by the gas fractional flow. Hydrate saturation in gas‐rich systems can be estimated by when Darcy flow dominates, where cl0 is the initial mass fraction of salt in brine, and cle is the mass fraction of salt in brine at three‐phase (gas, liquid, and hydrate) equilibrium. Hydrate saturation is constant, gas saturation and gas flux decrease, and liquid saturation and liquid flux increase with the distance from the gas inlet to the hydrate solidification front. The total gas and liquid flux is constant from the gas inlet to the hydrate solidification front and decreases abruptly at the hydrate solidification front due to gas inclusion into the hydrate phase. The advancing velocity of the hydrate solidification front decreases with hydrate saturation at a fixed gas inflow rate. This analytical solution illuminates how hydrate is formed by gas injection (methane, CO2, ethane, propane) at both the laboratory and field scales.

The influence of Fe2O3 doping on the pore structure and mechanical strength of TiO2-containing alumina obtained by freeze-casting

This work investigated TiO2/Fe2O3 doped alumina prepared by the freeze-casting technique and using camphene as the solvent. Dendritic pores were formed in the TiO2 doped alumina, a structure conferred by the frozen camphene. Contrary to this trend, further Fe2O3 doping of TiO2-containing alumina resulted in the formation of non-dendritic structures. This behavior was attributed to the higher density of α-Fe2O3 (5.24gcm−3) when compared to α-Al2O3 (3.95gcm−3) and anatase TiO2 (3.89gcm−3), which reduced critical solidification front velocity, thus forming material with different pore shape. Fe2O3 doping also improved the densification of TiO2–alumina and inhibited the formation of cracks, reflected by superior mechanical strength with best results ~150% higher for 10% Fe2O3 loaded samples as compared to TiO2–alumina samples.

In-situ observation and modelling of solidification and fluid flow on GTAW process

An experimental setup is presented in order to obtain experimental data during solidification of a static weld pool after arc extinction with a GTAW process. Several devices have been set up to extract three kinds of measurements: (i) solidification front velocity (ii) fluid flow velocity at the vicinity of the front (iii) temperature field in the solid part. A high-speed camera is used to film the interface during welding at microscopic level and an infra-red in order to take the temperature field around the weld pool in the solid part. After processing and calibration of the videos, the experimental results are compared to theoritical results founded on an adapted model from the KGT [1] and from the one of Gandin et al. [2]. All the tests are done thin plate of Cu-30wt.%Ni.

On the mesoscopic description of locally nonequilibrium solidification of pure substances

A phase field model of locally nonequilibrium solidification of a supercooled melt of a pure substance has been proposed. To derive thermodynamically consistent equations of the model, the method of extended irre� versible thermodynamics has been used. It has been assumed that Gibbs potentials describing thermodynam� ically equilibrium states in terms of experimental data are known. The limit of a sharp interface of the model has been considered and an analytical dependence of the velocity of a flat solidification front on the temper� ature has been obtained.

Solidification of Highly Undercooled Hypereutectic Ni-Ni3B Alloy Melt

The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔTp ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni3B dendrite, rod eutectic, and precipitates was obtained. If ΔTp < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni3B and L → Ni/Ni3B); if ΔTp ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni23B6, Ni23B6 → Ni/Ni3B, and Lr → Ni/Ni3B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as Lr → Ni/Ni3B; it also showed that this velocity decreases with increasing ΔTr, which can be explained by reduction of the residual liquid fraction and decrease of Ni23B6 decomposition rate.

Microstructural investigation of controlled short circuiting gas metal arc welding deposited aluminium–lithium alloy

Controlled short circuiting gas metal arc welding (CSC-GMAW) was investigated as a potential solid freeform fabrication (SFF) process for AA2199. The low heat input of the CSC-GMAW process resulted in a cooling rate on the order of 840–3500°C s−1 being realised during deposition. The solidification time was then calculated to range between 2–5 ms depending on the location within the weldment. The deposited material displayed a fine (4·3±1 μm) cellular structure, comparable to that previously reported for electron beam welding. Through comparison with the Kurz–Giovanola–Trivedi (KGT) model for microstructural development during solidification, the solidification front velocity (SFV) of the CSC-GMAW process was estimated to be ∼2–4·5×10−4 m s−1. Chemical analysis revealed lateral segregation of copper to the cell walls. TOF-SIMS revealed a homogeneous lateral lithium distribution, however depth profiling displayed some extent of lithium enrichment at the surface of the deposited material.On a examiné le soudage à l’arc sous protection gazeuse à court-circuit contrôlé (CSC-GMAW) comme processus potentiel de fabrication en forme libre solide (SFF) pour l’AA2199. Le faible débit de chaleur du processus de CSC-GMAW produisait une vitesse de refroidissement de l’ordre de 840 à 3500°C/s lors du dépôt. On a ensuite calculé que le temps de solidification variait de 2 à 5 ms, dépendant de l’emplacement dans la soudure. Le dépôt montrait une structure cellulaire fine (4·3±1 μm), comparable à la structure rapportée antérieurement pour le soudage par faisceau d’électrons. Par comparaison avec le modèle de Kurz–Giavanola–Trivedi (KGT) de développement de la microstructure lors de la solidification, on a estimé que la vélocité du front de solidification (SFV) du processus de CSC-GMAW était de ∼2 à 4·5×10−4 m s−1. L’analyse chimique a révélé une ségrégation latérale du cuivre sur les parois de la cellule. TOF-SIMS a révélé une distribution latérale homogène du lithium, cependant le profilage en profondeur a exposé une certaine zone d’enrichissement en lithium à la surface du matériel déposé.

In situ transmission electron microscopy of crystal growth-mode transitions during rapid solidification of a hypoeutectic Al–Cu alloy

The rapid solidification dynamics of a pulsed-laser-melted hypoeutectic Al–Cu thin-film alloy were monitored using in situ transmission electron microscopy with high spatial and temporal resolutions. Direct observation of the solid–liquid interface during the transformation allowed measurements of the time-evolving solidification front morphology and velocity. Transitions in the growth mode were detected and related to acceleration and instability at the solidification front. A banded structure that forms during instability at high solidification front velocities, previously only observed ex situ in post-mortem analyses, was imaged during formation, providing insight to the growth mechanisms of this banded morphology.

Non-equilibrium solute partitioning in a laser re-melted Al–Li–Cu alloy

Aluminum–lithium alloy AA2199 was rapidly solidified through the application of a laser re-melting process to determine the relationship between laser pulse energy and microsegregation during solidification. It was determined that laser pulse energies of the order of 0.125–0.5Js resulted in a fine cellular solidification structure. Through comparison of the measured cell spacing with that predicted by the Kurz–Giovanola–Trivedi (KGT) model it was possible to estimate that solidification front velocities (SFV) of between 3 and 25cms−1 occurred during solidification. The SFV calculated from the KGT model was then input into the continuous growth model for solute trapping developed by Aziz to predict the deviation from equilibrium partitioning during solidification for all pulse energy levels employed. The chemical profile of lithium within the re-melted samples was measured using X-ray photoelectron spectroscopy and compared with that expected for equilibrium segregation. Measurement of the lattice parameter via X-ray diffraction revealed that the solute trapping phenomenon resulted in the formation of a super-saturated solid solution, as is evident through a reduction of the lattice parameter from 4.0485Å for the starting material to 4.0399Å in the material re-melted with a pulse energy of 0.125J.

Interfacial morphology development and solute trapping behavior during rapid solidification of an Al–Li–Cu alloy

An aluminum–lithium–copper alloy was rapidly solidified via the electrospark deposition process. High-resolution scanning electron microscopy (HR-SEM), time-of-flight secondary-ion-mass-spectroscopy (TOF-SIMS) and atom probe tomography (APT) were employed to investigate the distribution of solute within the deposited materials. The TOF-SIMS data revealed evidence that solute trapping of lithium occurred during solidification, while SEM and APT revealed the presence of fine copper-rich cells within the microstructure (∼30–60nm in width). This morphology correlated directly with the microstructural morphology predicted by the Kurz–Giovanola–Trivedi (KGT) model for microstructural development during rapid solidification. The KGT model, which can be used to describe the planar–cellular transition within a microstructure, then predicted a solidification front velocity of ∼1ms−1 being realized during electrospark deposition solidification. This SFV corroborated the chemical mapping data, and therefore supported the solute trapping hypothesis, as the continuous growth model for solute trapping as developed by Aziz and Kaplan (Acta Metallurgica 1988; 36:2335) predicts significant trapping of lithium at a SFV of 1ms−1. Finally APT revealed the presence of Al3Li phase upon the copper-rich cell walls. It was then determined that the Al3Li was not formed during solidification, as predicted by a time-dependent nucleation model for phase prediction during rapid solidification, and therefore is the result of a subsequent aging process.

Effect of magnesium content and quenching rate on the phase structure and composition of rapidly solidified La2MgNi9 metal hydride battery electrode alloy

The phase structure and composition of La2MgNi9 metal hydride battery electrode alloy after rapid solidification at wheel surface speeds of 10.5ms−1 and 4.2ms−1, have been investigated by electron microscopy (SEM, TEM) and X-ray diffraction techniques. The alloy is multi-phase structured, containing rhombohedral La2MgNi9 as the main phase with small amounts of LaNi5, La3MgNi14 and LaMgNi4 as secondary compounds. The high vapour pressure and low melting point of magnesium compared with other constituent elements and its eventual loss from the melt prior to rapid solidification, as well as the peritectic mechanism of the formation of La2MgNi9, accounts for the formation of these secondary compounds during cooling of the melt. However, optimisation of the amount of Mg in the melt and the application of low cooling rates during solidification considerably increases the yield of the La2MgNi9 phase, which is required to achieve enhanced electrochemical properties of the metal hydride battery electrode. The lower alloy solidification front velocity and temperature gradient at the quenching wheel speed of 4.2ms−1, allow sufficient time for atomic diffusion and interaction of phases during the peritectic reaction and promote the formation of a nearly-homogeneous, single phase La2MgNi9 alloy.

Solid freeform fabrication of Al–Si components via the CSC-MIG process

The use of the controlled short circuit metal inert gas (CSC-MIG) welding for freeforming 4047 simple shapes was investigating in this work. The results demonstrate that when used with appropriate processing conditions and raster path approach, the CSC-MIG can be used to fabricate component possessing a fine primary Al dendrite with dendrite arm spacing (DAS) ranging between 2·0 and 4·8 μm. It was also observed that this approach is capable of refining the Si eutectic phase to a fibrous morphology, due to the solidification front velocity exceeding the transitional value of 0·9 mm s−1. The hardness was found constant throughout the height of the freeformed component (70 HV). The bending strength was measured to be similar to as cast components, 388±35 MPa for the freeform versus 357±11 MPa for the as cast, but the ductility was doubled, from 9±1% to 18±2%, and was associated with the presence of a fibrous eutectic as opposed to the flaky morphology observed in the cast samples.L’utilisation du procédé de soudage sous gaz inerte à court-circuit contrôlé (CSC-MIG) fut étudié pour la fabrication en forme libre de piéces simples en 4047. Les résultats démontrent que lorsque soumis à des conditions de fabrication appropriées ainsi qu'un tracé régulier, le procédé CSC-MIG peut être utilisé pour fabriquer des piéces possédant des dendrites primaires en Al, avec une distance inter-dendritique variant entre 2.0 et 4.8 microns. Il fut aussi observé que cette approche est capable de raffiner la phase eutectique Si vers une morphologie fibreuse, un changement causé par une vélocité du front de solidification excédant la valeur de transition de 0.9 mm/sec. La dureté est demeurée constante sur la hauteur de la pièce faite par fabrication libre (70 HV). La résistance à la flexion mesurée est significativement similaire entre les pièces fabriquées par mise en forme libre (388±35 MPa) et celles coulées (357±11 MPa), mais la ductilité a doublé, passant de 9±1% à 18±2%, et fut associée à la présence d’une structure eutectique fibreuse, contrairement à une morphologie floconneuse observée dans les échantillons coulés.

Structural morphologies of Cu–Sn–Bi immiscible alloys with varied compositions

The macroscopic morphology of Cu–Sn–Bi immiscible alloys with compositions varying from hypomonotectic, monotectic to hypermonotectic alloys was experimentally investigated by conventional casting method. It is shown that the hypomonotectic and monotectic alloys present a core–shell morphology composed of Bi-rich core and CuSn-rich shell. In contrast, the hypermonotectic alloys, as the fraction of Bi increased, transform from a triple-layer morphology, a core–shell morphology with CuSn-rich core and Bi-rich shell, to a uniform morphology with fine CuSn-rich phase dispersing in Bi-rich matrix. Besides, the effect of velocity of solidification front on formation of core–shell morphology for the critical composition in hypermonotectic alloys was studied by casting molten alloy into molds with different radii. The relation between the volume fraction of two immiscible liquid phases and morphology was clarified for hypermonotectic alloys. The solidification paths of alloys under all compositions were discussed.

Microstructural evolution of Al-20Si-5Fe alloy during rapid solidification and hot consolidation

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400°C/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43−1.2 cm/s and 15–28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 105 K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 107 K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural refinement and modification. The size of the primary silicon decreased from approximately 1 μm to 30 nm while the formation of iron-containing intermetallic compounds was suppressed. Supersaturation of the aluminum matrix in an amount of ∼7 at.% Si was noticed from the XRD patterns. During the hot consolidation process, coarsening of the primary silicon particles and precipitation of β-Al5FeSi phase were observed. Evaluation of the compressive strength and hardness of the alloy indicated an improvement in mechanical properties due to the microstructural modification.

The effect of Mg content on microstructure in Al‐12wt. %Zn‐x Mg Alloy

The effect of adding different Mg contents to an Al‐12wt.%Zn master alloy was experimentally investigated. The Al‐Zn‐Mg alloys were unidirectionally solidified as a function of solidification parameters, temperature gradient GL, solidification front velocity V, and composition C0. The alloys were solidified with a constant temperature gradient (GL=2500K/m) in the solidification front velocity range from 4X10‐6m/s to 1.7X10‐4m/s. The resulting microstructure was characterized to investigate the effect of solidification front velocities and composition on primary dendrite arm spacing, volume percentage of eutectic in interdendritic regions and τ intermetallic phase in α‐Al matrix. Theoretical models for the dendrite arm spacing and dendrite tip radius have been compared with the experimental observations.

Modeling rapid liquid/solid and solid/liquid phase transformations in Al alloys

Abstract A recently developed thermodynamic model for describing the ‘contact conditions’ (concentrations at an interface of the two adjacent phases) is implemented in a kinetic model for liquid/solid phase transformations. The resulting integrated model describes the concentration profiles in the liquid and solid phase during the phase transformation over a large range of solidification front velocities. Examples are shown for steady state solidification in aluminum alloys. The results are in good agreement with experimental data from the literature and the well known continuous growth model by Aziz. In addition to earlier models, transient states can be described. An example is shown for a solidification front that is momentarily disturbed, i. e. set off-equilibrium. One of the results is the time for local re-equilibration at the interface which is of the order of 20 μs for solidification. First results for describing melting are also shown and discussed.

Study of solidification behavior and splat morphology of vacuum plasma sprayed Ti alloy by computational modeling and experimental results

The microstructure of Ti–6Al–4V alloy manufactured by vacuum plasma spraying consists of individual lamellae, inter-lamellae boundaries, and porosity. Mechanical properties of the as-sprayed structure depend mainly upon the solidification behavior and resultant microstructure and morphology of the individual splats and cohesion between splats. Using a three-dimensional numerical model, the cooling rate and solidification behavior of a single Ti–6Al–4V droplet (50 μm) impacting on a titanium substrate under vacuum plasma spray conditions were investigated. Results were verified with experimental observations in single splats and as-sprayed microstructures obtained by vacuum plasma sprayed form of Ti–6Al–4V alloy. The average cooling rate of a single splat obtained from the numerical simulation was on the order of 10 8 °C/s and the solidification front velocity was approximately 63 cm/s which is in the range of the rapid solidification. The thickness of the splat was calculated to be around 3 μm and the deposition efficiency was approximately 70%. These results illustrated good agreements with those obtained from experiments.

Optimization of Weld Conditions and Alloy Composition for Welding of Single-Crystal Nickel-Based Superalloys

Calculations were carried out to identify optimum welding conditions and weld alloy compositions to avoid stray grain formation during welding of single-crystal nickel-based superalloys. The calculations were performed using a combination of three models: a thermal model to describe the weld pool shape and the local thermal gradient and solidification front velocity; a geometric model to identify the local active dendrite growth variant, and a nucleation and growth model to describe the extent of stray grain formation ahead of the advancing solidification front. Optimum welding conditions (low weld power, high weld speed) were identified from the model calculations. Additional calculations were made to determine potential alloy modifications that reduce the solidification temperature range while maintaining high gamma prime content. The combination of optimum weld conditions and alloy compositions should allow for weld repair of single-crystal nickel-based superalloys without sacrificing properties or performance.